The standardized ileal digestible lysine-to-net energy ratio in the diets of lactating primiparous sows to optimize maternal nitrogen retention is dynamic but does not impact piglet performance.

Fifty-five primiparous sows were recruited to evaluate the effect of increasing standardized ileal digestible (SID) Lys-to-net energy (NE) ratios on primiparous sow maternal nitrogen retention, milk nitrogen output, and piglet growth performance during each week of lactation. Sows were assigned to one of five diets with equally spaced and increasing SID Lys-to-NE ratios between 2.85 and 5.51 g SID Lys/Mcal NE. The intermediate ratios were achieved by blending the two extreme diets in varying proportions using a feeding system with feed blending capabilities and were provided to sows immediately after farrowing and until weaning (day 24 ± 1). Nitrogen (N) balances were conducted between days 4 and 7 (week 1), 12 and 15 (week 2), and 20 and 23 (week 3) with total urine and fecal grab sampling occurring during each N balance period. Linear (LBL) and quadratic broken-line and polynomial quadratic models were used to determine the optimum dietary Lys-to-NE ratios for maternal N retention in each week of lactation. The Bayesian information criterion was used to assess best fit. Dietary feeding program did not influence sow average daily feed intake in lactation, piglet average daily gain and body weight (BW) at weaning, or milk N output during any nitrogen balance period. Sows fed increasing SID Lys-to-NE ratios lost less BW and backfat by the end of lactation (linear; P < 0.05). Sow N intake, N excretion, whole-body N retention (N intake - N excretion in urine and feces), and maternal N retention (whole-body N retention - milk N output) increased as dietary SID Lys-to NE ratio increased within each N balance period (linear; P < 0.05). The LBL model showed the optimal SID Lys-to-NE ratio for maternal N retention was out of the range of the dietary treatments for week one, but maternal N retention was optimized at 4.74 and 4.85 g SID Lys/Mcal NE in weeks two and three, respectively. Therefore, a dynamic SID Lys-to-NE ratio during each week of lactation could be implemented to enhance maternal N retention of primiparous sows.