Ruminal and post-ruminal protein digestion in sheep fed on subterranean clover harvested at four stages of maturity

Abstract

Digestion of the protein of subterranean clover harvested at four stages of maturity was studied in Merino wethers fitted with re-entrant duodenal cannulae. The relative proportions of dietary and microbial nitrogen flowing into the duodenum were estimated from the incorporation into microbial protein of sulphur-35 from ruminal infusions of Na235SO4. Sheep given immature clover consumed more digestible organic matter (DOM) and retained more nitrogen per 100 g DOM intake than sheep given wilted and mature clovers. More protein of the immature clover was degraded in the rumen (73%) than of the mature clover (43–53%); post-ruminal digestibility of dietary protein escaping ruminal proteolysis was also higher for the immature clover (67–70% versus 51–53%). The quantities of clover protein (per 100 g DOM intake) digested post-ruminally were therefore similar at all stages of maturity. The yield of microbial protein (mean 19.9 g per 100 g OM digested in the rumen) was also similar on all treatments. However, ruminal digestion of the DOM of the immature clover was more complete (84%) than that of the mature clover (60–71%); consequently, microbial protein production per 100 g DOM intake was greater on the immature clover. Greater nitrogen retention by animals consuming immature clover was associated with a greater DOM intake. Whether the causal factor was the greater supply of microbial protein per 100 g DOM, or whether the causal factor was a greater DOM intake controlled by factors associated with ruminal digestion, is unresolved. It is also apparent that the greater faecal nitrogen losses per 100 g DOM intake on the mature clover compared with the immature clover were attributable entirely to differences in the amounts of dietary nitrogen excreted at different stages of maturity of the clover.