Growth, recovery, and yield of dual-purpose canola (Brassica napus) in the medium-rainfall zone of south-eastern Australia

The effect of grazing of vegetative canola (Brassica napus) with sheep on crop growth and yield was investigatedin two field experiments (Expts 1 and 2) in 2008 at Wagga Wagga, New South Wales, Australia. The experiments included arange of cultivars, sowing rates, and grazing periods to investigate the influence of these factors on grazing biomass, croprecovery, and grain yield. Three spring canola cultivars (representing triazine-tolerant, conventional, and hybrid types) wereused in both experiments and were sown at three sowing rates and grazed by sheep for 7 days in midwinter in Expt 1, while twodifferent grazing periods were compared in Expt 2. Supplementary irrigation was applied to Expt 1 to approximate averagegrowing season conditions, while Expt 2 received no irrigation. Increased sowing rate produced greater early shoot biomassfor grazing, but the-triazine tolerant cultivar produced less biomass than the conventional or hybrid cultivars in bothexperiments. Grazing reduced dry matter and leaf area by >50%, delayed flowering by 4 days on average, and reducedbiomass at flowering by 22'52%. However, there was no impact of cultivar or sowing rate on the recovery of biomass and leafarea after grazing. Grazing had no effect on final grain yield under supplementary irrigation in Expt 1, but reduced grain yieldunder the drier regrowth conditions in Expt 2.The results demonstrate that grazing canola is feasible under average seasonalconditions in a medium-rainfall environment (400'600 mm) without yield penalty, provided the timing and intensity ofgrazing are matched to available biomass and anticipated seasonal water supply to support grain production. More broadly,we suggest that grain yield reductions from grazing could be avoided if suitable conditions for regrowth (residual dry matter,length of regrowth period, and adequate moisture) can generate biomass levels in excess of a target value of ~5000 kg ha'1 atflowering. This target value represents a biomass level where >90% of photosynthetically active radiation was intercepted inour study, and in other studies represents a biomass level above which there is little further increase in potential yield. Such atarget provides a basis for more objective grazing management but awaits further confirmation with experimentation and modelling.