Impact of reducing dietary crude protein concentration on serum lysine concentration and lysine utilization efficiency in lactating sows
It was hypothesized that reducing dietary crude protein (CP) concentration while maintaining available Lys intake will improve Lys utilization efficiency for milk production in sows. Forty lactating multiparous Yorkshire sows were used to determine the effect of reducing dietary CP concentration and supplementing with crystalline amino acids on dietary Lys utilization efficiency during early (d3-7) and peak (d14-18) lactation. Sows were assigned to 1 of 4 diets:  16.0% CP (as-fed; analyzed contents; HCP);  15.7% CP (0.1% crystalline Lys; MHCP);  14.3% CP (0.2% crystalline Lys; MLCP);  13.2% CP (0.3% crystalline Lys; LCP); diet HCP was formulated using soybean meal and corn as the only Lys sources. Across diets, standardized ileal digestible (SID) content of Lys was 0.77%, based on analyzed content and estimated SID. Other essential amino acids were included to exceed requirements. Litters were standardized to 10 piglets within 24h of birth. Milk yield was estimated based on litter size and growth rate. Blood was collected from fasted sows on d3, 7, 14, and 18 for free amino acid analysis. The efficiency of using SID Lys intake for Lys output in milk (Klys) was calculated according to NRC (2012), accounting for maternal maintenance requirements and the contribution of maternal body protein mobilization based on sow BW change. Sow feed intake and litter growth rate during the 21d lactation period did not differ between dietary treatments (overall means: 5670±138 and 2263±94 g/d, respectively). Serum Lys concentration was influenced by day in lactation (P<.0001); there tended (P=0.08) to be a quadratic effect of dietary CP concentration (142.8, 105.6, 127.5, and 167.7±22.7, and 84.8, 63.2, 61.7, and 79.6±11.9 µmol/L; HCP, MHCP, MLCP, and LCP on d7 and d18, respectively). In early lactation, reduced dietary CP concentration did not affect Klys (67.5, 70.9, 64.2, and 66.7±6.63%; HCP, MHCP, MLCP and LCP, respectively; P>0.10). In peak lactation Klys was higher for MHCP than HCP (P<0.05), but a further reduction in diet CP concentration did not affect Klys (P>0.10; 59.8, 68.3, 65.1, and 67.6±2.92%; HCP, MHCP, MLCP and LCP, respectively). There appears to be an association between plasma Lys levels and Klys, which needs to be explored further. In peak lactation the use of up to 0.1% crystalline Lys to replace protein bound Lys in the diet improved Klys.
Keywords: amino acids, lactating sows, lysine utilization