Relationship between the milk yield response to short-term bovine somatotropin treatment and the lipolytic response to adrenaline in dairy cows.

The aim of this experiment was to determine if the milk yield response of dairy cows to short-term treatment with bovine somatotropin (bST) was correlated with the non-esterified fatty-acid (NEFA) response to an adrenaline challenge. Twenty-six multiparous Holstein cows (58+/-5.4 days postpartum) received daily sub-cutaneous injections of saline for 7 days followed by sub-cutaneous injections of 20mg/day of bST for 14 days. On day 7 of the saline treatment and day 14 of the bST treatment the cows were given an intravenous injection of adrenaline (1.4 microg/kg body weight). Blood samples were taken before and after the adrenaline challenge. The difference in milk yield between the saline and the second week of bST treatment (MYR) varied considerably between animals (from -0.4 to +8.0 kg/day). MYR was positively correlated with the change in the basal concentration of NEFA between the saline and second week of bST treatment, as well as with the change in the area under the profile of NEFA above basal values following the adrenaline challenge. It remains to be established whether the greater lipolytic responses to adrenaline of the cows with the greater MYR reflects the deeper negative energy that these animals also experienced or a fundamental difference in the physiology of their adipose tissue.