Sprayer early June Illinois

Editor’s note: The following was written by Aaron Hager, University of Illinois associate professor of weed science, for the university’s crop development Bulletin website.

Some may scoff at the notion of herbicide carryover after the often-saturated conditions in 2019, but do not overlook this as a possibility.

Delayed planting in 2019 often resulted in delayed application of soil-residual and foliar-applied herbicides. While soil moisture was not terribly limiting after planting, some portion of the Midwest (such as the east-central region of Illinois) experienced near drought-like conditions during July and August. Dry soil conditions can slow the rate of herbicide degradation and increase the potential for damage to rotational crops from herbicide carryover.

An indication of a herbicide’s soil persistence can sometimes be inferred from the crop rotation intervals listed on the respective product label. Herbicides that tend to persist longer in the soil generally have longer crop rotational intervals compared with herbicides with shorter persistence.

For example, labeled crops may be planted anytime following the application of carfentrazone (Aim), while 10 months must elapse between application of fomesafen (Flexstar) and planting corn. Rotational intervals for a particular herbicide sometimes vary by rotational crop, which also provides an indication of which rotational crops are more sensitive to herbicide residues remaining in the soil.

Be sure to adhere to all rotation crop intervals for all herbicides applied in 2019.

What can be done to minimize the risk of injury to rotational crops from residues applied during the previous growing season? In the simplest terms, herbicide degradation takes time and moisture.

Shallow tillage can help distribute herbicide more evenly across a field and is more likely to help enhance herbicide degradation when soil temperatures are warm and adequate soil moisture is present.

Early planting or planting a rotational crop that is very sensitive to the herbicide applied last season might further increase the likelihood of crop injury from herbicide carryover. Ultimately, the susceptibility of the rotational crop determines if persisting herbicide residues will cause any problems.

Planting the same crop in 2020 as was planted in 2019 would effectively eliminate the potential for crop injury from herbicide residues. This solution may not be feasible for every situation where herbicide carryover is possible, but it is an option that warrants some consideration.

If crop rotation must occur where there is concern for herbicide carryover, delaying planting as long as possible could allow additional herbicide degradation to occur.

Challenges associated with herbicide-resistant weeds will become even more acute in 2020. Waterhemp populations resistant to herbicides from multiple site-of-action groups populate millions of corn and soybean acres.

Many Illinois producers have experienced resistance caused by a modification of the herbicide-binding site (known as target-site resistance), whereas fewer producers have dealt with instances where herbicide resistance results from enhanced herbicide metabolism.

Many weed scientists consider metabolic herbicide resistance as the next frontier of challenge for successful crop production.

Last spring we announced the discovery of waterhemp populations resistant to Group 15 herbicides, which include acetochlor, dimethenamid, pyroxasulfone and metolachlor. This type of resistance is caused by the waterhemp plant’s ability to rapidly metabolize (i.e., break down) the herbicide before it causes damage.

Rapid metabolism is the same mechanism used by corn plants to survive Group 15 herbicides — it would seem waterhemp is mimicking the crop.

Compared with resistance to foliar-applied herbicides, resistance to soil-applied herbicides generally is more difficult to detect in the field. Resistance to foliar-applied herbicides is manifest as treated plants (assuming appropriate application rate and timing) not controlled, whereas resistance to soil-applied herbicides is manifest as a reduced duration of residual control.

It is not always possible to predict if residual control is reduced two days, eight days, 14 days etc., as populations vary in their response to individual Group 15 herbicides.

Remember, reduced residual control is not limited to Group 15 herbicide resistance. Resistance to soil-residual PPO-inhibiting herbicides also results in shortened residual control.

These examples do emphasize the necessity of applying full label-recommended rates instead of reduced rates, as reduced rates will further curtail the duration of residual control.