Ruminal volatile fatty acids and ammonia in cattle grazing dry tropical pastures

Abstract

SummaryTotal VFA concentrations, individual VFA proportions and ammonia concentrations in the rumen of cattle grazing tropical pastures for 7 months during the dry season were compared with concentrations of these metabolites in cattle grazing in the wet season and in cattle given two pasture hays in pens.Neither total VFA nor ammonia concentrations were altered by periods of new pasture growth within the dry season although responses in live weight occurred. Further evidence that concentrations of total VFA were insensitive to differences in feed quality was shown in the pen experiment.VFA concentrations were 53% higher in the rainy season than in the dry season and ammonia concentrations doubled from 62 mg/1 in the dry to 128 mg/1 in the wet season. Ruminal ammonia was related to ingested bolus N concentration (r= 0·58,P< 0.001) during the dry season.Individual VFA proportions did not vary greatly. The highest acetic (79·1%) and lowest butyric acid (5.10%) proportions occurred in the same sample. Iso-valeric and valeric acid proportions were related positively to ruminal ammonia (P< 0·001).In grazing cattle, concentrations of VFA and ammonia in their rumen taken at hourly intervals between 07.00 and 18.00 h did not vary greatly with time. Samples taken before 10.00 h had higher VFA and ammonia levels.