Effect of timing of defoliation on wheat (Triticum aestivum) in central Queensland -: 1.: Crop response and yield

Through reduction of water demand by defoliation during the vegetative growth period of wheat, it may be possible to optimize use of soil water and in-crop rainfall for enhanced grain yield and water use efficiency. Trials conducted in the field in the sub-tropical and semi-arid region of central Queensland (CQ), Australia, over 3 years (1997-1999) were set up under various water and fertility regimes to test this hypothesis. With an early maturity wheat (cv. Sunstate), under rainfed conditions with no fertilizer application, defoliation treatments at early (DE), or middle (DMT) tillering had not effect on grain yield while those imposed at late (DLT) tillering and booting (DB) reduced grain yield. In 1998, with 76 kg basal N ha(-1) and 100 mm irrigation during cropping, an early-sown late maturity wheat (cv. Batativa at the end of April) responded to DLT with an increased grain yield (by 7.3%) through an increase of harvest index (HI by 10.7%) and grain water use efficiency (WUE by 22%) compared to the control, but DB had negative effects on almost all components. Both defoliation treatments of a crop sown 2 months later resulted in yield reduction. In 1999, a similar late maturity wheat cultivar (Batavia 2) was sown on 27 April with 55 kg basal N ha-1 under entirely rainfed conditions. DMT had similar positive effects on grain yield (7.4%), HI (5.9%) and WUE (12.7) to DLT in 1998, but reduced protein content compared to the control. Reduction of plant density by 30% at middle tillering was without effect on grain yield; while DLT raised grain WUE it had no effect on grain yield due to a reduction of above-ground dry matter (AGDM). It also reduced grain protein content. Defoliation treatments of a crop sown one month later had no effect on grain yield. Analysis of physiological data collected in 1999 suggests that defoliation suppressed vegetative growth, optimized the ratio of AGDM at anthesis to that at final harvest (0.5-0.8, peak 0.66) and led to more AGDM post-anthesis for maximal grain production. Defoliation that led to a yield increase also increased the leaf area ratio and enhanced stomatal conductance and photosynthetic capacity at anthesis. It is concluded that defoliation of early (April)-sown wheat defoliated at the middle to late tillering stage could let to greater yield and WUE, and would not result in yield reduction. The value of foliage removed and overall economic and risk analyses are discussed in a further paper. (C) 2004 Elsevier B.V. All rights reserved.