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Effects of lysine intake and divergent selection for residual feed intake on nitrogen metabolism and lysine utilization in growing pigs
Residual feed intake (RFI) is defined as the difference between actual feed intake and that predicted on the basis of energy requirements for growth and maintenance. Little is known about the effect of divergent selection for RFI on efficiency of amino acid utilization in growing pigs. Our objective was to evaluate the effects of divergent selection for RFI and lysine (Lys) intake on nitrogen (N) metabolism and Lys utilization in growing pigs. Twenty four gilts (BW 66 ± 5 kg) were selected from generation 9 of the low RFI (LRFI; n=12) and high RFI (HRFI; n=12) Iowa State University Yorkshire RFI selection lines. Six pigs from each genetic line were assigned to each of two levels of Lys intake (70 and 100 % of estimated requirements; NRC, 2012) from diets in which Lys was first limiting among AA. Following 5 d of adaptation, whole body N-balance and apparent fecal nutrient digestibility (AFD) was determined during a 3 d period, after which pigs were anesthetized for determining body composition using Dual-energy X-ray absorptiometry (iDXA). Pigs were then euthanatized and ileal digesta was collected for measuring apparent ileal nutrient digestibility (AID). No interaction effects of line and Lys intake on N retention, AID and AFD of N, body lean and fat contents, and efficiency of Lys utilization for N retention were observed (P > 0.05). Line had no effect on retention, AID and AFD of N (P > 0.05). An increase in Lys intake improved N retention in both lines (from 15.0 to 19.6 g/d, SE 1.44, in LRFI; and from 16.9 to 19.8 g/d, SE 1.67 in HRFI pigs; P < 0.01). Lysine intake had no effect on AID and AFD of N (P > 0.05). At the low Lys intakes and when Lys clearly limited N retention the efficiency of using Lys intake (above maintenance requirements) for Lys in retained N was 77 and 86 %, SE 3.1, for the LRFI and HRFI pigs, respectively (P < 0.05). Body lean tissue content tended to be higher in LRFI than HRFI pigs (82 vs. 76 %, SE 2.7; P < 0.09). Collectively, these results suggest that genetic selection for low RFI is not associated with improvements in Lys utilization efficiency in growing pigs. USDA-NIFA grant number 2011-68004-30336.
Keywords: residual feed intake, lysine, pig