Effect of divergent selection for Residual Feed Intake on whole body protein turnover in growing gilts fed either adequate or lysine deficient diets

Abstract Text: Protein turnover, representing both protein synthesis and degradation, is one of the most energy consuming processes in growing pigs, and may be one of the main factors contributing to variation in residual feed intake (RFI). RFI is a measure of feed efficiency that reflects differences in the efficiency of the use of feed for maintenance and growth. We previously observed that genetic selection for low RFI did not improve efficiency of dietary Lys utilization above maintenance for protein retention. Little is known about the effects of genetic selection for RFI on whole-body protein turnover rate. The current study evaluated the effects of divergent selection for RFI on whole-body protein turnover of growing pigs fed either adequate or lysine deficient diets. Twenty four gilts (BW 66± 5 kg) were selected from generation nine of the low RFI (LRFI; n=12) and high RFI (HRFI; n=12) Iowa State University Yorkshire RFI selection lines, and were housed in metabolism crates at the Texas Tech University Swine Center. Six pigs from each genetic line were assigned to one of two levels of Lys intake (70 and 100 % of estimated requirements; NRC, 2012) from diets in which Lys was first limiting among amino acids. The “single-dose end-product” approach was used for the measurement of protein turnover. Whole-body N flux was determined using an oral dose of [¹⁵N]-glycine (7.1 ±0.29 mg/kg BW) and measurement of ^I5N enrichment in urinary ammonia or urea. There was no effect of line or of the interaction between line and Lys intake on N flux, protein synthesis, protein degradation, and protein deposition (P>0.05).  Lys intake increased N flux (from 119 to 149 SE 15.3 g/d), protein synthesis (from 98 to 117 g of N/d), protein degradation (from 84 to 100 g of N/d), and protein deposition (from 13 to 16 g/d; P < 0.05). The protein synthesis-to-retention ratio tended to be higher in the LRFI compared to the HRFI line (6.5 vs. 5.8 SE 0.62; P = 0.06). Collectively, these results suggest that divergent selection for residual feed intake is not associated with changes in whole-body protein turnover in growing pigs. Therefore, protein turnover does not appear to be one of the physiological mechanisms contributing to differences in RFI in these lines of growing pigs. USDA-NIFA grant number 2011-68004-30336.

Keywords: protein turnover, residual feed intake, growing pig