The effects of long-term administration of exogenous growth hormone (GH) on growth and carcass composition of pasture-fed, pre-pubertal dairy heifers were examined. Purified bovine GH (specific activity, 0·78 i.u. per mg) was administered daily for 21 weeks (0·6 mg GH per kg M0·75) to one member of each of 12 sets of twins. GH administration resulted in a significantly higher growth rate (0·58 kg/ day) compared with the control group (0·54 kg/day) and produced a heavier carcass (75·6 kg v. 69·5 kg). However, this production gain did not persist when GH treatment ceased. Plasma metabolite concentrations and carcass composition were not affected by GH treatment. GH tended to increase slightly the voluntary intake of freshly cut herbage dry matter (3·5 kg/day v. 3·7 kg/day; P < 0·05), but had no effect on food conversion efficiency. Serum somatomedin levels were not significantly increased by GH during week 13 of treatment. This experiment indicates that pre-pubertal heifers chronically treated with GH will increase their food intake to sustain an increased rate of growth. However, the production gains made over the treatment period were transient and within 5 weeks of the cessation of GH treatment there was no difference in the live weight of the two groups.

The effect of chronic administration of bovine growth hormone (GH) on milk production, food intake and live-weight change was evaluated in five sets of monozygotic twin cows on pasture. Purified bovine pituitary GH (specific activity, 0·78 IU/mg) was administered by daily subcutaneous injections (39 IU/day) for a period of 22 weeks (weeks 5 to 26 of lactation). GH treatment resulted in significantly higher yields of milk (23·3 kg/day), fat (0·97 kg/day), protein (0·74 kg/day) and lactose (115 kg/day) compared with the control group (19·8 kg/day, 0·79 kg/day, 0·63 kg/day, 0·99 kg/day). Milk composition did not differ between treatment groups. There was no difference in the intake of cut grass in week 8 but the voluntary intake of the GH group had increased by week 22 (controls, 15·4 kg dry matter (DM) per day and GH group, 17·5 kg DM per day). There were no differences in the rate of live-weight change for the two groups. Serum somatomedin concentrations were significantly elevated in the GH group on weeks 20 to 22 of treatment (0·043 v. 0·135 U/ml). This experiment indicates that cows chronically treated with GH were able to adjust their food intake upwards to sustain a substantial increase in milk production on a diet composed solely of grass.

The effects of long-term administration of bovine growth hormone (GH) on pre-pubertal mammogenesis and subsequent milk production were examined in pasture-fed dairy heifers. Purified bovine GH (specific activity 0·78 i.u. per mg) was administered daily for 21 weeks (0·6 mg GH per kg M 0·75) to one member of each of 12 sets of twins from 3·5 (range 2·5 to 4-5) months of age. GH administration resulted in a significantly smaller mammary gland compared with control heifers at slaughter at the end of treatment (717 v. 839 g; P < 005 (no. = 4)) and increased the proportion of mammary fat pad containing parenchyma (461 v. 383 g/kg; P < 0·01). Mammary parenchymal dry, fat-free tissue was increased in the GH-treated group (25·8 v. 22·5 g; P < 0·05), and fat-pad adipose tissue was reduced (323 v. 435 g; P < 005), compared with the control group. Morphometric analysis of the mammary gland revealed an increase in the volume fraction of connective tissue (P < 0·1) and a tendency toward less fat in GH-treated heifers compared with control heifers. Estimates of the composition of the whole gland confirmed that GH treatment reduced total mammary fat (372 v. 496 g; P < 0·05). At calving, udder volume was greater in the heifers treated with GH compared with the untreated group (14·8 v. 11·2 1; P < 0·05 (no. = 4)). However, there were no differences in the mean daily yields of milk or milk components between groups.

Eighty Holstein cows (first, second, or third lactation) were used to evaluate the efficacy of a prolonged-release formulation for sometribove (n-methionyl bovine somatotropin). Cows were fed ad libitum a complete mixed diet and milked twice daily. Cows were allocated randomly by parity to two treatment groups receiving 500 mg sometribove in a prolonged release formulation or excipient at 14-d intervals starting at 60 +/- 3 d postpartum and continuing for 36 wk. Treatment with sometribove increased FCM yield to a similar extent (11.4%, +3.1 kg/d) in primiparous and multiparous animals. Milk content of lactose, fat, ash, and Ca was not affected, but protein and phosphorous were slightly greater (less than 5%) in milk from sometribove-treated cows. Within a 14-d injection interval, animals treated with sometribove displayed a cyclic pattern in milk yield, but a similar pattern was not evident in feed intake.

Responses to daily injections of bovine growth hormone (GH, 0·15 mg kg-1 liveweight), beginning on day 10 of lactation, were measured in lactating ewes. Milk yields of GH-treated ewes increased soon after commencement of injections and continued to increase for some 25 days before reaching plateau levels. By comparison, yields of ewes injected with excipient (controls) decreased over the experiment. There was a tendency for contents of milk fat to be higher and milk protein to be lower for GH-treated than for control ewes during the first 15-20 days after injections were started.At the beginning and over the first 15-20 days of the experiment feed intakes of both groups of ewes were similar, but thereafter intakes of GH-treated ewes gradually increased to reach plateau levels some 2OG-300 g day-l higher than for control ewes by about day 35. Liveweights of both groups of ewes decreased during the first 2 weeks of treatment then increased, with GH-treated ewes losing, then gaining, more weight than control ewes. The efficiency of food utilization for milk production was higher for GH-treated than control ewes throughout the experiment but digestibility of food organic matter was not different during the eighth week of the experiment. At the end of the experiment, body composition, assessed by dilution of tritiated water, was similar for both groups of ewes. Differences in milk production were not sustained after withdrawal of GH injections.Measurements of tissue uptake of key metabolites were made on days 3 and 45 of GH treatment. On day 3, GH lowered uptake of glucose and non-esterified fatty acids by leg muscle tissue and increased mammary uptake of non-esterified fatty acids. By day 45 there were no apparent differences of tissue uptake of key metabolites.The results indicate that there is a biphasic response to exogenous GH in the lactating ruminant. It appears that initially GH affects nutrient partition thereby increasing supplies to the mammary gland of key nutrients for milk synthesis. In the longer term, GH increases feed intake, which provides sufficient nutrients to sustain increased milk production and also Iiveweight gain.Extra keywords: Milk yield and composition, feed intake and digestibility, Iiveweight and body composition, arterial metabolites and hormones, arterio-venous differences, leg muscle and mammary tissue, non-esterified fatty acid kinetics, tissue blood flow.