Responses of Lactating Ewes to Exogenous Growth Hormone: Short- and Long-term Effects on Productivity and Tissue Utilization of Key Metabolites

Abstract

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.

Effects of evaporative cooling on the regulation of body water and milk production in crossbred Holstein cattle in a tropical environment

“…Several investigations have shown that mammary gland glucose uptake depends on an increase in arterial plasma glucose concentration (Fullerton et al 1989;Sandles et al 1988), whereas other works have demonstrated no difference (McDowell et al 1987;Mepham 1993). The results of the present study support the latter observation in both cooled and non-cooled animals.…”

Effect of growth hormone treatment on the distribution of insulin-like growth factor-I between plasma and lymph of lactating sheep

“…Whether the effects on IGF-I transport observed here for mammary tissue were due to insulin remains to be established, although increase in plasma insulin concentration is observed after GH treatment of lactating ruminants in positive energy balance (e.g. Sandles, Sun, D'Cruz et al 1988). Interaction between insulin and IGF to regulate tissue metabolism according to nutritional status is an attractive concept considering the con¬ trasting roles of insulin and GH in lipid, carbohydrate and nitrogen metabolism (Vernon, 1989).…”

Hormonal control of late pregnancy and parturition in beef cattle and pigs

“…The role of GH in late pregnancy and early parturition in several species has been identified as supportive of processes that facilitate milk synthesis by the mammary gland Sandles et al, 1988;. In this study, GH concentrations were similar in both hysterectomized and pregnant gihs from day 99 until parturition.…”

“…The part played by growth hormone (GH) when administered over short periods of time during late pregnancy and/or lactation has been reported in cattle (Peel et al, 1982;McDowell et al, 1987), sheep ; Sandles et al, 1988), rats and pigs . In all these studies, there is a consistent finding that GH alters nutrient utilization in muscle and mammary tissue in a manner that favors increased nutrient supply for milk synthesis.…”

“…In the ewe, GH not only promotes blood flow in tissues to supply additional metabolites to the mammary gland, but it also stimulates processes that spare glucose oxidation, for example, increased oxidation of non-sterified fatty acids Sandles et al, 1988). When growth hormone-releasing factor (GRF) was administered to pigs in late pregnancy and early parturition , there was improved utilization of protein sources by lactating…”