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5 Thoughts on 2016 Soybean Weed Management

Don't let up on soybean weeds this year.

Don’t let weeds sink your soybeans in 2016. Here are five thoughts that Kevin Bradley, University of Missouri (MU) Extension weed specialist, wrote up this week regarding this year’s soybean weed management plan.

1. Even with low commodity prices, don’t skimp on weed management. Few things are more discouraging than listening to current commodity price and farm income predictions. During tight financial times, one of the first things many farmers determine is how they can reduce input costs. 

I certainly don't have all the answers to this one, but I would submit that we cannot afford to cut weed management costs. I have talked to many farmers who are tempted to cut costs by preemergence residual herbicide rates, or by choosing a cheaper, less effective herbicide than they had originally planned on using. 

Many studies show this doesn’t work in the long-term. For example, a recent economic modeling study sanctioned by the Weed Science Society of America showed that following good weed resistance best management practices like mixing effective herbicide sites of action are initially more expensive. Long-term, though, this provides better weed control, higher yields, and more revenue. 

Depending on the cropping system, farmer profits increased by 14% to 17% in this study over a 20-year period.

2. Prevented plant acres and fields with 2015 weed failures will almost certainly be high weed-pressure areas this year. 
We set all kinds of records last year for the number of corn and soybean acres that were never planted. Many prevented plant acres grew up into weedy messes. In some of those fields the weeds – mostly waterhemp or horseweed – produced viable seeds that were deposited back into the soil. 

Waterhemp produces about 300,000 to 500,000 seeds per plant. I estimate the average density in those fields was about 2 to 3 plants per square foot.

So, you can do the math. 

The bottom line is, the number of weed seeds in the soil waiting to germinate this spring may be unlike anything we've ever experienced before.

3. Continue to watch for Palmer amaranth. 
Waterhemp (Amaranthus rudis) is still the most common and troublesome weed in corn and soybean production throughout most of Missouri. I'm not sure if that will ever change. 

Palmer amaranth (Amaranthus palmeri) is currently the top weed to watch in most of Missouri and throughout most of the U.S. I say most of Missouri, since it has infested the bootheel for decades. It has not been present in the rest of the state until recently. 

Palmer amaranth is a much more aggressive and competitive pigweed than waterhemp. Over the past four to five years, we have watched this weed move northward into areas of Missouri where it did not previously occur. 

Palmer amaranth seed can be transported in used equipment. It also has been moved in feed, seed, or hay coming out of the southern U.S. A recent MU study has also shown that waterfowl can transport Palmer amaranth seed. 

One way to differentiate Palmer amaranth from waterhemp is by the presence of the leaf petioles that are usually as long as or longer than the leaf blades themselves. Palmer amaranth leaves are also more diamond-shape in outline, and often have a poinsettia-like leaf arrangement when viewed from above.

4. Herbicide resistance in waterhemp is here to stay.

I wish it weren't the case, but so far I haven't seen any evidence to the contrary. 

The fact is, so far we've never seen herbicide-resistant waterhemp disappear from a given population or geography. On the contrary, we've only seen herbicide resistance permeate throughout more waterhemp populations over a wider geography. 

A case in point is with the group 2 ALS herbicide resistance that started to appear in the late '80s/early '90s in waterhemp. This was a result of the continuous application of herbicides like Scepter and Pursuit at that time. 

Fast forward a couple of decades, to where we now can't find any Missouri waterhemp populations that don't resist group 2 ALS herbicides. None provide any appreciable waterhemp control. Glyphosate is another obvious example. We discovered the first glyphosate-resistant waterhemp population in Missouri in 2004. 

At that time, no other official glyphosate-resistant waterhemp populations had been identified in the U.S. By 2009, we conducted a survey of soybean fields at harvest and found that 69% of the waterhemp populations resisted glyphosate. 

By then, seven other states had also found glyphosate-resistant waterhemp. Five years later, we conducted another survey and found a similar or higher percentage of glyphosate-resistant waterhemp populations. By this time, most of the waterhemp populations exhibited resistance to two or three other classes of herbicides. Some even exhibit four- and five-way resistance. At this same time in 2014, 13 states besides Missouri now had glyphosate-resistant waterhemp. Most of these states were also seeing multiple resistance outbreaks. 

My fear is that we don't appreciate:

  • The significance of waterhemp resistance.
  • How quickly it can spread throughout a wide geography.
  • How quickly we can lose an entire herbicide site of action that once provided effective control of this weed. 

Consider the timelines I have described above when you decide on your 2016 waterhemp management program. We have very few effective herbicide sites of action left for waterhemp, so we have to preserve those herbicides sites of action that still work by using them appropriately.

5. We must preserve the new herbicide-resistant trait technologies, and use them wisely.

Most are aware by now that there will be Roundup Ready 2 Xtend soybean varieties commercially available during the 2016 season. The Xtend trait confers resistance to dicamba and glyphosate.

At this time, though, no label for over-the-top applications of any dicamba product to these varieties exists. Enlist soybean varieties (that include tolerance to 2,4-D and others) have not been approved for sale during the 2016 season

Whenever these traits and accompanying herbicides get approval, it is important that we preserve these technologies. We cannot afford to misuse the Xtend or Enlist traits right out of the gate and/or view them as the answer to all our weed-resistance problems. If they are being promoted as the solution to weed resistance, those who are doing so are wrong. 

Both traits offer another tool to help with resistant weeds like waterhemp. But if the following steps are not used, you will not be happy:

  • Use preemergence residual herbicides.
  • Mix effective herbicide sites of action at every application.
  • Make timely applications to small weeds.

Also, let's not forget that 2,4-D- and dicamba-resistant weeds already exist. In fact, we have recently confirmed a 2,4-D resistant waterhemp population in a corn/soybean field in Missouri, as have weed scientists in Illinois. And although there are no known dicamba-resistant pigweeds in the U.S., weed scientists in Arkansas selected for a dicamba-resistant Palmer amaranth in a greenhouse setting using less than labeled rates of dicamba over three generations. Although this was done in a controlled environment, this study proved that abusing the technology will result in weeds that resist dicamba as well. 

If we do get a label for the use of any dicamba product in Xtend soybeans in 2016, we must use these herbicides appropriately. This means using full-labeled rates on weeds that are less than 4 inches tall at the time of application, and preferably mixing more than one effective herbicide site of action at each application. It also means being aware of the risks of off-target dicamba movement.

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