GROWTH-HORMONE RELEASE IN CALVES SELECTED FOR MILK-FAT YIELD

In two experiments Red Danish dairy calves of both sexes, and of two lines selected for high (H) and low (L) milk fat production (proportional difference 0.22), were tested at 4 months and again at 10 months of age for their growth hormone (GH) release following intravenous administration of either thyrotropin releasing hormone (TRH, experiment 1: 0.15 mug/kg live weight, no. = 16 H + 10 L) or arginine hydrochloride (ARG, experiment 2: 0.10 g/kg live weight, no. = 19 H + 10 L). The GH response was measured in serial blood samples for 0.5 h prior to and for 2 h following intravenous injections. The response peak, measured as the geometric mean of the 5-, 10- and 15-min samples following TRH was greater in the line selected for high yield, at 10 months (H, 42.4 mug/l; L, 20.6 mug/l; P < 0.01) but not at 4 months of age (H, 25.4 mug/l; L, 18.6 mug/l; P > 0.05). The response peak following arginine measured in the 20-, 30- and 45-min samples was smaller than the peak following TRH and did not differ between selection lines, although there was a tendency for H calves to have a larger release at 10 months of age. After puberty (10 months) male calves responded more to both secretagogues than females, while there was no difference before puberty (4 months). These results suggest that GH release may be useful as a juvenile predictor of dairy merit, but results need to be confirmed in further and larger studies.