An extension model of lactation curves was used to determine the effect of recombinant bovine somatotropin (bST-r) on milk yield in Holstein dairy cattle. This model use the fitted values obtained by the Wood model, and was tested on the records of 66 cows. The milk yield predicted with the extension model and the observed yield were compared and no significant differences were observed (P>0.05). Once the extension model was validated, the milk yield tests of 199 cows were used. The cows received bST-r 500mg by subcutaneous injections. The injections were applied after 100 days in milk at 14-day intervals (seven injections). The observed milk yield was compared with the yield expected by the extension model. An increase of 5.3% was observed in milk yield in response to the bST-r. This increase is lower than that reported in the literature in response to the growth hormone in dairy cattle. It is concluded that extension model used in the present work is reliable for extending the lactation curve in Holstein cows, and the increase in milk yield in response to the application of bST-r, determined in the same animal using the extension model, was lower than that reported by other authors.
dairy cattle; bST-r; mathematical model
