202 Residual feed intake is not associated with muscle, fat, or liver expression of growth hormone receptor, insulin-like growth factor i, or beta-adrenergic receptor mRNA in Angus steers

The genetic and physiological basis of feed efficiency in beef cattle is unclear. The objective of this study was to test the hypothesis that more efficient cattle might have greater expression of growth hormone receptor (GHR) or beta-adrenergic receptor (ADRB) mRNA in skeletal muscle, fat, or liver, the major target tissues of GH and beta-adrenergic agonists. This hypothesis was based on the fact that both GH and beta-adrenergic agonists can improve feed efficiency in animals. Skeletal muscle, subcutaneous fat, and liver samples were collected at slaughter from top 10 high-residual feed intake (RFI) (1.03 ± 0.12) and top 10 low-RFI (−0.69 ± 0.02) steers selected from a population of 75 Angus steers (422 ± 14 days old). Abundances of GHR, insulin-like growth factor I (IGF1), IGF1 receptor (IGF1R), beta-1 adrenergic receptor (ADRB1), ADRB2, and ADRB3 mRNAs were quantified by real-time RT-PCR using validated primers. As expected, RFI was positively correlated with dry matter intake (DMI) (R = 0.63, P = 0.003) and feed conversion ratio (FCR) (R = 0.74, P = 0.0002) but not correlated (P > 0.05) with average daily gain (AVG); FCR was negatively correlated with ADG (R = 0.69, P = 0.0008) but not correlated with DMI (P > 0.05). Expression levels of GHR, IGF1, IGF1R, ADRB1, ADRB2, and ADRB3 mRNAs in muscle, fat, and liver were neither different (P > 0.05) between high- and low-RFI steers nor correlated (P > 0.05) with RFI. Expression levels of GHR and IGF1R mRNAs in muscle and liver and expression levels of IGF1, ADRB1, ADRB2, and ADRB3 mRNAs in muscle, fat, and liver were not correlated (P > 0.05) with FCR. However, expression levels of both GHR mRNA (R = 0.48, P = 0.009) and IGF1R mRNA (R = 0.47, P = 0.002) in fat were negatively correlated with FCR. Expression levels of GHR, IGF1, and GF1R mRNAs in muscle and fat were positively correlated with ADG (R = 0.52 to 0.65, P = 0.002 to 0.02), whereas expression levels of GHR mRNA (R = 0.50, P = 0.03) and IGF1 mRNA (R = 0.47, P = 0.04) in liver were negatively correlated with ADG. These results suggest expression of GHR, IGF1R, or ADRB mRNA in muscle, fat, or liver does not influence RFI in Angus steers. However, greater GHR and IGF1R mRNA expression in fat may improve feed efficiency, and increased GH and IGF-I in muscle and fat may stimulate body growth in beef cattle.