Crops
Still time to catch them
Post-emergent corn weed control especially critical this year
by Dr. Paul Tracy
We had a cool and wet spring, but hopefully the corn crop has reached mid-season growth and development. Considering delayed planting, I expect weed control to be especially critical this season. Over the past few years, our options for viable mid-season corn weed control have increased tremendously.
Integrated weed management starts with identifying the weed species present. You also need to know both the weed’s and the corn’s growth stages at application time. Individual herbicides have specific weed growth stages where they perform best. Generally, the smaller the weed, the more susceptible it is to a given herbicide. Most post-emergent herbicide labels list a recommended weed height, after which weed control efficacy weakens. If you are past the recommended weed height for a given weed population, consider changing herbicides.
Choice of spray additives becomes increasingly important later in the season. |
Herbicide labels list crop growth stage restrictions, which must be followed stringently. Many labels list application timing restrictions based on crop height and growth stage. I always recommend using the more restrictive of the two. Discrepancies between plant height and its corresponding growth stage are common. For example, a herbicide label may state “can be applied to corn up to 30 inches tall or up to the 8-leaf stage.” Under many environmental conditions, 25- to 30-inch tall corn may be in the 9- or 10-leaf growth stage. When in doubt, I always use the growth stage, as it relates to ear development, which is often what is affected if herbicides are applied too late.
When figuring growth stage for corn, include the thumb leaf (first rounded leaf to emerge) plus all leaves that are already showing a distinct collar. Also, remember that by the 6- to 8-leaf stage, the bottom leaf or two may have already dropped off. To determine the growth stage after lower leaf loss, split the lower stalk lengthwise and look at the internode area. The first node (lower leaf attachment area) above the first elongated stalk internode will generally be the fifth leaf. A photo of this stage can be found in the Iowa State University Report No. 48—How a Corn Plant Develops.
Anything that adversely affects ear development (fewer rows of kernels, or fewer kernels per row) has a great influence on final yield. After corn reaches the six-leaf stage, the young ear has already started to form and could be at risk to many influences, including herbicide activity.
Many herbicides have extended-use periods if drop nozzles are used. I know most folks despise using drops, but they often offer the only viable mid-season weed control option.
The choice of spray additives becomes increasingly important later in the season, especially as weeds mature. Weather also affects additive choice. The choice of spray additives is extremely herbicide dependent. Become familiar with all combinations of corn herbicides and their specific additive partners.
Choices of additives include methylated seed oil, ammonium sulfate, UAN solution, concentrated crop oil, non-ionic surfactants and drift retardants. There is a huge difference in spray additive quality and price. My experience is that using a poor quality additive is not much better than using no additive at all.
Another key issue with mid-season corn herbicides is the choice of tank mix partners in both herbicide-tolerant and conventional-genetic systems. Each herbicide-tolerant program (glyphosate tolerance, glufosinate tolerance and IMI tolerance) has strengths and weaknesses. These can perform well as stand-alone systems, unless specific weed species (or weed growth stages) require additional products.
Glyphosate (Roundup Ready) and glufosinate (Liberty-Link) systems sound alike but cannot be used interchangeably. While this appears obvious, every year I walk fields where the wrong product was used. In other words, please keep good records of what herbicide-tolerant system you are using. As we expand herbicide tolerance genetics, this will continue to be an issue.
The bottom line this year is that planting was delayed, early season herbicides may have been less effective, and weeds have had more time to develop. There will be increased pressure on our growers to use mid-season post-emergent corn herbicides. Select these herbicides based upon the weed species, weed growth stage, crop growth stage and environmental conditions present.
Paul Tracy is Director of Agronomy for MFA Incorporated
Click here to respond to this article
Top of Page
