Weed Management Considerations for 2009

Published: March 4, 2009

As you prepare for the 2009 season, there are a few issues related to weed management that you can begin planning for right now.

First, the floods and heavy rains last season meant some soybean fields never got sprayed with a postemergence herbicide at all. Other fields in low-lying areas were totally abandoned because the crop was lost to standing water. Weeds still came up in these fields and produced abundant supplies of weed seed that were eventually deposited onto the soil. These will be a problem to deal with in 2009. If you had either of these scenarios last year, it's a good idea to plan for a heavier-than-normal population of weeds this season. A preemergence residual herbicide in either corn or soybean is one way to reduce these weed populations dramatically.

Fields like this that received heavy rains and/or flooding last year are likely to have dense weed populations because postemergence herbicide applications were never made.

Another thing to be on the lookout for this season is any new weeds that may have "found their way" onto your land due to the floods from last year. Landowners with fields in the Missouri and Mississippi river bottoms should really be mindful of this possibility. In my travels at the end of last season, I was surprised to find palmer amaranth infestations (also called palmer pigweed) in a central Missouri field, and also in several west central and northwest Missouri fields. In all cases, the palmer amaranth was within site of the river and my guess is that past flooding events introduced this weed into those areas.

Palmer amaranth is on the left while waterhemp (our most common pigweed species in Missouri) is on the right. Palmer amaranth has much wider, diamond-shaped leaves with long petioles.

Palmer amaranth is a very competitive pigweed species, one that has evolved resistance to glyphosate in much of the southeastern U.S., and one that we don't want spread throughout Missouri! While we know we have populations of palmer amaranth in the boot heel and sporadically along the western edge of Missouri, we have not historically had a problem with this weed in any other areas of the state. If you have or think you may have palmer amaranth in your fields, please give me a call or send me an e-mail (bradleyke@ missouri.edu), we'd like to know more about the distribution of this weed in Missouri.

One final thing to consider is that the time for spring burndown applications is fast approaching, if not already here. Although we haven't had as much problem with horseweed or marestail in Missouri as many other states, there are areas in Missouri where horseweed is a real problem and I have been getting more and more calls about this weed each year. If you have this weed, it is very important to pay attention to the timing of your burndown herbicide application. Ideally, burndown herbicide applications should be made before horseweed plants reach 6-inches in height and 2,4-D ester should be added as a tank-mix partner with the primary burndown herbicide. Also, for our winter annual weed spectrum in Missouri, I believe that 2,4-D is a good addition to your burndown herbicide regardless of whether horseweed is present or not. A more detailed publication providing specific recommendations for the management of horseweed and glyphosate-resistant horseweed biotypes can be found at: http://www.glyphosateweedscrops.org/.