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Many herbicide labels contain application restrictions based on corn size using the leaf collar method and/or free-standing plant height. Additional label restrictions may exist when applying with or after some corn rootworm organophosphate (OP) insecticides. Product labels for herbicide tank-mix partners may have conflicting restrictions concerning corn plant size for application. Always follow the most restrictive product label.
Research has shown that good weed control within four to six weeks after planting is critical to maximize yield potential. Research conducted in Minnesota and Wisconsin indicated that if weeds are not removed by V3 to V4 (three to four leaf collars) growth stage, yields decrease by an average of 3 bu/acre per day, until the end of June.1
Before applying postemergence herbicides, scout fields to accurately determine the growth stage of the crop and the degree of variability that may exist across the field. Also, record weed species that are present and their height. Most herbicide labels define the maximum growth stage, and in some cases the minimum growth stage, for applications to prevent crop injury and facilitate good weed control.2 Typically, a corn growth stage is identified by the leaf stage or plant height. If a label has leaf stage and plant height restrictions, one should follow the more restrictive of the two recommendations. Similarly, when using a tank mixture, follow the recommendations for the most restrictive label language of the products in the tank mixture.
Figure 1. V3 growth stage corn plant showing 3 visible leaf collars and a plant height of 6 inches.
Counting leaf collars is generally considered the most consistent and accurate means of determining corn leaf stages (Figure 1). The collar is a visible, light colored band at the base of the leaf blade, separating the blade from the leaf sheath.3 Leaf stages are designated using a “V” (vegetative) to represent each leaf during vegetative development. The first true leaf on corn is the short, rounded leaf at the soil surface and is counted as V1. Each successive, visible leaf collar is counted as V2, V3, to VT (tasseling). Leaves within the whorl, not yet fully expanded, and with no visible leaf collar are not counted. The accepted practice is to count the first true leaf, but some herbicide labels ignore it. The herbicide label description of crop staging method should dictate the growth staging process. As corn plants age, the lower leaves die or are torn away. To stage older plants, dig up the plant without breaking the stalk, and then split the stalk down into the root ball. Leaves one through four originate from the stalk nodes at the base of the plant and when split, this area appears as a downward facing triangle. About 0.1 to 0.3 inch above the condensed area at the bottom of the stalk is the fifth node.3 Stalk splitting is useful for staging corn plants when wind blown soil or hail has removed leaves. This can help verify the exact stage of growth based on leaf nodes rather than just visible leaves.
Some herbicide labels use plant height to define application timing but the measuring method is not usually consistently defined. The generally accepted practice is to measure the height from the soil surface to the arch of the uppermost leaf (free-standing) that is at least 50% emerged from the whorl.2 There are other benchmarks that are used to determine plant height, but they can lead to different plant heights. Adverse environmental conditions, such as cool air or soil temperatures, hail, and other stresses can retard corn height resulting in plants that are physiologically older than their height would indicate.
Herbicide-Insecticide Interaction. Some herbicide labels restrict application if OP insecticides have or will be applied to corn, while other herbicide product labels prohibit application only after an in-furrow OP insecticide application. Potential injury is more likely to occur when insecticides are applied at planting in-furrow, rather than T-banded. Most research indicates that injury from a herbicide-insecticide interaction is likely to be more severe when rain is adequate to ensure effective insecticide and herbicide uptake and activity.4 Some studies have shown that significant rain during the week prior to the postemergence application of an ALS inhibitor increases the severity of injury. Injury may be more likely when the corn plant is under stress from weather or a previous herbicide application. However, conditions suitable for rapid crop growth following injury provides an opportunity for the crop to outgrow injury.
1 Rosenberg M. 2014. Early season weed control in corn is vital. South Dakota State University Extension. http://igrow.org/agronomy/corn/early-season-weed-control-in-corn-is-vital/. 2 Hager, A. 2012. Corn growth stages and postemergence herbicides: size is important. the Bulletin. No. 6 article 2. University of Illinois. http://bulletin.ipm.illinois.edu/. 3 Abendroth, L.J., Elmore, R.W., Boyer, M.J., and Marlay, S.K. 2011. Corn growth and development. Iowa State University. 4 Loux, M.M., Doohan, D., Dobbels, A.F., Johnson, W.G., Young, B.G., Legleiter, T.R., and Hager, A. 2015. Weed control guide for Ohio, Indiana, and Illinois. The Ohio State University. http://agcrops.osu.edu/. Web sources verified 05/12/15. 150514185652