Crop yield response to water - Cra

minimize water deficits during the three periods already mentioned: (a) tillering to stemelongation, (b) time of flowering, and (c) early to mid-grain filling. After the development of areasonable canopy, subsequent irrigations may be scheduled such that up to 50-60 percent oftotal available soil water (TAW) is depleted between applications without a notable negativeimpact on yield (Geerts and Raes, 2009).Over irrigation is common in wheat, even under supplemental irrigation: two or threeirrigations are sometimes applied in a short interval with little consideration of soil waterstatus or crop demand. Excessive water supply results in lower water productivity in termsof yield per unit of water applied. In extreme situations, excessive water (from rainfall orirrigation) results in waterlogging which, as mentioned earlier, can substantially depressgrowth and yield. Over-generous water supply during the vegetative period, particularly incombination with high fertility, produces luxuriant vegetation and may result in lodging afterhead formation. Lodging may also occur if an excessive amount of water is applied in a singleirrigation late in development, particularly with sprinkler irrigation.YieldWheat grain yields (at 11 percent moisture) can vary from crop failures in seasons with less than 100to 150 mm available water, to 1-3 tonne/ha in water limited rainfed conditions (Mediterranean,arid, dry-season subtropical environments) and 4-10 tonne/ha in rainfed temperate (westernand northern Europe) climates or irrigated systems (China). Exceptionally, grain yields can reacha maximum of 15 tonne/ha in cool, long season (life cycle of over 300 days) environments withhigh solar radiation input such as southern New Zealand, Southern Chile, Ireland, England andsome regions of China. In 2009, average country yields ranged from less than 0.5 tonne/ha inHonduras, Lesotho, Somalia, Venezuela and Eritrea to more than 9 tonne/ha in Belgium.A major factor contributing to the improvement in yield over the last century is the increases ofHI brought about by breeding for shorter statue. Under favourable conditions with no stress,HI ranges between 0.45 and 0.55 for modern wheat cultivars (Austin, 1999). However, whenthere is water stress after flowering or when the cultivar is poorly matched to the productionenvironment, HI can fall to as low as 0.20 to 0.30.The balance of water supply before and after flowering can have a substantial effect on grainquality. Water stress during grain filling leads to shrivelled grain with a low milling percentage(flour produced per grain input). On the other hand, high water supply late in the seasonleads to increased yield with low protein concentration. These changes alter the suitability ofthe grain for various end uses.wheat 99