Phenolic acids in the fibre of some tropical grasses, effect on feed quality, and their metabolism by sheep

A feature of the composition of dry-season tropical grasses, which may in part account for their lower digestibility, is the relatively high content of hydroxycinnamic acids covalently bound in the cell wall. The main phenolic acids liberated on mild alkaline hydrolysis in 19 species of native and introduced grasses in North Queensland were p-coumaric and ferulic acids, with the former predominating in 16 species. Caffeic acid was found in three species and sinapic acid in trace amounts. Total acids occurred at approximately 10 g/kg in most species, but were found at 50 g/kg in one species. When five of the species were fed as the sole diet to sheep, intake and digestibility were not clearly related to phenolic acid level in the diet. However, output of hippuric acid in the urine increased with the daily dietary intake of phenolic acids, and nitrogen excreted as hippurate was equivalent to as much as 17% of the total nitrogen in the diet. The effect of free acids on rumen function was tested by adding p-coumaric and ferulic acid directly to the diet. In contrast to the implications of numerous in vitro studies, intake and digestibility were not affected. Dry matter loss from intraruminal nylon bags was also not affected by administration of these compounds. In addition to hippuric acid, cinnamoylglycine was identified as a urinary metabolite. This appeared in animals on most of the grass diets, and was enhanced following administration of ferulic or p-coumaric acid, but not benzoic acid. It seems that phenolic acids liberated from the cell wall are unlikely to have an adverse effect on rumen microbial metabolism, but impose a serious nitrogen loss for animals on diets already deficient in nitrogen.