PROTEOLYSIS IN ENSILED FORAGE LEGUMES THAT VARY IN TANNIN CONCENTRATION

Extensive protein hydrolysis after harvest and during fermentation is characteristic of forage legumes preserved as silage. Differences in proteolysis among species have been observed, but plant characteristics associated with these differences have not been well defined. This study was conducted to determine if tannins play a role in modifying N transformations associated with the preservation of forage legumes as silage. In 1987, 12 legume genotypes (representing six species) were field grown and analyzed for tannin concentrations and pre- and post-ensiling dry matter (DM), pH, and N forms. In 1988, 18 genotypes (representing seven species) were evaluated in a similar manner. Tannin concentrations ranged from 0 to 27 g tannic acid equivalents kg-1 DM in 1987 and 0 to 31 g in 1988. Dry matter concentration, pH, and concentration of total N, soluble nonprotein N (SNPN), free amino acid N, and ammonia N of fresh and ensiled herbage were within the range of previous literature reports for forage legumes. The proportion of total N in the form of SNPN after 35 d ensiling ranged from 32 to 73% in 1987 and 26 to 69% in 1988 and was negatively related to tannin concentration (r2 = 0.75). Within sericea lespedeza [Lespedeza cuneata (DumCours) G. Don], the only species with substantial variation for both tannin and silage SNPN concentrations, r2 values were 0.81 in 1987 and 0.88 in 1988. Red clover (Trifolium pratense L.) and cicer milk-vetch (Astragalus cicer L.) did not contain measurable levels of tannins, but exhibited lower proteolysis than alfalfa (Medicago sativa L.). These results suggest that tannins play a major role in limiting proteolysis in some legumes during ensiling, but other factors also are involved.