In vitro solubility and degradability of nitrogen in concentrate ruminant feeds

Ten ruminant feeds were incubated either in situ or by four different in vitro methods: buffer alone (BF); buffer and protease (BP); strained and centrifuged rumen fluid (CR); strained rumen fluid (WR). The in vitro incubation lasted 0.5, 1, 2, 3, 4, 6, 9 and 24 h. Analysis of variance of the nitrogen degradability coefficients showed that the immediately soluble fraction a was not significantly affected by the analytical method; the potentially degradable fraction b was significantly (P < 0.01) higher with the in situ method than with BP and WR, which in their turn gave higher b values than BF and CR. A first comparison, between the nitrogen degradability coefficients a, b and c (the constant rate of degradation of b) measured in situ or in vitro, showed a general lack of correlation. The in situ effective degradability calculated with rumen outflow rates (k) of 0.02, 0.05 and 0.07 h-1 was correlated with the in vitro estimates, the r value being generally higher when k was assumed to be 0.07 h-1 for the in situ and 0.02 h-1 for the in vitro method (r=0.881, 0.811, 0.831 and 0.782 for BF, BP, CR and WR, respectively). The correlation coefficients between the in situ effective degradability and the percentage of nitrogen degraded in vitro at each time of incubation were always significant. For the BF, CR and WR methods, the highest coefficients were observed at 24 h of incubation (r=0.871, 0.812 and 0.750, respectively, P < 0.01), and for the BP method, after 6 h (r=0.889, P < 0.001). The results showed that protease degradability is superior in predicting in situ degradability of feeds, and suggested that a future in vitro technique would need to group feeds according to their degradability characteristics.