Nutritional evaluation of south-west Queensland pastures. 2. The intake and digestion of organic matter and nitrogen by sheep grazing on Mitchell grass and mulga grassland associations

Abstract

The intake and digestion of organic matter and nitrogen by sheep grazing two Mitchell and one mulga/ grassland association was studied under a variety of seasonal conditions. The in vitro digestibility of the diets selected by sheep grazing both pasture types was in the range 60-70% when seasonal conditions were good, but fell to 52.3% and 48.2% for the Mitchell and mulga/ grasslands respectively when the pasture were dry. Digestible organic matter intakes of intact sheep grazing the Mitchell grass pastures were 14.0-16.3 g (kg W)-1 day-1 when seasonal conditions were average to good and 13.5 g (kg W)-1 day-1 after the pastures had dried out. Comparable figures on the mulga/grassland pastures were 17.8-18- 1 g (kg W)- day- with good seasonal conditions and 11.8 g (kg W)-1 day-1 with dry pasture. The nitrogen concentrations in the diets varied from 7 to 26 g/kg OM on the Mitchell grass associations and from 17 to 34 g/kg OM on the mulga/grassland. Apparent digestion of dietary N within the rumen ranged from 70% to 90% on both sites when there was green forage in the diets, but fell to 58.5% and 37.6% on the Mitchell and mulga/grassland respectively when the pastures were drought affected. Under the latter conditions the quantity of amino acid N absorbed from the small intestines was depressed at both sites, as was the efficiency of microbial N production in sheep grazing the Mitchell grass pastures. These findings are discussed in relation to the protein and energy requirements of sheep grazing the two pasture associations.