The effects of ryegrass varieties differing in soluble sugar content on the rumen fermentation of amino acids and consequences for milk flavour chemistry

Abstract

A comparative study of cows grazing grasses differing in soluble sugar content was undertaken to explore whether sugar content improves ruminal plant protein utilisation and influences milk flavour chemistry. Lower concentrations of ammonia, and amino-acid degradation products including the flavour compounds skatole and indole were observed for rumen contents sampled from non-lactating fistulated cows grazing high sugar ryegrass relative to samples from cows grazing Italian or a standard perennial ryegrass, suggesting improved plant protein nitrogen utilisation in response to the high sugar grass diet. Concentrations of plant protein fermentation products in the rumen of the fistulated cows were significantly higher at noon, compared to morning and afternoon values. In milk collected twice daily from the lactating cows grazing the above ryegrass varieties, no differences in yields of milkfat or skatole were detected between cows fed the different grass varieties. However yields of skatole in milkfat were two times higher in afternoon than morning milk. The rumen response to morning feeding appears to influence the flavour chemistry of afternoon milk. Keywords: rumen metabolites, milkfat skatole, indole, perennial ryegrass, Italian ryegrass, water soluble carbohydrate