Gj Dean

Constraints to achieving high potential yield of wheat in a temperate, high-rainfall environment in south-eastern Australia

Average wheat yields in the high-rainfall zone (HRZ) of southern Australia are predicted to be around 10 t ha–1, yet most regions fall short through a lack of locally adapted cultivars or abiotic stress that constrains yield. Wheat yields in Tasmania can be variable but have exceeded this potential yield in some field trials and have thus approached that of other traditionally high-yielding HRZ environments such as northern Europe. A contributing factor to high wheat yields in Tasmania is the cool-temperate climate, which tends not to have extremes in temperature (cold, heat) as may be experienced in HRZ environments elsewhere. Hence an understanding of crop growth, development and yield of wheat of locally adapted wheat cultivars in Tasmania may improve our understanding of the basis of yield formation in other HRZ in Australia. This was evaluated by conducting an analysis for adaptive response of grain yield in 10 wheat genotypes to a range of 14 environments that were favourable for wheat production or experienced constraints to growth. Crop growth and yield formation was then examined in detail for all or a subset of these genotypes in three field trials with contrasting environments, two of which included a time of sowing (TOS) treatment. Environment accounted for around 90% of the sum of squares (SS) in the multi-site analysis of yield. Six environment groups were identified using cluster analysis, two of which were clearly separated in response to frost at flowering or putative biotic stress, which constrained yield to 1.8 and 6.8 t ha–1, respectively. Waterlogging was also a significant abiotic stress in one of the TOS field trials. The late-flowering cultivar Tennant had the highest yield in the presence of waterlogging and by avoiding frost at flowering, although it suffered a yield penalty of 35 and 66%, respectively, compared with the average across environments. The highest-yielding genotypes averaged 8 t ha–1 across environments and included Alberic, the breeding line K37.18 and the new release Revenue. In the detailed experiments on crop growth and development, high grain yields of 10 t ha–1 in Mackellar appeared to be due to increased grains ear–1, resistance to barley yellow dwarf virus and possibly higher radiation-use efficiency, although the latter needs to be confirmed. There was little genotype × environment interaction for grain yield, hence wheat breeders can have a relatively high level of confidence that genetic material with high yield potential should rank consistently across Tasmanian environments. Results presented in the paper will be useful in developing management and breeding strategies to increase potential yield across the HRZ of southern Australia.

Influence of end-grazing forage residual and grazing management on lamb growth performance and crop yield from irrigated dual-purpose winter wheat

The effects of end-grazing forage residual and continuous v. rotational grazing systems on prime lamb performance, grain yield and quality were examined in an irrigated dual-purpose winter wheat (cv. Mackellar) crop in Tasmania. The design was a two end-grazing residual (400 and 800 kg/ha of dry matter (DM) at Zadoks Growth Stage 30, Low and High respectively, 0.2 ha plots) · two grazing system (continuously, or rotationally grazed in four subplots) factorial, replicated three times. Mixed-sex, second-cross lambs (37 kg liveweight (LW), 2.5 body condition score, 45 kg DM/head initial feed allowance) grazed for a total of 46 days before removal. Initial feed availability was 1875 kg DM/ha, with final residuals of 520 � 57 and 940 � 70 kg DM/ha for the Low and High treatments respectively. Particularly for the Lowresidual,invitroDMdigestibilityandcrudeproteinatstemelongationwerereduced(P <0.05)byrotationalcompared with continuous grazing. The weekly lamb growth rate (g/day) during the first 5 weeks of grazing was linearly related to average weekly available DM in kg/ha (GR = 0.35 � 0.041 · DM - 194 � 49.0, P < 0.01, R 2 = 0.56). Total LW produced (336 � 11.7 kg/ha), and grain yield (6.9 � 0.21 t/ha), protein (11.4%), screenings <2.2 mm (10.9%) and 100 grain weights (3.82 g DM) were not different between treatments. There were no advantages of rotational grazing compared with continuousgrazing.Irrigateddual-purposewinterwheatcanbecontinuouslygrazedbylambsuptoa500kgDM/haresidual at stem elongation without compromising total LW produced, grain yields or grain quality.

Yield and water-use efficiency of wheat in a high-rainfall environment

Abstract. Yield, water use and water-use efficiency (WUE) in the high-rainfall zone of Tasmania are highly variable because of environmental and agronomic constraints to grain production that limit yield potential. The expansion of irrigation infrastructure in Tasmanian production systems with access to low-cost, plentiful irrigation sources will also influence these components in some areas. This paper reports on desktop modelling studies that aimed to benchmark wheat WUE and to explore the sensitivity of yield, water use and WUE to changes in management practice in a high-rainfall environment. Here, WUE was defined as: grain yield/(evapotranspiration + drainage + runoff). The crop simulation model APSIM-Wheat was used to quantify key water balance elements and estimate ‘attainable’ and ‘potential’ WUE and grain yield for 27 wheat trials. The upper limit for WUE was ∼30 kg/ha.mm in excess of 180 mm evaporation, which is 16% higher than previous estimates at this southerly latitude for wheat. Attainable WUE ranged from 58% to 100% of potential WUE and was limited by nitrogen supply and water loss through evaporation, drainage and runoff. Model scenarios showed that co-limitation of inputs of nitrogen and irrigation was an important driver of grain yield and WUE. The implications of this research on crop management and production in temperate, high-rainfall environments are discussed.