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Substantial research has been conducted to determine the concentrations of rumen ammonia (NH3) below which rates of fibre digestion (Miller 1973; Russell and Strobel, 1987) or microbial growth (Satter and Slyter, 1974) are limited. Because the NH3 concentrations for optimal dry matter (DM) fermentation and for microbial growth appear to differ, various N supplements were evaluated in terms of their ability to optimise microbial growth efficiency in sheep fed a basal diet of low digestibility roughage. Nine mature sheep fitted with rumen cannulas ingested a basal diet of 900 g/d wheaten hay (as fed) alone (13.1 g/d of N), or the basal diet with soluble protein (whey, 24.5 g/d of N) or urea (24.2 g/d of N). Microbial protein outflow from the rumen was estimated from daily urinary allantoin excretion (Chen and Ørskov, 2004), and volatile fatty acid (VFA) production was predicted from VFA concentrations using equations developed by Leng and Leonard (1965). It was assumed that the production of 1 mol of acetate, propionate or butyrate generates 2, 3 or 2 mol ATP, respectively. |
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