To Control Volunteer Corn In Soybeans
On July 17, 2020
To Control Volunteer Corn In Soybeans
As the soybean crops continue to mature, volunteer corn is rearing its head in some fields. Extension Agronomist Dwight Ligenfelter explains data from a few Midwestern universities shows that populations of volunteer corn can reduce soybean yields. University of Nebraska found that a volunteer corn density of 3500 plants/acre led to 10% yield reduction in soybean. Doubling the density to 7000 plants/acre led to a 27% yield reduction. South Dakota State University results were similar – a volunteer corn density of 5000 plants/acre resulted in a 20% yield reduction (12 bu/acre yield loss in 60 bu/ac soybean). In addition, it was noted that clumps of volunteer corn tends to cause more yield reductions in those areas as compared to scattered individual plants. Furthermore, volunteer corn attracts corn rootworms to that field which can further perpetuate insect problems if corn is rotated to that field next year. Regarding management, we assume that most of the volunteer corn is glyphosate- and glufosinate-resistant (Roundup Ready/LibertyLink) so we will not spend time going over all the options for killing the various types of volunteer corn. In general, the most logical choices to control volunteer corn in any kind of soybean are the post-grass herbicides (e.g., Assure II, Select, clethodim, Fusilade, etc.). Here are some rates to consider depending on the height of the volunteer corn. If using Select Max 0.97EC with corn less than 12 inches use 6 fl oz, for corn 12-24 inches use 9 fl oz. For Clethodim 2EC and corn 4-12 inches tall use 4 fl oz, for corn 12-18 inches, use 6 fl oz. When using Assure II/Targa 0.88EC and the corn is 10-12 inches use 4 fl oz, for corn 12-18 inches use 5 fl oz, for corn 18-30 inches use 8 fl oz. When using Fusilade DX 2EC and the corn is less than 12 inches use 4 fl oz, for corn 12-18 inches use 6 fl oz.
Always include the necessary adjuvants otherwise control will be impacted. In most cases, this will require the addition of a crop oil concentrate (COC) or methylated seed oil (MSO). If crop injury is a concern, a nonionic surfactant (NIS) may be used in place of an oil with Select Max only. Keep in mind if you plan to tank-mix a broadleaf-specific herbicide, the rate of the grass herbicide may need to be increased to overcome possible antagonism
Always include the necessary adjuvants otherwise control will be impacted. In most cases, this will require the addition of a crop oil concentrate (COC) or methylated seed oil (MSO). If crop injury is a concern, a nonionic surfactant (NIS) may be used in place of an oil with Select Max only. Keep in mind if you plan to tank-mix a broadleaf-specific herbicide, the rate of the grass herbicide may need to be increased to overcome possible antagonism.
To Evaluate Nodulation in Soybeans
Soybeans at the V2 stage revealing adequate nodulation. Photo credit: Dwane Miller.
Soybeans are a big user of nitrogen and need approximately 3.5 lbs. of nitrogen per bushel of grain produced. Extension Agronomist Dwane Miller explains an 80-bushel soybean crop will uptake 280 pounds of nitrogen. Fortunately, if we have adequate nodulation, we don’t need to be concerned with nitrogen deficiency; nature will take care of it for us. But we need to assess soybean fields for adequate nodulation.
About 5-6 weeks following planting of soybeans is a great time to scout plants and evaluate nodulation. At this time, nodules should be large enough to be active. When digging up plants in a field, be sure to use a shovel to insure you are carefully removing as many roots as possible. Simply pulling out the plant will likely detach roots and you will not get an accurate nodule count. Then put the plants into a bucket of water to rinse the soil from the roots. At the V2 growth stage (image1), nodules will be actively fixing nitrogen from the air.
Just because nodules are present does not mean they are actively working. Nodules that are active in fixing atmospheric nitrogen will be pink or red in color when cut open (image 2). If nodules are white in color they may be immature and haven’t yet begun to fix nitrogen. In this case, we would recommend visiting the field in another week to conduct another assessment. Nodules that are mushy and brown are not fixing nitrogen. Be sure to note the location of the nodules on the plant. Nodules that form on the tap root are likely a result of inoculation from this season. Nodules on lateral roots are from bacteria that are existing in the soil.
A properly nodulated soybean plant should have five to seven nodules on the tap root two weeks after emergence or twelve total root nodules per inch of tap root at flowering (R1) (R. Elmore 2007). While we evaluate nodulation early in the season, the number and size should continue to increase until the R5 growth stage.
Causes of poor nodulation can include improper soil pH. Soil pH should be between 6.5 and 7.0. If pH is below 6.5, the addition of Molybdenum will aid in infection and nodulation. This is a correction that needs to occur with seed treatment or very early seedlings (V1 stage). Another situation which could cause issues is planting soybeans into “virgin” ground, where soybeans haven’t been grown. Here, there are low bacteria populations in the soil, and we often see inadequate nodulation. Another problem could be inoculant that has heated in storage. Be sure to store inoculant in a cool place, out of direct sunlight. Also be mindful of the expiration date on inoculant. Flooding or very saturated soils lasting 7 days or longer will deprive the plants of oxygen and cause nodulation issues. Also compacted soils that limit available oxygen could contribute to the problem.
What can be done if your field doesn’t produce adequate nodules, or they are inactive? If fields are showing nitrogen deficiency (light green, stunted plants), we would recommend that up to 50 lbs. of a dry nitrogen fertilizer source be broadcast over the field prior to full bloom (R2 growth stage).
Image 2 – Inside of soybean nodule. Notice the pink color which represents active nitrogen fixation. Photo Credit: Dwane Miller
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