Effects of gender on protein requirements and the somatotropic axis in feedlot cattle

Two trials were conducted to evaluate the effects of gender on optimal CP concentrations (TRIAL 1) and gender and anabolic implants on the somatotropic axis in feedlot cattle (TRIAL 2). More specifically, the objective of TRIAL 1 was to examine the effects of 3 dietary CP concentrations on performance, carcass characteristics, and serum urea nitrogen (SUN) in finishing steers and heifers and the objective of TRIAL 2 was to evaluate the effects of ovariectomy (OVX) and implantation (200 mg of trenbolone acetate and 28 mg of estradiol benzoate; Synovex-Plus) on performance, serum urea nitrogen (SUN), serum IGF-1, and mRNA expression of hepatic IGF-1, GH receptor, and E receptor-alpha as well as pituitary GH, E receptor-alpha and GHRH receptor in feedlot heifers. TRIAL 1 results indicated that ADG was optimized when both steers and heifers were fed 12.5% CP and G:F was optimized for steers fed 12.5% CP but heifer G:F was optimal at 14.0% CP. Feeding diets containing 11.0% CP appears to cause a protein deficiency in both steers and heifers. TRIAL 2 data indicated that gender had no influence on performance or SUN in feedlot heifers. Serum IGF-1 is increased more in OVX heifers than intact heifers due to a greater response to implantation from the OVX heifers. However, the reason for the extra increase in serum IGF-1 is not clear, although trends in gene expression analysis suggest the possibility that the increased serum IGF-1 may be controlled outside of the somatotropic axis. Further research is warranted to examine the effects of OVX and anabolic implants on the somatotropic axis.