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Protein requirements of sheep in late pregnancy: partitioning of nitrogen between gravid uterus and maternal tissues.

The objective of this study was to quantify effects of maternal protein nutrition on N accretion or loss in conceptus and maternal tissues of ewes during late pregnancy. Ewes, pregnant with twins, were fed low (LP, 79 g CP/kg DM), medium (MP, 116 g CP/kg DM), or high (HP, 157 g CP/kg DM) protein diets, each with an estimated ME concentration of 2.7 Mcal/kg DM, between d 111 and 140 of pregnancy; all ewes had been fed the same diet (2.7 Mcal ME, 120 g CP/kg DM) for the previous 30 d (d 80 to 110). Dry matter intakes were varied (LP = 1.0, MP = 1.2, and HP = 1.4 kg/d) according to predicted energy costs of protein deposition for each diet. Nitrogen accretion was estimated by comparative slaughter (d 140 minus d 110) and by collection of excreta between d 120 and 130. Fresh weights of maternal and gravid uterine tissues were measured at slaughter, before proximate analysis of these components. Whole-body N retention was directly and linearly related to N intake, but efficiency of deposition of apparently absorbed N decreased linearly with increasing N intake (LP, .79; MP, .70; HP, .62). Nitrogen accretion in the gravid uterus, maternal viscera, and mammary gland was significantly less in LP than in MP or HP ewes. Nitrogen balance in maternal carcass tissues was linearly related to N intake, ranging from a negative value in LP ewes to a positive value in HP ewes (LP, -63 g; MP -39 g; HP, 55 g). These data provide the basis for estimating N requirements for protein accretion in the conceptus and in maternal tissues during late pregnancy. They also highlight the capacity of maternal carcass tissues to mobilize or deposit amino acids in response to variations in dietary protein supply.