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Herbicide carryover PDF Print E-mail
Thursday, April 18, 2013 1:57 PM

Ag Educator OSU-Extension Putnam County

The following article was summarized from a Strip Till Strategies article on herbicide carryover (April 12, 2013). After the drought of 2012, herbicide carryover could potentially be a problem in 2013. Herbicides that normally break down from microbial activity could still be around since most degradation occurs in the summer and early fall when the soil is warm and moist. If the soil is too hot or if moisture is limiting, microbial activity declines and then herbicides may carryover. Cold temperatures reduce microbial breakdown of herbicides, so cold winters may increase soil herbicide persistence. While warmer spring temperatures and moisture may help degrade the herbicides, there may not be enough time before the next crop is planted.

Due to glyphosate-resistant weeds, many farmers are using older herbicide chemistries with more soil persistence. For example, fomesafen is the active ingredient in Reflex, Flexstar and Prefix and has an average half-life of 100 days (relatively long half-life). Fomesafen includes herbicides like flumioxazin (Valor and others), sulfentrazone (Authority and Spartan products), and saflufenacil (Sharpen and others). These products are degraded by soil microbes so dry or hot summer soils may increase the chance of herbicide carryover and injury to the next crop.

Fomesafen (PPO mode of action) is used in soybeans to control many broadleaf weeds and the product label has a 10-month interval before planting corn. If the product was applied in mid to late June 2012; the earliest farmers could plant corn with a 10-month interval is mid to late April assuming adequate microbial breakdown. However, with hot or dry weather conditions, the 10 month interval may not be long enough. Carryover symptoms include corn buggy whipping, white leaves (chlorosis), and midvein leaf breakage. If growing conditions are good, corn can usually outgrow the problem, but if the weather is cold and corn is not actively growing, herbicide carryover may reduce corn yields.

Corn herbicide carryover may be possible with the ALS herbicides which include imazaquin (Scepter and others), imazethapyr (Pursuit, Lightning and others), and the clorimurons (Canopy, Classic, Cloak, and others). Soybean herbicide carryover may be caused by HPPD herbicides with mesotrione (Callisto) and Auxin herbicides with clopyralid (Curtail, Stinger).

So what can farmers do to prevent a problem? First, review each field and see what and when products were used. Look at the label restrictions to see if herbicide carryover may be an issue. Avoid planting early and follow all time restrictions on fields where persistent herbicides were used. Second, avoid stressing plants and try to get the plants off to a fast start so that they can outgrow any potential herbicide carryover problem. Avoid planting in compacted fields or cold wet soils. Third, if a persistent product was used late in the growing season, it may be best to plant the same crop again, rather than risk crop injury.

A bioassay may be conducted to see if crop injury is an issue. Use soil from the top 3-4 inches and plant the intended crop and look for emergence or herbicide carryover signs. While this sounds good, it may be difficult to determine herbicide carryover problems, so be careful. In addition, there may not be enough time left to do this bioassay. Sandy soils or soils low in organic matter tend to have more herbicide carryover problems than soils high in organic matter.

Plan to plant susceptible fields last; the ones with persistent herbicides applied late last summer. Late planted fields tend to have less stress and plants grow faster. The last two years, late planted crops had the best crops yields due to beneficial late summer rains. Some farmers may be tempted to till the soil to dilute the herbicide. Tillage will aerate and warm the soil but results have been mixed. Most research studies have not shown a major crop response to more tillage in regards to herbicide carryover.

Overall in 2013, we could see some issues with herbicide carryover. Review and know the spray history for each field and watch fields that have had persistent herbicides applied. Reduce plant stress to fields by not planting into compacted or wet cold soils early in the growing season. In some cases, it may be best to plant the same crop, especially on sandy soils with low organic matter or if a bioassay shows potential crop damage. Plan to plant susceptible fields last to avoid plant stress.


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