Effects of grazing on crop crown temperature: implications for phenology

In many regions, livestock are allowed to graze grain crops during their vegetative development, before grain is harvested at crop maturity. Little is known of the effects of grazing on crop microclimate, particularly the effects of defoliation on crown temperatures. Knowledge of such effects is important because temperature is the main factor underpinning crop ontogeny, and ontogeny drives dry matter allocation, leaf appearance rates and the timing of anthesis, which are key determinants of grain yield. The primary aim of this study was to examine the influence of grazing intensity and duration on the crown temperatures of winter wheat crops grown at Canberra, Australia. A secondary aim was to examine the association between crown temperature and phenology. In 2007, wheat cv. Mackellar was grazed at intensity–duration combinations of low–short (LS, 33 sheep/ha for 31 days), heavy–short (HS, 67 sheep/ha for 31 days) or low–long (LL, 33 sheep/ha for 62 days). In 2008, cvv. Mackellar and Naparoo were grazed at the HS intensity-duration. Cubic smoothing splines were fitted to crown temperature data measured between the end of grazing and anthesis to facilitate identification of long-term trends and statistical differences caused by the effects of defoliation on crown temperature. Grazing treatments with greater intensity or longer duration significantly elevated maximum daily crown temperature; differences of 6–7°C were common in the month following grazing. Cubic-spline analysis showed that long-term trends in maximum crown temperature of the HS and LL treatments were significantly greater than corresponding temperatures of controls for the entire post-grazing duration. By contrast, effects of grazing on minimum diurnal crown temperature were small. Increasing biomass removal significantly delayed stem elongation and anthesis. We demonstrate that although initial phenological delays caused by defoliation are large, greater diurnal crown temperature fluctuation in grazed crops leads to greater growing degree-day accumulation between the end of grazing and anthesis. This increases the rate of thermal time accumulation during the post-grazing–anthesis period and is likely prominent in driving greater development rates of grazed crops. We further demonstrate that delays in phenology associated with grazing can be largely accounted for by a thermal time constant, with the LS, HS and LL treatments delaying stem elongation by ~52, 141 and 214 degree-days, respectively, above a base temperature of 0°C. Results from these experiments and interpretations herein will be of use in designing crop-grazing regimes, and in studies examining implications of defoliation on vegetative microclimate and on physiological feedback effects caused by elevated temperature.