Abstract:
This study investigated the effect of urea or calcium nitrate with or without the inclusion of Acacia tannin extract on dry matter intake, nutrient digestibility, growth performance, and methane emission in growing lambs. Forty South African Mutton Merino lambs (95 days old, average body weight of 34.7 kg) were blocked by weight and sex and randomly allocated to four groups and fed diets supplemented with: urea (control); nitrate; urea + tannin; and nitrate + tannin. The concentration of urea and nitrate source in the diet was 10 g/kg and 32 g/kg, respectively, while the tannin was an extract of Acacia mearnsii added at 42 g/kg DM. Lambs were gradually adapted to the diets for 21 days after which growth performance was monitored for 60 days. At the end of the experimental period, methane was measured in open-circuit respiratory chambers, and digestibility was carried out inside metabolic cages. Dry matter and other nutrient intakes of lambs were not influenced by NPN source or the inclusion of tannin extract in the diet with the exception of the crude protein intake, which was higher (P ≤ 0.05) in lambs receiving urea-containing diets. Tannin inclusion reduced (P < 0.05) digestibility of dry matter, crude protein, neutral detergent fibre and acid detergent fibre of diets but did not affect methane emission of lambs. Tannin inclusion shifted N-excretion from urinary-N to faecal-N although overall N-retention was not affected. Tannin inclusion reduced acetate and increased propionate proportion in the urea-containing diet (P < 0.01), whereas, it had no such effect in the nitrate-containing diet. Higher (P ≤ 0.05) average daily gain and lower methane emission (P < 0.01) were recorded in lambs fed the nitrate-containing diets. In contrast, the inclusion of tannin reduced (P < 0.05) the average daily gain without any reduction in methane production. This study demonstrated that nitrate could be used as a source of non-protein nitrogen with the additional benefit of reducing enteric methane emission and improving the average daily gain of lambs. In contrast, tannin extract did not show any positive effect to justify its use under the dietary conditions of this study.
