The New Soybean Weed

When was the last time you ever heard anyone say corn is a weed?  Have you driven past a field of soybeans recently and seen random bunches of corn growing in a soybean field?  If you have, then you are witness to what some are calling the “new soybean weed.”

By definition,  a weed is a plant species growing where it is not wanted.  Therefore if corn is growing in a soybean field, it is a weed.  Although many people have seen this recent phenomena, few understand why they have not seen it before.  We’re going to solve that mystery right now.

First, if corn is grown on a field and some ears escape the combine, hungry migrating geese, foraging turkeys and deer, and the next spring’s tillage activity, they just may begin to sprout and grow.  In the industry, these are deemed “volunteer” plants.  They are not uncommon given the fact that every acre of corn is planted to 30,000 seeds more or less, and some ears will be knocked off by animals, weather or harvesting equipment and end up on the ground before or during harvest.

If soybeans are rotated onto the field that grew corn the previous year, then volunteer corn is likely to emerge.  This is nothing new, so why are we actually seeing this volunteer corn growing so big and so late in the season this year?  The answer has to do with new genetics and weed control strategies.

In previous columns we have described the widespread adoption of glyphosate resistant (eg. Roundup Ready) corn and soybean varieties by farmers.  These varieties contain a gene that makes them immune to this broad spectrum herbicide.  Herbicides can be broad spectrum or  selective. As the description implies, broad spectrum herbicides pretty much kill everything while selective herbicides target specific types of plants such as grasses or plants that have broad leaves, like dandelions.

Weed control becomes a snap with these glyphosate-resistant varieties because a single spray kills all plants except the ones that have this gene.  Previous weed control strategies included mixing two or more herbicides, each with a specific target in mind such as grasses or broad leaf weeds.

The answer to the  mystery of volunteer corn growing in soybean fields should be apparent now.  The “new soybean weed” is a glyphosate resistant volunteer corn growing in a field of glyphosate resistant soybeans.   The question now is, will this new soybean weed have enough economic impact to upset the world’s supply of soybean meal or soybean oil?  Don’t lose any sleep over it.  A Wisconsin study has shown that is takes at least 75 clumps of volunteer corn per acre to reduce soybean yield by one percent and none of what I have seen comes close to this level of weed pressure.

However, not everyone sleeps well with volunteer corn growing in soybean fields. Believe it or not, farmers take a lot of pride in how their crops look.  Although we generally think farmers are too busy working on the farm to see what other farmer’s crop look like, you’d be surprised by comments they make about crops growing in fields a long way from home.  To these crop-growing perfectionists, the color of the crop, the straightness of the rows, and the absence of weeds are every bit as important as yield.