Growth of an annual pasture on virgin land in South-Western Australia including effects of stocking rate and nitrogen fertilizer

Abstract

A pasture of Bromus mollis L. and Trifolium subterraneum L., sown on virgin land, was grazed by sheep for 4 years. Three rates of ammonium sulphate were factorially combined with two stocking rates.

Stocking rates of 8 and 12 sheep/ha had similar effects on growth rate of the sward and influenced botanical composition only in the second and third year of grazing.

Ammonium sulphate increased the proportion of B. mollis and volunteer grasses, and reduced T. subterraneum and dicotyledonous volunteers. B. mollis persisted only where ammonium sulphate was applied. T. subterraneum, in the absence of nitrogen fertilizer, was highly dominant for 3 years, then declined to 40% by the fifth year. Where nitrogen fertilizer was applied in large quantities the clover did not persist even at the higher stocking rate.

The annual dry matter production of the swards which did not receive the nitrogen fertilizer was poor (c. 6000 kg/ha); the possible causes of this are discussed. Ammonium sulphate increased winter growth in the early years of the experiment. In the later years, large quantities of straw accumulated on the ammonium sulphate plots, regeneration was poor, and the nitrogen fertilizer ineffective.

Growth curves of the sown species are presented and discussed.