The days of easy, simple weed control using all post applied herbicides are gone because of weed resistance rearing its ugly head across the entire nation. Continuous glyphosate applications over the top of Roundup Ready crops has resulted in a disaster in some regions of the country, mainly the South and Midsouth—but spreading fast into the main Corn Belt.

Midwest farmers need help to be aggressive against weeds showing up as glyphosate resistant. Farmers who haven’t been attacked by glyphosate-resistant weeds need to be proactive. But being proactive means that ag retailers have to step forward to assist with technical information and more.

Palmer pigweed is a major problem in the Midsouth, and Larry Steckel, associate professor, plant science, Univesity of Tennessee, points to it as an example of what can happen with the extremely rapid spread of glyphosate-resistant weeds.

How to help farmers with custom application is a major problem because the herbicides that are replacing glyphosate need sprayed much more timely than glyphosate has been sprayed. The size of the weed was not as critical as it is now.

Palmer pigweed, Steckel recently noted, will grow two to three inches per day above ground, even in 100-degree temperatures, and it will put down a tap root five feet long. He documented that the weed grew five feet tall in 20 days, or three inches per day, in at least one field during 2011.

The current control measures in soybeans typically include using post-emerge PPO mode-of-action herbicides such as Flexstar, Cobra and Blazer, but these need to be applied when the weed is two inches tall. Control of two-inch resistant Palmer pigweed usually results in 95 percent control, but if the weed is allowed to reach four inches before the herbicide application, then the result is only about 75 percent control. Naturally, control continues to decrease drastically every 24 hours that the pigweed is emerged.

“With only 75 percent control, you won’t combine that field of soybeans,” Steckel said.

Obviously, Steckel was stressing how important timing is for post-emergence herbicide applications for Palmer pigweed or other glyphosate-resistant weeds. He suggested there isn’t enough custom application equipment plus farmer equipment in use today to do the number of applications exactly when the applications need done. This problem could lead to more post-emerge application equipment being purchased.

The ag retailer’s concern has to be how to cover as many acres as possible but within a shorter window than the hay days of glyphosate herbicide use. The pay back for equipment has to be using it over the most acres possible.

Application equipment dependability becomes even more important, too. In general, new equipment has more dependability. 

Some herbicide manufacturers claim that using a pre-emergence herbicide applied at planting slows growth of weeds and could have weeds emerging at much different times in different fields. Recommending specific pre-emerge herbicides could end up extending the post-emerge season for custom application. 

A warning that has been issued by weed scientists is to not try and spray more acres per day late into the evening or quite early in the morning, even though it is possible with today’s precision equipment allowing spraying in the dark. Temperature inversions commonly occur in the Midwest and South, which can cause major problems with off-target crop injury.