Using a mathematical framework to examine physiological changes in winter wheat after livestock grazing 2. Model validation and effects of grazing management

Abstract

Previous work with crops used for livestock forage and grain production has shown that crop regrowth after grazing is influenced by grazing pressure, shoot dry matter (DM) removed and rainfall, though such findings are typically obtained using limited growing seasons. The aims of this paper were four-fold: (1) validate a wheat-grazing crop model (WHTGRAZ), (2) determine the effect of grazing regime on grain yield at the same grazing pressure, (3) determine the relationship between grazing intensity, yield and rainfall, and (4) determine how shoot DM at the start of grazing influences total DM consumption and grain yield. The ability of WHTGRAZ to predict post-grazing shoot DM accumulation was as good as or better than its ability to predict that of ungrazed crops. Aims 2-4 were investigated by simulating grazing while crops were vegetative. When the number of grazing days per hectare was predefined, yield of crops subjected to light grazing intensities for long periods were similar to yields of crops subjected to high grazing intensities for short periods. When the number of grazing days per hectare was unlimited and grazing was terminated only upon reaching a minimum shoot DM or maximum development stage, more grazing was obtained with lower grazing intensities. Irrespective of grazing intensity, crop yields were generally equal to or greater than those of ungrazed crops in years when growing season rainfall was low, but the converse occurred when growing season rainfall high. In very wet years, delaying the start of light intensity grazing decreased total shoot DM consumption and increased yield. Alternatively, delaying the start of high intensity grazing increased shoot DM consumed and decreased yield. Two major conclusions were deducted from this study. First, grazing of rainfed wheat at relatively light intensities for long durations allows more regrowth to occur during the grazing interval. This both increases removal of shoot DM and is less likely to penalise yield. Second, the probability of grazing increasing crop productivity is greatest in years with low rainfall, regardless of grazing intensity.