M.H. ANDREW

CSIRO Division of Tropical Crops and Pastures, PMB 44, Winnellie, N.T. 5789, Australia.

Abstract

The defoliation of pasture species by cattle was monitored for two wet-dry seasonal cycles in recently burnt native monsoon perennial tallgrass pastures. Three uncleared 50 ha paddocks were used. Alternate halves of each paddock were burnt each dry season. Relative defoliation ratings were combined with relative abundance (frequency of occurrence) values to derive, for each pasture species, a selectivity index, whose values could range from – 1 to + 1, with zero indicating non-selective grazing.
The mean index values for the four dominant perennial grass species Themeda triandra (formerly T. australis), Sehima nervosum, Sorghum plumosum and Chrysopogon fallax) were significantly different (respectively 0.49, 0.14, 0.04 and –0.44) and indicated selectivity for Themeda and avoidance of Chrysopogon. The cattle were least selective at the start of the rainy season when the post-burn growth was short, and were most selective at the end of the rainy season when herbage was stemmy and mature.
Four other pasture components (Eriachne obtusa, annual grasses, legum minous forbs and other forbs) were generally rejected by the cattle.
The approach described forms the basis of a rapid field survey technique for assessing diet selection.