Pest Patrol: Bluegrass management in winter wheat

Yield losses of more than 50 per cent can occur from bluegrass competition in winter wheat. As well, because bluegrass species often head at the same time as winter wheat, they could intercept and reduce the amount of fusarium head blight fungicide that actually reaches the grain head (Figure 1 at top).

There are several species of bluegrass found in Ontario. Annual, roughstalk and Canada bluegrass are the most common species in field crops. These competitive grassy weeds will emerge in the fall or early spring. Plants smaller than 10 centimetres (4 inches) in height are more easily controlled by herbicides. Based on preliminary Ontario data in winter wheat, April herbicide applications made to bluegrass are more effective than May applications.

Testing herbicide effectiveness

Figure 2: Roughstalk bluegrass was less than 10 cm (4 in.) tall at the time of herbicide applications. *photo:* Supplied

An on-farm research trial evaluated five different herbicides, three of which are currently available in Ontario. The field site contained primarily roughstalk bluegrass. Herbicides were applied when bluegrass was 5 to 7.5 cm (2 to 4 in.) tall (Figure 2 above) and the winter wheat crop was just at stem elongation. By contrast, the rest of the field was sprayed one month later when bluegrass was between 20 and 30 cm (8 and 12 in.) tall.

Key learnings:

Figure 3 (left): Axial, Simplicity GoDRI and Varro herbicides all provided excellent control of roughstalk bluegrass compared to the unsprayed check. Figure 4 (right): Roughstalk bluegrass control shown under the winter wheat canopy at 48 days after an April application was made to plants that were smaller than 10 cm (4 in.) in height. *photo:* Supplied

Axial, Simplicity GoDRI and Varro provided excellent control of roughstalk bluegrass that was less than 10 cm (4 in.) tall at application. (Figure 3, Figure 4 above)
Delaying herbicide applications into May, when bluegrass was greater than 20 cm (8 in.) tall, reduced control but did keep bluegrass from heading above the wheat canopy (as of June). However, there was plenty of bluegrass under the wheat canopy that was beginning to flower (Figure 5 at bottom). This will likely result in seed dispersal back into the field.
Varro herbicide has label restrictions against its use when temperatures are below 3 C and/or winter wheat has reached the first node stage of growth. Both those restrictions were violated in this experiment. Significant yellowing and slight stunting occurred as a result. However, plants fully recovered within 14 days of application.

Figure 5: Roughstalk bluegrass control shown under the winter wheat canopy at 28 days after a May application was made to plants that were taller than 20 cm (8 in.). This application has prevented bluegrass from emerging above the crop canopy, but has not stopped flowering. *photo:* Supplied

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