The effect in sheep of physical form on the sites of digestion of a dried lucerne diet: 2.* Sites of nitrogen digestion

Abstract

The effect of altering the physical form of a regrowth crop of dried lucerne (Medicago sativa L. Tar. Chartainvilliers) on the site of nitrogen digestion was studied with sheep fitted with a rumen cannula and re-entrant cannulas at the proximal duodenum and the terminal ileum. Chopped, cobbed, and ground and pelleted diets were prepared by processing the same high-temperature dried crop and given twice daily to sheep at a level of intake of 910 g dry matter/z+ h. The chopped diet had a slightly lower content of total N and amino acid N than the other forms but the amino acid composition was similar and there was no difference between the diets in the apparent digestibility of total N. For all three physical forms the amounts of total N reaching the small intestine were greater than those ingested. The disappearance of apparently digested N in the caecum and colon of the sheep was significantly higher with the chopped form of the diet than with the cobbed and ground, pelleted forms ( P < 0.01). Of the individual amino acids, the greatest (and significant) increases compared with the food at the proximal duodenum were found for methionine, lysine, tryptophan and cysteinel cystine, and there were significant differences between diets for methionine, tryptophan, valine and cysteinelcystine. The amounts of isoleucine were also significantly increased at the proximal duodenum. Significant losses at the duodenum occurred for aspartic acid (all diets) and for phenylalanine and proline (two diets). T h e amounts of tryptophan, methionine, cysteinelcystine and serine absorbed from the small intestine, relative to amounts ingested, were significantly greater on the chopped and pelleted diets than on the cobbed diet. Ditferences between the chopped and pelleted diets in relative amounts absorbed from the small intestine were observed for tryptophan ( P < 0.01) and for cysteinelcystine ( P < 0.05). Apparent digestibilities of total amino acid N within the small intestine (based on amounts entering the small intestine) were 71'5, 66.2 and 69.8 yo for the chopped, cobbed and pelleted diets respectively. Mean values for apparent digestibility of individual amino acids within the small intestine ranged from 47 % for histidine to 80 "/o for methionine. Of the eighteen amino acids examined, values for apparent digestibility in the small intestine of thirteen of them were lowest on the cobbed diet; the effect was particularly noticeable for histidine. Despite this variation, there was a very significant positive correlation between the amounts of individual amino acids reaching the small intestine and absorbed from it with all three diets (n = 54; r = 0.990; P < 0.001); the same was true for each individual diet. Significantly lower amounts of nucleic acid N entered the small intestine of sheep given the pelleted diet ( P < 0.01). From the results for nucleic acid N it was calculated that on the chopped and cobbed diets 44-46 yo of the total N entering the small intestine was of microbial origin; on the pelleted diet the value was 32%.