The influence of tropical adaptation on plasma concentrations of insulin-like growth factor-I in purebred and crossbred beef cattle.

In an effort to determine whether tropical adaptation influences circulating concentrations of the growth-related hormone IGF-I, 3-breed diallel matings were conducted using temperate Bos taurus (Angus), tropical Bos indicus (Brahman), and tropical Bos taurus (Romosinuano). Purebred Angus, Braham, and Romosinuano and crossbred Angus-Braham, Angus-Romosinuano, and Braham-Romosinuano heifers and steers were evaluated in 2 separate calf crops from 2003 and 2004. Blood samples were obtained from 10 heifers of each breed group (n = 90) for each year at weaning and on d 0 and 84 of postweaning trials. Samples were also taken from 10 steers of each breed group (n = 90) at weaning and on d 0 and 60 of individual finishing phase feeding trials for each year. Concentrations of IGF-I were determined by RIA. Analyses included effects of sire breed, dam breed, year of record, the age of the dam of the calf in years, and interactions. Age of calf in days was investigated as a linear and quadratic covariate. Separate analyses were conducted for steers and heifers. The direct effect of Angus was to reduce (P < 0.03) heifer concentrations of IGF-I at d 84 and in the repeated measures analysis. In the repeated measures analysis, the direct effect of Romosinuano was to increase concentrations of IGF-I (P = 0.01). Relative to the temperate Bos taurus breed, plasma concentrations of IGF-I were numerically greater in male and female tropically adapted breed groups.