Effect of slow-release somatotropin on the pattern of milk yield between and within injection intervals

Thirty-six Alpine goats (10 primiparous and 26 multiparous) were used to study the milk yield response induced by recombinant bST administered in a sustained-delivery vehicle. Eighteen goats were injected with recombinant bST at 4-wk intervals from about 7 wk postpartum for 12 wk. Patterns of milk yield were highly affected by injection of recombinant bST. Milk yield of treated goats increased from day of injection, peaked between d 3 to 5 after treatment, and then decreased progressively. This pattern of response was-well fitted by a curve containing tao exponential terms: Y-i = A x [exp(-k(1)d(i) - exp(-k(2)d(i))], where Y-i = daily relative response of treated goats to recombinant bST, computed as a deviation of least squares means of milk yield at day i (i = 0, ..., 26) of the treatment interval from least squares means of milk yield at d 27 of the treatment interval; A constant; k(1) and k(2) = response rate parameters; and d(i) = day from treatment (i = 0, ..., 27). Administration of recombinant bST also improved persistency of lactation, and the overall milk yield response to exogenous hormone was 300 g/d (13.9%) higher than the yield of untreated goats. This response was due to both the short-term response immediately after injection (53%) and the medium-term effect on lactation persistency.