There is renewed interest in several areas of soybean weed control. Some growers are asking what they can add to their Roundup Ready programs for resistance management. LibertyLink soybeans which will be commercially available for the first time in 2009. And there is renewed interest in conventional soybeans.

Intertwined in all three areas is an increase in the promotion and interest in soil residual herbicides for resistance management. The interest in soil residual herbicides has come full circle. Prior to the introduction of Roundup Ready soybeans, a soil-applied herbicide was used on the majority of the soybean acreage. After Roundup Ready soybeans were planted on most of the acres, soil applied herbicide use fell to essentially zero. Now the pendulum is swinging back.

With the exception of Valor, most of the active ingredients available for soil-applied use are the same as those we had prior to the introduction of Roundup Ready soybeans. The names have changed on some of them and there are a lot of different mixtures available, but the herbicides are essentially the same.

Perhaps those who were farming soybeans 15 or so years ago do not need a refresher course on soil-applied herbicides. Of course, some of you may be like me — you were farming then, but your memory isn’t too good.

There are a lot of farmers, though, whose only experience with soybean weed control has been with glyphosate in Roundup Ready soybeans.

When we were using soil-applied herbicides on the majority of the soybean acres, most were preplant incorporated herbicides instead of pre-emergence herbicides (those applied to the soil surface after planting). Most of the soybeans then were grown using conventional tillage systems and most were grown in wide rows and were cultivated.

Only after Roundup Ready soybeans were introduced (along with better no-till drills about the same time) did narrow row and reduced tillage systems really take off.

Most of the soil-applied herbicides were preplant incorporated for several reasons. Treflan, introduced in the 1960s and one of “Ford’s Hall of Fame” herbicides, became extremely popular and could only be used preplant incorporated because it would volatilize if left unincorporated.

We later got Lasso and Dual, which could be used both preplant incorporated and pre-emergence. However, they were much more effective on red rice when incorporated. Therefore, most farmers in Arkansas were using preplant tillage anyway and they learned very quickly how to use incorporated herbicides effectively.

As herbicide development continued, several effective herbicides, such as Sencor, Scepter, Pursuit, and Canopy, came along. They could be used either incorporated or pre-emergence.

In addition to the reasons cited earlier, incorporated herbicides continued to be much more popular in the South than pre-emergence herbicides because a soil-applied herbicide is useless until it is activated by moisture and available to be taken in by the weeds. If a residual herbicide is incorporated into moist soil, it is usually activated. If it is applied to the soil surface, it must have rainfall or overhead irrigation for activation.

With warm soil temperatures and erratic rainfall patterns in the South, weeds often emerge before the pre-emergence herbicide is activated. For this reason, pre-emergence herbicides have always been much more erratic in the South than in the Midwest, for example.

Preplant incorporated herbicides can be just as effective today as they were in the 1980s and before. However, I doubt that most farmers will go back to them. This means the increased interest in residual herbicides is going to be in those applied pre-emergence, and you must be realistic in what you expect from them.

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