The effect of bovine somatotropin in a sustained release preparation (Somidobove) on milk production of cows at pasture in New Zealand

Dairy cows in mid lactation were treated with a controlled release preparat ion of bovine somatotropin (bST; Somidobove) at each of two dose rates (320 or 640 mg). Thirty-six cows per dose rate were treated on four occasions a nd were compared with 36 control cows (vehicle only). The effect of Somidob ove on milk yield was sustained only for the first 14 d of each 28-d treatm ent cycle. The increase in milk solids output ranged from 188 to 352 g d(-1 ) over the first 14 d of each treatment cycle, the increase being relativel y higher in the 640-mg-dose group. In absolute terms the yield response dec lined as yield declined with advancing lactation, although the percentage y ield response was similar at all stages (mean 18% and 23%, respectively, fo r 320- and 640-mg-dose groups). Milk fat content was increased and milk pro tein content was decreased by both Somidobove dose rates. During a period o f drought, yield was adversely affected in the 640-mg-dose group relative t o control cows. This effect occurred as a carry-over response to treatment. There were no adverse effects of Somidobove treatment on any aspect of ani mal health, including calf birthweight and calving liveweight. Further work is required to optimise bST dosage levels and patterns and to establish ef fects of bST on feed intake for grazing dairy cows in New Zealand.