Assessing a simple technique to predict forage selection by cattle grazing Northern Queensland rangelands

This study is part of a major research project being conducted since 1994 in northern Queensland to develop a better understanding of the impact of selective grazing on rangelands in order to prevent a decline in their condition and productivity. Increased understanding of species selection, in particular, is needed to make sound grazing management decisions and prevent rangeland degradation. Diet selection in cattle was studied using a procedure based on Ivlev's (1961) selectivity index (SI) and modified by Andrew (1986). We evaluated the SI technique as a potential tool to monitor species selection. We measured pasture species frequency and defoliation rates between 1995 and 2000 on five commercial properties representing three major soil types. Frequency and defoliation were also calculated for four functional groups of pasture species (i.e., introduced, decreasers, increasers, and intermediates). The SI technique showed promising results for species selection by cattle grazing on the rangelands of northern Queensland. In Lakeview/Allan hills, for instance, cattle avoided increaser species (average SI was -0.378) and preferentially selected decreaser (average SI was 0.726) and introduced species (average SI was 0.657). Intermediate species were generally grazed in similar proportions to their occurrences. When only one or two pasture species were dominant (i.e., above 90% frequency), however, this technique provided erroneous results. Alternative techniques need to be used in conjunction with the SI technique when one or two species are dominant. We propose this technique as a best bet for producers' adoption as it is simple and requires little expertise. But the technique may not produce the expected outcomes when one or two pasture species dominate a site.