With good soil moisture conditions, warm soil temperatures and generally favorable weather, winter wheat is emerging quickly across Manitoba. However, those same conditions are also allowing volunteer canola to grow very well. And in some cases, volunteer canola will be present in establishing winter wheat fields in higher than wanted populations.
There are really two camps in regards to management of volunteer canola post-emergence in winter wheat in the fall. One is to wait for the first killing frost of the fall to control the volunteer canola, with the assumption the weed pressure is not sufficient to impact yield or crop establishment.
The other is to remove the early weed competition through herbicide application. There are a few products available for fall application after winter wheat emergence for control of volunteer canola. These include a bromoxynil/MCPA ester tank mix, Infinity (pyrosulfatole & bromoxynil) and Simplicity (pyroxsulam – does not control Clearfield volunteer canola). However, remember that a fall application of 2,4-D or dicamba products is not recommended (or registered) as it can cause crop injury only seen the following year at heading, as well impact yield potential (see photos below).
For more information on registered products, application timing and rates, refer to the Guide to Crop Protection at http://www.gov.mb.ca/agriculture/crops/guides-and-publications/pubs/crop-protection-guide-herbicide.pdf
If herbicide application is considered, are there economic thresholds available, i.e. what density volunteer canola will cause yield losses that are economically greater than the cost of control? Unfortunately, there is limited data available to assist producers and agronomists. In a 2-year study done in Ontario, yield response to increasing volunteer canola densities was variable in both years of the trial (Table 1). In 2004, the volunteer canola plant density of 760 plants/m2 was significantly lower than the other treatments. However, there were no statistical differences in winter wheat yield at the various volunteer canola densities in 2005. Therefore, ‘it is inconclusive as to the density of volunteer canola that will significantly reduce winter wheat yields’.
Table 1. Winter wheat yield at various densities of volunteer canola in Ontario (2004 & 2005).
Source: Controlling Volunteer Canola in Winter Wheat
by F. Tardif, P. Smith (University of Guelph) and M. Cowbrough, OMAFRA
Fertility Considerations – N Uptake. Another factor to consider with a significant growth of volunteer canola is the amount of nitrogen the canola is utilizing prior to being killed by fall frost or herbicide application. A former Manitoba Agriculture staff person based out of Stonewall did some investigating in fall of 2008 into how much N uptake by volunteer canola was occurring in one of his producer’s fields.
He collected and weighed volunteer canola plants from two locations (2.79 square feet area) in one winter wheat field in early October. The dry matter weight of volunteer canola was calculated to be 791 lbs of dry matter per acre. The samples were also submitted for tissue analysis and test results indicated the total nitrogen content at 5.02%. Using 5% for the total nitrogen content results in 39.5 lbs of nitrogen taken up by the volunteer canola to that point. However, much of that nitrogen would be released for the crop next year.
Some other points to consider is some of that nitrogen might be lost overwinter in wet conditions – so the canola is functioning as a ‘catch crop’. However, the bad news is if producers have applied N during seeding or later in the fall, the canola is tapping into ‘applied N’ which is not desirable. And as always, banding is better than broadcast, especially to limit weed uptake of N.
Submitted by: Pam de Rocquigny, Provincial Cereal Crops Specialist; Jeanette Gaultier, Provincial Weed Specialist; and John Heard, Crop Nutrition Specialist; Manitoba Agriculture
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