RUMINAL IN-VITRO DEGRADATION OF PROTEIN IN TANNIN-FREE AND TANNIN-CONTAINING FORAGE LEGUME SPECIES

Protein in forages often is poorly utilized by ruminants because of ex tensive degradation in the rumen. Our objectives were to assess the va riation in ruminal protein degradability among forage legumes and to d etermine whether degradability was reduced by tannins. Legumes represe nting a range of species adapted to temperate regions were harvested o n 29 Aug. 1989 and 31 Aug. 1990 from triplicate plots, then lyophilize d, ground, and analyzed for tannins by radial diffusion. Ruminal prote in degradation rates and escapes, estimated assuming a passage rate of 0.06 h(-1), were determined both years by an in vitro limited substra te inhibitor procedure, and on samples from 1990 by a Michaelis-Menten method. Protein degradation rates and escapes (limited substrate meth od) in 1989 ranged from 0.27 h(-1) and 18% for white clover (Trifolium repens L.) to 0.001 h(-1) and 96 and 97% for two cultivars of sericea lespedeza [Lespedeza cuneata (Dum.-Cours.) G. Don]. In 1990, rates an d escapes ranged from 0.21 h(-1) and 21% for alfalfa (Medicago sativa L.) to virtually 0 and 93 to 97% for two cultivars of sainfoin (Onobry chis viciifolia Stop.) and three of sericea lespedeza. More rapid degr adation rates were obtained with the Michaelis-Menten approach for the most slowly degraded entries. Differences in degradation rate and rum inal escape were proportional to tannin concentration. However, red cl over (Trifolium pratense L.), which does not contain tannins, had a pr otein degradability comparable with forages with low levels of tannin. These data suggest that differences in ruminal protein degradation am ong forage legumes are only partly explained by the presence of tannin s.