Injection Of bST Research Articles

Paradoxical increases of circulating nonesterified fatty acids in somatotropin treated cattle undergoing mild disturbances

Effects of various doses of bovine somatotropin (bST) on plasma concentrations of nonesterified fatty acids (NEFA) were studied in a 14-d Latin square with six Holstein heifers. Animals were given daily injections of excipient or bST at 12:00 p.m. and fed twice daily at 7:00 a.m. and 7:00 p.m. On Day 14, plasma NEFA remained low through the day except around the 7:00 p.m. feeding when they were substantially elevated. The elevation was significantly greater in bST-treated animals and corresponded to the excitement of the animals in anticipation of the evening feeding. To further investigate this phenomenon, a second experiment was conducted in which nine growing Holstein steers were fed hourly and received either daily intramuscular (i.m.) injection of excipient or bST (120 mg/kg BW) for 15 d in a crossover design. Daily profiles of NEFA were obtained under undisturbed conditions or concurrently with intensive handling. Although no elevations could be detected in any case in control animals, bST caused a substantial rise in NEFA concentration only when animals were subjected to intensive handling. This suggested that NEFA peaks noted in bST-treated heifers in the first experiment resulted from increased ability of adipose tissue to respond to adrenergic stimulation associated with the anticipation of feeding. Consistent with this hypothesis, plasma NEFA concentrations in bST-treated steers were increased to a greater extent during a challenge involving i.v. injection of epinephrine. This amplification of adipose tissue response by bST must be considered when conducting intensive studies. Even the minimal excitement associated with blood sampling can confound the results regarding lipid mobilization, and this may have contributed to the notion that ST is a lipolytic hormone.

Bovine Somatotropin Dose Titration in Lactating Dairy Ewes. 2. Dose Determination and Factors Affecting the Response

Seventy-four lactating dairy ewes were injected with recombinant bovine somatotropin (bst; sometribove) in a sustained-release formulation. Ewes received 0, 80, 160, or 240mg of bST/14 d from wk 3 to 8 of lactation (part 1) and 0, 80, or 160mg of bST/14 d from wk 11 to 18 of lactation (part 2). The optimal dose of bST was studied as well as the factors (lactation stage, lactation number, initial milk production, body weight, and body condition) possibly affecting the increase in milk production following bST injection. Using a quadratic regression model, the maximum theoretical dose was determined to be 181mg of bST/14 d during the first part of lactation. During the second part of lactation, 143mg of bST/14 d was the maximum theoretical dose. The increment of milk production did not vary with lactation number, but first lactation ewes, in general, responded better than did multiparous ewes. Relative to initial milk production, improvement was greatest for ewes with average milk production (1500 ml/d) that received a dose of 192.3mg of bST/14 d during the first part of lactation; improvement was also measured from the highest producers (2000 ml/d) during the second part of lactation. The best response was obtained from ewes with average body condition (score 3 on a five-point scale where 1 = thin to 5 = fat) and a dose of 200mg of bST/14 d during the first part of lactation; during the second part of lactation, body condition score had no effect. Body weight had no effect on the increment of production at any time.

Follicular Function in Lactating Dairy Cows Treated with Sustained-Release Bovine Somatotropin

The objective of this study was to determine the long-term, short-term, and residual effects of bovine somatotropin (bST) onovarian function. Twenty-six Holstein cows were assigned to one of four groups and given either bST (500mg) orsaline for seven injection cycles (14 d between injections). Groups were 1) seven injection cycles of saline (control), 2) seven injection cycles of bST, 3) three injection cycles of bST followed by four injection cycles of saline, and 4) three injection cycles of saline followed by four injection cycles of bST. Beginning in the middle of the fourth injection cycle, an estrous cycle was synchronized for study. Serum concentrations of bST and insulin-like growth factor I were lowest during saline injections and increased after bST injections. Growth of the first wave dominant follicle was similar; however, start of the second follicular wave was approximately 48h earlier in those cows treated with bST (groups 2, 3, and 4). Cows treated with somatotropin had a similar pattern of follicular growth, but tended to have more class 1 (3 to 5mm) follicles and had more class 2 (6 to 9mm) follicles than did the control cows. Peak concentrations of serum FSH were lower and occurred earlier for the cows treated with bST. There were no differences in the long-term or short-term effects of bST on ovarian function. Furthermore, cows that were switched from bST to saline (group 3) showed a residual effect of bST for up to 3wk.

Effect of Slow-Release Somatotropin on the Pattern of Milk Yield Between and Within Injection Intervals

Thirty-six Alpine goats (10 primiparous and 26 multiparous) were used to study the milk yield response induced by recombinant bST administered in a sustained-delivery vehicle. Eighteen goats were injected with recombinant bST at 4-wk intervals from about 7 wk postpartum for 12 wk. Patterns of milk yield were highly affected by injection of recombinant bST. Milk yield of treated goats increased from day of injection, peaked between d 3 to 5 after treatment, and then decreased progressively. This pattern of response was well fitted by a curve containing two exponential terms: Yi = A × [exp (–k1di – exp (–k2di)], where Yi = daily relative response of treated goats to recombinant bST, computed as a deviation of least squares means of milk yield at day i (i = 0, …, 26) of the treatment interval from least squares means of milk yield at d 27 of the treatment interval; A = constant; k1 and k2 = response rate parameters; and di = day from treatment (i = 0, …, 27). Administration of recombinant bST also improved persistency of lactation, and the overall milk yield response to exogenous hormone was 300 g/d (13.9%) higher than the yield of untreated goats. This response was due to both the short-term response immediately after injection (53%) and the medium-term effect on lactation persistency.

Effect of recombinant bovine somatotropin on follicular growth and quality of oocytes in cattle

Abstract The effect of exogenous bovine recombinant somatotropin (bST) on the recruitment of follicles and quality of oocytes was investigated in the bovine. Single injections of recombinant depot form of bST were given to nine heifers and seven cows at the time of synchronized estrus (day 0). Nine heifers and six cows served as controls. Three days after bST injection the animals were slaughtered, their ovaries dissected and follicles separated into two categories according to diameter (2–5 and 5–10 mm). A total of 196 (53.2%) healthy oocyte-cumulus complexes (OCC) was isolated and selected from 369 follicles (diameter 2–5 mm) of bST treated heifers and 137 (49.8%) OCC from 275 follicles of bST treated cows. After 25–28 h of culture, 164 (44.4%) and 124 (45.1%) oocytes from follicles of heifers and cows, respectively, had developed to the metaphase of the second meiotic division (MII). From control heifers, 122 (35.5%) OCC derived from 344 follicles (2–5 mm) and from control cows 87 (39.7%) OCC derived from 219 follicles were similarly selected for maturation in vitro and 109 (31.7%) and 78 (35.6%) oocytes in control heifers and cows, respectively, developed to the MII stage. The numbers of follicles developed in the experimental and control groups were similar (P > 0.05) but differences were evident in the healthy OCC and oocytes maturing in the treated versus control groups of heifers and cows (χ2 detected at 1% and 5% levels, respectively). The injection of bST resulted in significantly increased levels of growth hormone (GH) in peripheral blood and follicular fluid (FF), the GH levels in blood plasma correlating with the levels in FF. The level of insulin-like growth factor I (IGF-I) in sera from bST treated animals was significantly higher than in sera from control animals (353.6 vs. 155.5 ng ml−1). In conclusion, recombinant bST positively affected (at day 3) the proportion of OCC and oocytes maturing to MII.

Physiological responses to repeated endotoxin challenge are selectively affected by recombinant bovine somatotropin administration to calves

The study determined 1) whether the pretreatment of calves with recombinant bovine somatotropin (bST, sometribove) would alter the change in packed cell volume (PCV), rectal temperature (RT), and the plasma concentrations of Ca 2+, Fe 2+, glucose (G), urea N (PUN), nonesterified free fatty acids (NEFA), albumin (ALB), and blood cell populations after endotoxin challenge (EC) and 2) whether the natural development of physiologic tolerance to repeated EC was affected by bST. Twelve steer beef calves were assigned to either control (-bST) or +bST treatment in equal numbers. Calves were injected intramuscularly with either HCO 3 −-buffered saline or bST (0.1 mg/kg) daily for 5 d. On Day 6, the first EC was administered ( Escherichia coli, 055:B5, 0.2 μg/kg, intravenous bolus in pyrogen-free saline). Saline or bST injections were continued from Day 7 up to the repeat of EC on Day 11. RT and PCV were measured hourly through 12 and 6 hr, respectively. Jugular blood was obtained at 0, +1, 2, 3, 4, 6, 8, 12, and 24 hr relative to each EC. bST had no effect on the increase in RT, the hyperglycemic phase of the G response, the biphasic change in Fe 2+, or increases in NEFA and PUN. PCV increased after each EC only in -bST. The mean decrease in G during the hypoglycemic phase was less in +bST. Hypocalcemic responses were significantly less in +bST. ALB concentrations decreased after each challenge; the response was unaffected by bST. CD2 +, CD4 +, and CD8 + T-lymphocyte populations were unaffected by bST and EC. Overall, the magnitude of change in all plasma variables was less after the second EC compared with the first, either in terms of magnitude or duration. The data suggest that the treatment of calves with bST diminishes the magnitude of hypoglycemic, hypocalcemic, and PCV changes after EC and does not compromise fever response, changes in blood cell populations, or tolerance to repeated EC.

Effect of somatotropin on adipose tissue metabolism: Ontogeny of the enhanced response to adrenergic challenge in the lactating cow

Seven multiparous Holstein cows (>150 d postpartum) were used to evaluate the time course of the chronic adaptation in lipolytic response to adrenergic challenge with bovine somatotropin (bST) treatment. Cows received daily bST (sometribove; 40 mg/d) or excipient injections for 7 d (single reversal design) with a 7-d interim between periods. Epinephrine challenges (1.4 μg/kg body weight intravenously) were administered on Days −2, −1, 1, 2, 3, 5, and 7 of treatment at 10:00 a.m. (15 hr after bST or excipient injection). Frequent blood samples were collected, and concentrations of plasma glycerol (GLY) and nonesterified fatty acids (NEFA) were determined. Treatment with bST increased milk yield 23% (P < 0.05) and milk fat content 33% (P < 0.001) compared with controls. Somatotropin-treated cows entered negative energy balance by Day 3 and had higher basal plasma concentrations of GLY and NEFA than did controls by Day 2 and Day 3, respectively. Response to epinephrine, expressed as area under the response curve corrected for basal, was enhanced by bST treatment, regardless of energy balance. GLY response was greater than control by Day 1 of bST treatment (P < 0.01), and had plateaued by Day 2 (P < 0.001). The NEFA response area was higher than control and had plateaued by Day 1 of bST treatment (P < 0.001). Day 1 represented 15 hr after the first bST injection. Results illustrate that bST treatment results in enhanced in vivo lipolytic response to catecholamine challenge, and the metabolic adaptation is in place by 15 hr after the first bST injection.

Production Responses of Cows to Recombinantly Derived Bovine Somatotropin and to Frequency of Milking

To determine whether bST additively increases milk production in cows milked at different frequencies per day, 118 Holstein primiparous and multiparous cows were milked two or three times daily beginning at parturition and received either 14mg of bST or no injection beginning at d 75 of lactation. Increased milking frequency from two to three times daily increased 3.5% FCM in multiparous (4.7 kg/d) and primiparous (4.1 kg/d) cows over 305 d. Injection of bST increased FCM 4.3 kg/d in multiparous cows and 5.0 kg/d in primiparous cows over 230 d. Increased milking frequency from two to three times daily reduced milk fat and protein percentages in milk, but bST generally did not affect these percentages. Injection of bST prevented increases in body condition score as lactation advanced, but increasing milking frequency from two to three times daily did not. In general, bST and increased milking frequency additively increased FCM in multiparous and primiparous cows.

Effects of Calcium Salts of Long-Chain Fatty Acids Added to a Diet Containing Choice White Grease on Lactation Performance

We studied the effect of increased energy density of diets during early lactation on lactational performance of cows injected with bST. Total mixed rations contained 50% forage (DM) and 2.5% choice white grease during the first 14 wk of lactation. Thirty cows were fed diets with grease as the only supplemental fat, and 15 cows were fed grease plus 1.8% calcium salts of long-chain fatty acids. One-half of the cows fed grease and all cows fed grease plus calcium salts of long-chain fatty acids received 640 mg/28 d of sustained-release bST from 6 to 44 wk postpartum. Calcium salts of long-chain fatty acids did not increase milk yield, 3.5% FCM, or DMI during the first 14 wk postpartum or during 15 to 44 wk postpartum. Administration of bST increased milk fat concentration .4% units during the first 14 wk postpartum. Cows receiving bST had higher DMI and produced 18.5% more 3.5% FCM from 15 to 44 wk postpartum than did controls. Calcium salts of long-chain fatty acids or bST injections did not influence body condition score or BW. These data suggest that cows yielding 30 to 35kg of milk/d during the first 14 wk of lactation may not respond to added fat over 2.5%.

Recombinant bovine somatotropin blunts plasma tumor necrosis factor-alpha, cortisol, and thromboxane-B2 responses to endotoxin in vivo.

In vivo studies determined the effects of recombinant bovine somatotropin (bST; sometribove) administration (0.1 mg/kg.day, im) to calves on the increases in plasma immunoreactive tumor necrosis factor-alpha (TNF alpha), prostacyclin [6-keto-prostaglandin F1 alpha (6KP)], thromboxane-B2 (TXB), and cortisol (C) that occurred after endotoxin challenge (ET). Two ETs were administered 5 days apart to test the effect of bST on the natural attenuation of hormone and cytokine responses that occurs after repeated challenge with endotoxin. Calves (n = 6) were treated with bST for 5 days. On day 6, the first ET was administered (Escherichia coli; 055:B5; 0.2 microgram/kg, i.v.). Blood was sampled before and hourly after ET through 6 h. For the next 4 days, bST injections continued, and ET was repeated 1 day later. Six additional calves were treated with bicarbonate buffer as contemporary controls for the bST and were similarly challenged with endotoxin. Plasma TNF alpha, C, 6KP, and TXB were significantly increased after each ET. The increases in TNF alpha, C, and TXB were blunted after the second Et compared to those after the first in both untreated and bST-treated animals. The increases in plasma TNF alpha and C and peak plasma TXB and TXB/6KP ratios were smaller in bST-treated than in untreated after the first ET. TNF alpha receptor binding was studied in hepatic microsomal fractions from three bST-treated and three untreated calves. Microsomal fractions from bST-treated calves bound 40% less TNF alpha (5.97 vs. 9.96 pmol/mg) than similar fractions from controls. The data indicate that bST decreases TNF alpha, TXB, and C responses to ET and reduces the TNF alpha-binding capacity of hepatic membranes, suggesting a multiplicity of sites where bST might affect the physiological response to endotoxin.

Multi-Farm Use of Bovine Somatotropin for Two Consecutive Lactations and Its Effects on Lactational Performance, Health, and Reproduction

In a two-lactation study, 352 Holstein cows (124 primiparous) from six commercial dairy herds were assigned to daily injections of bST. Dosages were 0 (control), 5.15, 10.3, or 16.5mg/d of bST; injections began 28 to 35 d postpartum. During yr 1, FCM production increased linearly as bST dose increased for primiparous and multiparous cows. However, FCM production for primiparous cows that were retained for yr 2 was not increased by bST, and the increase in FCM for multiparous cows was only 67% of that observed during yr 1 for doses of 10.3 and 16.5mg/d. Milk SCC were not increased with the use of bST during either lactation. Days to conception of multiparous cows during yr 1 tended to increase linearly with increasing dose. Multiparous cows administered bST during yr 2 tended to have a lower conception rate than control cows (71 vs. 87%, respectively). Body condition was decreased linearly by bST dose at the end of yr 1 and 2. prior to receiving bST during yr 2, bST-treated cows regained less body condition than control cows. Number of health disorders at parturition and assisted births of cows that received bST during yr 1 were unaffected by previous treatment. General health, ratio of single to multiple births, and BW gain of calves was similar among treatments.

Effect of bovine somatotropin on reproductive function in lactating dairy cows

Several recent experiments have reported that chronic treatment with bovine somatotropin (bST) increased the number of days open without affecting the services per conception. The physiological basis for these effects was examined. Eleven lactating Holstein cows received daily injections of bST (40 mg) and 10 received daily injections of vehicle. Treatment was initiated between 32 and 85 d post partum and continued for up to 180 d. Eight of 11 bST-treated cows experienced at least one period of extended ovarian acyclicity during treatment. Only 3 of 10 control cows did so (P = 0.05). Concentrations of progesterone during luteal phases were lower in bST-treated cows than in controls (P = 0.06). Baseline concentrations of LH were suppressed in bST-treated cows compared with those of controls (P < 0.04). Neither the pulse frequency of LH nor the expression of estrous behavior was affected by bST (P > 0.30). These results indicate that chronic administration of a high dose of bST can reduce reproduction performance by promoting ovarian acyclicity.

Selective changes in oxidative xenobiotic metabolism in vivo and in vitro after parenteral administration of recombinant bovine somatotrophin to rats

Effects of recombinant bovine somatotrophin (r bst) on in vivo and in vitro oxidative drug metabolism were studied in male rats. r bst was given subcutaneously at a dose of 250 or 500 μg 100 g −1 bodyweight 24 h −1 in different dosage patterns. Sulphadimidine ( sdd) plasma clearance, urinary excretion of 6-hydroxy- sdd and the in vitro microsomal sdd-hydroxylations were only inhibited when r bst was given in three injections per 24 hours. The hepatic microsomal ethylmorphine N-demethylation rate and the testosterone hydroxylation rate at the 6β position were significantly reduced after one r bst injection per 24 hours. Microsomal testosterone hydroxylation rates at the 16α and 2α-positions were reduced depending on the frequency of r bst administration. It is concluded that the inhibition of in vivo and in vitro drug oxidation in rats by r bst is associated with selective changes in activity of cytochrome P450 enzymes in the liver.

Effects of administration of recombinant bovine somatotropin on the responses of lactating and nonlactating cows to heat stress

Summary An experiment was performed to determine whether bovine somatotropin (bst) increases the severity of responses to heat stress and whether this effect depends on an increase in milk yield. Lactating and nonlactating cows received daily sc injections of bst (25 mg) or sodium phosphate solution for 12 days. Rectal temperatures and respiration patterns were not affected by BST when cows were maintained in a thermoneutral environment on day 9 of treatment. On day 12, cows were heat stressed by exposure to direct solar radiation. Rectal temperature, respiration rate, and the frequency of open-mouth panting increased as a result of heat stress. For lactating and nonlactating cows, those treated with bst had higher rectal temperatures and rate of open-mouth panting than cows treated with sodium phosphate solution. Administration of bst can increase the severity of responses of cows to heat stress without changing milk yield.

Effects of Recombinant Bovine Somatotropin Under Conditions of High Production and Heat Stress

The effect of bST injection on milk production of Israeli Holsteins was tested under conditions of mean production >9000 kg/yr and climatic stress; mean maximum and minimum summer temperatures are 38 and 25°C, respectively, in the Jordan Valley, located 200m below sea level. In 1989, 111 cows were injected, and 115 cows were recorded as controls. In 1990, 108 cows were injected, and 93 cows were included as controls. Fifty-nine of the cows injected in 1990 were also injected in 1989. Production records were corrected for parity, calving month, days to first injection, and days in milk. Injection with bST increased total lactation milk production by 12%, fat production by 15%, and protein production by 13%. Injection also resulted in slight increases in fat and protein percentages. Daily milk production during the injection period was increased by 4.4kg. Injection during the previous lactation slightly decreased production of cows injected during the following lactation. Advancing the commencement of injection from the 4th to the 2nd mo in milk did not affect total lactation production. Weight gains and dry matter intake were higher for injected cows, but body condition score was higher for the control group. Injection had no discernible effect on fertility variables.

Ovarian Follicular Populations in Lactating Dairy Cows Treated with Recombinant Bovine Somatotropin (Sometribove) or Saline and Fed Diets Differing in Fat Content and Energy

Interactions of dietary energy and fat with recombinant bST (sometribove) injections were tested for their effects on ovarian function. Lactating dairy cows were fed one of three diets differing in energy concentration (NEL) and percentage of DM of calcium salts of long-chain fatty acids: diet 1, 1.68 Mcal/kg and 0%; diet 2, 1.68 Mcal/kg and 2.2%; and diet 3, 1.78 Mcal/kg and 2.2%. Cows were injected daily with bST or saline during one of two 19-d interestrous periods (crossover design) in which ovarian follicles were measured by ultrasonography. The bST-treated cows produced more FCM and were in lower energy balance than saline-treated cows. Before d 12 (first follicular wave, estrus = d 0), bST-treated cows had more ovarian follicles in classes 1 (3 to 5mm; cows on diet 3) or 2 (6 to 9mm; cows on diets 1 and 2) than saline-treated cows. After d 12 (preovulatory follicular wave), numbers of follicles in different size classes were similar for bST-treated and saline-treated cows, but cows fed diet 2 had larger preovulatory follicles. Lower dietary energy and bST treatment were both associated with larger subordinate follicles. Ovarian follicles less than 10mm were stimulated by bST, and calcium salts of long-chain fatty acids increased diameters of preovulatory follicles.

Effects of Treatment of Dairy Cows with Recombinant Bovine Somatotropin over Three or Four Lactations

Jersey, Dutch Red and White, and Friesian cows were subcutaneously injected with 640mg of recombinant bST at 28-d intervals from 87 to 115 d after calving through four successive lactations. A TMR (6.72 MJ of NEL and 168g of CP/kg of DM) was fed for ad libitum consumption. The bST effects per day were 3.3kg for milk yield, 189g for fat yield, 109g for protein yield, 157g for lactose yield, 4 MJ of NEL for feed intake, and –4kg for body weight. Responses in blood parameters measured 7 d after injection were –.007ml for glucose, –1.3mg of N/l00ml for urea, 221 μmol/L for 3-hydroxybutyrate, 59 μmol/L for NEFA, 65 ng/L for insulin, 2.8μg/L for thyroxine, and 26.7μg/L for somatotropin. Somatic cell count in milk was 75,000 cells/ml higher in treated cows. Concentrations of NEFA, Ca, Mg, and phosphorus were unaffected. Repeatability of the maximum response in milk yield after bST treatment was low: .2 within and .5 between lactations. Cows treated in the previous lactation had slightly more retained placentas, and birth weight of their calves was 2kg less. No differences were observed between treated and control cows in disease incidence. Six treated cows were culled in third and fourth lactations. No indications for tissue damage, inflammation, or stress after bST injections were detected.

Effect of somatotropin administration and duodenal infusion of methionine and lysine on lactational performance and nutrient flow to the small intestine

Lack of sufficient methionine and lysine delivered post-ruminally may limit milk production response to bovine somatotropin (bST). To test this hypothesis, four Holstein cows fitted with rumen and duodenal cannulas were used in a 4 x 4 Latin square design with 14 d periods. Treatments were: (1) control, (2) continuous duodenal infusion of 8 g methionine and 24 g lysine/d, (3) injection of 25 mg bST/d and (4) infusion of methionine and lysine plus injection of bST. Infusion of amino acids led to trends for small increases in milk (3%), fat (5.5%), and protein (3.7%) yield. Larger and significant increases (8.7, 14 and 6.9% for milk, fat and protein yield respectively) were achieved with bST administration which also increased milk fat content. Plasma levels of urea-N and essential amino acids were reduced with bST. Duodenal nutrient flow was generally unaffected by treatment. The production response to bST was not enhanced in cows producing an average of 34 kg milk when provided additional methionine and lysine post-ruminally in this short-term study.

Abomasal Infusion of Casein Enhances Nitrogen Retention in Somatotropin-Treated Steers

The effects of bovine somatotropin (bST) administration and abomasal protein infusion on nitrogen metabolism were investigated using four abomasally-cannulated Holstein steers in a 4 × 4 Latin square design. Treatments were factorially arranged to include daily abomasal infusion (water or casein, 0.9 g/kg body wt) and daily bST injection (0 or 200 μg/kg body wt). Each treatment period lasted for 23 d and consisted of an adjustment period (d 1 to 7), casein infusion (d 8 to 23) and bST treatment (d 10 to 23). Nitrogen metabolism and serum insulin-like growth factor-I (IGF-I) profiles were characterized on d 16–22. Animals were fed a total mixed diet balanced to achieve 0.75 kg gain/d and supplemented to provide 115% of requirements for crude protein and minerals. Nitrogen retention was increased by casein and bST individually (21 and 33%, respectively) or in combination (75%). Somatotropin treatment increased the biological value of absorbed N, which was consistent with reductions in plasma urea nitrogen. Circulating IGF-I was increased by bST and was highly correlated (r = 0.73) with N retention. Overall, bST altered requirements so that less absorbed N was required per unit of N retained. However, results also demonstrated that because of limitations in amino acids supplied from microbial and ruminal escape protein in young growing cattle, amino acid supply may limit the magnitude of N retention response to bST under many situations.

Factors Affecting Response of Cows to Biweekly Injections of Sometribove

Seventy-eight lactating Holstein cows (18 primiparous and 60 multiparous) were used to study certain factors affecting response in milk yields to biweekly injections of bST. Cows were intramuscularly injected with placebo or with 500mg of bST every 14 d for 36 wk beginning 60 d postpartum. Primiparous cows responded less to bST than did multiparous cows. Cows were separated into three yield groups within each treatment based on pretreatment yield: low (27.0kg/d), medium (32.0kg/d), and high (39.0kg/d). Response of low yielding cows to bST was numerically greater than that of medium or high yielding cows, but there was no significant interaction between treatment and pretreatment milk yield. Individual cow index for milk yield was used as a measure of genetic potential of cows, which were divided into two index groups: high cow index (greater than −23kg) and low cow index (less than −23kg). Cows in the high index group showed significantly higher milk yields than the low index group during treatment, but treatment by cow index interaction was not significant, suggesting that cows of widely ranging genetic potential respond similarly to bST. Response of cows to bST in mid to late lactation was slightly greater than in early lactation (15.9 and 10.6% vs. 17.8%, respectively), but no significant interaction of treatment by days postpartum was noted. Finally, even though cows treated with bST exhibited slightly higher rectal temperatures during the hot summer months, no significant difference in milk yield response to bST was noted during periods of moderate versus high ambient temperatures.