Effects of Daily Exogenous Oxytocin on Lactation Milk Yield and Composition S. D. NOSTRAND, D. M. GALTON, H. N. ERB,' and D. E. BAUMAN Department of Animal Science Cornel University Ithaca, NY 14853 ABSTRACT
Abbreviation key: IMI = intramammary infection, ME = mature equivalent, RCBD = randomized complete block design.
Eighty-four Holstein cows were used to determine effects of exogenous oxytocin on 305-:
24
:!!
22
:i
20 18 16 14 12 10 0
10
15
20
25
30
35
40
45
50
Week o. Lac1atlon
Figure 3. Lactation curves by treatment derived from Wood's 1actation model using least squares means for cows without mastitis. Journal of DaiIy Science Vol. 74, No.7, 1991
2124
NOSTRAND ET AL.
TABLE 2. Least squares means of Wood's coefficients for milk production and somatic cell linear scores by treatment and incidence of mastitis.
Treatments
Wood's coefficients
No. of cows
a
X
Somatic cell linear scores
c
b X
SE
X
SE
X
SE
.17 .19
.018 .018
.023 .029
.0022 .0023
2.43 2.58
.17
27.89 1.18 26.90 1.28
.182 .026 .0177 .028
.026 .026
.0033 .0036
3.00 3.19
.36 .32
2726 1.11 25.56 1.01
.158 .204
.020 .031
.0031 .0029
1.92
2.12
20 .16
All cows Oxytocin Control
35 38
27.58 26.23
Cows with mastitis Oxytocin Control
17 16
Cows without mastitis Oxytocin Control
18 22
SE .81 .82
change in the slope of the lactation curve. This mechanism is supported by studies with rats (28) and mice (6), in which biochemical and histological evidence suggested that oxytocin can maintain secretory cell integrity during late lactation. Rather than a direct effect on the secretory tissue, exogenous oxytocin may simply be preventing a normal decline in milk yield due to changes in endogenous oxytocin secretion. Wachs et al. (31) suggested that the sensitivity of the neuroendocrine reflex declines with advancing lactation. Therefore, less hormone is available for milk ejection as lactation progresses. This could result in decreased stimulus to eject milk and in an increase in the onset of alveolar involution. There was no sudden increase in milk pr0duction of treated cows with our data, but rather a slow increase in production as lactation progressed. However, the design in this study did not allow for the detection of a sudden increase in milk production because the use of exogenous oxytocin started soon after calving. Thus, a mechanism that can not be directly inferred from our data is that exogenous oxytocin may allow for a more complete evacuation of milk from the udder at each milking (sudden production increase), thereby lowering intramammary pressure, which may allow greater secretory activity between milkings. Evidence to support the udder evacuation mechanism is reported in the literature (4, 17, 21, 30). Alternatively, Henderson (14) indicated that secretion in goats may be limited by feedback Journal of Dairy Science Vol. 74, No.7, 1991
.025 .022
.18
from an unidentified chemical in the milk rather than by internal alveolar pressure. Tucker (30) suggested chemical feedback as a possibility in cows. Whether the limitation is due to intramammary pressure or chemical feedback, oxytocin may act to lower the amount of residual milk in the alveoli, thus decreasing the stimulus to lower secretion rate. Brandsma (5) reported that decreasing the amount of residual milk left in the udder after normal milking (by the use of exogenous oxytocin) resulted in an increase in milk production during the later part of lactation compared with cows without residual milk removed. Similar changes in milk yield (13 to 17%) were seen with increases in milking frequency (three times milking) (7). Thus, a reduction in the amount of milk left in the udder appears to influence total production. Although data from this study do not support mechanisms that produce an immediate change in volwne of secretion, small increases in milk yield during early lactation may not have been detectable with the treatment regimen. Therefore, these mechanisms remain plausible. If the onset of oxytocin treatment had been delayed, an initial step increase in milk yield may have been observed. The actual mechanism of exogenous oxytocin on milk production may be a combination of the reversal of the decrease in hormonal sensitivity and lowering of pressure or chemical feedback. Along with consideration of the possible mechanisms, the time interval between initiation of udder preparation and machine attachment may have a potential effect on milk
2125
EXOGENOUS OXYTOCIN AND Mll..K. PERFORMANCE TABLE 3. Number of new intramammary infections by cow and by treatment. Number of new intramammary infections by cow Treatment Oxytocin Control
Staphylococcus aureus
No. of total cows
Streptococcus
I
35
o
38
species
Environmental organisml
7
8
I
4
7
5
=
lEnvironmental organism Escherichia coli, Klebsiella. and Pseudomonas. Other yeast, Nocardia, Prototheca, Corynebacterium pyogenes, and Enterobacter.
2ainicaI
Total
Other
= Pasteurella, Proteus, Serratia,
mastitis with a bacteriological isolate.
production. The long-standing recommendation has been a I-min interval for continuous stimulation or 30 s of stimulation followed by 30 s of delay before machine attachment to achieve optimal milk ejection; however, recent findings indicate that this treatment may not be as necessary or that the degree of benefit may not be as great with high producing cows and modem milking equipment (11, 19, 23, 31). A recent lactation study (19) indicated no statistical difference for lactation milk yield with full stimulation, a 60-s routine, compared with a minimum stimulation, a 15-s routine, before machine attachment. In our study, machines were attached within 3 min after the initiation of a good udder preparation, with most machines attached within I min. Machine attachment varied for all cows at each milking because the treatment groups were milked together, and cows entered the parlor in a random order. Additional research is warranted to determine the possible interaction for the timing of machine attachment with and without the use of exogenous oxytocin. Milk Composition
Overall mean fat and protein percentages did not differ (P > .40) for oxytocin and
control cows during lactation. Fat averaged 3.59% ± .066 and 3.61 % ± .067 and protein 3.11% ± .040 and 3.15% ± .040 for oxytocin and control cows. Changes in milk fat content reported by others during short-term experiments (8, 11, 23, 26) were not seen in this full lactation study. The results suggest that chronic use of oxytocin does not alter milk content of fat and protein. However, production of total quantity of milk fat and milk protein varied proportionally with changes in lactation milk yield. Health No significant differences (P > .10) existed for somatic cell linear scores by treatment (fable 2). Numbers of cows by treatment and type of mastitis organisms are in Table 3. Number of total cows determined to have mastitis (both subclinical mastitis and clinical mastitis with an isolate) was 17 and 16 for oxytocin group and controls. Cows showing clinical signs with negative bacteriological culture numbered 4 for the oxytocin group and 1 for the control group. Although the presence of mastitis appeared to cause a nonresponse to exogenous oxytocin, the use of exogenous oxytocin did not appear to affect the incidence
TABLE 4. Reproductive parameters for all cows by treatment.
Treatment
cows
No. of pregnant cows
Oxytocin
35
28
7
Co~
~
~
9
No. of
No. of cows not
pregnant
Days openl
X
SD
136.7 130.0
58 54
IPregnant cows only; pregnant cows conf"mned by rectal palpation during lactation. Journal of Dairy Science Vol. 74, No.7, 1991
2126
NOSTRAND ET AL.
of mastitis, as evidenced by the even distribution of infections between groups and by the nonsignificance of the somatic cell linear scores. Previous work (3, 13) has shown that the use of exogenous oxytocin may affect the length of the estrous cycle when administered at 100 IU/d or greater. In this study, cows received 20 IU at each milking twice daily, and no apparent detrimental effects were observed (Table 4). No unfavorable reactions were noted to the twice daily injections. Several cows exhibited increased anxiety during udder preparation early in the study; however, cows adjusted within a few weeks. No abscesses were detected at the site of injections. CONCLUSIONS
Administration of exogenous oxytocin to cows for a full lactation increased milk yield by 11.6% over cows not receiving oxytocin, but no alteration in milk composition or apparent difference in reproductive or health parameters occurred. It appears that most of the increase in milk yield occurred during the declining phase of lactation. Possible mechanisms for this action were proposed, but future research is needed to understand these mechanisms and the importance of timing of machine attachment relative to initiation of udder preparation. Mastitis appeared to negate the effect of oxytocin administration. Warning
The use of exogenous oxytocin in herds for milk production reasons is presently prohibited
by the United States Food and Drug Administration; thus, its use as a management tool is illegal and not recommended. REFERENCES 1 Adams. H. P., and N. N. Allen. 1952. The value of oxytocin for reducing fluctuations in milk and fat yield. J. Dairy Sci. 35:1117. 2 Adams, H. P., and N. N. Allen. 1952. The effect of removal of residual milk by use of oxytocin upon the yield and fat content of subsequent milkings. J. Dairy Sci. 35:1121. 3 Armstrong. D. T.• and W. Hansel. 1959. Alteration of the bovine estrous cycle with oxytocin. J. Dairy Sci. 42:533. Journal of Dairy Science Vol. 74, No.7, 1991
4Benson, G. K., and S. J. Folley. 1957. The effect of oxytocin on mammary gland involution in the mt. J. Endocrinol. 16:189. 5 Brandsma, S. 1978. The relation between milking. residual milk and milk yield. Page 47 in Proc. Int. Symp. Machine Milking, Louisville, KY, Natl. Mastitis COUIlC., Washington, DC. 6 CaruoIo, E. V. 1971. Exogenous oxytocin and lactation in the mouse. J. Dairy Sci. 54:1207. 7 DcPeters, E. J., N. E. Smith, and J. Acedo-Rico. 1985. Three or two times daily milking of older cows and first lactation cows for entire lactations. J. Dairy Sci. 68:123. 8 Donker, J. D., J. H. Koshi, and W. E. Petersen. 1954. The effects of hourly milking with the aid of intravenous injection of oxytocin. J. Dairy Sci. 37:1261. 9 Ely, F., and W. E. Petersen. 1941. Factors involved in the ejection of milk. J. Dairy Sci. 24:211. 10 Gavin, W. 1913. On the effects of administration of extracts of pituitary body and corpus luteum to milch cows. Q. J. Exp. Physiol. 6:13. 11 Gorewit, R C., and R Sagi. 1984. Effects of exogenous oxytocin on production and milking variables of cows. J. Dairy Sci. 67:2050. 12 Graf, G. C. 1968. Effects of oxytocin injected intramuscularly and intravenously on milk ejection of bovine. J. Dairy Sci. 51(SuppI. 1):628.(Abstr.) 13 Hansel, W., and W. C. Wagner. 1960. Luteal inhibition in the bovine as a result of oxytocin injections, uterine dilatation, and intrauterine infusions of seminal and preputial fluids. J. Dairy Sci. 43:796. 14 Henderson, A. J., and M. Peaker. 1984. Feed-back control of milk secretion in the goat by a chemical in milk. J. Physiol. 351:39. 15 Hill, R. L., and S. Simpson. 1914. The effect of pituitary extract on the secretion of milk in the cow. Proc. Soc. Exp. BioI. Med. 11:82. 16 Knodt, C. B., and W. E. Petersen. 1942. The effect of the continuous injection of pitocin upon milk and milk fat production. J. Dairy Sci. 25:709. 17Knodl, C. B., and W. E. Petersen. 1944. The effect of complete evacuation of the mammary gland by pitocin upon milk and fat production. J. Dairy Sci. 27:449. 18 Koshi, J. H.• and W. E. Petersen. 1955. Complementary milk and its relationship to lactation. J. Dairy Sci. 38:788. 19 Merrill, W. G., R. Sagi, L. G. Petersson, T. V. Bui, H. N. Erb, D. M. Galton, and R. Gates. 1987. Effects of premilking stimulation on complete lactation milk yield and milking performance. J. Dairy Sci. 70:1676. 20 National Mastitis Council. 1981. Microbiological procedures for usc in the diagnosis of bovine mastitis. Natl. Mastitis Counc., Washington, DC. 21 Petersen, W. E., and T. V. Rigor. 1932. Relation of pressure to mte and quality of milk secreted. Proc. Soc. Exp. BioI. MOO. 30:254. 22 Premachandra, B. N., G. W. Pipes, and R. von Bersvordt-Wallmbe. 1959. Comparison of the intravenous and subcutaneous injections of oxytocin in the lactating cow. J. Dairy Sci. 42:918. 23 Sagi, R., R. C. Gorewil, and D. B. Wilson. 1980. Role of exogeDOns oxytocin in eliciting milk ejection in dairy cows. J. Dairy Sci. 63:2006.
EXOGENOUS OXYTOCIN AND MILK PERFORMANCE
24 SAS® User's Guide: Statistics, Version 5 Edition. 1985. SAS Inst, Inc., Cary, NC. 25 Shaw, J. C. 1942. The effect of oxytocin on milk and milk fat secretion. 1. Dairy Sci. 25:1051. 26 Smith, V. R. 1947. The effect of milking at short intervals with and without injections of oxytocin. 1. Dairy Sci. 30:703. 27 Sprain, D. G., V. R. Smith, W. 1. Tyler, and O. T. Fosgate. 1954. The effect on milk and fat production of injections of oxytocin at alternate 14-day periods during lactation. 1. Dairy Sci. 37:195. 28 Thatcher, W. W., and H. A. Tucker. 1970. Lactational performance of rats injected with oxytocin, cortisol21-acetate, prolactin, and growth hormone during prolonged lactation. Endocrinology 86:237. 29 Thompson, P. D., M. 1. Paape, and J. W. Smith. 1973.
2127
Residual milk yield as affected by dose and time of injection of oxytocin. J. Dairy Res. 40:221. 30 Tucker, H. A., R. P. Reece, and R. E. Mather. 1961. Udder capacity estimates as affected by rate of milk secretion and intramammary pressure. J. Dairy Sci. 44:1725. 31 Wachs, E. A., R C. Gorewit, and W. B. Currie. 1984. Oxytocin concentration of cattle in response to milking stimuli through lactation and mammary involution. Domest. Anim. Endocrinol. 1:141. 32 Wood, P.D.P. 1967. Algebraic model of the lactation curve in cattle. Nature (Lond.) 216:164. 33 Wildman, E. E., G. M. lones, P. E. Wagner, and R. L. Boman. 1982. A dairy cow body condition scoring system and its relationship to selected production characteristics. J. Dairy Sci. 65:495.
Journal of Dairy Science Vol. 74, No.7, 1991
Abbreviation key: IMI = intramammary infection, ME = mature equivalent, RCBD = randomized complete block design.
Eighty-four Holstein cows were used to determine effects of exogenous oxytocin on 305-:
24
:!!
22
:i
20 18 16 14 12 10 0
10
15
20
25
30
35
40
45
50
Week o. Lac1atlon
Figure 3. Lactation curves by treatment derived from Wood's 1actation model using least squares means for cows without mastitis. Journal of DaiIy Science Vol. 74, No.7, 1991
2124
NOSTRAND ET AL.
TABLE 2. Least squares means of Wood's coefficients for milk production and somatic cell linear scores by treatment and incidence of mastitis.
Treatments
Wood's coefficients
No. of cows
a
X
Somatic cell linear scores
c
b X
SE
X
SE
X
SE
.17 .19
.018 .018
.023 .029
.0022 .0023
2.43 2.58
.17
27.89 1.18 26.90 1.28
.182 .026 .0177 .028
.026 .026
.0033 .0036
3.00 3.19
.36 .32
2726 1.11 25.56 1.01
.158 .204
.020 .031
.0031 .0029
1.92
2.12
20 .16
All cows Oxytocin Control
35 38
27.58 26.23
Cows with mastitis Oxytocin Control
17 16
Cows without mastitis Oxytocin Control
18 22
SE .81 .82
change in the slope of the lactation curve. This mechanism is supported by studies with rats (28) and mice (6), in which biochemical and histological evidence suggested that oxytocin can maintain secretory cell integrity during late lactation. Rather than a direct effect on the secretory tissue, exogenous oxytocin may simply be preventing a normal decline in milk yield due to changes in endogenous oxytocin secretion. Wachs et al. (31) suggested that the sensitivity of the neuroendocrine reflex declines with advancing lactation. Therefore, less hormone is available for milk ejection as lactation progresses. This could result in decreased stimulus to eject milk and in an increase in the onset of alveolar involution. There was no sudden increase in milk pr0duction of treated cows with our data, but rather a slow increase in production as lactation progressed. However, the design in this study did not allow for the detection of a sudden increase in milk production because the use of exogenous oxytocin started soon after calving. Thus, a mechanism that can not be directly inferred from our data is that exogenous oxytocin may allow for a more complete evacuation of milk from the udder at each milking (sudden production increase), thereby lowering intramammary pressure, which may allow greater secretory activity between milkings. Evidence to support the udder evacuation mechanism is reported in the literature (4, 17, 21, 30). Alternatively, Henderson (14) indicated that secretion in goats may be limited by feedback Journal of Dairy Science Vol. 74, No.7, 1991
.025 .022
.18
from an unidentified chemical in the milk rather than by internal alveolar pressure. Tucker (30) suggested chemical feedback as a possibility in cows. Whether the limitation is due to intramammary pressure or chemical feedback, oxytocin may act to lower the amount of residual milk in the alveoli, thus decreasing the stimulus to lower secretion rate. Brandsma (5) reported that decreasing the amount of residual milk left in the udder after normal milking (by the use of exogenous oxytocin) resulted in an increase in milk production during the later part of lactation compared with cows without residual milk removed. Similar changes in milk yield (13 to 17%) were seen with increases in milking frequency (three times milking) (7). Thus, a reduction in the amount of milk left in the udder appears to influence total production. Although data from this study do not support mechanisms that produce an immediate change in volwne of secretion, small increases in milk yield during early lactation may not have been detectable with the treatment regimen. Therefore, these mechanisms remain plausible. If the onset of oxytocin treatment had been delayed, an initial step increase in milk yield may have been observed. The actual mechanism of exogenous oxytocin on milk production may be a combination of the reversal of the decrease in hormonal sensitivity and lowering of pressure or chemical feedback. Along with consideration of the possible mechanisms, the time interval between initiation of udder preparation and machine attachment may have a potential effect on milk
2125
EXOGENOUS OXYTOCIN AND Mll..K. PERFORMANCE TABLE 3. Number of new intramammary infections by cow and by treatment. Number of new intramammary infections by cow Treatment Oxytocin Control
Staphylococcus aureus
No. of total cows
Streptococcus
I
35
o
38
species
Environmental organisml
7
8
I
4
7
5
=
lEnvironmental organism Escherichia coli, Klebsiella. and Pseudomonas. Other yeast, Nocardia, Prototheca, Corynebacterium pyogenes, and Enterobacter.
2ainicaI
Total
Other
= Pasteurella, Proteus, Serratia,
mastitis with a bacteriological isolate.
production. The long-standing recommendation has been a I-min interval for continuous stimulation or 30 s of stimulation followed by 30 s of delay before machine attachment to achieve optimal milk ejection; however, recent findings indicate that this treatment may not be as necessary or that the degree of benefit may not be as great with high producing cows and modem milking equipment (11, 19, 23, 31). A recent lactation study (19) indicated no statistical difference for lactation milk yield with full stimulation, a 60-s routine, compared with a minimum stimulation, a 15-s routine, before machine attachment. In our study, machines were attached within 3 min after the initiation of a good udder preparation, with most machines attached within I min. Machine attachment varied for all cows at each milking because the treatment groups were milked together, and cows entered the parlor in a random order. Additional research is warranted to determine the possible interaction for the timing of machine attachment with and without the use of exogenous oxytocin. Milk Composition
Overall mean fat and protein percentages did not differ (P > .40) for oxytocin and
control cows during lactation. Fat averaged 3.59% ± .066 and 3.61 % ± .067 and protein 3.11% ± .040 and 3.15% ± .040 for oxytocin and control cows. Changes in milk fat content reported by others during short-term experiments (8, 11, 23, 26) were not seen in this full lactation study. The results suggest that chronic use of oxytocin does not alter milk content of fat and protein. However, production of total quantity of milk fat and milk protein varied proportionally with changes in lactation milk yield. Health No significant differences (P > .10) existed for somatic cell linear scores by treatment (fable 2). Numbers of cows by treatment and type of mastitis organisms are in Table 3. Number of total cows determined to have mastitis (both subclinical mastitis and clinical mastitis with an isolate) was 17 and 16 for oxytocin group and controls. Cows showing clinical signs with negative bacteriological culture numbered 4 for the oxytocin group and 1 for the control group. Although the presence of mastitis appeared to cause a nonresponse to exogenous oxytocin, the use of exogenous oxytocin did not appear to affect the incidence
TABLE 4. Reproductive parameters for all cows by treatment.
Treatment
cows
No. of pregnant cows
Oxytocin
35
28
7
Co~
~
~
9
No. of
No. of cows not
pregnant
Days openl
X
SD
136.7 130.0
58 54
IPregnant cows only; pregnant cows conf"mned by rectal palpation during lactation. Journal of Dairy Science Vol. 74, No.7, 1991
2126
NOSTRAND ET AL.
of mastitis, as evidenced by the even distribution of infections between groups and by the nonsignificance of the somatic cell linear scores. Previous work (3, 13) has shown that the use of exogenous oxytocin may affect the length of the estrous cycle when administered at 100 IU/d or greater. In this study, cows received 20 IU at each milking twice daily, and no apparent detrimental effects were observed (Table 4). No unfavorable reactions were noted to the twice daily injections. Several cows exhibited increased anxiety during udder preparation early in the study; however, cows adjusted within a few weeks. No abscesses were detected at the site of injections. CONCLUSIONS
Administration of exogenous oxytocin to cows for a full lactation increased milk yield by 11.6% over cows not receiving oxytocin, but no alteration in milk composition or apparent difference in reproductive or health parameters occurred. It appears that most of the increase in milk yield occurred during the declining phase of lactation. Possible mechanisms for this action were proposed, but future research is needed to understand these mechanisms and the importance of timing of machine attachment relative to initiation of udder preparation. Mastitis appeared to negate the effect of oxytocin administration. Warning
The use of exogenous oxytocin in herds for milk production reasons is presently prohibited
by the United States Food and Drug Administration; thus, its use as a management tool is illegal and not recommended. REFERENCES 1 Adams. H. P., and N. N. Allen. 1952. The value of oxytocin for reducing fluctuations in milk and fat yield. J. Dairy Sci. 35:1117. 2 Adams, H. P., and N. N. Allen. 1952. The effect of removal of residual milk by use of oxytocin upon the yield and fat content of subsequent milkings. J. Dairy Sci. 35:1121. 3 Armstrong. D. T.• and W. Hansel. 1959. Alteration of the bovine estrous cycle with oxytocin. J. Dairy Sci. 42:533. Journal of Dairy Science Vol. 74, No.7, 1991
4Benson, G. K., and S. J. Folley. 1957. The effect of oxytocin on mammary gland involution in the mt. J. Endocrinol. 16:189. 5 Brandsma, S. 1978. The relation between milking. residual milk and milk yield. Page 47 in Proc. Int. Symp. Machine Milking, Louisville, KY, Natl. Mastitis COUIlC., Washington, DC. 6 CaruoIo, E. V. 1971. Exogenous oxytocin and lactation in the mouse. J. Dairy Sci. 54:1207. 7 DcPeters, E. J., N. E. Smith, and J. Acedo-Rico. 1985. Three or two times daily milking of older cows and first lactation cows for entire lactations. J. Dairy Sci. 68:123. 8 Donker, J. D., J. H. Koshi, and W. E. Petersen. 1954. The effects of hourly milking with the aid of intravenous injection of oxytocin. J. Dairy Sci. 37:1261. 9 Ely, F., and W. E. Petersen. 1941. Factors involved in the ejection of milk. J. Dairy Sci. 24:211. 10 Gavin, W. 1913. On the effects of administration of extracts of pituitary body and corpus luteum to milch cows. Q. J. Exp. Physiol. 6:13. 11 Gorewit, R C., and R Sagi. 1984. Effects of exogenous oxytocin on production and milking variables of cows. J. Dairy Sci. 67:2050. 12 Graf, G. C. 1968. Effects of oxytocin injected intramuscularly and intravenously on milk ejection of bovine. J. Dairy Sci. 51(SuppI. 1):628.(Abstr.) 13 Hansel, W., and W. C. Wagner. 1960. Luteal inhibition in the bovine as a result of oxytocin injections, uterine dilatation, and intrauterine infusions of seminal and preputial fluids. J. Dairy Sci. 43:796. 14 Henderson, A. J., and M. Peaker. 1984. Feed-back control of milk secretion in the goat by a chemical in milk. J. Physiol. 351:39. 15 Hill, R. L., and S. Simpson. 1914. The effect of pituitary extract on the secretion of milk in the cow. Proc. Soc. Exp. BioI. Med. 11:82. 16 Knodt, C. B., and W. E. Petersen. 1942. The effect of the continuous injection of pitocin upon milk and milk fat production. J. Dairy Sci. 25:709. 17Knodl, C. B., and W. E. Petersen. 1944. The effect of complete evacuation of the mammary gland by pitocin upon milk and fat production. J. Dairy Sci. 27:449. 18 Koshi, J. H.• and W. E. Petersen. 1955. Complementary milk and its relationship to lactation. J. Dairy Sci. 38:788. 19 Merrill, W. G., R. Sagi, L. G. Petersson, T. V. Bui, H. N. Erb, D. M. Galton, and R. Gates. 1987. Effects of premilking stimulation on complete lactation milk yield and milking performance. J. Dairy Sci. 70:1676. 20 National Mastitis Council. 1981. Microbiological procedures for usc in the diagnosis of bovine mastitis. Natl. Mastitis Counc., Washington, DC. 21 Petersen, W. E., and T. V. Rigor. 1932. Relation of pressure to mte and quality of milk secreted. Proc. Soc. Exp. BioI. MOO. 30:254. 22 Premachandra, B. N., G. W. Pipes, and R. von Bersvordt-Wallmbe. 1959. Comparison of the intravenous and subcutaneous injections of oxytocin in the lactating cow. J. Dairy Sci. 42:918. 23 Sagi, R., R. C. Gorewil, and D. B. Wilson. 1980. Role of exogeDOns oxytocin in eliciting milk ejection in dairy cows. J. Dairy Sci. 63:2006.
EXOGENOUS OXYTOCIN AND MILK PERFORMANCE
24 SAS® User's Guide: Statistics, Version 5 Edition. 1985. SAS Inst, Inc., Cary, NC. 25 Shaw, J. C. 1942. The effect of oxytocin on milk and milk fat secretion. 1. Dairy Sci. 25:1051. 26 Smith, V. R. 1947. The effect of milking at short intervals with and without injections of oxytocin. 1. Dairy Sci. 30:703. 27 Sprain, D. G., V. R. Smith, W. 1. Tyler, and O. T. Fosgate. 1954. The effect on milk and fat production of injections of oxytocin at alternate 14-day periods during lactation. 1. Dairy Sci. 37:195. 28 Thatcher, W. W., and H. A. Tucker. 1970. Lactational performance of rats injected with oxytocin, cortisol21-acetate, prolactin, and growth hormone during prolonged lactation. Endocrinology 86:237. 29 Thompson, P. D., M. 1. Paape, and J. W. Smith. 1973.
2127
Residual milk yield as affected by dose and time of injection of oxytocin. J. Dairy Res. 40:221. 30 Tucker, H. A., R. P. Reece, and R. E. Mather. 1961. Udder capacity estimates as affected by rate of milk secretion and intramammary pressure. J. Dairy Sci. 44:1725. 31 Wachs, E. A., R C. Gorewit, and W. B. Currie. 1984. Oxytocin concentration of cattle in response to milking stimuli through lactation and mammary involution. Domest. Anim. Endocrinol. 1:141. 32 Wood, P.D.P. 1967. Algebraic model of the lactation curve in cattle. Nature (Lond.) 216:164. 33 Wildman, E. E., G. M. lones, P. E. Wagner, and R. L. Boman. 1982. A dairy cow body condition scoring system and its relationship to selected production characteristics. J. Dairy Sci. 65:495.
Journal of Dairy Science Vol. 74, No.7, 1991
