Modelling the physiological dynamics of winter wheat after grazing

Abstract

The use of cereal crops for the dual-purposes of livestock forage and grain production is increasing in farms of southern Australia. Field and modelling studies were conducted to investigate the agronomic and physiological responses of winter wheat crops to sheep grazing. Field experiments were carried out in 2007 and 2008, and growing season rainfall (GSR) in both years was below average. Light and heavy grazing intensities conducted for one month did not affect total shoot dry matter (SDM) accumulation or maximum post-grazing growth rates (GR) but light grazing for two months significantly reduced both variables. Mean grain yields (GYs) were approximately 380 g m-2 in both seasons and were not significantly affected by grazing treatment or cultivar. SDM production per unit evapotranspiration was not affected by grazing. Canopy light interception and post-grazing GRs were reduced by grazing. In 2008, mean post-grazing radiation-use efficiency (RUE, SDM produced per unit radiation intercepted) was 44% greater than that of ungrazed crops, but grazing did not affect RUE in 2007. A grazing-induced transient increase in photosynthesis for 3-4 weeks after the end of grazing was associated with a reduction of crop water stress. During this period, leaves on grazed plants had greater specific leaf area, stomatal conductance, intercellular C02 concentrations and Rubisco activity compared with leaves on ungrazed plants. A wheat-grazing model (WHTGRAZ) was derived and calibrated using the field measurements, then validated using data from four independent seasons. Validation showed that accurate modelling of ungrazed crop dynamics is a crucial prerequisite for accurately simulating the behaviour of grazed crops. A fundamental element of models designed to simulate crop grazing is the grazing-induced delay in phenological development. Grazing increased SDM allocated to leaves at the expense of stems, which was implemented in WHTGRAZ by delaying development. This delayed the onset and reduced the duration of stem DM accumulation, and decreased the stem DM retranslocated to kernels after anthesis. Developing kernels of grazed crops were thus more reliant on current photosynthesis whereas retranslocation from stems was more important in ungrazed crops. Simulation analyses using 121 years of weather data revealed that GY penalties were small if less than 90 g SDM m{u207B}{u00B2} was consumed during grazing. Above this level of SDM consumption, GY losses were greater as grazing-pressure increased. The effects of grazing intensity versus duration on GY were similar on average, provided grazing was terminated before stem elongation. Lower SDM removal rates during light grazing allowed longer grazing durations than heavy grazing. Consequently, more crop growth occurred so that total SDM removal was greatest for light grazing. Productivity of grazed crops was typically greater than controls when GSR was less than 300 mm, but progressively less than controls as GSR increased above 300 mm. Varying the SDM threshold for the start of grazing altered the trade-off between SDM consumption and GY, particularly in wetter seasons. These results advance current knowledge of (1) crop physiology after grazing and (2) crop-grazing models, and will be of use to farmers, scientists and policy-makers.