Is There Value in Using Residual Herbicides in Roundup Ready Corn?

Corn Yield is most affected by weeds when

1. They emerge before or with the crop
2. They are at high densities
3. They are mainly broadleaves which are more competitive than grass weeds.

Impact on Yield

To date, research trials conducted by the University of Guelph have shown that a single application of glyphosate made at the 3 leaf stage can protect Roundup Ready corn yields (Table 1). This is consistent with research results from Chris Boerboom, University of Wisconsin who concluded that "a single properly timed glyphosate application can protect Roundup Ready corn yields"¹.

Source: University of Guelph, Ridgetown Campus (6 sites)
*Corn Yields of all treatments are not statistically different

However, significant economic losses are possible if that single application cannot be properly timed due to inclement weather, machinery breakdowns or time constraints. For example, research conducted by the University of Guelph has shown that if glyphosate applications are delayed from the 3 leaf stage until the 5 leaf stage of corn, corn yields can be reduced by 5 bu/ac. To look at it another way, delaying glyphosate applications past the 3 leaf stage (the beginning of the critical period of weed control) can result in yield losses ranging from ½ bu/ac/day to more than 1 bu/ac/day. As a manager you will need to ask yourself the honest question: "Can I ensure that all of my Roundup Ready corn acres will receive a single application of glyphosate so that the corn crop is kept weed-free from the 3 to 8 leaf stage?" If the answer to that question is "no", residual herbicides should be a part of your weed management strategy in Roundup Ready corn.

Impact on Weed Control

Glyphosate provides no residual control. Although timing glyphosate at the 3 leaf stage of corn may protect the full yield potential of the crop, it may result in unacceptable weed control (Table 2) due to late flushes of annual weeds, and late emergence of perennial weeds (e.g. field bindweed and Canada thistle). The incorporation of a residual herbicide may improve harvesting efficiency and reduce the amount of weed seed returned to the soil.

Source: University of Guelph, Department of Plant Agriculture

Resistance Management

Biotypes of a number of common annual weed species in the United States have been selected that have resistance to glyphosate. These species include many competitive broadleaf weeds such as lambsquarters, common ragweed, giant ragweed, pigweed species and Canada fleabane. Tank-mixing residual herbicides that are from different chemical families and are equally effective on these weed species will reduce the likelihood of selecting for tolerant or resistant weed populations.

Which Residual Herbicides offer the Most Value?

On a per plant basis, broadleaf weeds are more competitive than grass weeds. In general, residual broadleaf herbicides are cheaper than residual grass herbicides. Therefore, residual broadleaf herbicides offer greater value. That said weed spectrum will determine the type of residual herbicide that you should invest in. It would make little sense to tank-mix glyphosate with atrazine, when you have tremendous grass weed populations (Primextra II Magnum would make more sense). Likewise, Primextra II Magnum as a residual herbicide partner makes very little sense when you are targeting velvetleaf and triazine resistant lambsquarters. A new online herbicide selection tool at www.weedpro75.com will assist in matching the appropriate residual herbicide with a particular weed spectrum.

Residual Herbicides as a Risk Management Tool

It is well known that weeds will cause significant yield losses and that keeping the corn crop weed-free from the 3 to 8 leaf stage will minimize these yield losses from weeds. The unknown will always be the environmental conditions. It could rain for a week, or wind speeds could be excessively high at the time you attempt to properly time the post emergent glyphosate application. Applying a preemergent residual herbicide to a Roundup Ready corn crop provides a level of insurance for early season weed control. In a best case scenario, the residual herbicide provides adequate season long weed control and there is no need for the postemergent glyphosate application. Most realistically, a preemergent residual herbicide will protect against early season weed competition, the glyphosate application is then used to "clean up" any weed escapes or late emerging perennial weeds.

¹Boerboom, Chris. 2003. ECONOMICS OF RESIDUAL HERBICIDES IN ROUNDUP READY CROPS. http://www.soils.wisc.edu/extension/FAPM/2003proceedings/Boerboom-1.pdf

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