Somatotropin, which can now be produced by biotechnology, could have an enormous impact on the dairy industry. Milk yield has been increased up to 40% with daily injections of somatotropin. Cows adjust their nutrient intake to support this increase. Somatotropin does not adversely affect cows' health, although all studies to date have been for less than one complete lactation. The search for a single biochemical or physiological event to account for the effects of somatotropin on milk production is elusive. Coordinated changes in many tissues and physiological processes occur to support the increases in the synthesis of lactose, fat, and protein in the mammary gland. Changes in the irreversible loss and oxidation rates of two key metabolites, glucose and free fatty acids, can quantitatively account for increases in lactose and milk fat during the short-term administration of somatotropin. Similarly, feed intake and live weight changes can account for increases in milk production in the longer experiments. Parallels between physiological changes that occur during somatotropin administration and differences between genetically high and lower yielding cows are highlighted, and the rates of improvement that can be expected from various new technologies are quantified. Existing data on the safety of somatotropin to both the consumer and the animal are evaluated.
Effects of bovine somatotropin (bST) on irreversible loss rate (ILR) and oxidation rate of glucose and nonesterified fatty acids (NEFA) were examined. Nine lactating cows received bST or excipient in a single reversal design using 14-d periods. Kinetic variables were estimated by compartmental analysis of blood metabolite and expired CO2 specific activity values obtained during infusion of [U-14C]glucose or [1-14C]palmitate. With bST treatment, milk energy yield increased by 31% but feed intake was unchanged. Blood glucose concentrations were not affected by treatment or correlated with any glucose kinetic variables. In the control period, glucose ILR was 12.1 mol/d with 66.5% utilized for milk lactose synthesis and 17.4% oxidized to CO2. Treatment with bST increased glucose ILR (+1.5 mol/d) and reduced glucose oxidation (-0.4 mol/d); this accommodated the additional glucose (+1.3 mol/d) required for the increase in lactose secretion. Increases in milk energy yield with bST treatment caused cows to be in a substantial negative net energy balance (-9.8 Mcal/d). No acute lipolytic response occurred with bST treatment, but plasma NEFA were chronically elevated (+104 mumol/L) and NEFA ILR increased (+2.3 mol/d). Increased NEFA turnover was primarily used for increased oxidation to CO2 (+0.5 mol/d) and 41% increase in milk fat (equal to approximately 1.3 mol fatty acids/d). For NEFA, plasma concentrations were correlated with ILR (r = +0.80), oxidation to CO2 (r = +0.74) and net energy balance (r = -0.78). Overall, bST resulted in an exquisite coordination of metabolism to meet nutrient needs for increased synthesis of milk components.
Bovine growth hormone (51.5 IU/day) and placebo injections were administered for 10 days to four Holstein cows in early lactation (wk 12) and again in late lactation (wk 35). Milk productions in the last 5 days of each period were compared. In early lactation, growth hormone increased milk yield by 15%, fat yield by 17%, protein yield by 14%, and lactose yield by 21%. In late lactation the respective increases were 31, 42, 18, and 35%. For responses of early and late lactation to growth hormone on a quantitative basis, increases for milk yield (4.3 versus 3.9 kg/day) and milk energy secretion (3.3 versus 3.4 Mcal/day) were similar. Concurrent with these increased milk yields, ad libitum intakes of a complete mixed diet declined during the period of growth hormone treatment by 3% in early lactation and 16% in late lactation. During the 6 h immediately following injections of growth hormone, blood plasma concentrations of growth hormone were elevated about 400% in early lactation and 700% in late lactation. Concentrations in plasma of free fatty acids were also higher during growth hormone treatment in late lactation but not in early lactation. Treatments did not affect plasma concentrations of glucose, insulin, glucagon, prolactin, tri-iodothyronine, thyroxine, or cortisol in either early or late lactation. Daily administration of growth hormone in early or late lactation resulted in similar and substantial increases of milk yield and efficiency of milk production.
Bovine growth hormone (bGH) was administered to high-yielding Holstein cows fed a complete mixed ration ad libitum. Commencing on day 74 of lactation, 10 cows averaging 34.4 kg milk per day were divided into two groups and received a daily subcutaneous injection of bGH (51.5 IU/day) or a placebo. Injections were continued for an 11-day period and differences in lactational performance, nitrogen balance and estimated energy balance between the two groups were compared for the last 5 days of the preinjection and injection periods. Growth hormone resulted in increases of 9.5% in milk yield, 22.7% in milk fat yield, 14.5% in milk lactose yield and a 17.1% increase in milk energy secretion. Feed intake was slightly reduced (-4.3%, nonsignificant) while milk protein secretion and nitrogen balance were unchanged. Serum growth hormone levels in the bGH group were maintained at the higher concentrations of the normal physiological range during the injection period. By 48 hours following the last injection, declining bGH concentrations approached control values, and milk production decreased to preinjection values. Serum prolactin levels and plasma concentrations of free-fatty acids were slightly increased during the injection period in the bGH group. Growth hormone clearly enhances milk synthesis in the high-yielding dairy cow.
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.
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