Wednesday, March 30, 2011
Glyphosate Resistant Volunteer Corn: How Do You Control It?
What a headache! You planted all of that Roundup Ready corn last year for better weed control. Now, all of those kernels that popped out when it was harvested are going to be germinating in a soybean field. And they will be immune to the glyphosate that you are going to be using on your soybeans. How in the sam hill do you kill volunteer corn that is genetically designed to withstand the herbicide that you need to use on your soybeans?
With just 16 corn plants in a square meter (10+ square feet) of soybeans, your soybean yield will drop by 40%. That is the calculation of Purdue weed specialist Bill Johnson who says volunteer corn has emerged as one of the top five unwelcome plants Cornbelt bean fields, behind lambsquarters, waterhemp, ragweed and horseweed, the last three of which are becoming glyphosate resistant. For the past 60 years, volunteer corn has not been the problem in soybeans as it is today.
Johnson and the Plant Management Network offer an on-demand webinar for farmers wanting help with controlling their glyphosate resistant volunteer corn. He says that if other Cornbelt farmers are like those in Indiana, help is certainly needed. He and his colleagues found that from 2003 to 2005, the percent of fields with volunteer corn rose from 3% to 12%. During that same time US adoption of Roundup Ready corn grew from 11% of acreage to 18% of acreage. Where volunteer corn was present, in one quarter of the fields, it was the only weed in the field. It was also twice as likely to found in tilled fields, compared to no-till, which would have reduced its vigor for germination.
An issue that may be even more serious than just having volunteer corn that is difficult to control is the fact that most of the volunteer corn also has a Bt gene, designed to control corn rootworms. However, that means that some of the corn rootworms in that field are being exposed to the toxin every year, which increases the potential for some of them to develop immunity to the Bt gene. That would shorten the usefulness of Bt genetics in controlling corn rootworm. One of the reasons is that the amount of Bt toxin being expressed by the corn is a low dose, and is not having a significant effect on controlling the rootworm adults emerging from the soil.
Johnson’s research on the competitiveness between beans and corn found that corn was the most competitive with beans when they emerged at the same time. When corn emerged as the soybeans were at the V3 stage in their growth cycle, the corn was not able to overtake them and have any impact on yield. In one year, Johnson said it took 4 corn plants per square meter to have a 20% impact on yield, but in a subsequent year it only two 2 corn plants per square meter to suppress bean yields by 20%.
Johnson said farmers should consider volunteer corn density before spraying. If the density of the corn is not sufficient to significantly reduce bean yields, then the cost of the spray may not be economical. He says an early post emergent spray on volunteer corn may help reduce the impact on the soybeans as they are competing for light and nutrients. He suggested May as being the time for the best control.
There are several control options according to Johnson, which amount to tank mixing a post emergent herbicide to kill the corn with glyphosate to kill everything but the soybeans. His research used Select, Fusilade, and Assure, both individually and in combination with Pursuit which was added to provide a residual control. All provided 90% or more control of the volunteer corn up to 12 inches tall, as did an individual spray of Raptor. Raptor did not do well with 24 inch corn, and neither did Pursuit by itself. And Johnson said there are other herbicide options in addition to those.
Summary:
Volunteer corn can result in yield loss when densities exceed 2-4 plants per square meter. It should be controlled before it reaches 12 inches in height. There are a number of control options for tank mixes with glyphosate, and lower rates can be used. Reducing the amount of volunteer corn in soybeans will not only preserve soybean yields, but will also increase the chance for Bt corn to continue working as a rootworm control agent.
Posted by Stu Ellis on 03/30 at 12:00 AM | Permalink
Comments
Posted by: sergio venanzi at March 31, 2011 12:12PM
excelent article!!!