By Ralph Anderson

Buffalo County Extension Educator


Continued dry weather has become a real challenge for all producers.

Crop water use predictions based on weather conditions indicate crop water use at over 0.4 inch per day. Many crop consultants believe that as we approach these high rates, the model tends to over-predict crop water use. Even if that is correct, we still are using in excess of one-third inch per day, or two and one-third inches per week. Corn roots probably are at three feet now and soybeans over two, which will help if you had any sub-soil moisture.

Even one-tenth inch moisture will not help very much, but crop water use will be down with the presence of cloud cover and high humidity. But the prospects for relief are not in the immediate forecast and crop water use remains high.

Hopefully, you have had an opportunity to apply your post-emergence herbicides by now. The dilemma that we have is that if we apply post-sprays too early, we miss late germinating weeds. If we wait too long, we may not get good control on big weeds.

Dry weather tends to reduce the performance of post-emergence herbicides, because the waxy cuticle on the leaf surface thickens and plant growth slows. To offset this, spray additives can be added to improve herbicide performance by increasing penetration of the leaf surface. Post-herbicide activity is strongly influenced by spray additives. Most, but not all, post-herbicides require the use of spray additives. There can be a "fine line" between increased weed control and crop injury. Consult the herbicide label for specific information.

The most commonly used spray additives include oils, surfactants and certain fertilizers. Oil concentrates include petroleum and seed-derived oils and usually are composed of at least 17% emulsifier plus oil. Methylation of seed oils (MSO) improves their performance. Oil concentrates generally are used at 1 to 1.25% by spray volume of one to two pints per acre, depending on the herbicide, oil and spray volume.

Surfactants are compounds that reduce the surface tension of liquids. Used with post-herbicides, they cause spray droplets to spread out and "wet" the leaf surface. This results in increased penetration of the leaf surface and increased activity. Nonionic surfactants (NIS), meaning no electrical charge on the molecule, are the most commonly used surfactants. Surfactants usually are used at 1/4% by spray volume or one quart per 100 gallons of spray mixture.

Surfactants and oils generally perform the same function--increasing penetration of the leaf cuticle. Generally, one or the other, but not both, would be used in a spray mixture. Where an oil and a surfactant are suggested with a particular herbicide, the oils tend to increase activity the most.

Ammonium containing fertilizers are effective spray additives with 28-0-0, 32-0-0 solutions and 21-0-0 spray grade ammonium sulfate, the most commonly used formulation. The ammonium in these materials enhances the uptake of certain herbicides by a mechanism not well understood. While oils and surfactants function primarily at the leaf surface, the ammonium ion functions inside the cell wall. Ammonium fertilizers are not surfactants and do not replace the need for surfactants or oils in a spray mixture.

Ammonium containing fertilizers are used at two to four quarts or two to four pounds per acre and can be used with equal effect with post-herbicides, except glyphosate. With glyphosate, only ammonium sulfate (AM) should be used as the additive. AMS is effective in countering the effect of calcium, iron and magnesium in our water, cautions that complex and deactivate glyphosate. The other ammonium containing fertilizers are not effective in counteracting the effect of these cations.

The benefit of using spray additives with post-herbicides is greatest under adverse conditions, when it is hot and dry and weeds are stressed. To give yourself an edge, use the best additive for the herbicide.

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