Concentrations of hormones in plasma from cattle with different growth potentials.

Blood samples were taken from four large- and four small-frame steers each month for 12 mo to study the effect of size on plasma hormone concentrations in cattle from 5 to 17 mo of age. The rate of gain and daily feed intake were 45% (P less than .05) and 51% (P less than .001) greater for the large steers. The large-frame steers had higher mean concentrations of growth hormone (P less than .001), insulin (P less than .06), thyroxine (P less than .06) and cortisol (P less than .002). The concentration of growth hormone was highest in the young cattle (P less than .01) and gradually declined with increasing age. Insulin concentrations were lowest in the young cattle (P less than .01) and gradually increased with age. The results of this study suggested that there are differences in hormone concentrations that are related to size rather than being the result of differences in physiological maturity of different breeds of cattle.