Winter Wheat Cropping System Response to Seed Treatments, Seed Size and Sowing Density.

There is disagreement in anecdotal reports regarding the influence of seed-applied fungicides, insecticides or combinations of both on winter wheat (Triticum aestivum) crop growth and vigour.  A study was designed and conducted at 7 sites across the Canadian prairies to determine the influence of dual fungicide/insectidal seed treatments, sowing density and seed size on fall stand establishment and overwinter survival of winter wheat.  The three factor experiment consisted of 2 levels of seed treatment 1) Check – no seed treatment, and 2) dual fungicide/insecticidal seed treatment: tebuconozole + metalxyl + imidacloprid (‘RaxilWW’); 2 levels of sowing density 1) 200 seed m^-2, or 2) 400 seeds m^-2; and 3 levels of seed size (small, medium, and large) as a proxy for seed vigour.  The combined factors create a range of agronomic systems from weak (low seed rate, small/thin seed, no seed protection) to superior (high seed rate, heavy/plump seed, dual seed treatment).  A combined mixed model analysis of all 7 sites indicates seed treatments strongly affected grain yield, and the main effect of seed size or interactions of seed size with sowing density weakly affected grain yield.  Crop growth and vigour responses were greatest in the weak agronomic system and tended to diminish with a stronger agronomic system.  There were gains observed to grain yield by using seed treatments or plump seeds but the difference was relatively modest ie. 4.4 vs. 4.3 Mg ha^-1, and 4.6 vs. 4.5 Mg ha^-1, respectively.  The overall stability of the entire system (grain yield vs. CV of treatment) was assessed and the stability and overall productivity of a weak agronomic system (thin seeds and low seeding rate) was improved with seed treatments.  However, if seed size was large and a higher seeding rate was used, there was no gain in grain yield but a slight increase to system stability did occur with the seed treatment.  The results suggest seed treatments could enhance productivity, particularly if the agronomic system is compromised with less than desirable seed lots, lower plant populations, or perhaps other components not assessed in this study.  Therefore, it is important for producers to carefully assess seed lot quality, seeding rates, stubble quality, planting date in fall, and soil moisture conditions.  If flaws to the system or less than desirable conditions exist, the use of seed treatments could reduce risk in winter wheat production systems.