Use of Late-Season Herbicides on Canola Yield and Seed Quality.

Canola (Brassica napus L.) was adopted into the southern Great Plains as a means to increase the productivity and decrease weed pressure in traditional monoculture wheat production.  While canola has been effective at achieving this task, the inability to apply herbicides in the spring following dormancy can result in high weed pressure within the canola system or potentially the successive wheat crop.  It has been indicated that a decreased viability of pollen can result from herbicide applications during or around flowering.  However, this information has been strictly noted in more northern locations, typically with spring canola.  To evaluate this phenomenon in the Southern Great Plains region a trial was initiated in Perkins, OK from 2012-2016. Experimental design was a 2 by 3 by 3 factorial, factor one being herbicide: glyphosate, quizalofop, and clethodim at a high or low rate, based on labeled rates, and factor two being canola growth stage: bolting, early flower, and late-flower. Impact of herbicide applications at each of these stages was evaluated for effects on yield, oil content, and percent germination.  Overall, the application of herbicides during reproductive growth negatively affected crop yield.  This was especially true for glyphosate, which decreased yields by an average of 700.7, 775.4, and 313.2 kg ha^-1 during bolting, early bloom, and late bloom, respectively.   Higher herbicide rates almost always resulted in decreased yield, but was only significant with the glyphosate applications.  Total oil production was significantly decreased; however, as percent oil did not differ this difference can be attributed to the loss in total tonnage.  Similar to yields, glyphosate significantly decreased percent germination of harvested seeds when applied at bolting, early bloom, or late bloom.  These results suggest that late-season herbicide application, especially glyphosate, should be avoided when possible as decreased yields and percent germination can be expected.