REGIONAL WINTER HARDINESS REQUIREMENTS
The Crop Development Centre (CDC) winter cereal program has conducted a large number of agronomy and physiology studies that have established the relationship between winter survival of wheat and weather conditions in western Canada. The relationship established in these studies has been used to create the computer program that estimates winterkill in wheat in the CERES crop growth simulation model that was developed as part of the United States Department of Agriculture's ARS - Wheat Yield Project and the multi-agency Resources Surveys Through Aerospace Remote Sensing (AGRISTARS) program. In cooperation with the CDC winter cereal program, Environment Canada researchers have analyzed 29 years of climatic data for 53 weather stations in western Canada utilizing the CERES model to determine the winterkill risk for properly managed stubbled-in Norstar winter wheat. These simulations indicated that when snow is maintained uniformly on winter wheat fields by snow trapping in standing stubble, a large area in western Canada has a winterkill risk level as good or better than the traditional production area in southwestern Alberta (
The CERES model predicted a low incidence of winterkill for the Wynyard east region and the southeastern tip of Saskatchewan, southern Manitoba and the Banff-Edmonton corridor in Alberta
The combined effects of drier air, which crosses the Rockies, and cold arctic invasions were evident in the higher winterkill risk predicted for the area that includes a large part of the Dark Brown soil zone in Alberta and Saskatchewan
Figure 1. Frequency (%) that more than 10% of the stubbled-in Norstar winter wheat crop is expected to winterkill as estimated by the CERES model (from Savdie, Whitewood, Raddatz and Fowler,1990).
Figure 2 . Frequency (%) that more than 50% of the stubbled-in Norstar winter wheat crop is expected to winterkill as estimated by the CERES model (from Savdie, Whitewood, Raddatz and Fowler,1990).
REGIONAL GRAIN YIELD POTENTIAL
The influence of weather on stubbled-in winter wheat grain yield have been studied in detail in Saskatchewan. Evaporation during the two week period immediately prior to heading, root zone extractable soil water at heading and evaporation during the last two weeks in July have been found to be the primary weather factors determining grain yield in these studies.
Most of the highly drought sensitive stem elongation to heading period for winter wheat occurs in June, which is usually the wettest month of the year
Evaporation rates during the average growing season in western Canada gradually increase from May to July and then drop off quickly in August
Figure 3. Mean monthly precipitation (25 mm = 1 inch) at the selected stations in Saskatchewan 1951-80 (from Guide to Farm Practice in Saskatchewan, 1984).
Figure 4. Calculated monthly lake evaporation (25 mm = 1 inch) for Swift Current, Wynyard, and Nipawin, Saskatchewan (from Canada Climate Normals, Vol. 9, Environment Canada, Atmospheric Environment Service).
As a re-crop, stubbled-in winter wheat is highly dependent upon precipitation that occurs between harvest of the previous crop (August) and heading (June). Field studies have demonstrated that soil water reserves only contribute approximately 20 percent of the total winter wheat annual water use, while 80 percent is derived from rainfall. These studies also established that stubbled-in winter wheat often exhausts most of its available soil water reserves by heading making later season growth even more dependent upon growing season rainfall. Since growing season precipitation can vary greatly, grain yield of stubbled-in winter wheat can also be expected to vary considerable from year to year
Large variation in average maximum grain yield was observed among years in the 1982 to 1991 period
Figure 5. Average grain yield trials grown between 1982 and 1991 in Saskatchewan. Total number of trials = 84. Values shown are the average of 10 yearly means for each soil zone. 1000 kg/ha = approx. 15 bu/acre.
Figure 6. Average (brown+dark brown+grey black soil zone)/3 grain yield of NOrstar winter wheat in trials grown in Saskatchewan. Total number of trials = 84. 1000 kg/ha = approx. 15 bu/acre.
The crop years 1984/85, 1985/86, and 1990/91 produced the highest average grain yields in research trials throughout Saskatchewan