PHENOLIC-ACIDS IN THE FIBER 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. T he main phenolic acids liberated on mild alkaline hydrolysis in 19 spe cies of native and introduced grasses in North Queensland were p-couma ric and ferulic acids; with the former predominating in 16 species. Ca ffeic acid was found in three species and sinapic acid in trace amount s. 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 fe d as the sole diet to sheep, intake and digestibility were not clearly related to phenolic acid level in the diet. However, output of hippur ic acid in the urine increased with the daily dietary intake of phenol ic 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 direc tly to the diet. In contrast to the implications of numerous in vitro studies, intake and digestibility were not affected. Dry matter loss f rom intraruminal nylon bags was also not affected by administration of these compounds. In addition to hippuric acid, cinnamoylglycine was i dentified 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 o n rumen microbial metabolism, but impose a serious nitrogen loss for a nimals on diets already deficient in nitrogen.