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Preplant herbicides return to importance

Preplant herbicides return to importance

Herbicide-resistant weeds. Preplant incorporated herbicides. Roundup Ready. Marestail and pigweed.

A farmer recently told me that anywhere groups of farmers are gathered, herbicide resistance is the main topic of discussion. Growers are exchanging a lot of ideas among themselves about how they can best solve or avoid the problem on their farms. This is huge step forward.

One important topic is preplant incorporated herbicides. In the days before Roundup Ready, when we were dependent upon soil residual herbicides, most were incorporated.

Some of the heavy hitters like Treflan had to be incorporated because they are volatile. Other herbicides such as Dual, Scepter, Canopy, and Sencor were often shallowly incorporated to get them into moisture and away from sunlight, and to make them more consistent than they would be applied to the surface after planting.

After Roundup Ready came along, we quit using the residual herbicides (which was a mistake), but we also made huge strides in conservation tillage. My comments on preplant incorporated herbicides should not be interpreted as me advocating a shift back to them and away from conservation tillage systems.

The reality is, however, that where resistant marestail and pigweeds are issues, crops must be planted into clean seedbeds or you should be positive the emerged vegetation can be controlled with a burn-down herbicide.

For this and other reasons, we are getting a lot of questions about incorporated herbicides — especially on sandy loam to silt loam soils. Uniform incorporation is much more challenging on clay soils.

With the yellow herbicides, Treflan was always somewhat more effective on pigweeds than Prowl. We had resistance to these herbicides also back in the day. Therefore, you will not know how they will perform until you try one of them.

They are root growth inhibitors, so you want them incorporated where and below most of the weed seed are. The best depth is 2 to 3 inches. There are a lot of ways to incorporate, but two passes with a field cultivator or other scratching tool (one pass at an angle to the other) works well.

There is a lot of Dual and generics being used again. These herbicides can also work well incorporated. They are shoot growth inhibitors, so they need to be in and above the weed seed zone. Incorporation depths of 1 to 2 inches are best for these herbicides.

If herbicides like Sencor, Prefix and Authority MTZ are used or added, they work best at the shallow depths as well.

There are advantages and disadvantages on preplant incorporated herbicides. There are trade-offs with conservation tillage. However, where soil moisture is good and the seedbed is smooth, incorporation can make some of the treatments more consistent.

I think they would best fit where tillage is going to be used to insure planting into a clean seedbed anyway. That way you can just combine the incorporation with the final seedbed preparation.

I am 100 percent on board with the increased use of residuals both pre-emergence, early preplant surface applied, postemergence and in some cases incorporated. There is a huge emphasis on them and we have no choice but to rely heavily on them again.

I was around when we had to depend on them. The first time we go through a two- to four-week dry spell when the majority of our crop is being planted, a lot of folks who were not around then are going to get graduate courses on residuals not working under dry conditions.

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