We are in the middle of a dry weather period that is enabling an early and rapid start to corn and soybean planting. Our concern in this type of situation is that continued dry weather can result in at least partial failure of pre-emergence herbicides, which are currently being applied at a rapid rate also. In a typical spring planting season, we receive rain often enough to ensure pre-emergence herbicide effectiveness, and it’s possible that the rain forecast for the end of the week will ensure that it occurs this year also. Pre-emergence herbicides reach maximum effectiveness only if rain moves them down into soil before weed shoots emerge. Once the shoots have emerged, herbicides that act strictly through residual activity have lost most of their effectiveness, unless you are a big believer in “reachback activity.”

In a situation where it is dry for the first several weeks after planting, we assume a subsequent rain will eventually activate pre-emergence herbicides, so that they will provide weed control, even if they fail to control the weeds that emerge soon after planting. One solution here is to apply a low rate of glyphosate or other postemergence herbicides to control the small weeds that emerge initially, with the hope that the pre-emergence herbicides will control the later-emerging weeds.

It may be worth reconsidering an application of pre-emergence herbicides when there is no rain in the foreseeable (week or more) forecast, and switching to an early postemergence application. An early postemergence application of foliar plus residual herbicides can be just as effective at preventing yield loss due to weed interference, compared with a program of consisting sequential applications of pre-emergence and postemergence herbicides. Most pre-emergence corn herbicides can be applied to emerged corn, and some of them have enough foliar activity to control small, emerged weeds without the need to add postemergence herbicides. In addition, the majority of the corn planted in 2010 is resistant to glyphosate and/or glufosinate (Ignite), and these can be combined with pre-emergence herbicides to control weeds emerged at the time of application.

For more information on this subject, see the article in the April 4, 2009 issue of corn, “Corn planted with no herbicide applied? Don’t panic”.