Genetic and nongenetic effects on the response of Holstein cows to slow-release somatotropin injection

Data on 89 multiparous Holstein cows were used to investigate the interaction between the genetic and nongenetic potentiai of cows and the administration of recombinant bovine somatotropin @ST) on milk yield and composition. Somatotropin was administered to treated cows (n = 50) in a slow-release formulation (Somidobove, Eli Lilly, 640 mg). Untreated cows (n = 39) did not receive any excipient injection. Data on milk yield and composition traits were analysed considering, simultaneously, regressions on cows’s estimated breeding value (EBV) and cows’s environmental ability (CEA) across and within treatment groups. CM was computed for each cow as the deviation of EBV from mature equivalent milk yield and composition. Data analysis provided some evidence of a possible EBV by rbST treatment interaction. Regression coefficients of milk yield on EBV were consistently lower for treated than for untreated cows, although statistical significance was found for fat yield only (P < 0.05). Results suggest that cows with lower EBV tend to respond better to rbST than cows of higher genetic merit when feed quality is not limiting yield. An implication of this is that, in order to maximize the return from the use of rbST, only cows of low breeding merit should receive rbST treatment, provided that feeding regime is suitable