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Soybean weed control drives variety selection

In 2010 many growers are thinking about changing weed control programs away from Roundup Ready. Weed resistance and cost-to-benefit ratios are what will drive the change.

Basically I am getting two questions from growers. First, “Can I grow conventional beans given the weeds that I have? ” Second, “Do I need to look at LibertyLink soybeans?”

I think the third question should be, “What are my weed control costs with each program?”

In regards to the first question, I think that we have a lot of soybean acres in Arkansas where 10 years of excellent weed control with the Roundup Ready system has resulted in some good clean fields in terms of weeds. If you are basically using glyphosate as a grass control material and most other weeds are under control, then you may want to compare the cost of conventional varieties and herbicides (probably a residual followed by a post option), to the cost of Roundup Ready varieties and your glyphosate program.

If you are burning down with glyphosate plus dicamba or 2,4-D plus a residual product like Authority MTZ, Canopy EX, Prefix, or Valor, and having to come back with a broadleaf tank-mix, either for horseweed or pigweed, then you may be able to simply add a grass material post such as Select and like that you go from a Roundup Ready program to a conventional program. This would be an example where you have some resistance, but it is not overwhelming you yet.

In either of the options above it is important to remember just how strong glyphosate is. Nothing else will control as broad a spectrum of weeds as glyphosate. Nothing else will control perennial vines and sicklepod. Be careful before you give up glyphosate as a tool and switch to conventional soybeans.

Also, keep in mind that conventional herbicides are pretty expensive for many weeds. Do your homework before you decide to go conventional soybeans.

Don’t forget that most conventional beans are going to need a residual herbicide upfront followed by at least one if not two post products. These post applications will need to be applied very early to be effective.

There are not that many acres of conventional soybeans available, so one other piece of advice is to book your varieties early if you intend to go this way. The same goes for LibertyLink soybeans.

A number of growers have expressed an interest in growing LibertyLink soybeans in 2010. For many it is not going to be an option as much as it will be a requirement. There are now six weed species in Arkansas known to be resistant to glyphosate: Palmer amaranth, horseweed, johnsongrass, common ragweed, giant ragweed and Italian ryegrass.

We have some fields in Arkansas where glyphosate-resistant pigweed and/or horseweed are so bad that LibertyLink soybeans and Ignite herbicide are the only really logical choice left.

In our trials the past two years, the LibertyLink soybean varieties have yielded as good as the good Roundup Ready varieties. I am comfortable recommending them from a yield standpoint, although there is probably still a lot left to learn in terms of nutrient, disease and insect issues.

I am also comfortable, based on our data, making the weed control recommendations.

First of all, Ignite is not Roundup. I do not believe that the LibertyLink system is as “simple” as the plant and spray system has been with Roundup, but it is the closest thing we have.

Residuals look good in the LibertyLink system and simplify the post timing of a limited amount of Ignite (44 ounces per acre per year).

One of the best pigweed control programs I have looked at is Prefix or Valor applied pre-emergence followed by 22 ounces per acre of Ignite applied on 2- to 3-inch pigweeds. If you do not use a pre in LibertyLink soybeans, then the timing of the first shot of Ignite becomes critical and will need to go out 10 to 14 days after the soybeans come up. Trust me, the weeds will be there and you cannot hit them too early, especially pigweed.

A tank-mix with Dual has looked good in plots and in the field with the first post shot of Ignite especially when no pre is used. In some cases where a residual is used, you may only make one post application of Ignite. You can go up to 36 ounces per acre in this situation if larger weeds are present.

For many growers the Roundup Ready system is working just fine and is very economical weed control. In those situations, I might recommend adding a residual to the burndown or pre for resistance management, but I certainly would not recommend that you switch from something that is working to an unknown for no reason.

I think the weeds you are dealing with and the economics involved should dictate the system you choose. I believe that choosing the right soybean weed control programs may dictate profit margins for many in Arkansas in 2010.


TAGS: Soybean
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