Abstract
Efficacy of a sustained-release formulation of recombinant bovine somatotropin (rbST) was evaluated for two consecutive lactations. In trial 1, 48 multiparous and 12 primiparous Holstein cows were allocated randomly by parity to four treatment groups receiving 0, 320, 640, or 960 mg rbST in a sustained-release formulation at 28-d intervals starting in early, mid-, or late lactation with the total treatment period being 252, 168, or 84 d for the three different starting periods, respectively. Fifteen cows receiving 640 or 960 mg rbST 28 d−1 in trial I received 640 mg rbST 28 d−1 in trial II for a full second lactation and were paired with cows receiving a placebo and which were never administered rbST. Cows were fed total mixed rations ad libitum and milked twice daily. Response in milk and milk component yield was positive and very consistent for multiparous cows between trials. Primiparous cows responded in both lactations with the same numerical magnitude as multiparous cows but with higher variability. No adverse effect of treatment on final body condition score and achievement of proper condition for a subsequent lactation was found because treated cows had lower condition scores for a longer period of time but regained body condition at a faster rate in late lactation. No effect of treatment was observed on health parameters in either lactation. Trends towards longer days open and more breedings per conception were apparent for both lactations. Key words: Somatotropin, milk production, health, dairy cows, primiparous cows
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