POSSIBLE ASSOCIATION OF GROWTH-HORMONE GENE POLYMORPHISM WITH GROWTH-HORMONE RELEASE IN CALVES FROM LINES SELECTED FOR HIGH AND LOW MILK-FAT YIELD

Fifty-seven calves from two lines selected for high (n = 36) and low ( n 21) milk fat yield were typed for a known restriction fragment lengt h polymorphism (RFLP) in the growth hormone (GH) gene. The genetic ori gin of the RFLP is an insertion (I)/deletion(D) in the 3'-region of th e growth hormone gene. The calves were tested at 3-4 months of age and again at 9-10 months of age for plasma GH concentration before and fo llowing three GH release tests: fasting-refeeding (n = 57), arginine i njection (n = 28) and thyrotropin-releasing hormone (TRH) injection (n = 24). The RFLP typing showed 36, 19 and 2 calves of the genotypes II , ID and DD at the I/D site, respectively. The frequency of the D-alle le was higher in the high than in the low selection line (0.29 vs. 0.0 5, P<0.01). At 9-10 months of age the GH release following TRH injecti on was larger in calves with genotype II (3 5.9 +/- 4.5 ng / ml) compa red with calves with genotype ID (23.6 +/- 3.8 ng/ml) (P < 0.05), and the calves from the high line had a higher GH release after TRH inject ion (42.9 +/- 5.5 ng/ml) compared to low-line calves (19.7 +/- 3.4 ng/ ml) (P < 0.001). These results may indicate that the I/D site may affe ct or be a genetic marker for TRH-induced GH release in dairy calves.