Voluntary intake and rumen digestion of a low-quality roughage by goats and sheep

Abstract

SUMMARYPrairie grass (Bromus catharticus) straw (13·7 g N/kg DM) was fed ad libitum to six goats and seven sheep kept in metabolism cages at Palmerston North, New Zealand in 1986 and 1987.Goats had greater voluntary feed intake (VFI) (56 v. 36 g DM/kg W0·75 per day), greater apparent DM digestibility (36·8 v. 32·6%) and a larger rumen pool of DM and liquid (W0·75) than sheep. Goats also had greater apparent digestibility of fibre, especially of lignin, and greater rumen fractional degradation rates (FDR) of cellulose, hemicellulose and lignin. Goats had a higher rumen ammonia concentration (115 v. 80 mg N/l), lower rumen pH (6·73 v. 6·90), a smaller proportion of large particles and greater proportion of small particles in rumen contents than sheep. Passage through a 1 mm sieve was established as the threshold particle size to have a high probability of leaving the rumen in both goats and sheep. Voluntary water intake/unit DM consumed was lower in goats than in sheep and, whilst rumen fractional outflow rate (FOR) of water and particulate matter also tended to be lower in goats, the difference was not significant. Irreversible loss rate of rumen NH3 and the amount of N calculated as recycled to the rumen, both expressed as mg N/kg W0·75 per day, were greater for goats than for sheep.It was concluded that the ability of goats to maintain a higher rumen NH3, concentration than sheep, their larger rumen pool and the higher proportion of small particles in rumen contents were all contributing factors to their greater VFI and fibre digestion of this low-quality roughage.