Controlling Weeds in Your Fields

April 13, 2017

Controlling Weeds in Your Fields

            Recently I ran into an article that addressed a topic that is a concern to many farmers going into this growing season... resistant weeds and the cost of controlling them. This information was in a blog from Bob Hartzler, extension weed specialist at Iowa State University. He had done some interesting research looking at the annual cost per acre of soybeans herbicide treatments starting in 2000.

            His work showed that average costs peaked in 2002 at about $30 per acre, then dropped to under  $20 per acre by 2004 as the price of Roundup and generic forms of glyphosate kept prices low. The average cost of herbicide programs for soybeans stayed in that $15 to 20 per acre range for the next 10 years as “glyphosate only” programs were commonplace.

            As more and more resistant weed problems developed, the cost of soybean herbicide programs increased dramatically, more than doubling between 2014 and 2017. His work estimated soybean herbicide programs in 2017 will average around $40 per acre as farmers look at other, more expensive alternatives to control herbicide resistant weeds. This isn’t just glyphosate resistant weeds, but weeds that may be resistant to several chemistries or families of herbicides.

            The reason this is important is, some fields will be planted to dicamba tolerant soybean varieties in 2017 and, if the past is any indication, the number of acres planted to varieties with any new source of herbicide tolerance will increase in following years. This cycle of relying heavily on any source of tolerance has led to natural selection of weeds that can overcome that family of herbicides.

            To keep this new trait viable as long as possible, it will be important for soybean growers to not rely just on dicamba to control their resistant weeds. They need to alternate dicamba with or use dicamba in combination with other herbicides with different modes of action to control whatever resistant weed problems exist in their field.

            Using glyphosate with dicamba is only using one mode of action, not two, if the weeds in a field are already resistant to glyphosate. I’m not picking on glyphosate, the same would hold true if using a Group 2 herbicide with dicamba against ALS Inhibitor resistant weeds or a Group 14 herbicide against PPO Inhibitor resistant weeds. The problem we run into is populations of weeds that are resistant to several families of herbicides.

            There are several other considerations when managing resistant weeds. First is you need to keep on top of any weeds that are emerging in your fields. Treat them when they are small, they are much easier to control. If you had a weed problem in your field last year, scout those areas and control weeds as soon as they appear.

            I’ve talked mostly about chemical control, but there are several other good practices to keep weed problems from developing. Tillage is very effective against small weeds. This doesn’t mean you have to cultivate your whole field, but use it where problem weeds are getting established.

            Right along with cultivation, hand pulling weed escapes, whether they escaped from herbicides or tillage, can prevent them from going to seed and your problem from multiplying in future years. We’ve gone away from “walking beans” in recent years. It may be necessary to go back to this to get on top of resistant weeds.

            There are other options such as changing your rotation, using cover crops, or narrowing your row spacing so crops canopy over quicker that will all help in the battle against resistant weeds. The worst thing you can do is try the same thing again this year that did not work last year and expect different results. Go on the offensive to get the upper hand on resistant weeds and protect new crop traits.

            For more information on controlling weeds in your fields, contact your local Nebraska Extension office.