Seasonal variations in the chemical composition and nutritive value of five pasture species in south-western Victoria

Abstract

The dry matter digestibility (DMD), grinding energy, and the concentration of neutral detergent fibre, crude protein, soluble carbohydrate, calcium, magnesium, sodium, potassium, and phosphorus in 5 pasture species were monitored over 2 years from pasture which was grazed at 14.8 sheep/ha in south-western Victoria. The species were: perennial ryegrass (Lolium perenne L. cv. Victorian), phalaris (Phalaris aquatica L. cv. Australian), Yorkshire fog grass (Holcus lanatus), subterranean clover (Trifolium subterraneum cv. Mount Barker), and capeweed (Arctotheca calendula). Perennial ryegrass and subterranean clover were the dominant species and contributed 50 and 20% DM, respectively, to the sward in winter and the opposite in spring. The DMD of the 5 pasture species did not vary greatly throughout the vegetative phase of growth (autumn-mid spring). In winter, fog grass was slightly less digestible (70-74% DMD), while the other grasses were similar (76-79% DMD). Subterranean clover was not digested (72-75% DMD) as thoroughly as the other species until after late spring; then it retained its digestibility while the digestibility of the others fell. None of the species provided digestible herbage above 65% DMD in the summer. The energy required for grinding herbage was lowest in the autumn-winter, and increased with a corresponding increase in neutral detergent fibre content as pastures matured in late spring. The crude protein content of all species declined from 27-30% in autumn to 18-20% in spring, while the soluble carbohydrate contents increased from about 54% in autumn to 10-13% by spring. The implications on animal production of these seasonal changes in nutritive value are discussed. Higher soluble carbohydrate contents in spring herbage than in autumn herbage possibly explain the better performance of animals when grazing spring pasture. The levels of magnesium, sodium and potassium were adequate for the dietary requirements of ruminants; however, on grass dominant pasture, shortages of calcium (0.14-0.25% DM) and phosphorus (0.11-0.24% DM) were likely in the summer when energy and protein were also deficient. The problem of a calcium deficiency is greatly reduced by the presence of legumes. Capeweed is an accumulator of minerals, but it is only occasionally eaten and is rarely present in summer; hence its presence is of marginal benefit to stock.