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

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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.