Rock and Roll Agronomy

Roundup-Ready Beans and Roundup-Ready Corn:  Are You Managing Weeds Appropriately With These Crops?

By Jason Hanson
Certified Crop Advisor
jlhanson@agriliance.com

It’s estimated that about 75% of soybeans grown in Minnesota have the Roundup-Ready trait, and North Dakota numbers are probably even higher. Corn acres have also started to catch on to the glyphosate trait, but at a slower pace.  It’s estimated that about 8% of corn acres in Minnesota are RR. Although I don’t have the exact numbers, North Dakota is much higher.

The University of Minnesota has reported that RR technology offers better “value” in soybeans than in corn. A key reason is the agronomic differences between the two crops, and how each competes with weeds.

Both corn and soybeans are warm season crops, corn being a monocot (plant with one cotyledon contained in the seed, characterized as a grass with narrow leaves) and soybeans a dicot (plant containing two cotyledons within the seed, characterized as broadleaf plants.)

Corn and soybeans both yield well when environmental conditions, fertility levels, and stand establishment are good. Yields for both are better when weed control is good versus poor or untimely. But they differ when it comes to weed pressure, weed species and application timing. The timing with weed control in corn is more critical early than with soybeans.

Soybeans and the RR trait are, in my opinion, a very good agronomic and economical decision. I remember the days of putting Treflan and Sencor down pre-plant, and then hoping to get a good shot in of Blazer or Basagran. Maybe some Poast or Fusilade too. This was quite a few trips and hours in the sprayer. Then Pursuit hit the market, and the use of pre-plant chemicals fell. But Pursuit had some holes in it after a couple of years, and so then we had to spike in some Pinnacle or Cobra along with surfactants and fertilizers. So now the timing became very critical, and you hoped the weather was favorable. Some people went back to putting pre-plants down in front again.

Then Roundup Ready came along, and changed many aspects of timing and rates. The increased acres drove the market value of glyphosate down, and more applications became more common. Soybeans can take weeds early longer than corn, and for a longer period of time. So now soybeans can be seeded without a pre-plant, sprayed multiple times and have a great tolerance to the herbicide.

As well, the ability of soybeans not to get set back five to seven days from a conventional herbicide can be a huge difference in the harvestability of that crop, as 2002 year proved. Soybeans are also better at shading out some of those later emerging weeds than corn.

Corn is somewhat different in its ability to handle weeds at different stages, and the time to take out those weeds. Corn cannot compete with weeds for as long as soybeans or as early. The critical time period for weed control is from three to eight weeks after planting. If weeds emerge with the corn, these weeds must be controlled within two to five weeks after they emerge. Soybeans can take longer weed pressure before they start to lose yield. They can handle weeds from four to six weeks after planting, giving them an extra week or two for application.

But RR corn does fit well both from a convenience standpoint and weed control. Grassy weed pressure is something that corn cannot handle very well.  I have had good luck spraying Accent and Steadfast on conventional corn, as timing is very critical for these products, as is weed species. But the only time I have had trouble is when Mother Nature cools the temperatures down on me. Then I have some issues with herbicide uptake and weed size if the wind has kept me out of the field for awhile. If the field needs a broadleaf herbicide product and the tank-mix is restrictive on the corn height, I could be looking at an injury issue.

RR corn has made it easier to move more acres into North Dakota, and also to plant more acres of corn on the farm without having to double incorporate herbicide and burn up diesel. Pre-plant herbicide will come back into more use with the acres of corn increasing, because of the need to keep those acres cleaner early, and to help avoid spraying so much in such a short period of time. There are many herbicide options with corn, such as Liberty Link or Clearfield and the conventional programs that can be used to keep weeds in check.

Many growers have settled into a glyphosate-based weed control program and who can blame them, since it makes farming easier and more economical.  But remember that we use glyphosate on many acres of canola, and also pre-harvest wheat and durum acres too.

Even though this herbicide technology is simple and easy, it needs to be managed appropriately. Farmers and/or their agronomists need to keep track of the amount and rate of glyphosate that’s going on their acres to prevent weed shifts and/or resistance in weeds.  We need to rotate our herbicide chemistries, and not use more than two applications of glyphosate in any given field over a two-year period.

You may have read that incidences of glyphosate-resistant marestail have been reported in Delaware, Maryland, New Jersey, Tennessee and Kentucky. The environmental conditions in those states may have been a factor, but so has the use of glyphosate-tolerant crops, mainly soybeans.

We do not have glyphosate-resistant weeds yet in the Northern Plains, and let’s keep it that way.  We don’t need weeds like marestail, wild buckwheat or common mallow taking over in the region, to the point where glyphosate is no longer a viable weed control weapon. Remember to manage your soybean and corn acres differently when it comes to weed control timing, and manage glyphosate strategically and appropriately so that it long remains an effective weed control tool on your farm.

For more good information on preventing weed resistance, check out the web site www.weedresistance.com

Hanson is a certified crop advisor, near Devils Lake, N.D.