The biggest issue most people associate with growing corn is the drought of 2012 that shortened the crop across the country and many parts of the world and raised prices. Hopefully the drought was a one year event but as corn growers heard at the annual Wisconsin Corn Conferences around the state over the past week, there are other issues that are going to have a much longer term effect on production.
      One of those is the spread of glyphosate (commonly known as Roundup) resistant weeds.  Vince Davis, UW Extension Weed Scientist, spoke to the corn growers about the ongoing challenge of resistant weeds and had both good news and bad news.  He told the growers “Wisconsin is actually right now in a pretty good place since we don’t have as many glyphosate resistant weeds as many states surrounding us.”  But he also said we’re not completely free of the problem since, as he told the growers, “We do now have giant ragweed that we have confirmed resistant in the state and that is a very competitive weed in both corn and soybeans that we are very concerned about.”  
       Davis said an area of resistant giant ragweed has been confirmed only in extreme southern Wisconsin in Rock county but researchers are investigating potential resistant cases in other parts of the state.  Currently glyphosate resistant giant ragweed is much more of a problem in states like Minnesota, Michigan, Ohio and Indiana as well as in Ontario, Canada.  He added that states like Iowa, Illinois, Michigan, Ohio and Indiana are also fighting other glyphosate resistant weeds like horseweed (marestail), Waterhemp and Palmer Ameranth.  So far Wisconsin has no reports of any resistant varieties of any of those weed species.
       In Wisconsin, Davis said they are dealing with resistant weeds in a  “two pronged approach”.  The first project, funded by the Wisconsin Corn Growers Association, has been a far-reaching survey across the state looking at all weed species that escaped control during the year in both corn and soybeans.  Last year that meant visiting over 160 fields across the state to gather treatment history as well as plant samples to evaluate them for possible resistance.  The other strategy, Davis explained, has been to go directly into three fields, two of which had confirmed cases of ragweed resistance, and look at different herbicide programs to how they worked to control the giant ragweed.  Davis said those field tests indicate the best control is with a two pass program across the field, whether it’s a corn or soybean stand. He also said that one size or product won’t get the job done. He told the corn growers, “It’s going to take diversification of multiple effective herbicide modes of action.”  Reliance solely on glyphosate is not the answer anymore, he emphasized to the growers.
       Another of the challenges is that because research and development programs cost companies so much money and take so long to yield market ready product, the industry left with reformulations of already available products.  But he did say there are enough available to do the job.  For corn he cited some of the hpp inhibitors like Calisto and Lotus, some of the growth inhibitors like Dicamba at various times during the growing season like when mixed with glyphosate later in the year as well as some older products like Atrazine with those hpp inhibitors whether it be pre or early post emergence of the weeds.  For growers using the Liberty Link system he advises tank mixing with other products like Atrazine or some other compounds, “So you have multiple modes of action for better and more effective control with fewer chances of escape.”
        Davis is also aware that 2012 was an anomaly for growing crops and controlling weeds.  Because of the early warm weather, the weeds got off to a fast start and “They got big early on us.”  Secondly, the dry weather didn’t allow a lot of the pre-emergence herbicides to work they way farmers had hoped and thirdly, the hot, dry conditions that persisted all growing season long made it hard for the post emergence products to do their job.  That also has Davis concerned as we get into the 2013 growing season because a lot of the weed seed banks escaped and increased in size and are going to be worse going into this cropping season.
         It’s not just weeds like giant ragweed Davis and fellow researchers are concerned about this year.  He told growers to also expect to battle more grassy weeds particularly those fields that were in corn production last year because of a lot of weed escapes from treatment.
         Davis hopes growers took good field notes last year that will be very useful for weed control this spring and summer and if they didn’t they should plan to spend extra time checking fields this summer to keep weeds under control and identify all the weeds they see, because as he said, “There may very well be some new weeds out there this year that we’re not familiar with and we may have to be very aggressive in controlling weeds early in the season so they don’t get out of hand.”
        To make matters more challenging for growers this year, Davis also said some not so familiar weeds could very well become a challenge this year.  He is especially on the lookout for some pigweed species like Waterhemp and Powell and Palmer Ameranth which all have a lot of glyphosate resistance in them as well as ALS inhibitor resistance and even in some cases, like in Iowa and Illinois, some hppd inhibitor resistance.  While none of those are major problems yet in Wisconsin, they are big problems in neighboring states and we need to keep them under control.  How they get from one location to another is also always a challenge, Davis said.  He emphasized they will use whatever source is available, from wind blown to hitching a ride on trucks and tractors, bird migration and some have even been shown to come in on cotton feed bi-products to dairy operations.  
           Another warning Davis had for the corn growers was to be on the lookout for carryover as many of the herbicides didn’t activate last year because of the dry weather and there could be some crop injury this year as a result. He said scouting and crop rotation are keys to controlling that potential situation.