Growth hormone response to infusion of somatostatin in dairy calves: testing its use as a diagnostic tool for genetic merit

This paper describes a two-part study evaluating the use of growth hormone (GH) response to infusions of somatostatin (SRIF) and GH-releasing factor ( GRF) as an indicator of genetic merit in dairy cattle. The first part of th e study involved the development of a standard test procedure. This was ach ieved in two sub-trials, the first testing three SRIF doses, 0.6, 3.0 and 1 5 mu g/kg per h, given over an infusion length of 3 h. The second involved infusing 3.0 mu g/kg per h SRIF for either 20 min, I h or 3 h. The GH respo nse was measured in serum during the infusion and for 2 h following cessati on, during which time a rebound response in GH occurred. As a consequence o f these trials, a total of 40 British Friesian calves from two lines geneti cally divergent for yield traits were infused with 3.0 mu g/kg per h for a period of I h and at the end of the infusion given 0.1 mu g/kg GRF Five hou rs later GRF was administered without SRIF The protocol was repeated on 2 d ays consecutively. Results suggested that calves of high merit had lower SR IF-induced baselines than their low-line contemporaries. There were no diff erences in the GH concentrations immediately following GRF administration, either with or without SRIF The results from this trial, together with prev iously published work, indicated that calves selected for high milkfat plus milk protein production may be more sensitive to SRIF such that GH is regu lated by lower concentrations of SRIF However, the practical use of GH resp onse to SRIF as an indicator of genetic merit for dairy cattle may be limit ed because of logistical difficulties in implementation.