THE PARTIAL DIGESTION AND RUMINAL VOLATILE FATTY-ACID CONCENTRATIONS IN WETHERS FED HIGH-AND LOW-FIBER DIETS

Twenty Dohne merino wethers with an average mass of 52 kg, were used t o study the effect of varying the fibre content of the diet on differe nces in ruminal concentrations of volatile fatty acids (VFA) and the p artial digestion of these diets. Dietary carbohydrate was provided as either structural (HF treatment) or readily fermentable (LF treatment) , while energy and nitrogen intakes remained constant. The wethers wer e fitted with simple 'T' cannulae in the rumen, abomasum and ileum. Th e diets were fed at half-hourly intervals. Cr-51-EDTA and (RuP)-Ru-103 were infused intraruminally for 14 days and spot samples were withdra wn from the various cannulae over the last four days, in order to dete rmine digesta flow and apparent digestion of the diets in the differen t gastrointestinal compartments. Changing the form of dietary carbohyd rate had no effect on organic matter (OM) or nitrogen (N) digestion. Q uantitatively more starch (p less than or equal to 0.05) was digested in the rumen (186 +/- 24.6 g.day(-1)) and small intestine (35.6 +/- i2 .6 g.day(-1)) of the LF than the HF wethers, whereas more fibre (p les s than or equal to 0.05) was digested in the rumen (84.2 +/- 10 g.day( -1)) and small intestine (7.4 +/- 5.1 g.day(-1)) of the HF than the LF wethers. Ruminal fermentation was significantly affected by altering the fibre : concentrate ratio in the diet. The molar proportions of th e VFA differed significantly (p less than or equal to 0.05) between th e two treatments. The ruminal proportions of acetate, propionate and b utyrate (mmol.mol(-1)) were 0.721, 0.208 and 0.071 +/- 0.01, respectiv ely, in the HF group and 0.645, 0.226 and 0.112 +/- 0.01, respectively , in the LF group. As a result the acetate : propionate ratio was lowe r (p I 0.05) in the LF compared to the HF wethers (2.86 and 3.48 +/- 0 .07, respectively). The main differences between the two treatments ap peared to be an increased supply of glucogenic precursors (viz, glucos e and propionate) to the host when the LF rather than the HF diet was fed.