Delayed preemergence and early postemergence herbicide applications in corn
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
It’s that time of year again where we are all trying to get as much corn planted as we can before the next rain. This year weather has played a major role in planting, and rain has kept many of us out of the field. Preemergence herbicides should be applied as soon after planting as possible because delayed application increases the risk of poor herbicide performance, especially for grass control. However, there are often challenges for herbicide application following planting, such as high winds and rainfall. Therefore, it is likely there will be situations this year where corn has emerged before preemergence herbicides can been applied. In fields with emerged corn and no herbicide applied, growers have two primary options: to use an herbicide program typically applied before emergence; or switch to a total postemergence strategy. There are several possible herbicide programs with each strategy.
When herbicide application is delayed until after crop emergence, rotary hoeing is recommended. The rotary hoeing may be done before or after the herbicide application, but must be done when the weeds are very small. If the delayed application includes Banvel/Clarity, Distinct, Marksman (or other herbicides containing dicamba) or Prowl/Prowl H2O/Pendimax, crop safety will be greater if the rotary hoeing is done before herbicide application.
Herbicides typically applied preemergence are listed in Table 1K (pg. 46) in the 2009 Weed Control Guide for Field Crops. Many of these herbicides are also labeled for application after corn emergence. Axiom and Princep should only be applied before corn emergence. To avoid confusion, tank mix combinations are not included in this table. Details on the specific restrictions for tank mixtures can be found on herbicide labels. Some herbicide labels allow application to emerged corn using 28 percent liquid nitrogen fertilizer as the herbicide carrier; however, we advise that all herbicide treatments to emerged corn be applied with water as the carrier. Applying herbicides to emerged corn with 28 percent liquid nitrogen as the carrier poses a risk of severe crop injury. This risk is increased under cool, cloudy conditions prior to application, a common occurrence this time of year in Michigan.
For several years, MSU has demonstrated the importance of timely herbicide applications to prevent the loss of yield in corn. We have shown that it is necessary to control weeds before they reach four inches of growth to prevent measurable yield reduction. Two-pass herbicide applications (preemergence followed by postemergence or two postemergence) or timely single postemergence applications with residual control are most effective at preserving yield and maximizing gross margin. Information about the effectiveness and economics of preemergence and post emergence herbicide programs can be found in the summary of Commercial Herbicide Program Comparisons found on the MSU Weed Science website at: www.msuweeds.com/publications
Herbicide options for total postemergence weed control are dependent on the hybrid planted in the field. Many postemergence herbicides can be used on any corn hybrid, while others such as Liberty and glyphosate require herbicide resistant hybrids. For details on postemergence herbicides programs in corn, see 2009 Weed Control Guide for Field Crops Extension Bulletin E-434.