Every growing season brings unique challenges to our cropping systems—2025 will be no exception. 

According to U.S. Drought Monitor data, abnormally dry conditions persist throughout much of central and northern Illinois, while southern Illinois is experiencing a prolonged wet spring that has delayed planting for most producers in the area. 

Crop production is a complex, carefully planned sequence of field operations—hybrid or variety selection, tillage, planting, spraying etc.—and natural components like rainfall, heat, sunlight, and time. We build our systems approach around these factors in hopes that the season will unfold as we expect it to. But as we all know, Mother Nature often has the final say. When that system is disrupted—such as by delayed planting due to excessive rainfall or by abnormally dry conditions—it can create ripple effects throughout the season and impact future plans.  

Herbicide carryover damage in cover crops (Photo – Abigail Peterson, CCA, IL Soybean Assoc.)

One of the more overlooked consequences of these disruptions is the increased risk of herbicide carryover, especially when herbicide application timing and/or environmental conditions depart from what we are expecting. This should be on the minds of all producers and consultants, even in a “normal” year, but especially when systems are shaken up as they are this spring particularly in the southern portions of the state. 

Herbicide carryover can be particularly problematic for producers intending to establish cover crops later this summer or fall. In southern Illinois, delayed planting will likely push post-emergence weed control applications back much later than originally planned. Meanwhile, in other parts of the state where soil moisture is just barely adequate or abnormally dry, applications may be timely—but without sufficient rainfall throughout the season, the risk of carryover injury to cover crops remains. 

How does herbicide carryover impact fall cover crops? 

Typically, we see: 

• Poor or inconsistent stand establishment 

• Weak fall growth, even under good conditions 

• Increased winterkill of species expected to overwinter well 

These symptoms may suggest that herbicide carryover has occurred, but by then, it’s too late. It’s helpful to understand some of the contributing factors to herbicide carryover, so we can make proactive adjustments and reduce the risk. 

Key Factors That Increase Herbicide Carryover Risk 

• Delayed post-emergence applications due to saturated field conditions shorten the degradation window for residual herbicides before fall cover crop planting. 

• Inadequate moisture during the season limits the degradation and movement of herbicides. 

• Soil properties—such as texture, pH(especially pH>7), and low microbial activity—also play important roles in the rate of degradation. Some of these we can influence; others we cannot. 

My advice for minimizing herbicide carryover risk:

1. Know your herbicides and their potential effects on cover crops.
Adjust your herbicide plan if appropriate, but don’t compromise weed control just for the sake of a future cover crop.
(Remember: Later Applications = Greater Risk of Carryover) 

• Broadleaf cover crops are most affected by products in: 
  • Group 14 (PPO) Ex. Flexstar, Sharpen 
  • Group 27 (HPPD) Ex. Callisto, Impact 
  • Group 5 (PS II) Ex. Atrazine, Metribuzin 
  • Group 2 (ALS) Ex. Classic, FirstRate 

• Grass residual products tend to cause the most injury to grass cover crops. 

2. Understand that herbicide sensitivity varies by cover crop family and species.

• Brassicas (radish, turnip, rapeseed) and legumes (Austrian winter pea, some clovers) are generally more sensitive. 

• Cereal rye tends to be the most tolerant. 

• Soil residual herbicides often affect small-seeded weeds the most and therefore small-seeded cover crops the most. 

• Your crop rotation will affect your herbicide package – species sensitivity will likely vary from corn to soybean fields due to the difference in herbicide chemistries used on those acres.  

3. Accept some uncertainty—it’s not always cut and dry. 

• Research is ongoing to better understand how timing, chemistry, and species interact. 

• Consider conducting your own bioassay to evaluate specific fields. 

• Use reliable resources to guide your decision-making: 
  • Cover Crops for Weed Management: Herbicide Persistence and Carryover to Cover Crops Fact Sheet – Take Action 
  • Herbicide Rotational Restrictions – UW Madison  
  • Soil Residual Herbicides And Establishment Of Cover Crops In The Fall | Purdue University Pest&Crop newsletter 

By keeping herbicide carryover in mind, especially in a season marked by delayed operations, we can better prepare our systems for successful cover crop establishment this fall.