Crop yield response to water - Cra

Lead AuthorSenthold Asseng(formerly CSIRO, Wembley,Australia; currently University ofFlorida, Gainesville, USA)Contributing AuthorsSteve Milroy (CSIRO,Wembley, Australia),Simona Bassu(University of Sassari,Sassari, Italy),Marie-Therese Abi Saab(formerly CIHEAM-IAMB, Bari,Italy; currently LARI,Fanar, Lebanon)AcknowledgmentsRossella Albrizio(formerly CIHEAM-IAMB,Bari, Italy;currently CNR-ISAFOM,Ercolano, Italy),John Angus(CSIRO, Camberra, Australia),Francesco Giunta(University of Sassari,Sassari, Italy),Gerrit Hoogenboom(University of Georgia,Athens, USA),Matthew Reynoldand Ivan Ortiz-Monasterio(CIMMYT, El Batán, Mexico),Victor O. Sadras(SARDI Waite Campus,Australia)WheatGeneral descriptionBread and durum wheat (Triticum aestivum and Triticum turgidumdurum, respectively) comprise the third largest crop in the world.In 2009, 226 million ha were sown to wheat, producing 685 milliontonne of grain at an average grain yield of 3 tonne/ha (FAO, 2011). Overthe last 50 years, the average yield per hectare has increased dramatically,particularly between the 1950s and 1980s. Since the area cropped hasremained relatively constant, global production has reflected the increasein yield (Figure 1).Wheat is a cool season crop originating in the Fertile Crescent but nowwidely spread around the world. It is grown in arctic and humid regionsas well as the tropical highlands and from sea level on the Dutch Poldersto 4 500 m altitude in Tibet. The growing conditions are very diverse, notonly because of the widespread climatic regions and altitudes, but alsobecause of variability of soil types and crop management. Currently, thecountries with the largest wheat production are China, India, the RussianFederation, the United States, France, Canada, Germany, Pakistan, Australiaand Ukraine in that order. These countries totally cover more than twothirdsof the global wheat production (Figure 2).In many annual cropping areas, wheat is grown in rotation with a varietyof other winter annuals such as other cereals, oilseed crops, and pulses,although wheat following wheat often occurs. In other cases it is sown asa second crop after summer cereals or cotton. Two systems of particularimportance are the rice-wheat and maize-wheat systems.Growth and developmentWinter wheat is sown in autumn, while spring wheat is sown in autumn orspring. Winter wheat requires a cold period or chilling (vernalization) duringearly growth for normal heading under long days. Wheat is usually sownat a depth of around 5 cm, although greater depths may be used underdry conditions, to attempt to place the seed into moist soil. While this canbe successful, it delays emergence and growth and in extreme situationsmay reduce stand density. Sowing is usually into moist soil but in some dryenvironments ‘dry sowing’ may be practised shortly before the expectedstart of the rainy season. Plant densities range from 50 to over 500 plant/m 292crop yield response to water