Wheat straw as ruminant feed. Effect of supplementation and ammonia treatment on voluntary intake and nutrient availability.

This thesis describes the results of experiments with goats, sheep and cattle fed untreated or ammonia-treated wheat straw. Aim of the experiments was to identify factors limiting voluntary intake and digestion of these low-quality feeds. Supplementation of urea to untreated wheat straw increased in vitro degradation if the ammonia-nitrogen concentration in the substrate was below 60-100 mg/l. No effect of urea supplementation to untreated wheat straw on digestion was observed in sheep and goats. Voluntary straw intake increased, however, in goats, but not in sheep when urea was supplemented to untreated wheat straw. Ammonia treatment increased intake and digestion in sheep as well as in cattle. The increased rumen turnover as a result of ammonia treatment was associated with an increased potentially degradable fraction of the ingested straw. The fractional rate of passage, estimated from the faecal excretion pattern of Cr-NDF and the fractional rate of degradation derived from in sacco studies were not much affected by ammonia treatment. Validation of these rate constants of passage and degradation by model estimates derived from a rumen evacuation study showed that the in sacco rate of degradation underestimated and that the rate of passage derived from the faecal excretion pattern of Cr-NDF overestimated the actual values. Digestible energy intake from untreated and ammonia-treated wheat straw by sheep and cattle were equalized by scaling to liveweight <sup>0.946</SUP>. There were strong indications that maintenance requirements Accrues sheep and cattle were related to liveweight to an exponent lower than 0.9. Cattle were therefore more efficient with regard to utilization of rations based on untreated and ammonia-treated wheat straw than sheep. Although ammonia treatment resulted in a considerably increased digestible energy intake, the availability of amino acids for absorption from the small intestine remained relatively low as a consequence of a low efficiency of rumen microbial protein synthesis and a low duodenal flow of rumen undegraded dietary amino acids. Supplementation of casein, a protein of a high rumen degradability or of potato protein of a relatively low rumen degradability, to ammonia-treated wheat straw increased the efficiency of microbial protein synthesis, whereas potato protein also increased the duodenal flow of undegraded dietary amino acids. Casein supplementation resulted in substitution of straw by the supplement intake, whereas digestible energy intake increased as a result of potato protein supplementation. It was concluded that voluntary digestible energy intake of rations based on wheat straw was primarily limited by the duodenal availability of protein and not by the rumen processing capacity