Effect of white clover (Trifolium repens), perennial ryegrass (Lolium perenne) and Lotus corniculatus on in vitro skatole and indole formation

Abstract

Consumers accustomed to eating meat and dairy products from animals raised on grain often describe products from New Zealand pasture-raised animals as having an undesirable flavour. This could adversely affect the acceptability and returns for meat and dairy products in certain oversea markets. This flavour, often described as pastoral, grassy or faecal, has been attributed in part to the high concentration of indole and skatole present in the meat of animals raised on fresh perennial ryegrass/white clover pasture. Skatole and indole are formed in the rumen from the microbial degradation of tryptophan. A pasture diet is high in protein that is both soluble and rapidly degraded, resulting in an abundance of free peptides and amino acids in the rumen. Condensed tannins (CT) in Lotus corniculatus have been shown to reduce the solubility and degradability of plant protein in the rumen. This study investigated the in vitro rumen fermentation characteristics of white clover (Trifolium repens; WC), perennial ryegrass (Lolium perenne; PRG) and Lotus corniculatus (LC) to determine their effects on skatole and indole formation. All incubations were carried out in the presence and absence of polyethylene glycol (PEG), which inactivates CT. In the absence of PEG, maximal skatole and ammonia concentrations were greater when incubating WC compared with PRG and LC (P<0.05). A similar trend was evident for indole concentrations, but statistical significance was not reached. Adding PEG to the incubations with LC increased the concentration of indole, skatole and ammonia (P<0.05), but did not affect those incubations with WC or PRG. Differences in peak concentrations by the in vitro method were comparable with those found in an in vivo study using the same forages. Thus, the in vitro method can be used to screen a range of forages to determine their effect on skatole and indole formation in ruminants, and to identify candidate forages for managing the flavour of products from forage-fed ruminants.