The effect of genetic selection for residual feed intake on nitrogen metabolism and lysine utilization in growing pigs

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2016-06

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ABSTRACT Residual feed intake (RFI) is a measure of feed efficiency which is based on energy intake and energy requirements of the animals. It is defined as the difference between observed and expected feed intake based on requirements for gain and maintenance. However, the knowledge about the effects of genetic selection for RFI on efficiency of dietary protein and AA utilization for body protein deposition/ retention is limited. Therefore the objectives of this thesis were to 1.) Evaluate the effects of genetic selection for RFI on the growth performance of growing pigs (Chapter III). 2.) To evaluate the effects of lysine (Lys) intake and genetic selection for RFI on nitrogen metabolism and Lys utilization for protein deposition in growing pigs (Chapter IV). 3.) To evaluate the effect of genetic selection for Residual Feed Intake on whole body protein turnover in growing gilts fed either adequate or lysine deficient diets (Chapter V). To achieve these objectives, we initially used a total of 28 pigs (BW 7 ± 1 kg) selected from generation 9 of the low RFI (LRFI; n =14) and high RFI (HRFI; n =14) Iowa State University Yorkshire RFI selection lines. Pigs were subjected to a twelve weeks performance study. They were initially housed in group pens and then transferred to individual floor pens at 39-53 kg BW. During the study pigs were fed a typical corn-soy based diet. In the IV and V chapter of this thesis, a total of twenty four gilts (BW 66 ± 5 kg) were used. Six gilts from each genetic line were randomly assigned to individual metabolic crates. All gilts had free access to water and were assigned to two levels of Lys intake (70 and 100 % of the estimated requirements from diets in which Lys was first limiting among AA. Following 5 d of adaptation, urine samples were collected from each animal in the metabolic crate to establish a baseline for natural abundance of 15N. Pigs were administered orally with [15N] glycine gelatine capsule, following this, whole body N-balance and apparent fecal nutrient digestibility (AFD) was determined during a 3 d period. At the end of the isotope tracer and N-balance study pigs were anesthetized for determining body composition using an Intelligent Dual-energy X-ray Absorptiometry (iDXA) machine. In chapter V, the end-product technique was used to measure whole body protein turnover, in which 15N enrichment of ammonia and urea were end product metabolism. Results from chapter III showed that genetic selection for RFI had no effect on average daily gain (ADG) in these two lines (P > 0.05). However the average daily feed intake (ADFI) was lower in LRFI by 10 % compared to their HRFI counterparts (P < 0.01). Interestingly, we discovered that the deviation in feed intake (FI) of these two lines occurs at a very early stage of life and as early as two weeks. The feed to gain (F: G) ratio was considerable lower in LRFI lines compared to the HRFI line, suggesting that LRFI are more efficient (P < 0.01). The findings from of chapter IV showed that there were no interaction effects of line and Lys intake on N retention, apparent ileal digestibility (AID) and apparent fecal digestibility (AFD) of N, body lean and fat contents, and the 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). Lastly, we reported in chapter V that 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 genetic selection for low RFI is not associated with improvements in dietary Lys utilization and whole-body protein metabolism in Yorkshire gilts.

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Protein turnover, pigs

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