By Mark Loux, Ohio State University

We are in the middle of a dry weather period that is enabling an early
and rapid start to corn and soybean planting in Ohio. Our concern in this type
of situation is that continued dry weather can result in at least
partial failure of preemergence herbicides, which are currently being
applied at a rapid rate also. In a typical spring planting season, we
receive rain often enough to ensure preemergence herbicide
effectiveness, and it's possible that the rain forecast for the end of
the week will ensure that it occurs this year also. Preemergence
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" (see C.O.R.N. article from April 21, 2009).

In a situation where it is dry for the first several weeks after
planting, we assume a subsequent rain will eventually "activate"
preemergence 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 preemergence herbicides will control the later-emerging

It may be worth reconsidering an application of preemergence 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 preemergence and
postemergence herbicides. Most preemergence 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 preemergence herbicides to control weeds emerged at the
time of application.

For more information on this subject, see the article in the May 4,
2009, issue of the CORN newsletter, "Corn planted with no herbicide applied? Don't panic".

SOURCE: Ohio State University.