Standardized ileal digestible lysine (protein) intake by primiparous sows should be increased in late gestation to maximize whole-body nitrogen retention, piglet birth weight, and subsequent milk yield.

One hundred fifty-two gestating primiparous sows were used to determine the standardized ileal digestible (SID) Lys (protein) intake in late gestation necessary to maximize whole-body N retention, piglet birth weight, and subsequent milk yield. On day 90 of gestation, primiparous sows were assigned to 1 of the 7 isoenergetic diets with equally spaced and increasing SID Lys (protein) contents using primarily soybean meal to supply additional Lys [70% to 160% of NRC-NRC-(2012) estimated requirements; 13.3 to 30.5 g SID Lys per day; n = ~ 21]. An N-balance was completed between days 105 and 108 of gestation. After farrowing, all sows received a standard lactation diet until weaning (day 20 ± 1). Using the Bayesian information criteria to assess best fit, the linear broken-line model was selected to determine optimal SID Lys (protein) intakes. Sow BW gain in late gestation increased, and BW loss in the subsequent lactation tended to increase with increasing SID Lys (protein) intake in late gestation (linear; P < 0.001 and P = 0.086, respectively). The gain in backfat depth during late gestation tended to decrease with increasing SID Lys (protein) intake (linear; P = 0.067), but backfat depth loss during the subsequent lactation was not influenced by SID Lys (protein) intake in late gestation. The N intake, excretion, and whole-body retention (N intake-N output in urine and feces) increased with increasing SID Lys (protein) intake (linear; P < 0.0001). Whole-body N retention was maximized at an SID Lys intake of 22.0 g/d during late gestation (115% of NRC-estimated requirements). Litter size and the number of stillborns per litter were not affected by dietary treatment, but piglet birthweight increased and then decreased with increasing SID Lys (protein) intake (quadratic; P < 0.01) and was maximized at an SID Lys intake of 22.0 g/d during late gestation. Estimated milk yield in the subsequent lactation increased (linear; P < 0.05) and litter growth rate tended to increase (linear; P = 0.057) with SID Lys (protein) intake in late gestation. Estimated milk yield was maximized at an SID Lys intake of 22.7 g/d during late gestation (119% of NRC-estimated requirements). Therefore, SID Lys intake in late gestating primiparous sows should be increased by 15% above current recommendations to maximize whole-body N retention in late gestation and piglet birth weight, and by 19% to maximize milk yield in the subsequent lactation.