Effect of mechanical defoliation on botanical composition and yield of a Townsville stylo-sabi grass sward

Abstract

A sabi grass (Urochloa mosambicensis)-Townsville stylo (Stylosanthes humilis) sward on cleared land at Katherine, Northern Territory, was mechanically defoliated for three seasons. The wet season defoliation treatments were no defoliation, late light, early heavy, and regular heavy. The sward's defoliation responses were measured in terms of dry matter production, quality, and botanical composition. In the non-defoliated treatment, sabi grass was highly competitive and this grass dominance was not reduced by any of the mechanical defoliation treatments studied. The legume dry matter proportion declined consistently from an initial all-treatment mean of 66 per cent to 9 per cent in the third season. Annual grasses failed to colonize the sward. Sabi grass dry matter yield was unaffected by defoliation in the first season, although plant vigour was reduced resulting in yield reduction in subsequent years. Townsville stylo and total dry matter were reduced by heavy wet season defoliation. Legume weighted mean phosphorus concentration was increased by heavy defoliation, but sabi grass mean phosphorus concentration, although higher than for Townsville stylo, was variable and seasonally dependent. Weighted mean nitrogen concentration was generally higher with heavy defoliation for both Townsville stylo and sabi grass, although the legume nitrogen concentration was approximately twice that of sabi grass.