Wheat is the principal crop grown in many Mediterranean-like climate zones around the world, including the 8.3 million acre dryland cropping region of the U. S. Inland Pacific Northwest (PNW). Farmers in the low and intermediate precipitation areas of this region know that planting spring wheat (SW) will help them control winter-annual grass weeds and that SW production can be profitable with adequate available water. However, farmers are often reluctant to plant spring wheat (SW) because grain yields are highly variable compared to winter wheat (WW) after summer fallow (SF). Our research had three objectives: (1) assess the relationship between available water and wheat grain yield based on dryland field experiments conducted from 1953 to 1957 and comparing it to findings in studies carried out from 1993 to 2005, (2) assess the relative importance of stored soil water and spring rainfall for both WW and SW during the 1993–2005 period, and (3) provide a tool for predicting SW grain yield that uses stored soil water at time of planting and expected rainfall during the individual months of April, May, and June. The results of statistical analysis in the 1953–1957 study of 90 replicated plots have shown that 4.0 inches of available water is required just for vegetative growth (before wheat reproductive development begins), whereas in the 1993–2005 study of 175 replicated plots, only 2.3 inches of available water were needed. In addition to water required for vegetative growth, statistical analysis showed that from 1953 to 1957 each inch of available stored soil water and spring rainfall produced 5.3 and 6.9 bushels per acre (bu/ac), respectively, compared to 5.7 and 6.6 bu/ac respectively, for the 1993–2005 study. Further statistical analysis done in the 1993–2005 studies showed that April rainfall contributed much less to grain yield than rainfall in May and June for both SW and WW. Winter wheat always produced more grain per unit of available water than SW did. Data reveal that modern semi-dwarf wheat varieties begin grain production with 1.7 inches less available water than the standard-height varieties of the 1950s. This, along with improved agronomic management, is a major contributor to the ever increasing wheat grain yields of the past 50 years.
This paper focuses on our research into the relationship between available water and wheat grain yield under the Mediterranean-like climatic conditions of the U.S. Inland PNW. Our research had three objectives: (1) assess the relationship between available water and wheat grain yield based on dryland field experiments conducted from 1953
Dryland wheat farming is widely practiced in Mediterranean-like climates, which include numerous countries surrounding the Mediterranean Sea, the U.S. Inland Pacific Northwest, parts of western and southwestern Australia, and central Chile. The Mediterranean climate is characterized by cool, wet winters and warm, dry summers. Dryland wheat production in these climates is heavily dependent on water stored in the soil during the winter, in addition to spring rainfall (Arnon 1972).
The dryland cropping region of the Inland PNW includes eastern Washington, north-central Oregon, and northern Idaho. Average annual precipitation in these areas ranges from 6-24 inches, with 60%-70% of it occurring from October through March. About 25% of the annual precipitation occurs from April through June, when most wheat growth occurs. Due to wide differences in the amount of precipitation, the Inland PNW can be divided into three annual precipitation zones: (1) low (<12 inches of precipitation), (2) intermediate (12 to 18 inches of precipitation), and (3) high (18 to 24 inches of precipitation).
In the low-precipitation zone, the dominant crop rotation is WW–SF, where only one crop is produced every other year. In the intermediate-precipitation zone, a 3-year WW–SW–SF rotation is commonly practiced, with spring barley sometimes substituted for SW. In the high-precipitation zone, annual crop ping is practiced, with WW mostly grown every third year in rotation with SW, spring barley, lentils, peas, and other spring-sown crops. Further details on crop rotations, soils, and climate in the Inland PNW can be found in Schillinger et al. (2006).
Increased cropping intensity (i.e., less SF) provides environmental benefits by reducing wind (Papendick 2004) and water erosion (Papendick et al. 1983). However, most farmers in the low and intermediate precipitation zones are reluctant to plant SW in lieu of SF because SW is riskier and grain yields less reliable compared to WW after SF (Schillinger et al. 2007). Of all the factors affecting crop growth, Brown et al. (1981) suggested using grain yield response to soil water at planting and expected growing season precipitation to help guide crop choices in flexible cropping systems in Montana and North Dakota that included SW, WW, barley, oats, and safflower. Nielsen and Halvorson (1991)