More herbicide carryover

More herbicide carryover

Symptoms of herbicide carryover injury showed up on some corn in Iowa this spring, caused by a soybean herbicide applied in the field a year ago. This carryover was caused by fomesafen, the active ingredient in Flexstar and Reflex soybean herbicides, says Virgil Schmitt, Iowa State University Extension field agronomist in southeast Iowa, who answered calls and visited farmers’ fields to check it out.

There are also “generics” that contain fomesafen, such as Battlestar, Dawn, Rhythm, Ringside, Rumble, Shafen, Topgun, etc., he notes. Fomesafen is also present in combination with other active ingredients in several products, such as Flexstar GT, Flexstar GT 3.5, Marvel, Prefix and Statement. Fomesafen was applied to many soybean fields in 2013.

The Flexstar label says corn should not be planted for 10 months after Flexstar application. In a number of fields this spring, corn was planted less than 10 months after fomesafen was applied. In addition, the dry weather in 2013 slowed the normal rate of chemical breakdown. The cold winter may also have reduced the amount of breakdown that would normally occur.

Key Points

Herbicide carryover symptoms showed up on corn this spring in some fields.

The corn injury symptoms were caused by a soybean herbicide applied in 2013.

Be careful which “rescue” herbicide treatment is used on soybeans.

Fomesafen carryover symptoms on corn show up as veinal chlorosis or yellowing. The veins are chlorotic, while the area of the leaf between the veins remains green. People sometimes confuse this carryover symptom with sulfur deficiency. Sulfur deficiency is the exact opposite, with the veins remaining green and the interveinal material becoming chlorotic or yellow. Sulfur deficiency is often seen in low organic matter areas without a history of animal manure and in instances where rapid top growth of the corn outpaces root development and the ability to move sulfur into the plant rapidly enough.

“Corn normally recovers quickly from fomesafen injury,” observes Schmitt. “I’m not aware of any instance where yield loss due to this injury has been documented.” But you need to watch it because fomesafen carryover can cause stand loss.

Bob Hartzler, ISU Extension weed management specialist, has also received calls regarding suspected fomesafen carryover injury to corn. Fomesafen is a Group 14 herbicide (PPO inhibitor) and it is the active ingredient in Reflex, Flexstar, Prefix, Marvel and other products applied to control weeds in soybeans. In certain conditions, fomesafen can carry over in soil and cause herbicide injury symptoms on corn planted in the field the following year.

This carryover is typically associated with applications made after late June or when the season turns dry following application, says Hartzler. Most product labels containing fomesafen specify the 10-month rotation interval for planting corn.

The primary symptom of fomesafen injury is striped leaves due to chlorotic or necrotic veins on the leaves. Other factors can cause striping on leaves, but fomesafen is unique as the veins are affected rather than interveinal tissue, says Hartzler. Some of the leaves may fold over midway due to loss of integrity of the leaf midvein. Usually only two or three leaves are affected, and injured plants recover quickly. But at times there can be stand loss, and the only way to determine potential impact is to determine the percentage of plants affected and closely monitor the rate of recovery.

Choosing a rescue treatment

Be careful which “rescue” herbicide you choose for use in soybeans. The spread of glyphosate-resistant waterhemp has led to an increase in rescue applications with Group 14 herbicides, such as fomesafen, lactofen (Cobra, etc.) and acifluorfen (UltraBlazer, etc.).

Of these products, only fomesafen poses a threat to rotational corn, says Hartzler. A switch to alternative herbicides later in the season can avoid this carryover risk. “However, keep in mind that late applications of Group 14 herbicides usually are ineffective against the large waterhemp present at these times,” he points out. “Ideally, you need to develop integrated weed control strategies that minimize the need for midseason rescue operations.”

It’s best to spray for waterhemp sooner rather than later, as this weed is much easier to control before it is 4 inches tall. Also, the later you spray, which is true for many herbicides, such as Flexstar and others, the greater chance of carryover problems when the field is planted to corn the next year. Especially in fields where soybeans were planted later this spring, you needed to keep the 10-month crop rotation interval in mind for next year’s corn, notes Hartzler.

Source: Iowa State University

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CARRYOVER SYMPTOMS: Fomesafen carryover injury typically only affects two or three leaves on corn plants. But it can cause stand loss, so fomesafen injury needs to be closely monitored.

This article published in the July, 2014 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2014.

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