PSVIII-5 - The effect of dietary protein (Lys) quantity and source in late gestation on mammary development in gilts

Abstract: Sixty-four Yorkshire × Landrace gilts (initial BW on day 75 of gestation 191.6 ± 9.6 kg) were used to determine whether protein (Lys) quantity and source influence mammary development in late gestation. Gilts were fed a conventional gestation diet until day 89 of gestation, then assigned one of four isocaloric (2,505 kcal/kg net energy) dietary treatments: [1] control [CTL; 0.70% standardized ileal digestible (SID) Lys, 16.6% crude protein (CP)], [2] additional soybean meal (SBM; 0.98% SID Lys, 21.1% CP), [3] casein (CSN; 0.98% SID Lys, 20.0% CP), or [4] crystalline lysine (LYS; 0.98% SID Lys, 16.9% CP) to supply Lys 40% above estimated requirements (n = 16). Gilts were fed 2.65 kg daily until day 110 of gestation, then slaughtered to assess mammary development. Blood samples were collected on days 90 and 110 of gestation to measure free fatty acids (FFA), plasma amino acids (AA), and serum urea concentrations. The BW (226.5 ± 10.1 kg) and backfat thickness (20.0 ± 2.9 mm) of gilts on day 110 of gestation were not influenced by dietary treatment (P > 0.10). Mammary composition, including parenchymal and extraparenchymal tissue masses, protein and fat contents of parenchymal tissue, and concentrations of DNA and RNA in parenchymal tissue were not influenced by dietary treatment (P > 0.10), unlike previous studies in gestating gilts. Total litter weight (18.56 ± 0.39 kg), fetal BW (1.21 ± 0.02 kg), total ovary weight (25.99 ± 0.49 g), and number of corpora lutea (21.03 ± 0.37) were not different among treatments (P > 0.10). Variables measured in blood did not differ between treatments on day 90 of gestation (P > 0.10), however, some differences were seen on day 110. Serum urea concentration was greater for CSN gilts versus CTL and LYS gilts (P < 0.01), with SBM being intermediate. The plasma Val concentration was greater in CSN gilts than CTL gilts (P < 0.05), with SBM and LYS intermediate. The daily SID Lys intakes were calculated using feed intake, analyzed amino acid contents, and estimated SID coefficients of the diets, and showed that the CTL, SBM, CSN, and LYS diets provided 14.3, 16.4, 20.0, and 14.3 g/day of SID Lys, respectively, whereas the estimated SID Lys requirement was 18.6 g/day in late gestation. Thus, the CTL, SBM, and LYS diets were below the estimated Lys requirements of gilts in late gestation, likely due to feed manufacturing issues. Furthermore, all diets with added Lys, either via whole protein sources or crystalline lysine, did not supply SID Lys at 40% above estimated requirements. Therefore, providing Lys up to 30% below estimated requirements in late gestation had no detrimental effect on mammary development with no apparent disadvantage attributed to dietary Lys source.