Dynamics of nitrogen retention in gestating gilts at two feeding levels.

Whole-body protein deposition (Pd) is a main determinant of AA requirements of gestating gilts; however, data on the dynamics of Pd is limited. The main objective of this study was to measure whole-body Pd (pregnancy-associated and maternal) during gestation in gilts at 2 feeding levels. Changes in BW and backfat (BF) throughout gestation and subsequent lactation performance were also evaluated. We hypothesized that the effect of feed intake on maternal Pd decreases toward the end of gestation due to increased fetal nutrient demands. Fifty-one pregnant Yorkshire gilts (initial BW and BF at d 28 ± 0.5 of gestation were 168.6 ± 2.2 kg and 16.9 ± 0.8 mm, respectively) were used. Gilts were assigned to 1 of 2 feeding levels (high feeding level and low feeding level; 2.54 and 1.87 kg/d, respectively) of the same diet (3.30 Mcal ME/kg, 17.8% CP, 0.82% standard ileal digestible Lys) from d 33 to 112 ± 0.5 of gestation. Nitrogen balance observations (based on total urine collection with urinary catheters and determination of fecal N digestibility using an indigestible marker) were made at 5 distinct time periods, each 4 d in length, starting at d 38, 52, 66, 87, and 108 ± 0.5 of gestation. Pregnancy-associated Pd (fetus, mammary gland, uterus, and placenta and fluids) was calculated for each gilt and N balance period using the gestating sow model, based on actual litter size (including stillborns) and mean piglet birth weight. Maternal Pd was calculated as the difference between whole-body Pd and pregnancy-associated Pd. Across all N balance periods, whole-body and maternal Pd were consistently 40.1 ± 5.2 g/d greater ( < 0.001) for the high feeding level compared with the low feeding level. At both feeding levels, whole-body Pd increased (linear and quadratic, < 0.020) with day of gestation and maternal Pd decreased (cubic, < 0.010) with day of gestation. During lactation, maternal voluntary feed intake and litter performance were not affected by gestation feeding level. Gilts on the high feeding level lost more BW and BF during lactation ( ≤ 0.030). In summary, the effect of feeding level on whole-body and maternal Pd was constant throughout gestation. However, the gradual decline in maternal Pd with day of gestation is in contrast to that reported by the and has important implications for the factorial estimation of AA requirements of gestating gilts toward the end of gestation.