Seeding Cereals this Fall? Be Aware of Herbicide Carryover Risks

 

Whether planting winter cereals for grain or as a spring or winter cover crop, there is always the potential for herbicides applied to previous crops to carry over and affect the cereal crop stand. The potential for herbicide carryover varies widely based on the product used, when and where it was applied, and what the weather has been like since the time of application.

 

 

Herbicides are degraded primarily by microbes in the soil or through chemical reactions with the soil itself. Soil water content is one of the most important factors affecting both of these processes. Other soil characteristics that also influence degradation include soil pH, organic matter content, cation exchange capacity, and soil texture. This is part of why some herbicide labels indicate rates based on soil characteristics. 

 

Soil temperature and soil water are the most influential factors on herbicide degradation rates. Warm weather with adequate, but not excessive moisture will typically provide the most rapid degradation, while cool and dry conditions will slow herbicide degradation rates. Even with warm soil temperatures, if the soil is water-logged, degradation via microbes can be slowed due to the lack of oxygen. 

 

 

Herbicide carryover is most concerning this year in northwest Minnesota, which has been dry for much of the summer. Despite the warm temperatures this year, the dry weather impacts herbicide degradation by limiting both microbial activity and the chemical reactions with the soil, likely resulting in slowed degradation rates. 

 

Southern Minnesota has been on the opposite end of the spectrum with excessive moisture. This is less concerning than dry weather, but many herbicide applications were delayed this summer, which means the time elapsed since herbicides were applied may not have been long enough to degrade the herbicide. Waterlogged soils can also decrease degradation rates due to a lack of oxygen, which slows microbial degradation. However, waterlogged soils are less concerning than dry soils as the wet soil allows some herbicide degradation via chemical reactions with the soil. Regardless of this year’s weather it is likely that there are areas of fields that will be affected more than others. 

 

In trying to determine what kind of crop injury potential you have this year, start with the herbicide label (searchable label database at www.cdms.net). Be sure that the rotational restrictions will allow you to legally seed a cereal.

 

Rotational intervals for common soybean and corn herbicides can be found in Table 1 and Table 2, respectively. Even when following the label restrictions, it is possible to get herbicide injury. Herbicides that are more prone to causing crop injury includes products containing atrazine, ALS-inhibitors (eg. Firstrate, Pursuit, etc.), and fomesafen (eg. Flexstar, Reflex, etc.). If you are unsure which active ingredients were in the herbicides applied this year, check the label or refer to the herbicide classification chart (https://www.iwilltakeaction.com/uploads/files/57229-6-final-ta-hrm-classificationposter-lr.pdf).

 

 

If you are concerned about whether or not you will have herbicide carryover issues, you can conduct a bioassay. A bioassay is very easy to do, and can prevent wasting money seeding an entire field before learning that herbicide carryover is a concern. To conduct a bioassay, collect soil samples from across the field 2-3 weeks prior to seeding. Then seed the winter cereal crop into the collected soil and grow up in pots or buckets. Seed at least one pot using soil from an area you know does not have herbicide carryover concerns for comparison. If the winter cereal has good germination and emergence without signs of herbicide injury, then you have very low risk of having herbicide carryover issues. 

 

Even if you heeded the rotation restrictions on the herbicide label and the bioassay showed no risk of crop injury, there is always the potential for some surprises when seeding cereals. Spray overlaps during herbicide application can result in curious crop injury patterns in seeded fields. The weather immediately after seeding can also affect crop injury potential. Warm weather results in quick emergence and often minimizes crop injury potential, while cold weather slows growth and can increase crop injury. Finally, not all varieties within a species have the same sensitivity to herbicide carryover, and can result in varietal differences. 

 

It is the interactions among all of these factors that ultimately determines whether or not you will have issues with herbicide carryover:

These interactions are what makes herbicide carryover difficult to predict. 

 

 

If you intend to graze fall-seeded cereals, it is important to check all herbicide labels for additional restrictions on grazing or harvesting rotated crops as forage. A rotational interval chart for grazing cover crops can be found at http://mccc.msu.edu/wp-content/uploads/2016/10/WI_2015_Herbicide-Rotation-Restrictions.pdf.