Herbicide resistant weeds are not a new subject. Southeast Missouri became familiar with resistance in the 1990's with ALS resistant cocklebur and Palmer amaranth. More recently, glyphosate resistance has been appearing in the United States. Glyphosate-resistant horseweed is now widespread, glyphosate-resistant common ragweed has been found in two states, glyphosate-resistant Palmer amaranth has been discovered in Georgia and North Carolina and glyphosate resistant waterhemp has been discovered in Northern Missouri.

With Roundup Ready soybeans and cotton being grown on more than 95% of our acres and Roundup Ready corn acres expanding steadily, much of our weed control depends on glyphosate. Repeated use of the same herbicide is a resistance prone use pattern; although, glyphosate itself has a lower resistance potential.

The standard recommendations for resistance are to rotate herbicide modes of action, and to include other cultural practices including tillage. However, this recommendation is easier to make than it is to follow. Preemergence herbicides are commonly recommended as alternatives; however, if a good activating rainfall does not occur, preemergence herbicides will fail. Additionally, some preemergence herbicides can cause crop injury.

Postemergence tank mixes can help- especially residual tank-mix partners. However, many tank mix herbicides use the ALS mode of action. In the case of pigweed, many pigweed species are already ALS resistant, so these tank mixes will not help with pigweed. Residual tank mix partners will not work if activating rain does not occur. In cotton, the use of traditional layby chemistry is important. With Flex cotton being available, there is a temptation to make all applications over-the-top glyphosate or a glyphosate tank mix. However, we urge growers to still make a layby with any of the residual herbicides. Flex cotton makes layby herbicide applications extremely easy.

In soybeans, the old preemergence herbicides generally work well. The use of tank mix partners appears to happen somewhat regularly also, and this can help.

Resistance prevention for Roundup Ready corn is very straightforward, as Atrazine exists as an herbicide that is every bit as effective as glyphosate and every bit as cheap. Roundup ready corn should receive an atrazine treatment, either PRE in a post tank mix.

In the past year we have discovered an ALS resistant nutsedge in rice. Fortunately, the herbicide propanil appears to provide good control.

2006 Field Day Report