10 June 2013

9 minute read

Uncontrolled grass weeds such as the foxtails, different species of Panicum, and various species of crabgrass are particularly powerful in preventing corn from attaining its maximum yield.

Primary Herbicides Families for Grass Control in Corn

• Chloroacetamides (acetochlor, s-metolachlor, dimethenamid, flufenacet, and related a.i.’s)
• Sulfonylureas (nicosulfuron, rimsulfuron)
• Glyphosate

Growers rely primarily on 3 herbicidal modes of action for grass weed control in corn. The first mode of action inhibits very long chain fatty acid synthesis. This mode of action includes the chloroacetamide herbicides (products containing acetochlor, dimethenamid, flufenacet, s-metolachlor, and chemically related active ingredients and is effective for preemergence grass control.

The second mode of action inhibits 5-enolpyruvylshikimate-3-phosphate synthase (EPSP), which eventually leads to the depletion of the essential aromatic amino acids, tryptophan, tyrosine and phenylalanine. Glyphosate is the only active ingredient with this mode of action and is very effective for postemergence grass weed control in glyphosate-tolerant corn.

The third mode of action inhibits acetolactate synthase, a key enzyme in the production of the essential amino acids isoleucine, leucine and valine. This mode of action contains the sulfonylurea herbicides (products containing nicosulfuron, rimsulfuron, and chemically related active ingredients). Nicosulfuron is effective for postemergence grass control in corn. In addition to its postemergence grass weed activity, rimsulfuron is also effective for preemergence grass weed control in lower organic matter soils.

A sulfonylurea-sensitive corn hybrid with a crop response warning for the use of unsafened sulfonylurea herbicides following postemergence application with unsafened (left) and safened (right) sulfonylurea herbicide.

A diversified weed control program that includes rotating or combining multiple herbicide modes of action is a core principle for successful long-term weed-resistance management. Relying only on a single mode of action for weed control dramatically intensifies the selection pressure for that particular mode of action which can accelerate development of weed resistance. The number of grass weed species with resistance to glyphosate and the spread and impact of these weed populations on corn yield will continue to grow if resistant weeds are not controlled by some other means - either chemical, mechanical or cultural practices.

DuPont Crop Protection and DuPont Pioneer strongly support the use of multiple herbicidal modes of action as part of a comprehensive program to control weeds in corn. This can include a 2-pass herbicide program in which a chloroacetamide herbicide is applied for preemergence grass weed control. DuPont herbicides containing nicosulfuron and/or rimsulfuron can be applied postemergence with or as an alternative to glyphosate for those corn growers who desire to rely on an additional mode of action for postemergence grass weed control.

Sulfonylurea Selectivity in Corn

Corn shows tolerance to the sulfonylurea herbicides because corn rapidly metabolizes the herbicide molecules via a cytochrome P450 degradation pathway to herbicidally-inactive products (Koeppe et al. 2000). Sensitive weeds (grass and broadleaf) do not rapidly metabolize these molecules and are therefore controlled.

The vast majority of corn hybrids produce a sufficient amount of this cytochrome P450 enzyme to rapidly metabolize these molecules under a wide range of environmental conditions for crop safety. However, a very small percent of the corn genetic background contains an insufficient amount of the necessary cytochrome P450 enzyme to metabolize intact herbicide molecules rapidly enough for acceptable crop tolerance under some conditions.

Cytochrome P450s

• Cytochrome P450s are the largest family of enzymatic proteins in plants.
• A plant species may have hundreds of different types of these proteins which are involved in a range of physiological processes, including detoxification of chemicals harmful to the plant.
• Cytochrome P450s play a primary role in the metabolism of several types of herbicides, including sulfonylureas.
• Corn has the necessary cytochrome P450 enzymes to rapidly metabolize sulfonylurea molecules, whereas susceptible weed species do not.

DuPont Crop Protection has developed the "Q" herbicides - Accent® Q, Realm® Q, Resolve® Q and Steadfast® Q - which include a safener for improved corn crop safety. The safener reduces the risk of corn crop injury for those hybrids and in those environments that may have previously resulted in crop response.

How Does the Safener Work?

Herbicide safeners are products applied in conjunction with a herbicide to help protect the crop from herbicide damage without reducing efficacy on target weed species. Safeners work by reducing the ability of herbicide molecules to reach and interact with their target site in the crop plants, which can be accomplished by:

• Direct interaction between the safener and target site
• Reduced translocation of the herbicide to the target site
• Accelerated breakdown of the herbicide into inactive metabolites

The safener included in the DuPont "Q" herbicides improves corn crop tolerance by increasing the rate of metabolism of active sulfonylurea herbicide molecules to inactive metabolites (Bunting et al. 2004 and Davies 2001). The safener selectively increases the amount of cytochrome P450 in the corn plant resulting in faster metabolism of herbicide molecules (Figure 1).

The herbicides safener increases metabolism by increasing the concentration of P450.
Figure 1. Corn hybrids are tolerant to the sulfonylurea (SU) herbicides because active ingredients are rapidly metabolized.

For the safener to work, some concentration of a specific cytochrome P450 enzyme complex must already be present in the plant. Sensitive weeds either do not contain or contain such low quantities of the correct cytochrome P450 that they cannot metabolize the herbicide molecules before these molecules bind to the herbicidally-active sites and kill these weeds. The safener does not protect sensitive grass or broadleaf weeds from the sulfonylurea herbicides because the specific cytochrome P450 enzyme necessary is not present in these plants (Figure 2).

Figure 2. The safener included in the DuPont "Q" herbicides has little effect on the rate of metabolism of sulfonylurea (SU) herbicides in sensitive weeds.

The DuPont “Q” Herbicides Dramatically Reduce the Risk of Corn Injury

The DuPont-safened "Q" herbicides enhance the corn plant's ability to rapidly degrade the sulfonylurea herbicide under a wide range of environmental conditions and across a wider range of corn genetic backgrounds. DuPont Crop Protection offers 4 safened herbicides for grass and broadleaf weed control in corn (Table 1).

University Research

Independent researchers at Cornell University conducted a field study in 2007 to compare corn crop response to Steadfast (nicosulfuron + rimsulfuron) with and without the safener and to Resolve (rimsulfuron) with and without the safener. The studies were conducted on Pioneer® hybrid 34A15, a hybrid that rapidly metabolizes sulfonylurea herbicides (tolerant) and on Pioneer hybrid 3941, a hybrid that slowly metabolizes sulfonylurea herbicides (sensitive). The tolerant hybrid, 34A15, does not require the safener for acceptable crop safety, whereas the safener dramatically improved the crop tolerance of the sensitive 3941 hybrid to both herbicide products (Table 2).

Table 2. Crop response of a tolerant hybrid (34A15) and a susceptible hybrid (3941) to postemergence treatments of 2X label rates of safened and non-safened versions of Steadfast and Resolve 2 weeks after treatment (Hahn and Stachowski 2007).

DuPont Research

A field study conducted in 2012 at the DuPont Rochelle, Illinois Midwest Field Research Station evaluated the effects of 1x and 2x labeled rates of 2 safened herbicides, Realm Q and Accent Q, on a Pioneer hybrid family with known susceptibility to sulfonylurea injury (P0916). Treatments were applied at the V4 growth stage and injury ratings were taken 7, 14 and 28 days after treatment.

Neither of the safened herbicides resulted in significant injury when applied at labeled rates (Table 3). Measurable but non-significant injury was observed with the 2x rate of Accent Q 28 days after treatment. The 2x rate of Realm Q resulted in minor but statistically significant crop response 14 and 28 days after treatment.

What if I choose to use a generic sulfonylurea herbicide that does not include a safener for postemergence weed control in my corn?

For a vast majority of the corn hybrids, you will observe postemergence weed control with excellent crop tolerance. The risk of phytotoxicity to corn is present in only a very low percent of corn hybrids. However, if you unknowingly planted one of these hybrids and then apply a product containing an unsafened sulfonylurea herbicide to this hybrid, phytotoxic response can vary from a temporary yellowing of leaf tissue in the whorl when the herbicides were applied to improper unfurling of corn leaves and shortened internodes to substantial stunting of the main stalk, depending on the corn growth stage, the environment, and other stress factors when the herbicides were applied. The potential for injury from sulfonylureas can be greater when a sensitive corn hybrid is sprayed after the plant is 10 to 12 inches tall and/or if the plants are exposed to environmental stresses.

Hybrid Ratings for Sulfonylurea Tolerance

DuPont Pioneer routinely screens new corn hybrid families for crop tolerance to several herbicide families, including the sulfonylurea herbicides.* Hybrid families are given 1 of 3 ratings:

Adequate Tolerance – For this particular hybrid family, available research and/or field observations suggest this herbicide is unlikely to result in material crop injury under normal circumstances.

Requires Careful Management – For this particular hybrid family, available research and or field observations suggest this herbicide may exhibit crop injury in challenging environments such as heavy rainfall during seed germination or seedling emergence, sandy soils, soils low in organic matter, high pH soils, or during periods of excessively cold, hot, dry or wet weather. University research indicates products within a herbicide class may vary in their degree of crop selectivity. The potential for herbicide interaction may also be impacted by the labeled herbicide rate used and the method or timing of application.

Crop Response Warning – Phytotoxic responses have been observed for this particular hybrid family in field observations and/or in research trials.

The variability in corn genetics allows corn breeders to continuously improve hybrid selections so that grain yields continue to increase as new hybrid families are developed. These same corn genetics allowed researchers to discover herbicides that selectively control weeds while maintaining good crop tolerance.

The chloroacetamide and sulfonylurea herbicides were discovered because of the corn plant's innate ability to metabolize these materials relative to the inability of the susceptible weed species. The strength of these herbicides is their ability to control grass weeds. Many products that contain either a chloroacetamide herbicide or a sulfonylurea herbicide as an active ingredient now contain a safener to protect the small proportion of corn hybrids that would otherwise be susceptible to crop injury under certain conditions.

Growers who properly use products with multiple modes of herbicide action as part of a greater management plan for weed control in corn will obtain the highest potential for maximum corn grain yield while keeping the risk of weed resistance to a minimum.

Summary

• Effective weed resistance management requires a corn producer to choose the proper tools, including using herbicides with different modes of action.
• Grass weed control in corn is achieved primarily with herbicides having 1 of 3 modes of action: the chloroacetamide herbicides, glyphosate and the sulfonylurea herbicides.
• Corn hybrids can vary in their sensitivity to some postemergence sulfonylurea herbicides under certain weather conditions and with certain adjuvant systems.
• DuPont's postemergence corn herbicides Realm® Q, Resolve® Q, Steadfast® Q, and Accent® Q all include a crop safener that accelerates herbicide metabolism by the corn plant and reduces risk of injury.
• DuPont Pioneer continues to evaluate new corn hybrid families to determine tolerance to several classes of herbicides.

June 2013