Effect of Slow-Release Somatotropin on the Pattern of Milk Yield Between and Within Injection Intervals

¹ Department of Animal Science, University of Padova, Agripolis, 35020 Legnaro (PD), Italy

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 two exponential terms: Y_i = A x [exp (–k₁d _i – exp (–k₂d_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; k₁ and k₂ = 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.

Key Words: goats • recombinant bovine somatotropin • milk yield

Submitted on November 6, 1995
Accepted on May 17, 1996