Milk Response to Bovine Somatotropin of High Producing Dairy Cows with Three Different Feeding Systems Combining Pasture and Total Mixed Rations

Abstract

Forty-five Holsteins cows (15 primiparous; 30 multiparous) were used to evaluate the milk response to bovine somatotropin (bST) injections with three feeding systems combining pasture and total mixed ration (TMR) in a 21-wk repeated measures experiment. The three treatments were 1) pasture plus concentrate (PC), 2) pasture plus TMR (pTMR), and 3) TMR (non-pasture). Mean total DMI, estimated using chromic oxide as a fecal marker, was 21.6, 25.3, and 26.8 kg/d for cows fed PC, pTMR, and TMR, respectively. Milk production was greatest for cows fed TMR (38.1 kg/d), least for cows fed PC (28.5 kg/d), and intermediate for cows fed pTMR (32.0 kg/d). Cows fed pTMR and TMR had greater milk fat (3.33% vs 3.13%) and true protein (2.97% vs 2.82%) percentage than cows fed PC. Cows on PC gained less BW and lost more body condition than cows on pTMR and TMR. Ten bST injections were administered during the 21-wk trial. Cows had a milk peak different (P<0.05) from d 1 (injection day) for six of the injections on the PC treatment, for four of the injections on the pTMR treatment, and for nine of the injections on the TMR treatment. Day of peak milk yield postinjection and milk response per day were d 6.4 and 2.6kg for the cows on the PC treatment, d 7.3 and 2.4kg for cows on the pTMR treatment, and d 7.1 and 4.0kg for cows on the TMR treatment, respectively. Overall, milk response to bST was greater for cows on the TMR treatment than for cows on the PC and pTMR treatments (344 vs. 259kg per cow).