Dual-purpose cereals: can the relative influences of management and environment on crop recovery and grain yield be dissected?

Growing cereal crops for the dual-purposes (DP) of livestock forage during the early vegetative stages and harvesting grain at maturity has been practised for decades. It follows that scientific experiments using DP crops are nearly as old. A survey of more than 270 DP crop experiments revealed that the average effect of crop defoliation on grain yield (GY) was −7 ± 25% (range –35 to 75%). In light of these results, the first purpose of this review was to assess how alternative crop and grazing management regimes affected forage production and GY. Management techniques in order of decreasing importance likely to maximise grain production include (i) terminating grazing at or before GS 30, (ii) matching crop phenology to environment type, (iii) sowing DP crops 2–4 weeks earlier than corresponding sowing dates of grain-only crops, and (iv) ensuring good crop establishment before commencement of grazing. The second aim was to identify the environmental and biotic mechanisms underpinning crop responses to grazing, and to identify crop traits that would be most conducive to minimising yield penalty. A variety of mechanisms increased GY after grazing. Under favourable conditions, increased GY of grazed crops occurred via reduced lodging, mitigation of foliar disease and rapid leaf area recovery after grazing. Under stressful conditions, increased yields of grazed crops were caused by reduced transpiration and conservation of soil water, delayed phenology (frost avoidance at anthesis), and high ability to retranslocate stem reserves to grain. Yield reductions caused by grazing were associated with (i) frost damage soon after grazing, (ii) poor leaf area development or (iii) delayed maturation, which led to water or temperature stress around anthesis, culminating in increased rates of green area senescence and decreased duration of grain-filling. The third aim was to examine the role of simulation models in dissecting the effects of environment from management on crop physiology. Simulation studies of DP crops have extended the results from experimental studies, confirming that forage production increases with earlier sowing, but have also revealed that chances of liveweight gain increase with earlier sowing. Recent modelling demonstrates that potential for inclusion of DP crops into traditional grain-only systems is high, except where growing-season rainfall is <300 mm. Prospective research involving crop defoliation should focus on crop recovery, specifically (i) the effects of defoliation on phenology, (ii) the time-course of leaf area recovery and dry matter partitioning, and/or (iii) development of crop-grazing models, for these three areas will be most conducive to increasing the understanding of crop responses to grazing, thereby leading to better management guidelines.