Nutritional evaluation of south-west Queensland pastures. 3. The amino acid composition of pasture plants in a Mitchell grass association and their digestibility by grazing sheep

Abstract

The amino acid composition of the plants in a Mitchell grass pasture was determined, together with estimations of the rate of intake and extent of digestion of amino acids by fistulated and intact sheep grazing these pastures; two grazing systems were examined, one where the sheep were grazed at the district average stocking rate of one sheep to 1.5-2 ha, and the other at a 20% higher stocking rate. Plant amino acid compositions were reasonably constant, although the cyst(e)ine content of the green leaf fraction of Queensland blue grass (Dicanthium sericeum), Flinders grass (Iseilema spp.) and Enneapogon spp., and the green stem and dead leaf fractions of curley Mitchell grass (Astrebla lappacea), was low. The amino acid concentrations in the oesophageal fistulae boluses were higher than the concentrations found in the green fraction of any plant growing in the pasture. The quantity of amino acids flowing into the small intestines of the sheep in the higher stocking rate group exceeded by 16.4 g/kg DOMI the quantity consumed, while in those in the lower stocking rate group the flow was 24.8 g/kg DOMI less than the quantity ingested; reduced amino acid flow into the small intestines was associated with a higher forb content in the diet.