Early Weed Warm-Up

January 10, 2009 05:49 AM



If you're frustrated with your soybean weed control program, you're not alone. After 13 years of relying almost exclusively on glyphosate, repercussions are becoming more frequent. Early season yield loss from weed competition, increased glyphosate costs, glyphosate-resistant weeds and a limited number of viable herbicides remaining due to resistance are dishing up a reality check.

"The Roundup Ready system is easy and simple and it still works," says Rick Cole, Monsanto Company chemistry technology development director. "Some growers occasionally think they can trim rates or get by with one application. Even depending on two
applications can put a strain on the system when you run into weather delays and try to get over lots of acres.

"If we learned anything from the challenges of the 2008 growing season, it is that you have to be proactive and flexible to get the best weed control timing," he adds. "Early season weed management is critical to maximize the yield potential of the crop, and that means many will need to add a soil residual product to their plans, followed by an in-crop application."

What it also means is that growers need to revisit best management practices to optimize weed control. "We have farmers that have only used total post programs," Cole explains. "Some have never applied a residual. The challenge in soybean weed control right now is to bring growers back to thinking about a total package or strategy for their weed control program."

University of Arkansas Extension weed scientist Dick Oliver likes to keep this simple rule in mind: Give the advantage back to your soybean crop. That means focus less on what chemical will control the weed and more on basic weed ecology and biology.

"The most important part of a weed control program is to know the weed, how the weed grows and when you can and can't manage it," he says.

In general, Southern growers have more weed species and weed pressure to deal with because of the warmer climate. But Oliver says the principles are the same regardless of region.

"Any practice that promotes rapid soybean stand establishment, proper plant density and rapid canopy closure will increase the ability of soybeans to compete with weeds and thereby increase the effectiveness of a given herbicide program," he says.

Know the enemy. The cornerstone of any weed control program is to properly identify the problems. University of Illinois Extension weed specialist Aaron Hager says the last scouting of the season is usually a windshield survey from the combine.

"Referencing the species that were prevalent and where they were located in the field will give you a good idea of what to expect next season. That's particularly important for 2009, as we had wet holes that were filled with weeds after water drowned out the crop," Hager notes.

In the spring, start scouting about seven days after crop emergence to determine weed species and plant density. Weeds can be tough to identify in the seedling stage, but it is important to figure out what you have before deciding what treatment is needed. Many excellent weed identification references are readily available.

Keep scouting through the season. Weeds such as waterhemp and Palmer amaranth can be sneaky, as they can emerge early in the season but also have flush after flush. "On the other hand, I've never seen hophornbean copperleaf emerge until the first week in June," Hager says.

Proper weed identification is also important when it comes to selecting which soil-applied herbicide to apply for the following crop. Weed scientists urge farmers to immediately identify any new or strange weed that shows up in a field for the first time. Keep in mind that weeds that survive despite repeated herbicide applications are a good indication of a possible herbicide-resistant population.

Weeds that are the most difficult to kill and the most competitive with the crop are those that come up first, Oliver says. "The perception is we have to let weeds come up before we control them. But those that come up early are getting most of the moisture and nutrients," he says. "It pays to get clean early."

University of Illinois research shows soybeans lose about 1 bu. for every inch of weed height more than 6". Oliver suggests growers have fields weed-free by 14 days after emergence for the greatest return on herbicide investment.

Some weeds are more competitive than others. For example, Arkansas research shows it takes only five common cocklebur plants per 20' of row to reduce soybean yields by 30% if the weed is allowed to remain unchecked for the entire season. The same density of velvetleaf has been found to reduce yields by only 9% if left for the season. Waterhemp is competitive with soybeans and can cause up to 40% yield losses if left unchecked, Hager says.

Soil-residual herbicides reduce weed density and also help reduce the variability of overall weed height prior to a post herbicide application, control a wide spectrum of grass and broadleaf weeds and allow greater flexibility for timely post applications.

Use a weed control program that fits the weeds in your field, says Dick Oliver, University of Arkansas Extension weed scientist.
Timing matters. "The three most important factors influencing post-emergence herbicide activity is timing, timing and timing," Oliver says.

"Applying a postemergence herbicide too late is the No. 1 cause for failure in soybean weed control programs," he adds. "Proper timing for postemergence herbicides is from 10 to 14 days after weed emergence," he notes.

The timing issue intensifies in total post programs, Hager adds. "We can suggest a time interval during which it is critical to remove weed interference," he says. "But we can't be precise enough to specify which day during that interval yields begin to be negatively impacted. That's why combining weed control tactics helps hedge your bets and why one-pass weed control systems carry the most risk.

"How many times have you been ready to spray, only to receive enough rain that day to keep you out of the field three to five more days? Weeds continue to compete with the soybean during that time." Hager says that scouting fields before post applications gives you a much better idea of what rate to use.

Since soybeans and the first flush of weeds usually emerge together, Oliver suggests timing the post application to soybean emergence—generally 10 days after soybean emergence. Waiting to apply post products between the V3 and V5 stage often results in disappointing weed control.

That 10-day time period is especially critical with the new LibertyLink soybean system, now officially commercialized for the 2009 planting season, Oliver adds. "Ignite gives the crop a chance because it works early, but I'm a little concerned that growers will get busy doing other things and miss that timing. A residual also makes sense in this program," he says. Keep a map of where LibertyLink crops and Roundup Ready crops reside; sharing that information with your custom applicator can mean life or death for the crop.

Environmental conditions are the next most important factor in determining herbicide activity. Soil moisture, temperature and relative humidity are all critical (in that order).

Soil moisture and application timing often go together. "Another reason the 10-days-after-emergence application is effective is because weeds are seldom drought-stressed at this time," Oliver says.

Rain or overhead irrigation is needed within five to seven days to activate most pre-emergence herbicides. If you don't get adequate rain, Oliver says, you can make a post-emergence application two to three days earlier than intended. If soil moisture is still marginal, apply just before dark to take advantage of higher relative humidity and dew formation.

Think process. There's more to soybean weed control than spraying away your problem. Proper crop rotation, seedbed preparation, row spacings, good standards and mechanical cultivation play crucial roles.

"Simply upping rates doesn't provide long-term sustainability either," Cole says. "Soybean weed control doesn't have to be complicated, it just needs to be calculated."

You can e-mail Pam Smith at

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