What to consider to avoid herbicide carryover

With the new year's arrival, farmers are buying inputs for next season's crops. Now is the time to think about purchasing herbicides, but also to think about how to avoid herbicide carryover, especially if planting cover crops. Factors that increase herbicide carryover include dry weather, late application especially on herbicides with long half-lives, low soil microbial activity, low soil organic matter (SOM), and cooler and cloudy days. Soil texture (especially sandy soils with low SOM) and soil pH also affect herbicide breakdown.

In soybeans, there are five major types of herbicides to watch to avoid herbicide carryover. Flexistar/Reflex and Warrant Ultra (fomesafen) soybean herbicides have a long half-life with up to 18 months planting restriction for small seeded legumes and clovers, brassicas (radish, kale, rape), and even some grasses (oats 4-18 months, rye, wheat, barley 4-11 months). The mode of action is a Group 14 which is a PPO inhibitor (causes reactive oxygen). Authority, Optill, Zidua and Pursuit (imazethapyr) also have a similarly long half-life of 18-month planting restriction for brassicas, oats, sorghum sudan, legumes and clovers (4-18 months), and grasses (rye, barley, wheat, 4-11 months). Its mode of action is a Group 2 ALS inhibitor of several key amino acids. Soils with low pH tend to have higher herbicide carryover on this soybean herbicide. Farmers should be aware that several herbicides contain these chemicals in various mixes, formulations, and under various trade names.

Other soybean herbicides to watch include Authority Supreme and Fierce (pyroxasulfone) which have planting restrictions of 18 months for brassicas (radish, kale, rape), 10-12 months on clovers, legumes, oats, and rye and wheat (4 months). This herbicide inhibits long chain fatty acids (VLCFA) in seedlings. Warrant and Warrant Ultra contain the chemical acetochlor with 9-18 month cropping restrictions for legumes and clovers and 4 months for most grasses (oats, rye, wheat). The Ultra Warrant formulation has 2X longer cropping restriction for this herbicide.

Spartan and Warrant MTZ with sulfentrazone have cropping restrictions of 18 months for brassicas and oats, clovers and legumes (12-18 months), and 4 months for other grasses (rye, barley, wheat). Some herbicide formulations contain 2-3 of these long-lasting soybean herbicides which tends to increase the cropping planting restrictions. Notice, even planting wheat can be affected by some of these soybean herbicides.

On corn, the same rules tend to apply. Overall, corn herbicides tend to have more impact on crop planting restrictions for clovers and legumes since the corn herbicide is trying to control broadleaf weeds while soybean herbicides tend to have more effect on grass plants. There are five major corn herbicides that cause most problems with herbicide carryover in corn. Callisto, Harness Max, Maverick, Lumax, and Investigator formulations contain mesotrione while Resicore has clopyralid. They have similar cropping restrictions of 18 months for brassicas, clovers, and legumes and up to 4 months for most grasses (rye, barley, and wheat).

Depending upon the product and the formulation, oats and sorghum planting restrictions may be 0-18 months. Mesotrione is a Group 27 herbicide that inhibits a 4-HPPD enzyme. Clopyralid inhibits auxin, a growth regulator hormone (Group 4). Some herbicides like Zidua and Fierce can be used on both soybeans and corn with similar crop rotation and planting restrictions.

Corvus and Balance Flex (isoxaflutole) have cropping restrictions of 18 months on brassicas, legumes, clovers, oats, sorghum, peas, with 6-9 months on barley, and 4 months on rye and wheat. Accent, Steadfast, and Stout contain nicosulfuron which have cropping testrictions of 10 months for brassicas, legumes, clovers, and sorghum, 8 months for oats, and 4 months for rye, barley, and wheat. Again, many corn herbicide formulations contain several chemicals that can increase the planting restriction time.

While many herbicides have long cropping restrictions because they take a long time to break down, there are several herbicides that have a short half-life. Generally, herbicides like Sharpen (saflufenacil), 2-4D and Roundup products (glyphosate) break down fairly quickly in the soil and cause less problems with herbicide carryover. The long lasting, slow to breakdown herbicides generally help with those hard to control weeds, but there are other alternatives. Some cover crops, if planted early; like rye, barley, radish, sorghum and Sudan varieties, have natural weed suppressing chemicals that suppress small seeded weed germination. The Ohio State University Weed Control Guide Bulletin 789 is a great resource for herbicide recommendations.