Horseweed, commonly called marestail, is the most troublesome weed at burndown for most farmers in north Alabama but ryegrass has become public enemy number two. Most ryegrass has developed a resistance to glyphosate (Roundup). Ryegrass also has resistance to Hoelon, Osprey and cross-resistance to Axial has been confirmed in Arkansas and Mississippi.
Ryegrass has allelopathic traits, which allows it to suppress other plants. This is an advantage when it suppresses weeds but detrimental to corn. The allelopathic activity of ryegrass can reduce corn’s seed germination and seedling growth. Ryegrass should to be burned down at least three weeks before planting corn to avoid the negative effects of ryegrass’s allelopathy.
Dr. Joyce Tredaway, Alabama Extension weed scientist, says, “There are so many types of glyphosate resistance ryegrass in north Alabama that a grower can’t just spray glyphosate during burndown and expect control. They need to add clethodim (Select Max or a generic clethodim) at the mid-to-high rate.” Clethodim has a plantback restriction for corn (most labels state six days).
The glyphosate plus clethodim option is not effective on horseweed. Dr. Tredaway notes, “Horseweed usually emerges in late February or early March in north Alabama so this is the optimum time to spray for horseweed. Verdict plus Atrazine and glyphosate is an excellent corn burndown option that controls horseweed and has a long residue. Leadoff plus Sharpen and glyphosate is also very effective and popular but more expensive. The Sharpen must be added to Leadoff for horseweed control.”
Dr. Tredaway warns that a methylated seed oil (MSO) at one pint per acre must be added to Verdict and Sharpen to activate them and for them to be effective.
Herbicide Resistance Info
If both horseweed and ryegrass are a present then Dr. Tredaway suggests Verdict plus Atrazine and either Gramoxone or glufosinate (Liberty) at a higher rate of spray volume.
Atrazine is the backbone of morningglory weed control and offers an alternative mode of action to help prevent herbicide resistance in Palmer amaranth. The maximum corn use rate for atrazine 4L is 2.5 quarts per acre so one quart per acre is advisable for burndown or preemergence use and followed up about thirty days after planting with 1.5 quarts per acre.
Atrazine can breakdown late in the season even when the higher rate is applied with the second application and morningglories can be a problem especially where there is not good soil shade from the corn plants. This should not discourage growers from using the split application of atrazine since morningglories would be much worse if this approach is not taken.