Robinson, M. A., Letarte, J., Cowbrough, M. J., Sikkema, P. H. and Tardif, F. J. 2015. Winter wheat (Triticum aestivum L.) response to herbicides as affected by application timing and temperature. Can. J. Plant Sci. 95: 325-333. Field studies were conducted to determine the effects of cold temperatures and physiological growth stage at the time of application on the tolerance of winter wheat (Triticum aestivum L.) to 10 herbicides used in Ontario, Canada. Herbicides were applied: early during a frost event (when forecasted temperatures ≤ 0°C); at a normal timing (Zadoks 21-29); and a late timing (Zadoks 39). Visible injury, yield, plant height at maturity, test weight and 1000-kernel weight were measured to determine if environmental conditions or growth stage at the time of herbicide application influenced wheat tolerance. Cold temperatures at the time of herbicide application resulted in injury with three treatments: 2,4-D, dicamba/MCPA/mecoprop and dichlorprop/2,4-D. Visible injury was greatest at 14 d after treatment (DAT); it was, however, transient and yield, plant height, test weight and 1000-kernel weight were not affected. The herbicides 2,4-D, dichlorprop/2,4-D, and fenoxaprop-p-ethyl caused visible injury 14 DAT when applied at the normal timing, while 2,4-D at this timing, also caused injury 7 DAT. Dicamba/MCPA/mecoprop was the most injurious herbicide, causing 4% injury at the normal timing and 11% injury at the late application timing (42 DAT). Dicamba/MCPA/mecoprop caused yield reductions of 11 to 24% at two locations in 2010 when applied at the normal timing. Dicamba/MCPA/mecoprop reduced yield at 6 of the 8 site-years when applied late, and also reduced plant height. Cold temperatures at the time of application did not affect tolerance of winter wheat; however, visible injury was more likely to occur when herbicides were applied at later growth stages. In most cases, herbicide injury was transient and no impact on yield was observed. Dicamba/MCPA/mecoprop was the most injurious herbicide, causing prolonged injury at all application timings and reducing yields when applied at the normal timing. In addition, yield and plant height were affected negatively when this herbicide was applied late.
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Vol. 95 • No. 2