Effect of diet forage to concentrate ratio on rumen degradability and post-ruminal availability of protein from fresh and dried lucerne

The ruminal degradation of dry matter (DM) and crude protein (CP) and the intestinal availability of CP of four lucerne samples were measured on two diets with lucerne hay to concentrate ratios of 2 : 1 (diet F) and 1 : 2 (diet C). Two samples of fresh lucerne (third cut) harvested after 2 (FL1) or 8 (FL2) weeks from the previous cut were used together with a sample of lucerne hay (LH) and another of dehydrated lucerne (DL). Rumen degradability, measured by the nylon bag technique, and rumen outflow rates were determined on three rumen cannulated wethers. Intestinal digestibility was determined by the mobile bag technique on three duodenal fistulated wethers. For CP, significantly lower values were observed with diet C than with diet F for the potentially degradable insoluble fraction (0.334 v. 0.397) and its degradation rate (0,093 v. 0.134 per h). As a consequence, the effective degradability was also lower with diet C (0.746 v. 0.821; P = 0.059). Effective degradability of DM was also apparently lower with diet C (0,596 v. 0.634). With both diets, the intestinal digestibility decreased in all the samples with increase of ruminal incubation time according to a simple exponential equation. The undegraded CP digested in the gut (Di) and therefore the effective intestinal digestibility (EID) were derived from this exponential function according to the rumen outflow of undegraded CP. Mean values of Di (expressed as proportion of food CP content) were respectively 0.091 and 0.142 for F and C diets and 0.084, 0.115, 0.116, and 0.152 for FL1, FL2, LH and DL samples. Lower rumen degradability was partially compensated for by higher Di values resulting in a close correlation between both parameters (r = -0.965; P < 0.001). The change of the digestion site associated with the reduction of the effective degradability of CP produced also an increase in the undigested CP as a proportion of food CR So, these values are respectively 0.087 and 0.112 for F and C diets and 0.053, 0.109, 0,096, and 0,141 for FL1, FL2, LH, and DL samples. No difference in EID between F and C diets was observed (0,529 v. 0.563). For samples, the only effect (P < 0.05) was recorded between FL1 (0.618) and the other samples (0.509, 0.544 and 0.512 for FL2, LH, and DL, respectively).