Skatole and indole formation in comparison to rumen ammonia and volatile fatty acids in sheep fed fresh forages

Abstract

Skatole and indole concentrations in the rumen were compared with the rumen concentrations of ammonia and volatile fatty acids (VFA) in sheep fed fresh white clover (WC; Trifolium repens), perennial ryegrass (PRG; Lolium perenne) or the condensed tannin-containing forage, Lotus corniculatus (LC). Six rumen fistulated Romney wethers were fed the cut forages over two 2-hour meal periods in the morning and afternoon, with rumen contents sampled at intervals after the start of the morning meal. Maximum concentrations of skatole, indole and ammonia were highest when feeding WC (P<0.05). Ammonia accumulates in the rumen when there is rapid protein degradation with unsynchronised energy supply. Condensed tannins in LC slowed protein degradation and consequently maximal ammonia, indole and skatole concentration were lower when feeding this forage (P<0.05). This indicates that skatole and indole formation will be greater when there is high protein degradability. Ammonia, indole and skatole concentrations in sheep fed PRG were intermediate, and not significantly different from either WC or LC. Maximum concentration of straight-chain volatile fatty acids (VFA) were not significantly different for the forages, but branchedchain VFA concentrations tended to be higher for WC (P=0.103) compared with LC. Straight-chain VFAs are a by-product of microbial carbohydrate fermentation in the rumen, but branched-chain VFAs are formed from deamination of amino acids. This suggests that skatole and indole production are not related to carbohydrate digestion in the rumen. In order for meat and milk producers in pastoral systems to ameliorate undesirable flavours and increase demand in certain markets, they will need to consider using alternative forages to reduce protein solubility and degradability.