Effects of Bovine Somatotropin on Physiologic Responses of Lactating Holstein and Jersey Cows During Hot, Humid Weather' J. W. WEST, 8. G. MULLINIX? and T. G. SANDIFER Department of Animal Science University of Georgia Coastal Plain Experiment Station Tifton 31793

to minimize the effects of heat stress. (Key words: bovine somatotropin, heat stress, acid-base balance)

ABSTRACT

Thirty-one lactating Holstein and Jersey cows were used to determine the effects of daily injections of 0 or 20 mg of recombinant bST on physiologic responses during hot, humid weather. Body temperature was determined by measuring milk temperature at each milking. Jugular blood was sampled for serum analysis of selected hormones, blood metabolites, and fatty acids, and arterial blood was sampled for blood pH and blood gas analysis. Milk was characterized for fatty acid composition. Blood pH was unchanged, but partial pressure of blood C02, blood bicarbonate, base excess, and total C02 declined with administration of bST. Serum triglycerides increased 89% in cows receiving bST. Blood urea nitrogen tended to decline in cows receiving bST. Serum cortisol, triiodothyronine, and thyroxine did not change, but insulin-like growth factor-1 increased 128% with bST use. Reduced milk shortthain fatty acids, increased milk long-chain fatty acids, and increased blood serum c18:1fatty acid content occurred in cows administered bST and probably reflected tissue mobilization. Cows administered bST in hot weather had higher milk temperatures. Alterations in physiologic and metabolic measures in association with higher milk temperature suggest an interaction of bST use with hot, humid weather and reflect the need

Abbreviation key: ALT = alanine aminotransferase, BUN = blood urea N, IGF-1 = insulinlike growth factor-1, pbST = pituitary-derived bST, pCOp = partial pressure of C02, RH = relative humidity, SP = standardization period, THI = temperature-humidity index, TP = treatment period, T3 = triiodothyronine, T4 = plasma thyroxine. INTRODUCTION

Received May 9, 1990.

Accepted October 17,1990. lSllpported by state and Hatch funds allocated to the Georgia Agricultural Experiment Station and by The Upjohn Company, Kalamazoo, MI. %tatistical and computer Services. 1991 J Dairy Sci 748404351

High environmental heat and relative humidity (RH) can lead to reduced performance and altered physiologic status in lactating dairy cows. Exposure to extreme heat stress by preventing access to shade in hot weather lowered feed intake and milk production in dairy cows (19). The hyperventilation of unshaded cows during hot weather increased blood pH and reduced blood bicarbonate, a measure of blood buffering capacity (19). In addition, exposure to heat stress disturbed metabolic processes. Heatstressed cows had elevated serum cortisol (24) and lower plasma thyroxine (T4) and triiodothyronine (T3) (12) compared with cows in a thennoneutral environment. Recombinant bST increases the milk production of lactating dairy cows, but the effects of bST and heat stress on productive performance of the cow are potentially antagonistic. Zoa-Mboe et al. (25) reported that unshaded cows had higher rectal temperatures compared with shaded cows (40.2 vs. 39.6'C) and that administration of bST increased rectal temperature slightly (39.99 vs. 39.81'C). Fat-corrected milk yield was increased 9.2% with administration of bST, but DMI was not affected However, West et al. (23) found that the p.m. milk temperature of Jerseys was 39.6 and 402°C and

840

PHYSIOLOGIC RESPONSES TO bST IN HOT WJUTHER

that of Holsteins was 39.9 and 40.2'C, respectively, for controls and cows administered bST. The higher milk temperature was associated with increased milk yield but unchanged DMI (23). Cows in environmental chambers had lower plasma T3 when exposed to heat stress, but administration of pituitaryderived bST (pbST) during heat stress caused no additional changes in plasma T3 (15). Plasma T4 was not altered by the first heat phase. The heat stress environment was 28.9'C and 55% RH in the environmental chambers (15), below the 34.6.C and 59.8% R H mean maximum daytime conditions reported in the study by West et aL (23). Extremely hot and humid environmental conditions in conjunction with administration of bST may impact on metabolic processes of the lactating daily cow. The objective of this study was to determine the effect of bST use in a hot, humid environment on blood acid-base chemistry, blood serum hormones and metabolites, and blood and milk fatty acids of lactating dairy cows.

841

daily milk production during the SP was used to rank cows from highest to lowest milk production within breed. Cows were blocked into groups of two by milk production rank within breed for assignment to experimental treatments. Upon assignment to treatment, cows began the 80 d treatment period (TP), which lasted from June 13 through August 31. Daily injections of 2 ml of vehicle or 20 mg bST were administered intramuscularly following the morning milking. The bST and the method of administration have been described by West et al. (23). Sampllng

Blood samples were taken by jugular venipuncture once during the SP (June 7) and on d 3,31, 59, and 80 (June 15, July 13, August 10, and August 31) of the TP. Blood serum was analyzed the day of sampling for selected components using a Ciba Corning Impact 400-E with Gilford reagents as described in Gilford Diagnostics clinical chemistry procedures workbook (Ciba-Corning Diagnostics Corp., Gilford Systems, Oberlin, OH>.Blood serum MATERIALS AND METHODS was frozen until analyzed for T4 by radioimmunoassay and for T3 by resin uptake (OrgaManagement non Teknika Corp., Durham, NC), insulin-like Sixteen multiparous Holstein cows and 18 growth factor-1 (IGF-1) as described by Dahl multiparous Jersey cows were used. A Holstein et al. (4), and cortisol (20). An additional alicow receiving the control treatment was omit- quot of blood serum and a I d composite of ted for sore feet. A Holstein cow receiving the milk (sampled within 2 d of the blood sample) bST treatment developed severe mastitis fol- were frozen for later analysis of fatty acid lowing a period of heavy rain during which the composition following lipid extraction (7) and free stalls became wet. Environment was proba- subsequent methylation (13). Fatty acid methyl bly a contributing factor. A Jersey cow receiv- esters were separated and quantified using a gas ing the control treatment had very erratic feed chromatograph (Hewlett-Packard 5890A gas intake, which was related to her inability to chromatograph; Hewlett-Packard Co., Avonoperate the magnetic gate feeder consistently. dale, PA) fitted with a wide bore capillary All data from these cows were omitted, and 31 column (Megabore DB-225; J and W Scientific, cows completed the study. Average DIM at Folsom, CA). Arterial blood from the coccygeal artery was initiation of the study were 187 and 178 d for the Holstein and Jersey cows. Feeding, man- sampled once during the SP (May 31) and on d agement, and housing of research cows were as 9, 37.67, and 79 (June 21, July 19, August 18, described by West et al. (23). Cows were and August 30) of the TP. Samples were obshaded with no supplemental cooling and were tained between 1300 h and 1500 h, generally in total confinement. when ambient temperatures were highest. SamCows were adapted to the experimental con- ples were taken using heparinized syringes. ditions for 19 d, followed by a 14 d standardi- Care was taken to exclude all air from the zation period (SP) from May 30 through June syringes, which were then capped with rubber 12. The SP was used to collect pretreatment caps, put on ice immediately, and analyzed data for covariate analysis. In addition, average within approximately 30 min for blood pH and Journal of Dairy Science Vol. 74, No. 3, 1991

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WEST ET AL.

gases using a blood gas analyzer (Corning 168 blood analyzer, Ciba-Corning Diagnostics Corp., Medfield, MA) and for blood Na and K using an electrolyte analyzer with ion specific electrodes (Coming 902 Na/K system, CibaComing Diagnostics Corp., Medfield, MA). The body temperature of individual cows was determined by measuring the milk temperature at each milking. A thermocouple in the short milk tube of each milker recorded the milk temperature (Temp-Sense, Udder Health Systems, Bellingham, WA). A comparison of rectal and milk temperatures of 131 cows in our herd yielded an r value of .89. The relationship was highly significant, indicating the strong relationship between the rectal and milk temperatures. Statistical Analysis

A randomized complete block design with repeated measures was used to analyze data. The model was

+ Bi + Tj + BTij + bXij + C(BT)ip + Sk + BS& + TSp + BTSi$ + &ju

Yijkl = p

where

p = the mean intercept; Bi = effect of breed i, i = 1,2; T, = effect of bST treatment j, j = 132; BTij = breed by treatment interaction; b = regression effect of the covariate; Xi, = variable measured during the SP, covariate (XijTX. .); C(BT)ip = effect of cow k within breed i, treatment j less the effect of the covariate (error a); Sk = effect of sample k, k = 1,. .4; BSk = breed by sample interaction; TSjk = treatment by sample interaction; BTSijk = the three-way interaction; and &w = the repeated measures error (error b). Data were analyzed by the general linear models procedure of SAS (18). Journal of Dairy Science Vol. 74, No. 3, 1991

RESULTS AND DISCUSSION

Environmental conditions during the study are shown in Figure 1. The mean maximum and minimum dry bulb temperature and temperature-humidity index (THI) during the TP were 34.6'C, 22.2'C and 85.8, 71.5, respectively. Temperatures exceeding 20'C may result in a reduction in feed intake and an increase in maintenance costs in lactating dairy cows (16). and milk production starts to decline when the THI exceeds 71 (8). Minimum THI during the TP also rose to values that were adequate to produce heat stress, at least for periods of 3 to 5 d, and this is certainly true for mean maximum TM (Figure 1). The milk temperature of cows administered bST was greater compared with controls at both a.m. and p.m. milkings, and a breed by treatment interaction was detected (Table 1). The milk temperature of all cows increased with rising environmental temperature, but the increase in temperature of Jerseys administered bST was greater compared with the Holsteins receiving bST treatment, whereas control Jerseys were cooler than control Holsteins (Figure 2). In other reported heat stress studies, rectal temperature was not increased for cows receiving pbST (15, 21) or was increased for cows receiving bST (25). In these studies, daily milk production of bST-treated cows increased over controls approximately .8 kg (15), 2.4 kg (21), and 1.1 kg (25), whereas production of bST cows improved over controls by 4 kg on average in our work, although the change in production was highly dependent upon production prior to administration of bST (23). Tyrrell et al. (22) reported greater heat energy production during thermoneutral conditions in lactating cows administered pbST compared with controls. This was associated with a 3.3 kg (11.9%) increase in milk production. Johnson et al. (8) reported that milk production declined more rapidly in higher producing cows in comparison with lower producing cows as the THI increased from 71 to 91, an indication that the higher producing cows were subject to greater heat stress. Increasing environmental temperature minimizes the temperature differential between the cow and the environment, thus reducing the effectiveness of radiation and conduction of heat to the surrounding environment. Reliance upon evaporative cooling (respi-

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PHYSIOLOGIC RESPONSES TO bST IN HOT WEATHER

Treatment Period

*.

I

I --- Minimum I

Moy 23

Jun 6

Jun 20

Jul 4

Jul 18

Aug 1

Aug 15

Aug 29

Calender Date Figure 1. Environmental coditions during the study.

Journal of Datry Science Vol. 74, No. 3, 1991

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WEST ET AL.

TABLE 1. Least squares means for milk temperatures.

Component

0 mgld

am. temperature, 'C Holstein Jmey p.m. temperahm, 'C Holstein Jersey

39.3 39.5 39.1 39.7 39.9 39.6

IB = reed;

Treatment significance

bST 20 mgfd

39.7 39.7 39.6 40.2

ration and sweating) increases sharply as ambient temperature rises from 1WC to 3572 (9). However, high humidity minimizes the effectiveness of evaporative cooling. The mean minimum RH was 59.8% (daytime), and the mean maximum RH was 100% (nighttime). Heat production in association with increased milk production from bST use in conjunction with the inability to dissipate excess body heat effectively may have resulted in the higher milk temperatures observed (Table 1; Figure 2). Blood pH was not changed by treatment, but blood partial pressure of C02 (pCOz), bicarbonate, base excess, and total C02 were lower in cows administered bST (Table 2). Schneider et al. (19)observed that cows with no shade in hot weather had lower blood pC@, bicarbonate, and total C02 as well as increased blood pH compared with shaded cows. The unshaded cows had a higher rectal temperature and more rapid respiratory rate. As the milk temperature of cows increased, blood pC02 declined (Figure 3), indicating a more rapid respiratory rate. Blood pC02 was lower in cows administered bST compared with controls, probably as a result of the higher milk temperatures observed (Table 1; Figure 2). h e breed by treatment interaction detected for pC02 content supports this concept. Holsteins had blood pCOz contents of 31.82 (control) and 30.03 @ST) mm Hg,whereas Jersey pC02 was 33.78 (control) and 28.85 (bST) mm Hg. Jerseys administered bST had lower blood pC0, compared with Holsteins. Hyperventilation in response to heat stress causes a reduction in blood pC02 so that less C02 is available for carbonic acid formation. Jerseys administered bST had a greater increase in milk temperature over controls compared with Holsteins. The decrease in Journal of Dairy Science Vol. 74, No. 3, 1991

(P 4

other effects1

.I

.01

B,BxT

.O1

B X T

.1 .1

.1 .1

40.2 40.2

T = treatment; effects significant at P