Tolerance of Winter Wheat to Co-application of Postemergence Broadleaf Herbicides Using Urea Ammonium Nitrate as the Carrier

Spring application of post-emergence (POST) broadleaf herbicides in a urea ammonium nitrate (UAN) as the carrier may provide winter wheat growers with a one-pass option for weed control and nitrogen application. Winter wheat tolerance to bromoxynil/MCPA, dichlorprop/2,4-D, thifensulfuron/tribenuron + MCPA amine or pyrasulfotole/bromoxynil applied using 28% UAN at various carrier volumes (0, 100, 200, 300 and 400 L ha-1) was evaluated in field experiments planted in the fall of 2008, 2009 and 2010 at Exeter and Ridgetown Ontario, Canada. Generally, winter wheat injury was greater as the carrier volume of UAN increased. Injury due to bromoxynil/MCPA, dichlorprop/2,4-D, thifensulfuron/tribenuron + MCPA amine and pyrasulfotole/bromoxynil was as much as 8, 11, 18 and 9% at 7 days after treatment (DAT); the injury observed was transient and was 1, 2, 4 and 1% at 28 DAT, respectively. Winter wheat height and yield were not reduced by the herbicides evaluated, excepting bromoxynil/MCPA which reduced yield by 2%, and thifensulfuron/tribenuron + MCPA amine, which reduced height 3% and yield 2%. The establishment of underseeded red clover was decreased by at least 50% for all herbicide treatments assessed, except bromoxynil/MCPA. Based on this study, there is potential for spring co-application of UAN and bromoxynil/MCPA, dichlorprop/2,4-D, thifensulfuron/tribenuron + MCPA amine or pyrasulfotole/bromoxynil in winter wheat. However, growers that underseed red clover may not fully benefit as carrier UAN significantly reduced clover establishment.