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January 2013 Archive for Ask an Agronomist

RSS By: Farm Journal Agronomists, Farm Journal

Have your agronomic questions answered by a Farm Journal agronomist. E-mail us directly at, and we’ll respond on this blog to provide an interactive dialogue.

How Do I Control Volunteer Corn?

Jan 31, 2013

Question: I’m having problems with volunteer corn in my corn crop. How do I control it?

Answer: I’m not sure of your specific agronomic practices, seed traits etc., so I’m going to offer a couple of thoughts for your consideration. If the previous crop you grew was conventional corn, you have more options. The obvious one is to rotate to soybeans this year. Another option (depending on what seed you purchased) is to plant Roundup-tolerant corn and use Roundup herbicide in a postemergence application. If you grew glyphosate-resistant corn last year, plant LibertyLink corn and apply a postemergence application of Liberty. It gets tricky, though, when you grow continuous corn with hybrids that contain both Roundup Ready and LibertyLink traits. Those traits stacked together can be good in a corn-soybean rotation, but in a corn-on-corn rotation you really need to separate those traits, so you can use one or the other to clean up weeds. Another option that I acknowledge is probably next to impossible for someone in the northern corn-growing areas is to plant corn later than normal after controlling the volunteer corn. Basically, you want to get out and get your seedbed ready early. Level up that ground and then wait for the right soil temperature. At 50°F corn germinates, and at 55°F it gets really active. You want to let that volunteer corn sprout up to about two collars and then spray Fusilade or Poast or work the ground again with secondary tillage, such as with a field cultivator. One caution, be aware that today’s vertical-till harrows are not very good at removing volunteer corn and can make it worse. It can shatter those corn ears and cause reseeding. I don’t know what your harvest was like last year, but if you had a 20-bu. or 30-bu. harvest loss when you went through the field last fall, you potentially will have a mess this spring. Even if 90% of it was killed by frost last fall, that other 10% will have to be managed. Get on top of it as soon as you can.

Will Last Year's Drought Impact Soybean Production in 2013?

Jan 23, 2013

Question: Is there anything you’d advise me to consider before planting soybeans after last year’s drought?

Answer: Extreme hot, dry conditions like last year can lead to reductions in the soil of bradyrhizobium populations. This microorganism is essential for nitrogen fixation in soybeans. I doubt this will be an issue in most fields that have had soybeans for many years. However, it may be an issue on fields that have had only one or two years of soybeans or have other issues, such as low pH. That said, if you’re planting soybeans after failed corn, there may be quite a bit of residual nitrogen still in the soil profile. That is not a problem; the soybeans will use it. However, excess nitrogen may inhibit nodulation early in the season, thereby potentially reducing soybean production later in the season. If you think that is the case in your fields, non-legumes may be a more efficient option for you. Another issue some farmers saw last season was increased soybean shatter due to the drought and quick drydown. If the drought persists, a bit later planting can help mitigate shattering, all other things being equal. With later planting, soybeans should mature a bit later in the fall and face conditions that are less likely to lead to shattering.

Thanks to Kraig Roozeboom, Kansas State University Extension soybean specialist, for supplying this answer.

How Concerned Should I Be About Carryover?

Jan 07, 2013

Question: How concerned should I be about herbicide carryover if I’m planning to rotate from soybeans to corn this spring?

Answer: It’s certainly a concern in those areas where we had dry, hot conditions last year and herbicides weren’t activated and where there’s been little moisture. One of the best ideas we’ve heard for checking on the potential for carryover came to us recently from Kevin Bradley, University of Missouri Extension weed scientist. He offers an easy and inexpensive idea you can use to tell if carryover herbicides will be a problem this coming spring.

Roughly a month prior to planting corn, he recommends that you pull half a dozen or so soil samples from across a field and then place a handful of dirt from each sample in a cup or jar. Plant some corn seed in each cup and watch the corn sprout. In about a week’s time you will know whether any significant herbicide residue is left from the previous year in that soil.

Bradley says if the corn shows signs of injury you’ll know it before you plant and you can keep from making a crop-rotation mistake. He adds that while many agronomic laboratory tests provide an accurate analysis on soils containing carryover herbicides, it’s difficult to know what the test results mean in practical terms.

Notes Bradley: "It’s hard to know whether 15 parts per billion of something in the soil will hurt your corn. A simpler thing is to just do a soil bioassay, and it’s the cheapest thing to do." Read on to learn more about how you might want to address carryover in 2013.

What's the Potential for Herbicide Carryover?
Recent dry soil conditions could make herbicide a bigger threat next year.

Herbicide Carryover Concerns for 2013 Corn and Soybeans Prompted by Drought
Iowa State Univesity agronomists look at the potential impact on corn/soybean rotation. No water, no herbicide breakdown.

What’s the Best Way to Calibrate My Yield Monitor?

Jan 02, 2013

Question: What is the best way to calibrate a yield monitor, and why is it that a poorly calibrated monitor can still match scale tickets?

Answer: A poorly calibrated yield monitor can still match scale tickets, but it doesn’t mean it always will. And just because your yield monitor matches scale tickets for the field, doesn’t necessarily mean the monitor is calibrated correctly. A poorly calibrated monitor can under-estimate high yields just as much as it over-estimates low yields, which means you could be pretty accurate on a whole field basis.

Yield monitors generate a source of geo-referenced yield data that can enable growers to document the extent of spatial yield variability within fields. That’s what we’re after—spatial variability. A well calibrated yield monitor will show more definition in our field maps, making it easier to see the management zones and the variations within those fields.

We have even seen accuracy decrease slightly with a good calibration, but the maps will show better spatial variability. This can happen because now instead of over-estimating by 2% and then under estimating by 2% creating a wash, we over-estimate by 2% all the time. This will create a map that better shows variation and at the end of the year can be accurately scaled down 2% to show real yields.

As for actually calibrating the monitor correctly, you will need to calibrate at various crop flows. Pick the most consistent yield spot in your field and run multiple loads at differing speeds. Doing so will ensure that you are calibrated for differing flows (i.e. differing yields). If your monitor will allow, run four to five different calibration loads. If your monitor will only allow for two calibration loads, like a John Deere monitor, for instance, make sure you are running both a normal flow and a low flow for increased precision and spatial variability.

All calibration loads should be between 5,000 lb. and 8,000 lb. and should not be unloaded on the go. You should also avoid passing waterways or any inconsistent yielding areas in the field. Also, check the accuracy of the yield monitor calibration throughout the season by harvesting and weighing loads and recalibrating as necessary.

For more specific calibration steps, consider the information provided here: Calibrating a yield monitor by Bob Nielsen at Purdue University.

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