However, specific simulation studies to predict biomass, yield, drainage and nitrate leaching are now possible for wheat crops on the tested soil types and rainfall zones in Western Australia. In particular, grain protein tended to be overpredicted at high protein levels and underpredicted at low levels. Simulation of grain protein, and depth to the perched water table showed limited accuracy when compared with field measurements. Yields tended to be underestimated during terminal droughts due to insufficient pre-anthesis stored carbohydrates being remobilised to the grain. Grain yields were well predicted with a coefficient of determination r 2(1:1)=0.77, despite some underestimation during severe terminal droughts.
Table 5 R2 and MAE (t/ha) values from a fit of observed (ABS) vs predicted values for each method at 6 selected shires. The overall APSIM model predictions of shoot growth, root depth, water and N uptake, soil water, soil N, drainage and nitrate leaching were found to be acceptable. The performance of the APSIM-Wheat model was comparable to or better than the Stress and Drought Indices in 3 of the 4 Queensland shires while TACT performed poorly in both Western Australia shires. The field experiments covered 10 seasons, with variations in sowing date, plant density, N fertiliser, deep ripping and irrigation. Model outputs were compared with detailed field experiments from four rainfall zones, three soil types, and five wheat genotypes. The model was used to simulate above- and belowground growth, grain yield, water and N uptake, and soil water and soil N in wheat crops in Western Australia. Crop and Pasture Science CSIRO Publishing APSIM-wheat is a crop system simulation model, consisting of modules that incorporate aspects of soil water, nitrogen (N), residues, and crop development. Overall, earlier sowing in a Mediterranean-type environment appears to be an important management strategy for maintaining wheat production in semiarid cropping regions into the future, although this has to be balanced with other agronomic considerations such as frost risk and weed control. For a future climate, sowing earlier limited the impact of hotter, drier conditions by reducing pre-anthesis plant growth, grain set and resource depletion and shifted the grain-filling phase earlier, which reduced the impact of future drier conditions in spring.
The simulated effect of eCO 2 alone (FACE conditions) increased average yields from 18 to 38% for the semiarid regions but not in the HRZ and there was no evidence of haying-off. Within the HRZ of the Western District crops were not water limited during grain filling, so no evidence of haying-off existed where average crop yields increased by 5% under a future climate (6.4–6.7 t/ha). Heavy textured soils exacerbated the impact of a future climate on haying-off within the Wimmera. Sowing earlier (1 May) reduced the impact of a future climate on haying-off where decreases in yield/anthesis biomass ratio were 24, 28 and 23% for the respective regions. This translated to a reduction in yield from 1.6 to 1.4 t/ha (northern Mallee), 2.5 to 2.2 t/ha (southern Mallee) and 3.7 to 3.6 t/ha (Wimmera) under a future climate. A pair-wise comparison of the yield/anthesis biomass ratio across climate scenarios, used for assessing haying-off response, revealed that there was a 39, 49 and 47% increase in frequency of haying-off for the northern Mallee, southern Mallee and Wimmera, respectively, when crops were sown near the historically optimal time (1 June). Mean annual minimum and maximum temperature over the four regions increased by 1.9 and 2.2☌, respectively. For down-scaled climate predictions for 2050, average rainfall during October, which coincides with crop flowering, decreased by 32, 29, 26, and 18% for the semiarid regions of the northern Mallee, the southern Mallee, Wimmera, and higher rainfall zone, (HRZ) in the Western District, respectively.
PERFORMANCE OF THE APSIM WHEAT MODEL IN WESTERN AUSTRALIA FREE
A wheat model, APSIM-Nwheat, was performance tested against three datasets: ( i ) a field experiment at Wagga Wagga, NSW ( ii ) the Australian Grains Free Air Carbon dioxide Enrichment (AGFACE) experiment at Horsham, Victoria and ( iii ) a broad-acre wheat crop survey in western Victoria. We used a modelling approach to assess the impact of an expected future climate on wheat growth across four cropping regions in Victoria.
Under a future climate for south-eastern Australia there is the likelihood that the net effect of elevated CO 2, (eCO 2 ) lower growing-season rainfall and high temperature will increase haying-off thus limit production of rain-fed wheat crops. ‘Haying-off’ in wheat is predicted to increase under a future climate in south-eastern Australia ‘Haying-off’ in wheat is predicted to increase under a future climate in south-eastern Australia
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