Rumen volatile fatty acids on six pasture species and relations with digestibility, voluntary intake and chemical composition

Abstract

The total concentration and the relative proportions of the volatile fatty acids (VFA) were measured in rumens of sheep fed 78 single species pasture samples. The pastures consisted of regrowths of white clover (Trifolium repens cv. Grasslands Huia), Manawa regrass (Lolium perenne x multiflorum cv. Grasslands Manawa), Ariki ryegrass (Lolium (multiflorum x perenne) x L. perenne cv. Grasslands Ariki), perennial ryegrass (Lolium perenne cv. Tasmania No. 1) , Currie cocksfoot (Dactylis glomerata cv. Currie) and Apanui cocksfoot (Dactylis glomerata cv. Grasslands Apanui), cut eight times between May, 1969 and August, 1970. Sheep fed cocksfoot had lower VFA concentrations, higher proportions of acetate (HAc) and lower proportions of butyrate (HBu) than sheep fed white clover and ryegrass (P < 0.05). They also had lower proportions of propionate (HPr) than sheep fed ryegrass (P < 0.05). Sheep fed white clover had higher proportions of valerate (HVa) than those fed the grasses (P < 0.05). There were no significant (P < 0.05) seasonal differences in total VFA concentration but HAc was lowest with spring pasture (P < 0.05) and HBu was lowest with autumn pasture (P < 0.05). Total VFA concentration was positively related to digestibility (DMD) (r = +0.52, P < 0.01) but species differences occurred and at similar DMD, white clover produced higher concentrations than ryegrass (P < 0.05) which produced higher concentrations than cocksfoot (P < 0.01). The overall relation between total VFA and intake (DMI) was weak but seasonal differences occurred and at similar levels of DMI, winter pasture produced higher VFA concentrations than spring-summer pasture (P < 0.01). The proportion of HPr was positively related to VFA concentration (r = +0.53, P < 0.01) and to water soluble carbohydrate (WSC) content (r = +0.51, P < 0.01) and relations with DMD and DM1 were weak and non-significant (P > 0.05).