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  • Writer's pictureRachel

Top 6 Spray Considerations for Post Applications in Corn

Updated: May 18, 2020

As we are beginning to see many operations close in on the end of planting and start looking to the next operation, we wanted to discuss some important topics in regards to spraying. As we go about getting ready for herbicide applications, three things are foremost on our minds, what weeds are present, how big are they, and how big is the crop. These three things drive our decision-making process for all post-emergence applications. The following six points are topics agronomist Jeremy Olson emphasizes with many growers.

1. Use recommended rates of adjuvants—stay on label! What is the role of adjuvants in our spray applications? We generally think of adjuvants as increasing the wettability on a leaf and helping the mixture adhere to the leaf surface. Adjuvants also serve as penetrants, solvents, and stabilizing agents. An article from Purdue points out that all surfactants and wetting agents are adjuvants, but not all adjuvants are surfactants or wetting agents. So, what is an appropriate definition of adjuvant? They propose “adjuvants are materials that facilitate the activity of herbicides of that facilitate or modify characteristics of herbicide formulations of spray solutions.” There are three main groupings for adjuvants: surfactants, crop oil concentrates, and ammonium fertilizers.

Because adjuvants have a variety of roles, it is important to use the recommended rate and stay on label. Additionally, some herbicides may have specific relationships with various types of adjuvants and specify which work best with their specific formulation. Be sure to follow these recommendations as adjuvants off label may interact with the herbicide formulation, allow too much uptake by leaves, or cause volatility in spray mix. Using the correct adjuvant will increase the performance of the herbicide. An incorrectly used adjuvant will reduce performance of herbicide and potentially cause damage to your crop.

2. Watch the use of oils. While crop oils can do an excellent job of improving the surface area coverage of plants, they can also pose more of a risk for too much absorption of herbicides by plants in conjunction with certain herbicides. In fact, use of crop oils is prohibited with certain products like Resicore. Be sure to check labels to insure you don’t inadvertently damage your crop.

Even on label, some visible damage may occur. If applied correctly, this is often cosmetic, and will not cause permanent damage to the crop. These symptoms are often visible as chlorotic spots and necrosis on the leaves.

3. Adjust your rates for the size of corn you are applying. Knowing the stage of crop is critical before applying post emerge herbicide applications. If applied outside of this range, extensive damage can be done to the crop. Additionally, rates of many herbicides change in accordance with plant size. Here are a few general guidelines for height restrictions by product:

  • Products containing atrazine should be applied before the plant is 12 inches tall. This includes Basis Gold, Liberty ATZ, and Buctril+atrazine

  • Maximum corn size when applying dicamba is 24 inches when nearby soybeans are over 10 inches tall or blooming. If soybeans are shorter, then corn size can be up to 36 inches. Remember, there are a wide variety of restrictions for Dicamba based products including timing restrictions mandated by states, as well as temperature and wind restrictions as well.

  • Roundup shouldn’t be applied past V8 or 30” or 48” if using drop nozzels.

  • Products like status shouldn’t be applied before the corn is 4” tall and not after 36” tall.

We also need to consider rate changes as plants grow. For instance, with Resicore, we can apply 2.25 quarts when corn is spiking through V2. From V2 through 11 inches, we should change that rate to 1.25 quarts. Always keep in mind both the size of corn and the size of weeds when making rate adjustments.

4. Be on the lookout for rec changers. What do we mean by this? There are certain weeds that will cause us make some changes to our standard herbicide plan. If we have any levels of field pansy, ragweed, lambsquarters, or marestail, we might be looking at a formulation or rate change. It is critical to get an accurate look at what weeds are present in the field before pulling the trigger to ensure we are adequately geared up to kill those specific species.

5. Use caution when double stacking growth regulators. In fact, some herbicides specifically prohibit it. For example, stacking Resicore with Status will often put too much stress on a stand of corn. Be sure to check your mode of action and read labels to ensure you aren’t using a mix that will do more damage than good.

6. Timing Matters. The environmental conditions you spray during can impact how the plant processes the herbicides. If there is several days of cool weather after application, corn plants may not be able to metabolize the herbicide quickly enough to avoid injury. The ideal temperature for spray applications is between 65 and 85 degrees.

Our agronomists are great resources for your questions on appropriate adjuvants, timing and rate adjustments, and killing those challenging weeds. Give them a call with your questions so you can get your crop off to a weed free start.


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