Winter survival and competition in a mixture of winter wheat cultivars

For winter wheat (Triticum aestivum L.) in western Canada, cold resistance is a major factor in winter survival ability (WSA) and yield stability. The few genotypes that display the highest levels of WSA are tall, low in yield potential and weak strawed. While improved types with good WSA have been developed, none of these are as hardy as the elite lines. This study examines the role of natural selection in the development of winterhardy wheats with desirable plant types. A mixture of six botanically distinguishable cultivars with wide differences in plant type and WSA was started from equal numbers of seed and grown repeatedly in southwestern Alberta over several environments and years. 'Kharkov 22MC', 'Westmont', 'Jones Fife', and 'Gaines' have all been cultivated commercially in southern Alberta while 'Alabaskaja' and 'Bezostaya' are also adapted. Under severe cold stress, tender cultivars killed out and winterhardy cultivars dominated the mixture in 1 yr. In a second experiment, six cycles of grain harvest and reseeding through milder winters produced similar changes (r s = 0.94, d.f. = 4, P< 0.01) with no evidence of differential winterkill. For five cultivars, high reproduction in the second experiment was associated with the ability of pure stands to compete with adjacent rows (r s = 1.0, d.f. = 3, P < 0.0001). The exception (Jones Fife) was tall and competitive as a pure stand but reproduced poorly in the mixture. Unlike most winter wheats which become prostrate in the fall, Jones Fife resembles spring wheat and keeps its leaves upright. Increased defoliation from freezing due to non-prostrate plant habit may be expected to reduce the competitive ability of seedlings in mixtures during spring recovery. Jones Fife notwithstanding, tall and winterhardy cultivars with low yield survived best in the mixture while higher yielding, short-strawed types survived poorly. In the absence of winterkill, changes in the mixture were attributed to competition. To minimize the advantage of competitive types, mass selection for WSA should be conducted in high-stress environments for a few generations only.