Cover Crops and Residual Herbicides: A Balancing Act
For Midsouth soybean producers who are planning to plant cover crops [CC] this fall, it is important that they know how the different CC species might be affected by the residual herbicide(s) that may have been applied PRE and/or POST to a preceding soybean crop. The United Soybean Board has compiled an article titled Cover Crops for Weed Management: Herbicide Persistance and Carryover to Cover Crops [edited by Essman and Loux of The Ohio State Univ. and Johnson of Purdue Univ.] that provides information pertaining to this subject. Pertinent points from that article follow.
• Because of herbicide-resistance [HR] issues that have arisen in soybean weed control, producers have increasingly relied on residual herbicides that are applied both PRE and POST to mitigate the HR weed problem.
• CC’s are being used by some soybean producers to reduce the density and size of weeds early in the growing season. Early establishment of a desired CC species or species mix is important when CC’s are used for this purpose.
• A residual herbicide that was applied to a soybean crop that is grown before CC planting in late summer/early fall should be considered when selecting a CC species that is to be planted.
• Residual herbicide properties that should be considered are 1) water solubility of the used herbicide, 2) properties that will influence the herbicide’s ability to be degraded by soil microbes and chemical reactions in the soil, and 3) herbicide half-life.
• Soil properties such as organic matter and clay content, CEC, pH, and soil microbial activity will influence herbicide carryover and persistence.
• Warm air temperatures and high rainfall or irrigation can result in an increased rate of herbicide degradation.
• Residual herbicides applied at the time of cash crop planting [PRE] should interfere less with CC establishment than if they are applied POST.
• No-till production systems can increase the amount of a residual herbicide present at shallow soil depths.
• Herbicides are typically more effective on small-seeded weeds, which means that they are more likely to adversely affect establishment of small-seeded CC species.
• Residual herbicides typically used for soybeans vary in their injury potential to a following CC planting. See the information in Table 1 of the above-linked article.
• Producers who plan on using fall-seeded CC’s following a soybean crop that received a residual herbicide application are encouraged to access the information in the above-linked article.
The above points and details in the linked article indicate that CC’s that are used for weed control in soybean production systems should be considered when developing a weed control strategy for that system. Thus, since optimum CC establishment is paramount for their success as a weed suppression tool, it is imperative that residual herbicides should be selected based on 1) their expected control of targeted weed species, and 2) their potential carryover impacts on CC seedings.
Composed by Larry G. Heatherly, Aug. 2025, larryh91746@gmail.com