Robinson, M. A., Cowbrough, M. J., Sikkema, P. H. and Tardif, F. J. 2013. Winter wheat (Triticum aestivum L.) tolerance to mixtures of herbicides and fungicides applied at different timings. Can. J. Plant Sci. 93: 491–501. Farmers commonly tank-mix herbicides and fungicides to reduce application costs. In the spring of 2008, there were reports of winter wheat injury with the application of herbicide–fungicide tank-mixes early in the growing season. This study was established to determine the tolerance of winter wheat to herbicide–fungicide mixtures as influenced by time of application. Field studies were conducted at four Ontario locations in 2009 and 2010 with three herbicides and four fungicides. Herbicide–fungicide tank-mixes were applied early, under cold conditions, and late at growth stage Zadoks 37–39. Dichlorprop/2,4-D mixed with tebuconazole caused up to 15% injury when applied early and up to 29% injury when applied late. Bromoxynil/MPCA mixed with tebuconazole injured wheat up to 15% when applied early but only 10% when applied late. Other herbicide and fungicide mixes caused a lower level of injury. Visible injury was transient and did not reduce winter wheat yields. The likelihood of tank-mixes causing injury was greater when they were applied late. The fungicide tebuconazole caused the highest level of injury when mixed with herbicides and injury was particularly high with dichlorprop/2,4-D.

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Underseeding red clover in winter wheat is a beneficial agronomic practice. Still, many growers tend to forgo this approach. One reason is that herbicides used on winter wheat may injure underseeded red clover, reducing its biomass and the subsequent benefits it provides. Therefore, the effect of winter wheat herbicides on underseeded red clover needs to be evaluated. The objectives of this research were to assess the crop tolerance of underseeded red clover to ten winter wheat herbicides used in Ontario, Canada and determine if red clover tolerance differed when the herbicides were applied at various winter wheat growth stages. Experiments were conducted in 2009 and 2010 at four different Ontario locations. Each herbicide treatment was either applied at an early, normal or late timing. Overall, red clover was not affected by herbicides applied at the early timing. The likelihood of herbicides causing injury and reducing biomass of underseeded red clover increased when they were applied at the more advanced winter wheat growth stages. If timing is a constraint, the three herbicides bromoxynil/MCPA, tralkoxydim, and fenoxaprop-pethyl are the safest to use on red clover underseeded to winter wheat. The remaining herbicides 2,4-D, dicamba/MCPA/mecoprop, dichlorprop/2,4-D, thifensulfuron/ tribenuron + MCPA, fluroxypyr + MCPA, pyrasulfotole/bromoxynil, and prosulfuron + bromoxynil are more injurious, with the last two being the most harmful. By having identified the least damaging herbicides on underseeded red clover in winter wheat and the optimal timing for herbicide application, growers are more likely to adopt this beneficial agronomic practice, save on fertilizer costs and improve soil quality.