The effects of administration of recombinantly derived bovine somatotropin (somidobove) in a sustained-release vehicle on the profiles of concentrations of bovine somatotropin (bST) in the blood plasma and on the milk yield of dairy cows of three herds were examined. Cows (36-87 days post partum') were treated subcutaneously with recombinant bST at 28-day intervals. In control animals, basal concentrations of bST averaged 1.4 ng.ml1 in first-calf heifers and 1.5 ng inl1 in multiparous cows. In somidobove treated first-calf heifers, the concentration of bST was increased to 10.7, 14.5, and 27.0 ng.ml'1 at 24 h postinjcction and in multiparous cows to 6.6, 11.0, and 11.7 ng.ml'1 on day 2 postinjcction of 320, 640, and 960 mg of somidobove, respectively. On day 8 postinjection the average plasma bST levels of both parity groups are similar (on the average 3.4, 8.6, and 12.5 ng.ml'1 for three doses of somidobove respectively) and for the two highest doses being still significantly increased. During the 2nd week postinjection plasma bST concentration declined returning to control levels on day 15 postinjcction. Somidobove-treated first-calf heifers produced 10.9, 16.7 and 17.9 % and multiparous animals 25.5, 24.2 and 32.5 % more milk than the controls when given 320, 640 and 960 mg somidobove, respectively. The cyclic pattern in milk yield within each 28-day injection interval was observed consistently in all herds. The milk yield increased to a maximum between day 4 to 8 postinjection and then slowly declined. Milk composition was not affected by somidobove treatment.
Milk production, plasma bovine somatotropin (bST) and insulin-like growth factor I (IGF-1) were measured in dairy cows following a single subcutaneous injection of a slowly released preparation of either recombinant enterokinase linker bST (somidobove: 640 mg) or recombinant methionyl bST (sometribove: 500 mg). There was a 3-7-fold increase in plasma bST concentrations during the first three postinjection hours in cows treated with both sometribove (from 3.4±0.8 to 11.2±3.0 ng ml-1) or somidobove (from 2.3±0.3 to 17.5±2.6 ng ml-1). In the next 8 days the bST concentration in the bST-treated cows varied, but was still significantly increased above the controls. In the following days, the concentrations of bST did not differ from the controls. Plasma concentrations of IGF-1 increased nearly 2-fold as early as 24 h following recombinant bST administration and then continued to rise so that by 48 h postinjection they were nearly four times higher (control 16.2, bST-treated 61.7 ng ml-1). From 48 h after sometribove injection, IGF-1 concentrations remained at a plateau (varying between 60.4 and 85.7 ng ml-1) till day 11. Then it decreased slowly, but still remained higher on day 14 than those in placebo-treated cows (44.4±17.8 ng ml-1 in bST-treated animals; 12.2±7.5 ng ml-1 in the controls). Although IGF-1 level was increasing in all bST-treated animals, the absolute IGF-1 increase was not related to the increase in milk production.