Optimal crude protein in diets supplemented with crystalline amino acids fed to high-yielding lactating sows1.

The objective of the current study was to determine the requirement of standardized ileal digestible (SID) CP for maximal litter gain in high-yielding lactating sows due to insufficient supply of either His, Leu, Val, Ile, or Phe. The content of SID Lys was formulated at 95% of the recommended level, while that of Met, Met+Cys, Thr, and Trp was formulated at 100% of the recommended level or slightly greater using crystalline AA. A total of 540 parity 1 to 5 sows (L×Y, DanBred, Herlev, Denmark) were included in the study from day 3 after farrowing until weaning at day 26. Sows were allocated to six dietary treatments increasing in SID CP content (96, 110, 119, 128, 137, and 152 g/kg). Litters were standardized to 14 piglets at day 3 ± 2 after farrowing. At day 3 ± 2 after farrowing and at day 26 ± 3, sow BW and back fat, and litter weight were recorded. On a subsample of 72 sows (parity 2 to 4), litters were also weighed at days 10 and 17 ± 3, and milk and blood were sampled at day 3 ± 2 d, and 10, 17 and at 24 ± 3 d in lactation. Sow body pools of protein and fat were determined on the 72 sows at days 3 ± 2 and 26 ± 3 d using the D2O dilution technique. All data were subjected to ANOVA, and to linear and quadratic polynomial contrasts. Variables with quadratic effects or days in milk × treatment interactions were analyzed using linear regression or one-slope linear broken line using the NLMIXED procedure of SAS. Average daily litter gain reached a breakpoint at 125 g SID CP/kg (as-fed). Multiparous sows had a greater litter gain than primiparous sows (3.33 vs. 3.02 kg/d above the breakpoint; P < 0.001) but litter size (13.1 ± 0.1) at weaning were unaffected by dietary treatment (P = 0.62). Sow BW loss was minimized at 102 g SID CP/kg. Concentrations of protein and casein in milk increased linearly with increasing SID CP (P < 0.001). Milk urea reached a minimum at 111-118 g SID CP/kg (P < 0.05) and milk fat a maximum at 116 g SID CP/kg (P < 0.05). In conclusion, 125 g SID CP/kg feed was required to maximize litter gain.