Changes in characteristics of pastures in the coastal subtropics when grazed by cattle during years of low rainfall

Abstract

Summary. The effect of continuous grazing on pasture composition and quality was assessed on 2 pasture types in a subtropical environment. The pastures were native stocked at 1 cow/2.5 ha, and previously improved stocked at 1 cow/1.6 ha. In August 1990, carpet grass (Axonopus affinus) was the major species in both the native (49.8%) and improved (61.3%) pastures. However, the proportion of carpet grass decreased during the study to 33% in the native pasture and 30% in the improved pasture in February 1994. In the native pasture, blady grass (Imperata cylindrica) increased at the expense of carpet grass, whereas in the improved pasture higher quality grasses, and to a lesser extent Giant Parramatta grass (Sporobolus indicus), increased at the expense of carpet grass. Overall, dry matter on offer and green dry matter depended on rainfall and these peaked in late summer. Dry matter on offer differed between non-drought and drought periods in both pastures and was ≤3500 kg/ha in drought months in the native pasture and ≥3500 kg/ha in comparable months in the improved pasture. Green dry matter was higher in pastures towards the end of the summer growing season (83%, March 1992) and lower during many of the drought months (12%, May 1991). Grazing pressure (liveweight 0.75/t green dry matter) was highest in August, the crucial period for weed incursion. Organic matter digestibility was highest following rain in late spring (November 1990; 69%, improved pasture) but lowest in the absence of rain (November 1993; 38%, native pasture). The proportion of green dry matter in the native pasture was not related to organic matter digestibility nor to the nitrogen content of plucked pasture samples. However, in the improved pasture the proportion of green dry matter was related to organic matter digestibility on 3 occasions but not to nitrogen content.