Effects of Protein Level and Forage Source on Milk Production and Composition in Early Lactation Dairy Cows' C. A. ZIMMERMAN, A. H. RAKES, R. D. JAQUETTE.*
B. A. HOPKINS. and W. J. CROOM, JR. Depkment of Animal Science North Carolina State University Raleigh 276957621
ABSTRACT
rumen acetate:propionate ratios were unaffected by treatment. Lowering fiber level resulted in an increased milk CP percentage regardless of treatment. Grass hay appeared to be more effective than alfalfa hay in preventing depression in milk fat test upon the change to a low fiber diet. (Key words: crude protein, milk fat depression, low fiber)
Twenty multiparous and 12 primiparous Holstein cows were assigned at calving to one of three grass hay-based diets containing either 14, 18, or 22% CP or an alfalfa hay-based diet containing 22% CP to examine the effect of protein level and forage source on milk yield and composition. The diets contained 23% ADF during wk 1 to 4 postpartum, which was lowered to 11% for wk 5 to 12 postpartum. Cows fed the 18 and 22% CP grassbased diets produced higher yields of milk, 4% FCM, fat, protein, and SNF than those fed the 14% CP diet during the high fiber period. In addition, cows fed the 22% CP grass-based diet had higher milk fat tests than those fed the 14%CP diet during the high fiber period, due primarily to an increase in short-chain fatty acid synthesis. Milk fat depression was more severe when cows were changed to low fiber diets while fed the 22% CP alfalfa-based diet than when fed the 22% CP grass-based diet. Depression in milk fat content was 15.0, 17.0, 15.6, and 27.0%for 14, 18, and 22% CP grassbased and 22% CP alfalfa-based diets, respectively. Cows receiving the 18 and 22% CP grass-based diets exhibited higher blood NEFA during the high fiber feeding period than those fed the 14% CP diet. After fiber was lowered, changes in
Abbreviation key: HF = high fiber, HPA = high protein alfalfa hay diet, HPG = high protein grass hay diet, LCFA = longxhain fatty acids, LF = low fiber, LPG = low protein grass hay diet, MPG = medium protein grass hay diet, PUN = plasma urea nitrogen, SCFA = short-chain fatty acids, VLDL = very low density lipoproteins. INTRODUCTION
Received March 8. 1990. Accepted October 5, 1990. 'Paper Number 12577 of the Journal Series of the North Carolina Agricultural Research Service,Raleigh, NC 27695-7621. The use of trade names m this publication does Wt imply endOrsaent by the North Ci~olinaA g i i tural Research Service of the products named nor criticism of similar ones not mentioned. 'fiesent address: central soya, Decatur. IN 46733. 1991 J Dairy Sci 74:98&990
High producing cows in early lactation, in an attempt to meet their energy requirements, often are not fed enough effective fiber to maintain milk fat tests. This is of particular concern in the southeastern US where high ambient temperatures and low forage supplies contribute to this problem. Numerous studies have shown that feerling high levels of dietary CP can partially alleviate the problem of milk fat depression caused by feeding low fiber diets (8.9, 10, 11,12,15). High CP rations also have been shown to increase milk fat test in some studies on diets adequate in fiber (6, 20, 22, 28). Forage type could alter the effectiveness of high dietary CP in alleviating milk fat depression. Alfalfa or alfalfa-grass hay mixtures were the main forages used in most experiments to examine the effects of dietary CP on milk fat depression. However, in one study (9) corn silage replaced alfalfa hay. Although still present, the CP effect was less with corn silage than
980
D m A R Y PROTEIN AND MILK FAT DEPRESSION
with alfalfa hay-based diets. Protein content of the forage should have a relatively minor effect on the CP response because forage CP, especially on the low fiber diets, contri’butes only a small portion of the total dietary CP. The high CP diets used in previous studies at our university have ranged from 21 to 24% CP (8,9, 10). The objectives of our study were to examine the effect of forage source (alfalfa versus grass hays) and an intermediate level of dietary CP on milk fat synthesis when daj. cattle were fed a high fiber diet and then switched to a low fiber diet. MATERIALS AND METHODS
Twenty multiparous and 12 primiparous cows were assigned randomly to one of four dietary treatments at calving. Three diets utilized orchardgrass or endophyte-free tall fescue as the forage source and contained protein percentages of 1) 14% low protein grass hay diet (LPG), 2) 18% medium protein grass hay diet (MPG), and 3) 22% high protein grass hay diet (HPG). The fourth diet was 22% CP with alfalfa hay as the forage some, a high protein alfalfa hay diet (HPA). Tall fescue was substituted for orchardgrass during the study due to inadequate supplies of orchardgrass. Tall fescue was selected as a substitute forage because of its compositional similarity to orchardgrass. The fescue hay was Johnstone variety, which is low in perloline alkaloids in addition to being endophyte-free. All hays were chopped in a bedding chopper to similar lengths (10 to 15 cm). After calving, cows were weighed on 2 consecutive d and gradually adjusted to a diet consisting of their respective chopped hay and concentrate. Hay and concentrate were fed simultaneously but not mixed and fed for 5% refusal. Concentrate and hay were fed at a ratio to maintain the ration ADF at 11.5%. All cows were fed high fiber (23% ADF) diets during wk 1 to 4 postpartum and gradually switched in five equal increments over a 5 4 period to low fiber (11.5% ADF) diets for wk 5 to 12 postpartum. Cows remained on their respective hay and CP rations for both the high fiber (HF) and low fiber (LF) periods. Hay samples were taken at the beginning of the trial, and concentrates were sampled biweekly. Chemical analyses of hays and concentrates are listed in Table
981
1. Nutrient compositions of experimental diets are in Table 2. Throughout the study, cows were housed in a tie stall barn and milked twice daily; milk weights were recorded at each milking. Cows received exercise on a dirt lot for 2 h daily. Cows were fed at 0800 and 1400 h, and individual feed intakes were recorded daily. Body weights were recorded on 2 consecutive d weekly throughout the study. Milk samples were collected at a.m. and p.m. milkings for 6 d during wk 3 to 4 of the €IF period and twice weekly during the LF period. A composite sample was taken from the am. and p.m. milk samples and analyzed for fat by the Babcock method, for CP by a dye-binding assay (29), and for SNF using a Watson lactometer (A. Daigger and Company, Richmond, CA). Jugular blood samples, collected by venipuncture, and rumen fluid samples, collected via stomach pump, were taken 5 h after the a.m. feeding twice weekly during wk 3 to 4 of the J3F period and once weekly throughout the LF period. Plasma was analyzed for urea N using a phenol-hypochlorite procedure (3) and for glucose with a YSI Model 27 Industrial Analyzer, using membrane-immobilized glucose oxidase (Yellow Springs Instrument Co., Yellow Springs, OH). Nonesterifid fatty acids were determined using an enzymaticcolorimetric method (WAKO Pure Chemical Industries, Ltd., Osaka, Japan). Rumen fluid supernatant was acidified with 25% metaphosphoric acid and analyzed for molar proportions of VFA by gas chromatography (2) and for N H 3 N using a phenol-hypochlorite procedure (5). Milk fat was collected from Babcock bottles after Babcock analysis and analyzed for relative percentage of fatty acids using the procedure of P a r d (18). Relative area under the curve from the gas chromatograph was used as the quantitative measure in lieu of a standard. Due to inadequate separation of saturated and unsaturated fatty acids, fatty acids of the same carbon length were reported as one value despite their degree of saturation (Le., c16 includes c16:O and C16:lr and c18 i d u d e s C18:0, C18:19 C18:2 and C18:3). Data were analyzed as a randomized complete block using the general linear models procedure of SAS (21). Parity (primiparous versus multiparous) served as the blocking factor. The model employed for statistical analysis was observation = overall mean + treatment effect + Journal of Dairy Science VoI. 74, No. 3, 1991
982
ZhrIMERMAN ET AL.
TABLE 1. Chemical analyses of hays and concentrates. DM
FetdStuffl
CP
(% of Dh4)
(96) =YS
Alfalfa Orchardgrass Fescue Alfalfa diet concentrates' High fiber HPA Low fiber HPA Orchardgrass diet concentrates' High fiber
HPG MPG LPG Low fiber
HPG MPG LPG Fescue diet concentrates' High fiber HPG
m
LPG Low fiber HPG
MPG LPG ~
~
NDF
ADF
91.6 87.0 92.1
19.1 16.8 12.6
31.0 32.5 31.8
44.5 60.1 58.1
90.0
30.8
5.4
19.9
88.9
23.6
4.9
21.3
90.1 89.0 88.9
32.5 20.9 8.9
5.3 4.8 4.0
19.0 23.7 28.5
88.9 89.2 88.7
23.8 18.1 12.8
5.0 5.0 4.5
20.6 22.8 23.9
89.4 88.9 89.0
39.3 28.6 17.7
5.4 4.1 3.8
19.8 21.0 23.0
89.4 89.5 89.2
25.7 20.2 16.0
5.5 5.1
22.6 24.6 22.6-
~
~~
~
~~
4.4 ~
~
~
~
~
'HPA= ~ i g protein h a ~ hayadier HPG = high protein grass b y diet; MFG = medium protein grass hay diet; LPG = low protein grass hay diet. kom po~ tiowsoybean meal, corn grain, dicalcium phosphate, limestone, magnesium oxide, potassium chloride, tracemineral salt, and vitamins.
TABLE 2. Nutrient compositions of experimental diets.' Diet
CP
ADF
NDF =Y (5% of DM)
Concentrate
91.3 88.8 89.1 88.6
22.2 22.3 18.2 14.1
23.1 22.8 22.5 22.3
37.3 45.4 47.9
71.8 64.8 65.3 64.6
28.2 35.2 34.7 35.4
89.6 88.8 89.4 89.0
22.5 22.5 18.1 14.3
11.0 11.1 11.2 10.8
26.8 29.5 31.8 32.3
23.8 21.9 22.9 22.8
76.2 78.1 77.1 772
DM (%)
High fiber HPA2
HPG MPG LPG Low fiber HPA
HPG MPG LFG
46.4
on actual DM intales. = W ~ Iprotein I alfatfa hay diet; HPG = high p t e i n grass hay diet; MPG = medium protein grass hay diet; LPG = low protein grass hay diet. 1~~~
2~~
Journal of Dairy Science Vol. 74, No. 3, 1991
983
DIETARY PROTEIN AND MILK FAT DEPRESSION
TABLE 3.Intakes by period @M basis) with contrasts to detect high fiber period treatment differences and percentage change differences when switched from high to low fiber diets. Intakes
coacentrate
Item
High fiber HPAl HPG
MPG LPG
SEM contrasts Low fiber HPA HPG MPG LPG SEM contrasts
10.4 8.5 8.8 7.8
4.1 4.6 4.7 4.3
.1
.I
4.6 4.1 4.6 4.1 .1
14.8 14.6 15.6 13.9 .1
CP
ADF
NDF
DM
3.23 2.93 2.46 1.72 .03
3.36 3.00 3.04 2.71 .03
5.42 5.% 6.25 5.82 .05
14.5 13.1 13.5 12.2 .1 NS
4.36 4.19 3.66 2.58
2.14 2.07 2.27 1.95 .01
5.20 5.53 6.43 5.82 .03
19.4 18.7 20.2 18.0
.M
.1
NS
'HPA = High protein alfalfa; HFG = high protein grass; MPG = medium protein grass; IPG = low protein grass.
parity effect + treatment x parity interaction + experimental error. Means were calculated by period (fiber level) for each dependent variable using data from the last 2 wk of the HF period and the last 6 wk of the LF period. The following single df contrasts were used to detect treatment differences within the HF period: HPA vs. HPG, HPG vs. MPG,HF'G vs. LPG, and MPG vs. LPG. The percentage change from HF to LF also was calculated for each dependent variable as
Milk Production and Composition
Milk yield and composition are listed in Table 4. Milk yield was higher for HPG and MF'G than for LPG during the HF period (Figure 1). This was due to the less than optimal dietary CP content of LPG. When cows were switched to LF. LPG milk yield remained lower than that of the other treatments. Low dietary CP has resulted in reduced milk production in other studies (8, 13). Yield of FCM paralleled uncorrected milk yields on HF. Fat yields also were higher for HPG and MPG versus LPG on HF. When cows were where: A = percentage change in dependent switched to LF, fat yield decreased more for variable, B = low fiber mean for dependent HPA than for HPG.This was due to a larger variable, and C = high fiber mean for depen- decrease in milk fat content when cows were dent variable. The same single df contrasts as switched to LF for HPA than HPG.Milk CP used to detect €IF treatment differences were and SNF yields also were higher for HPG and used to detect percentage change treatment dif- MPG on HF than for LFG. No significant ferences. Effects were considered to be differ- treatment differences were associated with the switch to LF in either CP or SNF yield. ent based on significant (P < .lo) F ratio. A linear increase in milk fat test (Figure 2) occurred with increasing dietary CP on HF. The RESULTS AND DISCUSSION HPG diet resulted in a significantly higher milk Dq matter intakes are listed in Table 3. fat test than LPG during the HF period, in Cows on HPA tended to consume more DM agreement with earlier reports (6, 20, 22, 28). during the HF period than HPG cows; however, Although the HPA diet exhibited a slightly the difference was not statistically significant. higher milk fat test compared with HPG on HF, When switched to LF,differences for intake of the alfalfa diet appeared not to maintain fat test DM due to treatment were not signifcant. as well as the grass hay diet upon the change to Journal of Dairy Science Vol. 74, No. 3, 1991
984
ZIMMERMAN ET AL..
TABLE 4. Milk production and milk composition by period with contrasts to detect high fiber pehiod treatment differences and percentage change differences when switched from high to low fiber diets. MiIk
Item ~
~~
PCM
Fat
SNF
CP
Fat
CP
SNF
3.18 3.06 3.11 3.16 .02
8.49 8.47 8.35 8.28
~
~~~~~
~~
Wd)
High fiber HPAl
HPG
m
LPG SEM COntrastS2 Low fiber HPA HPG
m
LPG SEM c0ntra~t~3
4.33 4.17 3.% 3.69
27.1 25.9 26.0 22.5 .2 ct dt
28.3 26.5 25.8 21A .4 c* d*
1.16 1.07 1.03 .83 .02 c* d*
.86 .79 .80 .71 .01 ct dt
2.29 2.20 2.16 1.86 .03 c* d*
Ct
NS
NS
32.0 30.4 31.8 28.0 .1
28.0 28.0 28.4 24.4 .3
1.12 1.09 1.09 .% -01
2.75 2.64 2.73 2.43 .02
3.14 3.52 3.30 3.15 .03
NS
NS
NS
At
3.53 3.62 3.49 3.45 .02 C*
8.63 8.75 8.64 8.72 .02
NS
1.01 1.os 1.05 .88 .o1 At
.05
.a?
NS
1
HPA = High protein alfalfa; HPG = high protein grass; MPG = medium protein grass; LPG = low protein grass. kontrasts among hi& fiber treatments: 8 = HPA vs. HFG,b = HPG vs. MPG, c = HPG vs. LPG.d = MPG vs. LPG. 3~crcentagcc-e contrasts w t m switcw high fiber to LOWfiber diets: A = HPA vs. HPG, B = HPG vs. MPG, C = HPG VS. LPG, D = MPG vs. LPG. t P c .lo. *P c .os. **P < .01. .e* P < .001.
32 30 34
28
26
-
-
22 -
-
24
20
-
18 2
Low fiber
lnltlated
. 4
I
I
I
6 8 10 WEEK OF LACTATION
HPG MPG LPG I
1
12
Figure 1. Average daily milk yields by week of lactation.HPA = High protein alfalfa; HPG = high protein grass; MPG = medium protein grass; LPG = low protein grass.
Journal of Dairy Science Vol. 74, No. 3, 1991
985
DIETARY PROTEIN AND MILK FAT DEPRESSION
TABLE 5. Rumen VFA molar percents by period
with contrasts to detect high fiber period treatment differences and percentage change diffexences when switched from high to low fiber diets.
Item
APl
c22
c3
c4
m
4
c5
ISOC~
1.38 1.02 .97 .87 .03
1.82 1.66 1.46 1.16
(moV100 mol)
High fiber HPA3
HPG MPG LFG SEM contrasts4
3.95 4.03 4.00 4.02 .03 NS
68.5 69.4 69.6 70.4 .2
NS
17.5 17.3 17.5 17.6 .1 NS
9.17 9.19 9.09 8.82 .06
NS
1.61 1.44 1.38 1.16 .03 C* d*
a*** Ct
.01 C*** cd*
Low fiber HPA
HPG MPG LPG
SEM cOntrasts5
2.15 2.46 2.43 2.27 .05 NS
56.2 59.2 59.5 58.8 .3 At
27.4 25.1 25.6 27.6 .4
NS
10.60 1054 10.38 9.81 .I4 NS
1.52 1.52 1.43 1.14 .03 At
2.13 1.64
1.38 127 .oQ NS
2.09 2.03 1.73 1.38 .04
NS
'Acetate:propionate ratio. 2Length of VFA carbon chain
%PA = mgh protein alfalfa; HPG = hi@ protein grass; MPG = medium protein grass; LPG = low protein grass. 4C0ntrasts among high fiber treatments:a = €PA vs. Hpc,b = HPG vs. MPG,c = HPG vs. LFG,d = MPG vs. LPG.
'P-&age change contrasts when switched from high fiber to low fiher diets: A = HPA vs. HPG,B = HPG vs. MPG, = HPG VS. LFG. D = MPG vs. LFG. t P < .lo. *P < .05.
**P < .01. ***P < .001.
low fiber feeding regimen, probably due to lower NDF content of the a l f a a diet (Table 2) or a different pattern of rumen digestion. Perhaps this also was due to decreased rumen residence time (27) and decreased chewing time. Also, because the alfalfa was higher in CP than the grass hays, less soybean meal was supplemented in the HPA diets, which would have resulted in a lower contribution of soybean meal amino acids to the diet. Because soybean meal has been used in previous milk fat depression studies (8, 9, 10, 11). we suggest that the amino acid contribution of soybean meal may be important in minimizing milk fat depression. Treatment did not affect milk CP percentage during the HF period, however, when cows were switched to LF, milk CP percentage increased more for HPG than for LPG. Interestingly, milk CP percentage increased when cows were switched to LF, which is in agreement
with other studies (26, 30). This may suggest a partitioning of energy from milk fat to milk CP production. Milk SNF percentages were similar among treatments on Hp.
0
HPA
HPG MPG TREATMENT
HIGH FIEER LOW FIBER
LPG
Figure 2. Average milk fat percentages by treatment and fiber level. HPA = High protein a l f a l f ~ HpG = high protein grass; MPG = medium protein grass; LpG = low protein glass. Journal of Dairy Science Voi. 74, No. 3, 1991
986
ZIMMERMAN ET AL.
TABLE 6. Rumen N H 3 N and blood metabolites by period with contrasts to detect high fiber period treatment differences and percentage change differences when switched from high to low fiber diets. Item
RumenNH3N
Urea
Plasma Glucose
w4L)
(mg/dl) High fiber €PA1
HPG
m
LPG SEM contrasts'
12.1 12.8 8.6 4.0 .4
b*** c*** d***
NEPA
24.5 23.5 17.1 8.6 .6 b*** c*** d***
49.8 54.6 51.9 49.7 .6 at c t
24.9 23.6 19.1 9.8 .5
62.1 64.4 60.0 58.9 .3
NS
NS
600
536 550 360 21 ct dt
Low fiber
HPA HPG
m
LPG SEM
contra~d
15.9 14.9 8.7 3.8 .5 NS
132 136 120 117 3 Ct D**
~ H P A= protein HPG = high protein grass; MPG = medium protein grass; LPG = low protein grass. kontrasts among hi@ fiber treatments:a = HPA vs. HFG, b = HPG vs. MPG,c = HPG vs. LPG,d = MPG vs. LPG. 3Percentage change contrasts when switched from high fiber to low fiber diets: A = HPA vs. HPG.B = HPG vs. MPG, C = HPG VS. LFG, D = MPG vs. LPG. tP < .lo. *P < .05. **P < .01. ***P < .001.
Rumen Fluld Measures
the LF period. One would expect the branchedchain VFA and valerate to increase with inRumen VFA data are listed in Table 5. creasing dietary true protein. Isobutyrate and Rumen acetatepopionate ratios were not sig- isovalerate are degradation products of the esnificantly different between treatments both sential amino acids, valine and leucine, respecwithin the HF period and as a percentage of tively. Valerate is produced mainly from carbochange when cows were switched to LF. Cows hydrates but also from the amino acids proline, fed HFG exhibited less depression in rumen arginine, lysine, and methionine (1). Jaquette et acetate molar percentage than those fed HPA al. (7) found that increasing dietary CP inwhen they were switched to LF, suggesting creased isobutyrate. differing patterns of nuninal fiber digestion for Rumen NH3 N levels (Table 6) mimicked the two hays. Molar percentages of rumen pro- dietary CP level on HF, because there was a pionate or butyrate did not differ significantly signrficant positive linear relationship between due to treatment. Isobutyrate was higher for dietary CP and NH3 N levels. When cows were HPG and MPG than for LPG on HF. When switched to LF, there were no percentage cows were switched to LF, isobutyrae levels change differences due to treatment. These decreased for HPA and increased for HPG.For values are all probably somewhat low due to valerate, HPA was significantly higher than method of sampling and possible dilution with HPG,and HF'G was significantly higher than water. LPG during the HF period. This pattern continued during the LF period. Isovalerate was Blood Parameters higher for HPG and MPG than for LFG during Plasma urea nitrogen (PUN) is likely a more the HF period. This trend also continued during accurate measure of protein status because this Joumal of Dairy Science Vol. 74. No. 3, 1991
987
DIETARY PROTEIN AND MILK PAT DEPRESSION
TABLE 7. Milk fatty acids by period with contrasts to defect high fiber period treatment differences and percentage change differences when switched from hi@ to low fiber diets. Item
C4l
c6
c8
c10
c12
c14
(peak ara %) High fiber HPA2
HPG MPG LPG
SEM c~ntra~ts~ Low fiber HPA HPG MPG
LPG SEM
conea~ts~
High fiber HPA
HPG MPG
LPG
4.60 5.09
4.38 4.10 .ll
bt c*
Low fiber HPA
HPG MPG
LPG SEM contrast&
.M a* bf c**
NS
Ct
9.22 9.68 928 9.20 .18
NS
At Ct
NS
c186
ScpA-147
LCFA-168
SCFA-169
LCFA-186
44.71 44.52 46.78 47.17
21.06 23.5 1 21.40 20.02
78.93 76.49 78.60 79.98
5528 55.48 53.22 52.83 .48
44.71 44.52 46.78 47.17 .48
C16l5
34.22 31.97 31.82 32.81 .25
.08
..@!
a c
2.30 2.64 2.44 2.18 .09
13.46 13.71 13.98 13.49 -08 NS
3.65 3.71 .05 Ct
3.92
1.88 2.34 2.11 1.78
.% 1.24 1.06 .89
4.44 4.41 4.78 3.98 .05 At
2.66 2.67 2.65 2.44 .03 At Ct
3.%
SEM c0~tra~ts3
2.10 2.52 2.14 1.88
1.64 1.68 1.73 1.52
.02
.44
.44
.48
5.32 5.29 5.66 4.85 .07
a*
NS
C*
C*
NS
NS
35.76 34.84 34.46 35.19 .19
32.73 33.48 33.05 34.81 .27
31.48 31.67 32.45 30.00 .19
67.24 66.5 1 66.90 65.19 .27
32.73 33.48 33.05 34.81 27
NS
NS
NS
68.49 68.32 67.50 70.00 .19 NS
NS
NS
'Length of fatty acid carbon chain, i.e., C4 = butyrate, four-carbon chain. *HPA = ~ i g protein h atfalfa; HPG = high protein grass; MPG = medium protein grass; LPG = low protein grass. 3C0ntrasts among high fiber treatments: a = HPA vs. HF'G, b = HPG vs. MPG. c = HPG vs. LF'G, d = hWG vs. LPG. 4P-ntage change contrasts when switched &omhigh fiber to low fiber diets: A = HPA vs. HPG,B = HPG vs. MF'G, C = HPG vs. LF'G, D = MPG vs. LPG. 'Patty acids c 1 6 9 and c16:1. bng-chain fatty acids: C1gfi c18:1. C182. and C18:3. 7 ~ ~ t short-chain al fany acids: c4to ~ 1 4 . '+om long-chain fatty acids: ~ 1 6 0C16:l. . c189. CIS.. c1s2. and C18:3. TOM short-chain fatty acids: ~4 to ~ 1 6 : 1 . t P < .lo. *P < .05. **P < .01. ***P < .001.
measure is unaffected by the problems of sample site and dilution associated with rumen NH3 N, which was sampled via stomach pump. In agreement with other studies (9, 10, ll), PUN displayed a significant positive correlation with CP intake (Table 6). No significant changes in
PUN occurred when cows were switched to LF. Blood glucose levels (Table 6) were higher for HPG than for HPA or LPG during the HF period. Blood NEFA levels on the HF diets were higher on HPG and MPG than on LPG. Using actual BW, BW change, milk producJournal of Dairy Science Vol. 74, No. 3, 1991
ZMMERWW ET AL.
988
560 540
5 I-
5-
520
500
i i -
w
480 -
460 0
m 440 420
I
-
Low flber lnltiated
I
,
tion, feed intake data, and 1988 NRC feed energy values (14), the HPA and HPG cows were determined to be in negative energy balance during the HF period (HPA = -.8 McWd and HPG = -1.8 MCd/d). COWSfed MPG and LFG were in positive energy balance when fed HF (+3.1 and +2.7 Mcal/d), respectively. Various studies have shown that increasing protein supply to the small intestine can increase body fat mobilization (8,16, 17,28). This could have contributed to the higher milk fat test for the HPG diet during the HF period. When switched to LF, cows on HPG and MPG experienced a greater depression than those on LPG in blood NEFA levels. During the LF period, all groups were in positive energy balance with values of +1.3, +.6, +1.8, and +1.3 McaVd for €PA, HPG, MPG, and LPG,respectively. Jaquette et al. (10) found that high blood NJFA levels during the HF period were associated with the inability of high dietary CP to alleviate milk fat depression when cows were switched to LF diets. Whether this phenomenon is cause or effect is not known, however, as e v i d e n d by the present study; high blood NEFA levels during the HF period did not hamper milk fat synthesis during the HF period. Journal of Dairy Science Vol. 74, No. 3, 1991
I
I
I
HPA
HPG
MPG LPG I
1
Milk Fatty Acids
High Fiber. Milk fatty acid composition is listed in Table 7. Values are listed as peak area percentages, which are similar to weight percentages. Milk butyrate was higher for HPG than for MPG or LFG.Milk c6 was higher for HPG versus HPA, MPG,or LPG. Levels of c8 were higher for HPG than for HPA and LPG. Milk Cl0 was higher for HPG than for LPG. Milk c16 was significantly higher for HPA than for HPG. Cows fed HPG exhibited higher levels of short-chain fatty acids (SCFA) C4 to C14 than those fed LPG. The reverse was true for long-chain fatty acids (LCFA) c16 to c18. Milk SCFA synthesis appeared to be enhanced by increasing CP level within the grass hay diets. This may have been due to either an increase in de novo fatty acid precursors, acetate and P-hydroxybutyrate, or perhaps an inhibition of the mammary enzyme acetylcoenzyme A carboxylase by low CP diets. Actual rumen VFA production rates and blood P-hydroxybutyrate levels were not measured. Cows fed HPG were in negative energy balance when fed HF, whereas LPG cows were in positive energy balance as stated previously. Milk LCFA were expected to be higher for the
DIETARY PRO'K" AND MILK FAT DEPRESSION
higher CP diets because they had higher NeFA during this perid however, this was not evident. Jaquette et al. (10) have postulated that high CP diets may increase fat test by increasing the amino acid supply available for the synthesis of the apoprotein fraction of lipoproteins. Because very low density lipoproteins (VLDL) are the major carriers of preformed fatty acids to the mammary gland for milk fat synthesis, an increased incorporation of LCFA into milk fat should occur as a result. This did not appear to be the case &ring the HF period. Another hypothesis (10) is that the amino acids of soybean meal are required for the foxmation or activation of the enzyme lipoprotein lipase that cleaves preformed fatty acids from chylomicrons and VLDL at the mammary gland to be used for milk fat synths sis. Again, this did not appear to be the case in our study. The increased fat production on the HPG diet may be due to increased de novo synthesis of SCFA within the mammary gland. Lao Fiber. When cows were switched to the LF diets, milk C4 was decreased to a significantly lesser degree for LPG than for HPG. Milk c6 increased more for HPA and LPG than for HPG. Mille cg increased more for LPG and HPA than for HPG. All other percentage changes in milk fatty acid parameters were not significant. When cows were switched to the LF diets, the percentage of milk fatty acids formed from de novo synthesis in the mammary gland increased. The fatty acids involved were C4 to C14 and some of the c16.This may have been primarily a stage of lactation effect, because cows on the LF diets were in a later stage of lactation, losing less BW, and therefore circulating levels of LCFA were decreased (19, 25). Milk C4 acted differently, however, actually decreasing on LP.This probably was due to the ability of milk C4 to be synthesized exclusively from p-hydroxybutyrate without the malonyl pathway of synthesis. Milk C4 tends to react differently than the other SCFA when dietary alterations are performed (4, 23, 24). When cows were switched to the LF diets, DM intakes and BW began to increase (Figure 3); therefore, blood P-hydroxybutyrate levels should have decreased, leading in turn to a decrease in milk C4. Body Weights
Body weights are shown in Figure 3. Cows on all treatments lost weight through wk 6 of
989
lactation; however, no large differences in BW loss occurred due to treatment. All cows gained weight from wk 6 to 12 of lactation and returned to their approximate starting weights. The HPA and MPG groups did weigh more throughout the experiment than the HPG and LPG p u p s . In contrast to previous studies (8, 9, lo), there were no parity x treatment interactions. Previous studies have shown that primiparous cows respond better to high dietary CP than multiparous cows (8, 9). This may be due to a differing partitioning of nutrients between parities. Despite the lack of parity response in this uial, this question deserves further research. CONCLUSIONS
The HPG and MPG diets resulted in higher yields of milk and milk components compared with the LFG diet during the H F period. Also, the HFG diet resulted in higher fat tests in cows fed HF diets; however, the MPG diet was not as effective as the HPG diet in attaining a high fat test during the HF period. The mechanism for increased fat test on the HF diet is not understood, but it appears to be due mainly to an increase in the SCFA of milk, Cows on HPG produced more milk SCFA on HF diets than LFG cows. High CP diets also increased body fat mobilization during the HF period, as indicated by increased blood NEFA levels. This extra body fat did not appear to be transferred to milk fat. It is difficult to draw conclusions about the effectiveness of M P diets in alleviating milk fat depression in cows switched to LF diets because we did not observe a difference in milk fat depression between the HPG and LFG diets. Rations utilizing alfalfa hay as the forage source increased the severity of milk fat depression in cows switched to LF diets. The effect of dietary CP on milk fat synthesis appears to be postruminal because ruminal acetate:propionate ratios did not differ. However, feeding high levels of dietary CP does result in increased m e n molar percentages of isobutyrate, valerate, and isovalerate. Lowering dietary fiber increased milk CP percentage. Perhaps this would be a reliable method for increasing milk CP in the future. More work is necessary to study the effects of parity on the response of milk fat synthesis to dietary CP. Journal of Dairy Science Vol. 74, No. 3. 1991
ET AL.
990 REFERENCES 1 Andries, J. I., F. X. Buysse, D. L. De Brabander, and B. G. Cottyn. 1987. Isoacids in mminant mlritiox~ their role in raminal and intermediary metabolism and possible influences on @ormark%s-a review. Anim. Feed Sci. Technol. 18169. 2Anonymous. 1975. GC separation of VFA’s C 2 a . Bull. 510. Supelco, Inc., Bellefonte, PA. 3 Anonymous. 1981. The colorimetric determbation of urea nitrogen. Bull. 640. Sigma Chemical Co.. St.
Louis, MO. 4Banks, W., J. L. Clapperton, A. K. Girdler, and W. Steele. 1984. Effect of inclusion of different f a n s of dietary fatty acid on the yield and composition of cow’s milk. J. Dairy Res. 51:387. SBroderick, G. A., and J. H. Kung. 1980. Automated simultaneous determination of ammonia and total amino acids in ruminal fluid and in vitro media. J. Dairy Sci. 6364. 6 Grieve, D. G., G. K. MaCLeod, and J. B. Stone. 1974. The effect of diet protein level for lactating dairy cows. J. Dairy Sci. 57:633. 7 Jaquette, R D. 1984. The effects of dietary protein level on milk production and composition in dairy cattle fed low-fiber diets. M.S.ntesis, North Carolina State Univ., Raleigh. 8 Jaquette, R D., A. H. Rakes, and W. J. Groom, Jr. 1986. Effects of dietary protein on mi& rumen, and blood parameters in dairy cattle fed low fiber diets. J. Dairy Sci. 691026. 9Jaquette, R D., A. H.Rakes, and W. J. Groom, Jr. 1987. Effect of amount and source of dietary nitrogen on milk fat depression in early lactation dairy cows. J. Dairy Sci. 7a1202. lOJaquette, R D.,A. H. Rakes, and W. I. Croom, Jr. 1988. Effects of body condition and protein on milk fat depression in early lactation dairy cows. J. Dairy Sci. 71:2123. 11 Kiam, M.,H.Neumark, Y. Folmao,and W.Kadbwm 1987. The effect of two concenmtiofls of dietary protein and of formaldehyde-treated soya-bean meal on the performance of high-yielding dairy cows. Anim. Prod. 44:333. 12leonard, M., and E. Block. 1988. Effect of ration protein content and solubility on milL production of primiparouS Holstein heifem. J. Dairy Sci. 71:2709. 13Lundquist, R G., D.E. Otterby. and J. G. Liuu 1986. Mluenct of formaldehydetreated soybean meal on milk production. J. Dairy Sci. 69:1337. 1 4 N a t i o d Research Council. 1988. Nutrient nqOirements of dairy cattle. 6th ed. Natl. Acad. Sci, Washington, Dc. 15 Oldham, J. D., W. H.Broster, and J. W.SMter. 1978.
J o d of Dairy Science Vol. 74, No. 3, 1991
The effects of a low-protejn diet on milk yield and plasma metabolites in Friesian heifers during early lactation. Proc. Nutr. Soc. 37:44A.(Abslr.) 16f&skov, E. R, D. A. Grubb, and R.N.B. Kay. 1977. Effect of postnuninal glucose or protein supplementation on milk yield and composition in Friesian cows in early lactation and negative energy balance. Br. J. Nutr. 38:397. 17&skov, E. R, G. W. Reid, and C.A.G. Tait. 1987. Effect of fish meal on the mobilization of body energy in dairy cows. Anim. Rod.45345. lSParodi, P. W. 1970. Fatty acid composition of Australian butter and milk fats. Aust. J. Dairy Technol. 25: 200. 19 Pmdi, P. W. 1974. Variation in the fatty acid composition of milkfat: effect of stage of lactation. Aut. J. Dairy Techwl. 29145. 20Rakes, A. H.,K.Vaughn, and D. G. Davenport. 1972. Effect of prepartum dietary energy and postpartum dietary protein on postpamun performance of dairy cows. J. Dairy Sci. 55:397. 21 SAS@User’s Guide. 1982. SAS Inst, Inc., Cary, NC. 22 Sparrow, R C., R. W. Hemken, D. R Jacobson, F. S. Button, and C. M. Enlow. 1973. Three protein percentages on nitrogen balance, body weight change, milk production and composition of lactating cows during early ktation. J. Dairy Sci. 56(Suppl. 1):664.(Abstr.) 23 Stony, J. E. 1970. Reviews of the progress of dairy science. Section A. Physiology. Ruminant metabolism in relation to the synthesis and secretion of milk fat. J. Dairy Res. 37:139. 24 Storry, J. E.,A. J. Hall, and V.W. Johnson. 1973. The effects of increasing amounts of dietary tallow on milk-kt secretion in the cow. J. Dairy Res. 40:293. 25 Stull, J. W., W. H.Brown, C. Valdez, and H.Tucker. 1966. Fatly acid composition of milk- Ill. Variation with stage of lactation. J. Dairy Sci. 491401. 26 Sutton, J. D. 1989. Alteriug milk composition by feeding. J. Dairy Sci. 72:2801. 27 Thornton, R. F.,and D. J. Minson. 1973. The relationship between apparent retention time in the nunen, voluntary intale, and apparent digestibility of legume and grass diets in shecp. Aust. J. Agric. Res. W889. 28 Trigg, T.E., C. R. Parr, A. M.Day, and B. W. Parsons. 1982. Effects on milk production and energy m e t a b lism of abomasal infusions of protein to lactating pasture-fed dairy cows. Page 42 in Energy metabolism of fann animals. A. ELem and F. Sundstol, ed. Agric. Univ. Norway, Lillehammer, Norway. 29Udy, D. C. 1956. A rapid method for estimating total protein in milk. Nature (Loud.) 178:314. 30Yousef, I. M.,J. T. Huber. and R. S. Emery. 1970. Mill;protein synthesis as aEected by high-grain, lowfiber rations. J. Dairy Sci. 53:734.
B. A. HOPKINS. and W. J. CROOM, JR. Depkment of Animal Science North Carolina State University Raleigh 276957621
ABSTRACT
rumen acetate:propionate ratios were unaffected by treatment. Lowering fiber level resulted in an increased milk CP percentage regardless of treatment. Grass hay appeared to be more effective than alfalfa hay in preventing depression in milk fat test upon the change to a low fiber diet. (Key words: crude protein, milk fat depression, low fiber)
Twenty multiparous and 12 primiparous Holstein cows were assigned at calving to one of three grass hay-based diets containing either 14, 18, or 22% CP or an alfalfa hay-based diet containing 22% CP to examine the effect of protein level and forage source on milk yield and composition. The diets contained 23% ADF during wk 1 to 4 postpartum, which was lowered to 11% for wk 5 to 12 postpartum. Cows fed the 18 and 22% CP grassbased diets produced higher yields of milk, 4% FCM, fat, protein, and SNF than those fed the 14% CP diet during the high fiber period. In addition, cows fed the 22% CP grass-based diet had higher milk fat tests than those fed the 14%CP diet during the high fiber period, due primarily to an increase in short-chain fatty acid synthesis. Milk fat depression was more severe when cows were changed to low fiber diets while fed the 22% CP alfalfa-based diet than when fed the 22% CP grass-based diet. Depression in milk fat content was 15.0, 17.0, 15.6, and 27.0%for 14, 18, and 22% CP grassbased and 22% CP alfalfa-based diets, respectively. Cows receiving the 18 and 22% CP grass-based diets exhibited higher blood NEFA during the high fiber feeding period than those fed the 14% CP diet. After fiber was lowered, changes in
Abbreviation key: HF = high fiber, HPA = high protein alfalfa hay diet, HPG = high protein grass hay diet, LCFA = longxhain fatty acids, LF = low fiber, LPG = low protein grass hay diet, MPG = medium protein grass hay diet, PUN = plasma urea nitrogen, SCFA = short-chain fatty acids, VLDL = very low density lipoproteins. INTRODUCTION
Received March 8. 1990. Accepted October 5, 1990. 'Paper Number 12577 of the Journal Series of the North Carolina Agricultural Research Service,Raleigh, NC 27695-7621. The use of trade names m this publication does Wt imply endOrsaent by the North Ci~olinaA g i i tural Research Service of the products named nor criticism of similar ones not mentioned. 'fiesent address: central soya, Decatur. IN 46733. 1991 J Dairy Sci 74:98&990
High producing cows in early lactation, in an attempt to meet their energy requirements, often are not fed enough effective fiber to maintain milk fat tests. This is of particular concern in the southeastern US where high ambient temperatures and low forage supplies contribute to this problem. Numerous studies have shown that feerling high levels of dietary CP can partially alleviate the problem of milk fat depression caused by feeding low fiber diets (8.9, 10, 11,12,15). High CP rations also have been shown to increase milk fat test in some studies on diets adequate in fiber (6, 20, 22, 28). Forage type could alter the effectiveness of high dietary CP in alleviating milk fat depression. Alfalfa or alfalfa-grass hay mixtures were the main forages used in most experiments to examine the effects of dietary CP on milk fat depression. However, in one study (9) corn silage replaced alfalfa hay. Although still present, the CP effect was less with corn silage than
980
D m A R Y PROTEIN AND MILK FAT DEPRESSION
with alfalfa hay-based diets. Protein content of the forage should have a relatively minor effect on the CP response because forage CP, especially on the low fiber diets, contri’butes only a small portion of the total dietary CP. The high CP diets used in previous studies at our university have ranged from 21 to 24% CP (8,9, 10). The objectives of our study were to examine the effect of forage source (alfalfa versus grass hays) and an intermediate level of dietary CP on milk fat synthesis when daj. cattle were fed a high fiber diet and then switched to a low fiber diet. MATERIALS AND METHODS
Twenty multiparous and 12 primiparous cows were assigned randomly to one of four dietary treatments at calving. Three diets utilized orchardgrass or endophyte-free tall fescue as the forage source and contained protein percentages of 1) 14% low protein grass hay diet (LPG), 2) 18% medium protein grass hay diet (MPG), and 3) 22% high protein grass hay diet (HPG). The fourth diet was 22% CP with alfalfa hay as the forage some, a high protein alfalfa hay diet (HPA). Tall fescue was substituted for orchardgrass during the study due to inadequate supplies of orchardgrass. Tall fescue was selected as a substitute forage because of its compositional similarity to orchardgrass. The fescue hay was Johnstone variety, which is low in perloline alkaloids in addition to being endophyte-free. All hays were chopped in a bedding chopper to similar lengths (10 to 15 cm). After calving, cows were weighed on 2 consecutive d and gradually adjusted to a diet consisting of their respective chopped hay and concentrate. Hay and concentrate were fed simultaneously but not mixed and fed for 5% refusal. Concentrate and hay were fed at a ratio to maintain the ration ADF at 11.5%. All cows were fed high fiber (23% ADF) diets during wk 1 to 4 postpartum and gradually switched in five equal increments over a 5 4 period to low fiber (11.5% ADF) diets for wk 5 to 12 postpartum. Cows remained on their respective hay and CP rations for both the high fiber (HF) and low fiber (LF) periods. Hay samples were taken at the beginning of the trial, and concentrates were sampled biweekly. Chemical analyses of hays and concentrates are listed in Table
981
1. Nutrient compositions of experimental diets are in Table 2. Throughout the study, cows were housed in a tie stall barn and milked twice daily; milk weights were recorded at each milking. Cows received exercise on a dirt lot for 2 h daily. Cows were fed at 0800 and 1400 h, and individual feed intakes were recorded daily. Body weights were recorded on 2 consecutive d weekly throughout the study. Milk samples were collected at a.m. and p.m. milkings for 6 d during wk 3 to 4 of the €IF period and twice weekly during the LF period. A composite sample was taken from the am. and p.m. milk samples and analyzed for fat by the Babcock method, for CP by a dye-binding assay (29), and for SNF using a Watson lactometer (A. Daigger and Company, Richmond, CA). Jugular blood samples, collected by venipuncture, and rumen fluid samples, collected via stomach pump, were taken 5 h after the a.m. feeding twice weekly during wk 3 to 4 of the J3F period and once weekly throughout the LF period. Plasma was analyzed for urea N using a phenol-hypochlorite procedure (3) and for glucose with a YSI Model 27 Industrial Analyzer, using membrane-immobilized glucose oxidase (Yellow Springs Instrument Co., Yellow Springs, OH). Nonesterifid fatty acids were determined using an enzymaticcolorimetric method (WAKO Pure Chemical Industries, Ltd., Osaka, Japan). Rumen fluid supernatant was acidified with 25% metaphosphoric acid and analyzed for molar proportions of VFA by gas chromatography (2) and for N H 3 N using a phenol-hypochlorite procedure (5). Milk fat was collected from Babcock bottles after Babcock analysis and analyzed for relative percentage of fatty acids using the procedure of P a r d (18). Relative area under the curve from the gas chromatograph was used as the quantitative measure in lieu of a standard. Due to inadequate separation of saturated and unsaturated fatty acids, fatty acids of the same carbon length were reported as one value despite their degree of saturation (Le., c16 includes c16:O and C16:lr and c18 i d u d e s C18:0, C18:19 C18:2 and C18:3). Data were analyzed as a randomized complete block using the general linear models procedure of SAS (21). Parity (primiparous versus multiparous) served as the blocking factor. The model employed for statistical analysis was observation = overall mean + treatment effect + Journal of Dairy Science VoI. 74, No. 3, 1991
982
ZhrIMERMAN ET AL.
TABLE 1. Chemical analyses of hays and concentrates. DM
FetdStuffl
CP
(% of Dh4)
(96) =YS
Alfalfa Orchardgrass Fescue Alfalfa diet concentrates' High fiber HPA Low fiber HPA Orchardgrass diet concentrates' High fiber
HPG MPG LPG Low fiber
HPG MPG LPG Fescue diet concentrates' High fiber HPG
m
LPG Low fiber HPG
MPG LPG ~
~
NDF
ADF
91.6 87.0 92.1
19.1 16.8 12.6
31.0 32.5 31.8
44.5 60.1 58.1
90.0
30.8
5.4
19.9
88.9
23.6
4.9
21.3
90.1 89.0 88.9
32.5 20.9 8.9
5.3 4.8 4.0
19.0 23.7 28.5
88.9 89.2 88.7
23.8 18.1 12.8
5.0 5.0 4.5
20.6 22.8 23.9
89.4 88.9 89.0
39.3 28.6 17.7
5.4 4.1 3.8
19.8 21.0 23.0
89.4 89.5 89.2
25.7 20.2 16.0
5.5 5.1
22.6 24.6 22.6-
~
~~
~
~~
4.4 ~
~
~
~
~
'HPA= ~ i g protein h a ~ hayadier HPG = high protein grass b y diet; MFG = medium protein grass hay diet; LPG = low protein grass hay diet. kom po~ tiowsoybean meal, corn grain, dicalcium phosphate, limestone, magnesium oxide, potassium chloride, tracemineral salt, and vitamins.
TABLE 2. Nutrient compositions of experimental diets.' Diet
CP
ADF
NDF =Y (5% of DM)
Concentrate
91.3 88.8 89.1 88.6
22.2 22.3 18.2 14.1
23.1 22.8 22.5 22.3
37.3 45.4 47.9
71.8 64.8 65.3 64.6
28.2 35.2 34.7 35.4
89.6 88.8 89.4 89.0
22.5 22.5 18.1 14.3
11.0 11.1 11.2 10.8
26.8 29.5 31.8 32.3
23.8 21.9 22.9 22.8
76.2 78.1 77.1 772
DM (%)
High fiber HPA2
HPG MPG LPG Low fiber HPA
HPG MPG LFG
46.4
on actual DM intales. = W ~ Iprotein I alfatfa hay diet; HPG = high p t e i n grass hay diet; MPG = medium protein grass hay diet; LPG = low protein grass hay diet. 1~~~
2~~
Journal of Dairy Science Vol. 74, No. 3, 1991
983
DIETARY PROTEIN AND MILK FAT DEPRESSION
TABLE 3.Intakes by period @M basis) with contrasts to detect high fiber period treatment differences and percentage change differences when switched from high to low fiber diets. Intakes
coacentrate
Item
High fiber HPAl HPG
MPG LPG
SEM contrasts Low fiber HPA HPG MPG LPG SEM contrasts
10.4 8.5 8.8 7.8
4.1 4.6 4.7 4.3
.1
.I
4.6 4.1 4.6 4.1 .1
14.8 14.6 15.6 13.9 .1
CP
ADF
NDF
DM
3.23 2.93 2.46 1.72 .03
3.36 3.00 3.04 2.71 .03
5.42 5.% 6.25 5.82 .05
14.5 13.1 13.5 12.2 .1 NS
4.36 4.19 3.66 2.58
2.14 2.07 2.27 1.95 .01
5.20 5.53 6.43 5.82 .03
19.4 18.7 20.2 18.0
.M
.1
NS
'HPA = High protein alfalfa; HFG = high protein grass; MPG = medium protein grass; IPG = low protein grass.
parity effect + treatment x parity interaction + experimental error. Means were calculated by period (fiber level) for each dependent variable using data from the last 2 wk of the HF period and the last 6 wk of the LF period. The following single df contrasts were used to detect treatment differences within the HF period: HPA vs. HPG, HPG vs. MPG,HF'G vs. LPG, and MPG vs. LPG. The percentage change from HF to LF also was calculated for each dependent variable as
Milk Production and Composition
Milk yield and composition are listed in Table 4. Milk yield was higher for HPG and MF'G than for LPG during the HF period (Figure 1). This was due to the less than optimal dietary CP content of LPG. When cows were switched to LF. LPG milk yield remained lower than that of the other treatments. Low dietary CP has resulted in reduced milk production in other studies (8, 13). Yield of FCM paralleled uncorrected milk yields on HF. Fat yields also were higher for HPG and MPG versus LPG on HF. When cows were where: A = percentage change in dependent switched to LF, fat yield decreased more for variable, B = low fiber mean for dependent HPA than for HPG.This was due to a larger variable, and C = high fiber mean for depen- decrease in milk fat content when cows were dent variable. The same single df contrasts as switched to LF for HPA than HPG.Milk CP used to detect €IF treatment differences were and SNF yields also were higher for HPG and used to detect percentage change treatment dif- MPG on HF than for LFG. No significant ferences. Effects were considered to be differ- treatment differences were associated with the switch to LF in either CP or SNF yield. ent based on significant (P < .lo) F ratio. A linear increase in milk fat test (Figure 2) occurred with increasing dietary CP on HF. The RESULTS AND DISCUSSION HPG diet resulted in a significantly higher milk Dq matter intakes are listed in Table 3. fat test than LPG during the HF period, in Cows on HPA tended to consume more DM agreement with earlier reports (6, 20, 22, 28). during the HF period than HPG cows; however, Although the HPA diet exhibited a slightly the difference was not statistically significant. higher milk fat test compared with HPG on HF, When switched to LF,differences for intake of the alfalfa diet appeared not to maintain fat test DM due to treatment were not signifcant. as well as the grass hay diet upon the change to Journal of Dairy Science Vol. 74, No. 3, 1991
984
ZIMMERMAN ET AL..
TABLE 4. Milk production and milk composition by period with contrasts to detect high fiber pehiod treatment differences and percentage change differences when switched from high to low fiber diets. MiIk
Item ~
~~
PCM
Fat
SNF
CP
Fat
CP
SNF
3.18 3.06 3.11 3.16 .02
8.49 8.47 8.35 8.28
~
~~~~~
~~
Wd)
High fiber HPAl
HPG
m
LPG SEM COntrastS2 Low fiber HPA HPG
m
LPG SEM c0ntra~t~3
4.33 4.17 3.% 3.69
27.1 25.9 26.0 22.5 .2 ct dt
28.3 26.5 25.8 21A .4 c* d*
1.16 1.07 1.03 .83 .02 c* d*
.86 .79 .80 .71 .01 ct dt
2.29 2.20 2.16 1.86 .03 c* d*
Ct
NS
NS
32.0 30.4 31.8 28.0 .1
28.0 28.0 28.4 24.4 .3
1.12 1.09 1.09 .% -01
2.75 2.64 2.73 2.43 .02
3.14 3.52 3.30 3.15 .03
NS
NS
NS
At
3.53 3.62 3.49 3.45 .02 C*
8.63 8.75 8.64 8.72 .02
NS
1.01 1.os 1.05 .88 .o1 At
.05
.a?
NS
1
HPA = High protein alfalfa; HPG = high protein grass; MPG = medium protein grass; LPG = low protein grass. kontrasts among hi& fiber treatments: 8 = HPA vs. HFG,b = HPG vs. MPG, c = HPG vs. LPG.d = MPG vs. LPG. 3~crcentagcc-e contrasts w t m switcw high fiber to LOWfiber diets: A = HPA vs. HPG, B = HPG vs. MPG, C = HPG VS. LPG, D = MPG vs. LPG. t P c .lo. *P c .os. **P < .01. .e* P < .001.
32 30 34
28
26
-
-
22 -
-
24
20
-
18 2
Low fiber
lnltlated
. 4
I
I
I
6 8 10 WEEK OF LACTATION
HPG MPG LPG I
1
12
Figure 1. Average daily milk yields by week of lactation.HPA = High protein alfalfa; HPG = high protein grass; MPG = medium protein grass; LPG = low protein grass.
Journal of Dairy Science Vol. 74, No. 3, 1991
985
DIETARY PROTEIN AND MILK FAT DEPRESSION
TABLE 5. Rumen VFA molar percents by period
with contrasts to detect high fiber period treatment differences and percentage change diffexences when switched from high to low fiber diets.
Item
APl
c22
c3
c4
m
4
c5
ISOC~
1.38 1.02 .97 .87 .03
1.82 1.66 1.46 1.16
(moV100 mol)
High fiber HPA3
HPG MPG LFG SEM contrasts4
3.95 4.03 4.00 4.02 .03 NS
68.5 69.4 69.6 70.4 .2
NS
17.5 17.3 17.5 17.6 .1 NS
9.17 9.19 9.09 8.82 .06
NS
1.61 1.44 1.38 1.16 .03 C* d*
a*** Ct
.01 C*** cd*
Low fiber HPA
HPG MPG LPG
SEM cOntrasts5
2.15 2.46 2.43 2.27 .05 NS
56.2 59.2 59.5 58.8 .3 At
27.4 25.1 25.6 27.6 .4
NS
10.60 1054 10.38 9.81 .I4 NS
1.52 1.52 1.43 1.14 .03 At
2.13 1.64
1.38 127 .oQ NS
2.09 2.03 1.73 1.38 .04
NS
'Acetate:propionate ratio. 2Length of VFA carbon chain
%PA = mgh protein alfalfa; HPG = hi@ protein grass; MPG = medium protein grass; LPG = low protein grass. 4C0ntrasts among high fiber treatments:a = €PA vs. Hpc,b = HPG vs. MPG,c = HPG vs. LFG,d = MPG vs. LPG.
'P-&age change contrasts when switched from high fiber to low fiher diets: A = HPA vs. HPG,B = HPG vs. MPG, = HPG VS. LFG. D = MPG vs. LFG. t P < .lo. *P < .05.
**P < .01. ***P < .001.
low fiber feeding regimen, probably due to lower NDF content of the a l f a a diet (Table 2) or a different pattern of rumen digestion. Perhaps this also was due to decreased rumen residence time (27) and decreased chewing time. Also, because the alfalfa was higher in CP than the grass hays, less soybean meal was supplemented in the HPA diets, which would have resulted in a lower contribution of soybean meal amino acids to the diet. Because soybean meal has been used in previous milk fat depression studies (8, 9, 10, 11). we suggest that the amino acid contribution of soybean meal may be important in minimizing milk fat depression. Treatment did not affect milk CP percentage during the HF period, however, when cows were switched to LF, milk CP percentage increased more for HPG than for LPG. Interestingly, milk CP percentage increased when cows were switched to LF, which is in agreement
with other studies (26, 30). This may suggest a partitioning of energy from milk fat to milk CP production. Milk SNF percentages were similar among treatments on Hp.
0
HPA
HPG MPG TREATMENT
HIGH FIEER LOW FIBER
LPG
Figure 2. Average milk fat percentages by treatment and fiber level. HPA = High protein a l f a l f ~ HpG = high protein grass; MPG = medium protein grass; LpG = low protein glass. Journal of Dairy Science Voi. 74, No. 3, 1991
986
ZIMMERMAN ET AL.
TABLE 6. Rumen N H 3 N and blood metabolites by period with contrasts to detect high fiber period treatment differences and percentage change differences when switched from high to low fiber diets. Item
RumenNH3N
Urea
Plasma Glucose
w4L)
(mg/dl) High fiber €PA1
HPG
m
LPG SEM contrasts'
12.1 12.8 8.6 4.0 .4
b*** c*** d***
NEPA
24.5 23.5 17.1 8.6 .6 b*** c*** d***
49.8 54.6 51.9 49.7 .6 at c t
24.9 23.6 19.1 9.8 .5
62.1 64.4 60.0 58.9 .3
NS
NS
600
536 550 360 21 ct dt
Low fiber
HPA HPG
m
LPG SEM
contra~d
15.9 14.9 8.7 3.8 .5 NS
132 136 120 117 3 Ct D**
~ H P A= protein HPG = high protein grass; MPG = medium protein grass; LPG = low protein grass. kontrasts among hi@ fiber treatments:a = HPA vs. HFG, b = HPG vs. MPG,c = HPG vs. LPG,d = MPG vs. LPG. 3Percentage change contrasts when switched from high fiber to low fiber diets: A = HPA vs. HPG.B = HPG vs. MPG, C = HPG VS. LFG, D = MPG vs. LPG. tP < .lo. *P < .05. **P < .01. ***P < .001.
Rumen Fluld Measures
the LF period. One would expect the branchedchain VFA and valerate to increase with inRumen VFA data are listed in Table 5. creasing dietary true protein. Isobutyrate and Rumen acetatepopionate ratios were not sig- isovalerate are degradation products of the esnificantly different between treatments both sential amino acids, valine and leucine, respecwithin the HF period and as a percentage of tively. Valerate is produced mainly from carbochange when cows were switched to LF. Cows hydrates but also from the amino acids proline, fed HFG exhibited less depression in rumen arginine, lysine, and methionine (1). Jaquette et acetate molar percentage than those fed HPA al. (7) found that increasing dietary CP inwhen they were switched to LF, suggesting creased isobutyrate. differing patterns of nuninal fiber digestion for Rumen NH3 N levels (Table 6) mimicked the two hays. Molar percentages of rumen pro- dietary CP level on HF, because there was a pionate or butyrate did not differ significantly signrficant positive linear relationship between due to treatment. Isobutyrate was higher for dietary CP and NH3 N levels. When cows were HPG and MPG than for LPG on HF. When switched to LF, there were no percentage cows were switched to LF, isobutyrae levels change differences due to treatment. These decreased for HPA and increased for HPG.For values are all probably somewhat low due to valerate, HPA was significantly higher than method of sampling and possible dilution with HPG,and HF'G was significantly higher than water. LPG during the HF period. This pattern continued during the LF period. Isovalerate was Blood Parameters higher for HPG and MPG than for LFG during Plasma urea nitrogen (PUN) is likely a more the HF period. This trend also continued during accurate measure of protein status because this Joumal of Dairy Science Vol. 74. No. 3, 1991
987
DIETARY PROTEIN AND MILK PAT DEPRESSION
TABLE 7. Milk fatty acids by period with contrasts to defect high fiber period treatment differences and percentage change differences when switched from hi@ to low fiber diets. Item
C4l
c6
c8
c10
c12
c14
(peak ara %) High fiber HPA2
HPG MPG LPG
SEM c~ntra~ts~ Low fiber HPA HPG MPG
LPG SEM
conea~ts~
High fiber HPA
HPG MPG
LPG
4.60 5.09
4.38 4.10 .ll
bt c*
Low fiber HPA
HPG MPG
LPG SEM contrast&
.M a* bf c**
NS
Ct
9.22 9.68 928 9.20 .18
NS
At Ct
NS
c186
ScpA-147
LCFA-168
SCFA-169
LCFA-186
44.71 44.52 46.78 47.17
21.06 23.5 1 21.40 20.02
78.93 76.49 78.60 79.98
5528 55.48 53.22 52.83 .48
44.71 44.52 46.78 47.17 .48
C16l5
34.22 31.97 31.82 32.81 .25
.08
..@!
a c
2.30 2.64 2.44 2.18 .09
13.46 13.71 13.98 13.49 -08 NS
3.65 3.71 .05 Ct
3.92
1.88 2.34 2.11 1.78
.% 1.24 1.06 .89
4.44 4.41 4.78 3.98 .05 At
2.66 2.67 2.65 2.44 .03 At Ct
3.%
SEM c0~tra~ts3
2.10 2.52 2.14 1.88
1.64 1.68 1.73 1.52
.02
.44
.44
.48
5.32 5.29 5.66 4.85 .07
a*
NS
C*
C*
NS
NS
35.76 34.84 34.46 35.19 .19
32.73 33.48 33.05 34.81 .27
31.48 31.67 32.45 30.00 .19
67.24 66.5 1 66.90 65.19 .27
32.73 33.48 33.05 34.81 27
NS
NS
NS
68.49 68.32 67.50 70.00 .19 NS
NS
NS
'Length of fatty acid carbon chain, i.e., C4 = butyrate, four-carbon chain. *HPA = ~ i g protein h atfalfa; HPG = high protein grass; MPG = medium protein grass; LPG = low protein grass. 3C0ntrasts among high fiber treatments: a = HPA vs. HF'G, b = HPG vs. MPG. c = HPG vs. LF'G, d = hWG vs. LPG. 4P-ntage change contrasts when switched &omhigh fiber to low fiber diets: A = HPA vs. HPG,B = HPG vs. MF'G, C = HPG vs. LF'G, D = MPG vs. LPG. 'Patty acids c 1 6 9 and c16:1. bng-chain fatty acids: C1gfi c18:1. C182. and C18:3. 7 ~ ~ t short-chain al fany acids: c4to ~ 1 4 . '+om long-chain fatty acids: ~ 1 6 0C16:l. . c189. CIS.. c1s2. and C18:3. TOM short-chain fatty acids: ~4 to ~ 1 6 : 1 . t P < .lo. *P < .05. **P < .01. ***P < .001.
measure is unaffected by the problems of sample site and dilution associated with rumen NH3 N, which was sampled via stomach pump. In agreement with other studies (9, 10, ll), PUN displayed a significant positive correlation with CP intake (Table 6). No significant changes in
PUN occurred when cows were switched to LF. Blood glucose levels (Table 6) were higher for HPG than for HPA or LPG during the HF period. Blood NEFA levels on the HF diets were higher on HPG and MPG than on LPG. Using actual BW, BW change, milk producJournal of Dairy Science Vol. 74, No. 3, 1991
ZMMERWW ET AL.
988
560 540
5 I-
5-
520
500
i i -
w
480 -
460 0
m 440 420
I
-
Low flber lnltiated
I
,
tion, feed intake data, and 1988 NRC feed energy values (14), the HPA and HPG cows were determined to be in negative energy balance during the HF period (HPA = -.8 McWd and HPG = -1.8 MCd/d). COWSfed MPG and LFG were in positive energy balance when fed HF (+3.1 and +2.7 Mcal/d), respectively. Various studies have shown that increasing protein supply to the small intestine can increase body fat mobilization (8,16, 17,28). This could have contributed to the higher milk fat test for the HPG diet during the HF period. When switched to LF, cows on HPG and MPG experienced a greater depression than those on LPG in blood NEFA levels. During the LF period, all groups were in positive energy balance with values of +1.3, +.6, +1.8, and +1.3 McaVd for €PA, HPG, MPG, and LPG,respectively. Jaquette et al. (10) found that high blood NJFA levels during the HF period were associated with the inability of high dietary CP to alleviate milk fat depression when cows were switched to LF diets. Whether this phenomenon is cause or effect is not known, however, as e v i d e n d by the present study; high blood NEFA levels during the HF period did not hamper milk fat synthesis during the HF period. Journal of Dairy Science Vol. 74, No. 3, 1991
I
I
I
HPA
HPG
MPG LPG I
1
Milk Fatty Acids
High Fiber. Milk fatty acid composition is listed in Table 7. Values are listed as peak area percentages, which are similar to weight percentages. Milk butyrate was higher for HPG than for MPG or LFG.Milk c6 was higher for HPG versus HPA, MPG,or LPG. Levels of c8 were higher for HPG than for HPA and LPG. Milk Cl0 was higher for HPG than for LPG. Milk c16 was significantly higher for HPA than for HPG. Cows fed HPG exhibited higher levels of short-chain fatty acids (SCFA) C4 to C14 than those fed LPG. The reverse was true for long-chain fatty acids (LCFA) c16 to c18. Milk SCFA synthesis appeared to be enhanced by increasing CP level within the grass hay diets. This may have been due to either an increase in de novo fatty acid precursors, acetate and P-hydroxybutyrate, or perhaps an inhibition of the mammary enzyme acetylcoenzyme A carboxylase by low CP diets. Actual rumen VFA production rates and blood P-hydroxybutyrate levels were not measured. Cows fed HPG were in negative energy balance when fed HF, whereas LPG cows were in positive energy balance as stated previously. Milk LCFA were expected to be higher for the
DIETARY PRO'K" AND MILK FAT DEPRESSION
higher CP diets because they had higher NeFA during this perid however, this was not evident. Jaquette et al. (10) have postulated that high CP diets may increase fat test by increasing the amino acid supply available for the synthesis of the apoprotein fraction of lipoproteins. Because very low density lipoproteins (VLDL) are the major carriers of preformed fatty acids to the mammary gland for milk fat synthesis, an increased incorporation of LCFA into milk fat should occur as a result. This did not appear to be the case &ring the HF period. Another hypothesis (10) is that the amino acids of soybean meal are required for the foxmation or activation of the enzyme lipoprotein lipase that cleaves preformed fatty acids from chylomicrons and VLDL at the mammary gland to be used for milk fat synths sis. Again, this did not appear to be the case in our study. The increased fat production on the HPG diet may be due to increased de novo synthesis of SCFA within the mammary gland. Lao Fiber. When cows were switched to the LF diets, milk C4 was decreased to a significantly lesser degree for LPG than for HPG. Milk c6 increased more for HPA and LPG than for HPG. Mille cg increased more for LPG and HPA than for HPG. All other percentage changes in milk fatty acid parameters were not significant. When cows were switched to the LF diets, the percentage of milk fatty acids formed from de novo synthesis in the mammary gland increased. The fatty acids involved were C4 to C14 and some of the c16.This may have been primarily a stage of lactation effect, because cows on the LF diets were in a later stage of lactation, losing less BW, and therefore circulating levels of LCFA were decreased (19, 25). Milk C4 acted differently, however, actually decreasing on LP.This probably was due to the ability of milk C4 to be synthesized exclusively from p-hydroxybutyrate without the malonyl pathway of synthesis. Milk C4 tends to react differently than the other SCFA when dietary alterations are performed (4, 23, 24). When cows were switched to the LF diets, DM intakes and BW began to increase (Figure 3); therefore, blood P-hydroxybutyrate levels should have decreased, leading in turn to a decrease in milk C4. Body Weights
Body weights are shown in Figure 3. Cows on all treatments lost weight through wk 6 of
989
lactation; however, no large differences in BW loss occurred due to treatment. All cows gained weight from wk 6 to 12 of lactation and returned to their approximate starting weights. The HPA and MPG groups did weigh more throughout the experiment than the HPG and LPG p u p s . In contrast to previous studies (8, 9, lo), there were no parity x treatment interactions. Previous studies have shown that primiparous cows respond better to high dietary CP than multiparous cows (8, 9). This may be due to a differing partitioning of nutrients between parities. Despite the lack of parity response in this uial, this question deserves further research. CONCLUSIONS
The HPG and MPG diets resulted in higher yields of milk and milk components compared with the LFG diet during the H F period. Also, the HFG diet resulted in higher fat tests in cows fed HF diets; however, the MPG diet was not as effective as the HPG diet in attaining a high fat test during the HF period. The mechanism for increased fat test on the HF diet is not understood, but it appears to be due mainly to an increase in the SCFA of milk, Cows on HPG produced more milk SCFA on HF diets than LFG cows. High CP diets also increased body fat mobilization during the HF period, as indicated by increased blood NEFA levels. This extra body fat did not appear to be transferred to milk fat. It is difficult to draw conclusions about the effectiveness of M P diets in alleviating milk fat depression in cows switched to LF diets because we did not observe a difference in milk fat depression between the HPG and LFG diets. Rations utilizing alfalfa hay as the forage source increased the severity of milk fat depression in cows switched to LF diets. The effect of dietary CP on milk fat synthesis appears to be postruminal because ruminal acetate:propionate ratios did not differ. However, feeding high levels of dietary CP does result in increased m e n molar percentages of isobutyrate, valerate, and isovalerate. Lowering dietary fiber increased milk CP percentage. Perhaps this would be a reliable method for increasing milk CP in the future. More work is necessary to study the effects of parity on the response of milk fat synthesis to dietary CP. Journal of Dairy Science Vol. 74, No. 3. 1991
ET AL.
990 REFERENCES 1 Andries, J. I., F. X. Buysse, D. L. De Brabander, and B. G. Cottyn. 1987. Isoacids in mminant mlritiox~ their role in raminal and intermediary metabolism and possible influences on @ormark%s-a review. Anim. Feed Sci. Technol. 18169. 2Anonymous. 1975. GC separation of VFA’s C 2 a . Bull. 510. Supelco, Inc., Bellefonte, PA. 3 Anonymous. 1981. The colorimetric determbation of urea nitrogen. Bull. 640. Sigma Chemical Co.. St.
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