Abstract
Two male cattle (live weight 240 kg) fitted with rumen and simple T-piece duodenal cannulas were given a basal diet of 12 kg of silage (227 g/kg DM, 25.7 g N/kg DM), 1 kg of rolled barley and 100 g of mineral mixture. In addition to the basal diet, 0, 450 or 900 g/d of either sucrose or xylose was continuously infused intraruminally. The amount of organic matter (OM) entering the duodenum (P < 0.001) and excreted in the faeces (P < 0.01) was linearly increased with the increased sugar infusion. The proportion of digestible OM apparently digested in the rumen averaged 0.694, and did not differ (P > 0.05) with the level or type of sugar. Rumen ammonia concentration and molar proportions of isovalerate were decreased (P < 0.001) with increased sugar level. The decrease in the ammonia concentration was slightly less (P < 0.05) with xylose than with sucrose. Molar proportions of acetate, propionate and butyrate were not affected by the sugar level but sucrose produced a lower (P < 0.05) proportion of acetate and higher (P < 0.01) proportion of butyrate than did xylose. There was a net loss of N (11.6g/d) between the mouth and duodenum when the basal diet was consumed alone but a net gain (21.6 g/d) with the high level of sugar infusion. Microbial N flow at the duodenum, measured on the basis of RNA purine bases, increased linearly (P 0.05) for sugar levels of 0, 450 and 900 g/d, respectively. The amount of microbial N produced was closely related to rumen ammonia concentration (r—0.86; P < 0.001) and outflow of water at the duodenum (r 0.83; P
Highlights
The change that occurs in nitrogenous components of grass during ensiling is often extensive and varies with the type of fermentation that occurs (McDonald 1982)
The Agricultural Research Council (1984) reported a value of 23 g N/kg organic matter apparently digested in the rumen (Omadr) for the efficiency of microbial protein synthesis for silages as compared to 32 g N for hays and grasses
The reason for the low efficiency of microbial synthesis is not entirely understood, but in part it may be due to the presence of silage fermentation products and to the lower yield of ATP/kg Omadr
Summary
The change that occurs in nitrogenous components of grass during ensiling is often extensive and varies with the type of fermentation that occurs (McDonald 1982). The efficiency of microbial protein synthesis in the rumen in animals given silage diets has been reported to be lower than in animals given hay or mixed diets. The Agricultural Research Council (1984) reported a value of 23 g N/kg organic matter apparently digested in the rumen (Omadr) for the efficiency of microbial protein synthesis for silages as compared to 32 g N for hays and grasses. The reason for the low efficiency of microbial synthesis is not entirely understood, but in part it may be due to the presence of silage fermentation products and to the lower yield of ATP/kg Omadr. It has been suggested that one reason for low efficiency of microbial synthesis is poorly matched rates of ammonia and energy release from silage. Reducing the proportion of NAN in silage from 0.56 to 0.40 by increasing the application of formic acid from 0 to 5.9 litres/t had little influence on rumen ammonia concentration, bacterial N synthesis or passage of undegraded dietary protein to the small intestine (Chamberlain et al 1982). Rooke et al (1985a) suggested that the nature of the nitrogenous substances available to rumen microbes may limit the efficiency of microbial synthesis with silage diets
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