The planting window this spring is wide open. Weather in the last few days of March and the first two weeks of April provided an opportunity to get more than half of the area corn and soybean fields planted.

Since then, scattered rain and below normal temperatures have slowed planting to a crawl. The planted crop is off to a good start but cool weather has prevented the rapid growth we expect in May.

With that said, warm days are coming and will provide conditions set for the fast growth of corn and soybeans and the weeds that accompany them. Some fields have been sprayed for as long as six weeks.

This is about the length of time we expect soil residual herbicide products to prevent weed emergence. Therefore it is time to begin prepping for post-emergence herbicide applications in corn and soybean fields.

There are three keys to successful season-long weed control results from the post-emergence herbicide application. First, it is important to know the crop stages required for the herbicide products you intend to use.

Herbicides used in corn range from maximum crop size of four leaves up to 48-inches-tall corn.

For soybeans, it can be tricky. Some soil residual products allow up to two days after planting but before emergence while others are only allowed between the first and third leaf trifoliate. That can be a narrow window if crops are growing fast.

Timing the herbicide application according to the product to prevent injury is critical. Second, it is important to recognize the weeds present to ensure the product will kill everything that has emerged.

Be aware of the maximum weed size allowed by the label. Some of the products are capable of killing weeds larger than described by the label but larger weeds provide an opportunity for a failed kill and increased susceptibility of resistance.

There really is no reason to wait to spray because it just allows the weeds to grow larger. The third and most important key is using a soil residual product in both corn and soybean fields to prevent later weeds from emerging.

A weed that never emerges is one that doesn’t have to be killed later. If you think the field will be ready to spray in a week, it is probably ready that day as long as the crop is within the required growth stage.

In soybeans, good control with a residual might be the difference between one and two post-emergence herbicide applications. This year, the soybean population is less than we anticipated due to cool damp weather and the fields are not growing very quickly. Do not count on crop canopy shade being a late-season weed emergence prevention tool.

For corn or soybean weed control, the days of adequate weed control costing less than $50 per acre are gone. Most soybeans are either Roundup Ready Xtend which are resistant to glyphosate and dicamba or Enlist which are resistant to glyphosate, glufosinate, and 2,4-D.

All of these programs work better on smaller weeds and soil residual herbicide should always be included. Rainfall following application is important. Up to an inch of rain is required to completely activate the soil residual product in heavy residue conditions. Efficacy is reduced after 7 to 10 days have passed without rain. This is true for corn and soybean soil residual products.

Another consideration is to avoid using the same mode of action in the burndown program and post-emergence application. A benefit of Xtend beans or Enlist beans is that there is no wait to plant soybeans with those traits after application.

If you use the product with the burndown, avoid using it post-emergence if at all possible. Dicamba and 2,4-D are the standards to include for soybean and corn burndown programs.

If they are used consistently for burndown and post-emergence weed control there is a real risk of quickly building resistance. If that happens glufosinate would be the only active ingredient with the ability to kill marestail, palmer, and waterhemp.

Likewise, if you know you intend to apply glufosinate post-emergence, leave it out of the burndown program. For more information regarding herbicide programs of corn or soybeans, contact the Daviess County Extension Office at 270-685-8480.

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