1837
Effects of selenium supply, maternal plane of nutrition, and physiological stage on nitrogen flow, microbial efficiency, and metabolizable protein in primiparous ewes
Primiparous Rambouillet ewes (n = 84, age = 240 ± 17 d, BW = 52.1 ± 6.2 kg) were allocated to 2 × 3 × 2 factorial arrangement to evaluate dietary Se (adequate Se [11.5 µg/kg BW] or high Se [77.0 µg/kg BW]), nutritional plane (60% [restricted], 100% [control], or 140% [high]), and physiological stage at necropsy (parturition or d 20 of lactation) effects on nitrogen digestion and microbial CP production. At parturition, lambs were removed from all ewes and 42 ewes (n = 7 per treatment) were necropsied. Remaining ewes were transitioned to a common lactation diet to meet NRC requirements and machine milked for 20 d. Differences between treatments were determined by the GLM procedure in SAS. No three way interactions were present (P > 0.10), but many 2-way interactions were found (P < 0.05). Selenium did not influence N intake (P = 0.23). Nitrogen intake increased with plane of nutrition during gestation and lactation (P < 0.001; 13.3 vs. 20.2 vs. 31.4 ± 0.6 g during pregnancy and 40.4 vs. 41.6 vs. 42.2 ± 0.5 g during lactation for restricted, control, and high ewes, respectively). Microbial efficiency of ewes on restricted diets (7.0 ± 1.04 g microbial nitrogen/kg truly fermented) was decreased compared with control and high ewes (13.4 and 14.9 ± 1.04 g microbial nitrogen/kg truly fermented) but these differences were gone after 20 d of lactation (P > 0.15). Supranutritional Se supply and restricted and adequate nutrient planes increased microbial efficiency regardless of physiological stage (P = 0.04). Selenium supplementation increased (P = 0.04) total tract N digestion during gestation (69.3 and 71.4 ± 1.0 %, respectively) but decreased N digestion during lactation (74.7 and 72.9 ± 0.9 %, respectively). Ruminal N digestion was greater (P = 0.01) during gestation in restricted ewes compared with control or high ewes (59.1 vs. 44.9 and 39.9 ± 2.8 %, respectively); however, post-ruminal N digestion was less (P = 0.02) in restricted ewes compared with control and high nutritional planes (13.1 vs 26.1 and 28.1 ± 2.8 %, respectively). Metabolizable protein increased (P < 0.001) during gestation with increasing nutritional intake but did not differ during lactation. Nutritional plane and Se supplementation had different effects on nitrogen digestion and microbial efficiency but those effects were dependent on physiologic stage.
Keywords: ewes, microbial efficiency, selenium