Friesian cows (25 per group) were treated with excipient or recombinantly-derived bovine somatotropin (bST, 25 mg/day) by two- weekly injection of a controlled release formulation for 26 weeks commencing 7 to 11 weeks after calving. Responses in fat yield were significant during weeks 1 to 13 of treatment (spring/early summer) and weeks 21 to 25 (autumn) but not during the intervening dry summer period when pasture allowance was low. Ten randomly-selected cows per group were blood sampled (tail vein, 2 hours off pasture) in weeks 5, 9, 13, 17, 21 and 25 of treatment. Treatment of cows with bST significantly elevated plasma concentrations of immunoreactive bST (12.4 vs 18.5 ng/ml,Pooled se = 0.8 ng/ml, P<0.001) and insulin-like growth factor-1 (IGF-1) (178.5 vs 246.1 ng/ml, Pse = 7.5 ng/ml, P<0.001). Treatment x sampling date interactions were non-significant for plasma levels of both bST and IGF-1, reflecting parallel changes in the concentrations of these hormones across the treatment period. It is concluded that the inability of cows to respond to exogenous bST during periods of low pasture availability does not reflect a diminished IGF-1 response to bST at this time.
Proceedings of the New Zealand Society of Animal Production, Volume 51, , 189, 1991
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