Don’t Let a Compressed Spring Force Bad Weed Control Decisions

By Travis Legleiter, Extension Weed Specialist & JD Green, Extension Weed Specialist

When a spring season is compressed, it puts pressure on things around it.  This is true if you are talking about a spring in the physical form or the season of spring in the agricultural world.  The spring of 2018 in Kentucky has not been friendly for planting corn and soybeans, and thus, we are looking at strenuous situations of getting multiple weeks of work into only a few.

As we approach the time in which we will be switching from corn planting to soybeans, this compression still holds true as we strive to get soybean into the ground.  Under these strenuous conditions there is a tendency to cut out things or skip steps, especially when it comes to weed control and use of pre-emergence herbicides.  The temptation has only been sweetened with the introduction of dicamba-tolerant soybeans to compliment Liberty Link soybeans, providing two systems in which we have more viable post-emergence options for fields that are infested with herbicide-resistant weeds.

We would warn you, though, that even with the strains of a compressed spring and the temptations of dicamba and glufosinate [e.g. Liberty], we must stick with pre-emergence herbicides, especially if fields have any of the pigweed species or marestail.

Benefits of Pre-emergence Herbicides

Pre-emergence herbicides provide numerous benefits for weed control in soybean:

• By applying a pre-emergence soil-residual herbicide, you are allowing your soybean crop to emerge without competition and grow for several weeks without weed completion.
• Pre-emergence herbicides reduce the number of weeds or density of plants you must control when it does come time to make a post-emergence application.  This leads to two advantages: coverage with those applications is easier to achieve to assure the success of those applications and selection pressure on post-emergence herbicides is effectively reduced by applying to fewer plants.
• The use of pre-emergence herbicides in soybean allows a producer to introduce additional sites of action into the rotation, such as Groups 15, 5, and 3. It is vitally important that we use as many sites of action as possible on acres that are infested with herbicide-resistant Palmer amaranth, waterhemp, and marestail.

Recommendations for Fields with Palmer Amaranth and Waterhemp

Specific recommendations for pre-emergence herbicides in fields with Palmer amaranth and waterhemp are as follows:

• Use products that contain at least two of the following effective sites of action: Group 14 (flumioxazin, sulfentrazone, and fomesafen), Group 15 (S-metolachlor, pyroxasulfone, dimethenamid-P, or acetochlor), or Group 5 (metribuzin).
• Apply full rates of your chosen pre-emergence product for your soil type; this assures you are getting the maximum utility out of your pre-emerge application.

Recommendations for Fields with Marestail

Specific recommendations for pre-emergence herbicides in fields with infestations of marestail:

• Products that include the active ingredient metribuzin can provide good-to-excellent suppression of spring and summer emerging marestail, depending of the rate of metribuzin in the product.
• Products containing the active ingredients flumioxazin (Valor) or sulfentrazone (Authority) can provide good suppression of marestail.
• Herbicides that contain saflufenacil (Sharpen) can be effective on emerged marestail (less than 2” tall) and provide some pre-emergence control.
• The active ingredients cloransulam (Firstrate) and chlorimuron (Classic) can provide good suppression of marestail populations that are ALS-sensitive.
• There are numerous products that contain mixtures of the above active ingredients that we would highly recommend. As always the more sites of action we can get involved the better.

Herbicide Information

An extensive list of the herbicides available for use pre-emergence in soybean, product rates, efficacy tables, and label highlights of each product can be found in the 2018 Weed Control Recommendations for Kentucky Grain Crops (AGR-6).