Responses of lactating ewes to exogenous growth hormone: short- and long-term effects on productivity and tissue utilization of key metabolites.

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 200-300 g day-1 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 liveweight gain.