One hundred fifty-seven gestating gilts were used to determine the standardized ileal digestible (SID) lysine (protein) intake in late gestation necessary to maximize whole-body N retention, piglet birth weight, and subsequent milk yield. On d 90 of gestation, gilts were assigned to one of seven isoenergetic diets with equally spaced and increasing SID lysine (protein) contents that ranged from 70 to 160% of NRC-(2012) estimated requirements, using soybean meal to supply additional lysine (13.3 to 30.5 g SID lysine per day; n=~22). Between d 105 and 108 of gestation, a N-balance was completed using total urine collection and fecal grab-sampling. After farrowing, all sows received a standard lactation diet until weaning (d20±1). Colostrum was collected (n=~14) ~24h after birth of the first piglet, confirmed by video recordings, and milk was collected on d 18 of lactation (n=~15). Linear and quadratic contrasts were constructed. Using the Bayesian information criteria to assess best fit, the linear broken line model was selected to determine optimal SID lysine (protein) intake. Gilt body weight gain in late gestation increased and body weight loss in the subsequent lactation tended to increase with increasing SID lysine (protein) intake (linear; P<0.001 and P=0.086, respectively). The gain in backfat depth during late gestation tended to decrease with increasing SID lysine (protein) intake (linear; P=0.067), but backfat depth loss during the subsequent lactation was not influenced by SID lysine (protein) intake. Nitrogen intake (52.5 to 79.5±0.9 g/d), excretion (29.6 to 43.8±2.1 g/d), and whole-body retention (N intake – N output; 21.1 to 36.9±2.3 g/d) increased with increasing SID lysine (protein) intake (linear; P<0.0001). Nitrogen retention was maximized at SID lysine intake of 22.0 g/d during late gestation (115% of NRC-estimated requirements). Litter size at birth (14.8±0.9) and the number of stillborns (0.7±0.3) were not affected by dietary treatment but piglet birth weight increased then decreased with increasing SID lysine (protein) intake (quadratic; P<0.01) and was maximized at SID lysine intake of 22.0 g/d during late gestation. Estimated milk yield and litter growth rate increased with SID lysine (protein) intake in late gestation (linear; P<0.05 and P=0.057, respectively). Estimated milk yield was maximized at SID lysine intake of 23.0 g/d during late gestation (120% of NRC-estimated requirements). Milk composition was not influenced by SID lysine (protein) intake in late gestation, but the dry matter content of colostrum decreased with increasing SID lysine (protein) intake (linear; P<0.01). Lactation feed intake tended to decrease then increase with increasing lysine intake (quadratic; P=0.067). Therefore, SID lysine supply in late gestating gilts should be increased by 15% above the current recommendations to maximize whole-body N retention in late gestation and piglet birth weight, and by 20% to maximize milk yield in the subsequent lactation.

Kloostra, V., C. Farmer and L. Huber. 2025. Standardized ileal digestible lysine intake by gilts should be increased in late gestation to maximize whole-body nitrogen retention, piglet birth weight, and subsequent milk yield. 2025 American Society of Animal Science Midwest Section meeting, Abstract 195.