Effect of Leucine Supplementation to a Reduced Crude Protein Diet on Nitrogen Utilization in Lactating Sows

The objective of the study was to test the hypothesis that partial substitution of dietary CP with crystalline amino acids (CAA) increases nitrogen (N) utilization and milk production, and with CAA in combination with Leu decreases N utilization and milk production. Lactating multiparous Yorkshire sows (36) were used in the experiment and were assigned to 1 of 3 diets: 19.60% CP (CON); 14.22% CP (OPT); 14.56% CP (OPT+LEU). Sow body weight (BW) and backfat were measured on d 1 and 21 of lactation while piglet BW was measured on d 1, 4, 7, 14, 17 and 21 of lactation. Nitrogen balance was measured on sows between d 4 and 7 (early) and d 14 and 17 (peak) of lactation. Overall mean of sow BW change (− 4.5 ± 4.47 kg), backfat change (− 2.2 ± 0.79 mm), average daily intake (5.36 ± 0.19 kg), and litter growth rate (2.5 ± 0.13 kg/d) did not differ across diets over the 21-d lactation period. Nitrogen intake decreased as dietary CP concentration decreased (149.5, 112.0, and 104.9 ± 3.29 g/d; 206.3, 156.6, and 152.9 ± 5.29 g/d for CON, OPT and OPT+LEU, in early and peak lactation, respectively; P < 0.01). In early lactation, maternal N retention (NRm) (N intake – fecal and urinary N) did not differ between diets (overall mean: 84.5 ± 4.04 g/d) and total N retention (NRt) (N intake – fecal and urinary N – milk N) differed (P < 0.05) across diets (32.9, 20.7, and 17.0 ± 3.57 g/d). In early lactation, as dietary CP decreased, NRt as percentage of N intake (61.0%, 74.6%, and 73.0% ± 0.02%) or percentage of N absorbed (N intake – fecal N) (69.3%, 84.8%, and 82.7%) increased (P < 0.01). In peak lactation, NRt (65.0, 14.4, and 29.3 g/d) and NRm (147.0, 116.7, and 125.2 ± 5.27 g/d) differed across diets (P < 0.01). As percentage of N intake, NRm (71.0%, 74.5%, and 82.0% ± 0.02%) or of N absorbed (80.7%, 84.6%, and 93.2% ± 0.03%) differed across diets (P < 0.05). Feeding lactating diets reduced in CP from 19.6 to 14.2% with CAA inclusion as partial replacement for limiting AA, with or without Leu, improved N utilization efficiency.