I’m thankful that farmers and agronomists share their ideas, experiences and questions with me as it relates to killing weeds. During the winter months I’ll test out ideas in the greenhouse to see if they’re worth taking to the field. I’m fortunate that we have some great public-sector researchers (such as Dr. Page, Dr. Sikkema and Dr. Tardif on the field crop side) who are willing to take promising greenhouse results to the field.
Gap in management: In some field crops (e.g. dry beans) there are fewer options to control glyphosate-resistant weeds (e.g. Canada fleabane) prior to planting since many of the effective herbicides that can be used in herbicide-tolerant corn and soybean cultivars would be too injurious.
Field observations: A significant amount of foliar leaf burn occurs to Canada fleabane when 28 per cent urea ammonium nitrate (UAN) is applied. Often the plant will grow out of the injury, but smaller rosettes are sometimes controlled (Figure 1 at top). Liberty, a contact, non-selective herbicide that has no soil residual activity, would be a good candidate as a potential pre-plant herbicide because it would not cause crop injury to crops planted after application. However, Liberty has not proven to be overly effective on glyphosate-resistant fleabane, even when applied under hot and humid greenhouse conditions (Figure 2 below).
The idea: The addition of 28 per cent UAN to Liberty has been shown to improve the control of certain weeds. It is also common for dry bean producers to apply 40 to 60 lbs. N/ac. What if 28 per cent UAN was used as a carrier for Liberty herbicide? Would this dramatically improve control of glyphosate-resistant Canada fleabane, while supplying the dry bean crop with the necessary amount of nitrogen?
Greenhouse results: Treatments were applied to Canada fleabane that were greater than 15 cm in diameter and/or in height. Liberty applied at one 1/ac. did not provide adequate control for Canada fleabane. When applied at twice the labelled rate (two l/ac.) Liberty did eventually control Canada fleabane. Control of Canada fleabane was achieved when one l/ac. of Liberty was mixed with 20 U.S. gal./ac. of 28 per cent UAN (~60 lbs. N/ac.) or when mixed with UAN and water at a 1:1 ratio for a total carrier volume of 20 U.S. gal./ac. (~30 lbs. of N/ac.).
To the field? The greenhouse study provides some optimism that mixing the labelled rate of Liberty with either 28 per cent UAN as a carrier or a combination of water and 28 per cent UAN will control glyphosate-resistant Canada fleabane when applied as a pre-plant treatment. Hopefully, there will be an opportunity to explore it in the field during the 2020 growing season.