Fifty-two multiparous sows (average parity 3.1 +/- 0.9 and initial BW 245.6 +/- 32.5 kg) were used to evaluate the effects of dietary standardized ileal digestible (SID) Lys-to-net energy (NE) ratios on nitrogen (N) utilization throughout a 24-d lactation period. Sows were randomly assigned to one of five isoenergetic feeding programs that provided equally spaced and increasing SID Lys-to-NE ratios between 2.79 and 5.50 g SID Lys/Mcal NE. The feeding programs were generated by blending the two extreme diets in varying proportions and were provided to sows immediately after farrowing (day 1) and until weaning at day 24 +/- 1. Nitrogen balances were conducted between days 4 and 7, 12 and 15, and 20 and 23 +/- 1 of lactation to represent weeks 1, 2, and 3, respectively, using total urine collection and fecal grab sampling. Contrast statements were used to determine the linear and quadratic effects of increasing Lys-to-NE ratios. Linear and quadratic broken-line and polynomial quadratic (QPM) models were used to determine the optimum dietary Lys-to-NE ratios for N retention in milk. The Bayesian information criterion was used to assess the best fit. Feeding program did not influence sow average daily feed intake (5.8 +/- 0.1 kg), BW change (-8.2 +/- 3.1 kg), or change in back fat thickness (-2.6 +/- 0.7 mm) over the 24-d lactation period, but piglet average daily gain increased with dietary SID Lys-to-NE ratio (linear; P < 0.05). Sow N intake increased with increasing dietary Lys-to-NE ratio in weeks 2 and 3 (linear; P < 0.001). Whole-body N retention (N intake - N output in urine and feces) increased with increasing dietary Lys-to-NE ratio in all weeks (linear; P < 0.05). The N retention in milk tended to increase then decrease with increasing dietary Lys-to-NE ratio in weeks 1 and 2 (quadratic; P = 0.051 and P = 0.081) and the QPM showed optimal milk N retention at 4.28, 4.42, and 4.67 g Lys/Mcal NE for weeks 1, 2, and 3, respectively. Maternal N retention (N intake - N output in urine, feces, and milk) decreased and then increased in week 1 (quadratic; P < 0.01) and increased in weeks 2 and 3 (linear; P < 0.01) with increasing dietary Lys-to-NE ratio. Therefore, the SID Lys-to-NE ratio necessary to optimize milk N output is dynamic throughout lactation. A two-diet feeding program could be created to match optimal weekly or daily SID Lys-to-NE ratios, which could lead to improved piglet ADG and body weights at weaning. Lay Summary Despite significant changes in nutrient and energy requirements as well as voluntary feed intake during lactation, sows are typically fed a single diet with a static nutrient and energy composition throughout the entire lactation period, which may not optimize milk output. Fifty-two sows were used to explore how various ratios of standardized ileal digestible (SID) Lys to net energy (NE) in lactating sow diets affect the growth of piglets and nitrogen utilization during a 24-d lactation period. Sows were randomly assigned to one of five feeding programs that contained equal amounts of energy and provided equally spaced and increasing Lys-to-NE ratios between 2.79 and 5.50g SID Lys/Mcal NE immediately after farrowing. The dietary Lys-to-NE ratio did not influence sow daily feed intake, body weight change, or change in backfat thickness over the 24-d lactation period; however, piglet growth rate and body weight at weaning increased with increasing Lys-to-NE ratio. The SID Lys-to-NE ratio nec-essary to optimize milk nitrogen output was 4.28, 4.42, and 4. 67g SID Lys/Mcal NE during weeks one, two, and three of lactation, respectively. Therefore, it is possible to create a two-diet feeding program offering dynamic SID Lys-to-NE ratios as lactation progresses, which could lead to improved piglet average daily gain and body weights at weaning
