When planning our crop rotations, including cover crops, we need to consider how the weeds will be controlled in each crop and if there is any risk of herbicide carry-over that will affect the next crop in rotation. Read herbicide labels and look for listed rotational restrictions for the crop that you plan to plant next in your rotation.
Residual herbicides are often applied to crops to provide season-long weed control. However, herbicidal activity is beneficial only for the time it is needed; longer herbicide activity can cause injury to subsequent crops. The length of time that an herbicide remains active in the soil is called soil residual life or soil persistence. Herbicide carryover is a term used to describe the presence of an herbicide in soil after its weed control function has been accomplished.
In a diverse crop rotation where a variety of herbicides are being used, record keeping becomes even more important. Many of today’s herbicides have more than season-long residual effect. Keeping a detailed notebook of the herbicides applied and at what rates will help you to identify any establishment problems in the following crop. Keep the small herbicide booklets that come with containers of herbicide.
The key is to think ahead, become familiar with the herbicides you are using and be aware of the rotational restrictions.
Often times we make decisions about replanting or planting another crop in rotation on the fly. Record keeping is important to be able to take into consideration what herbicide was applied to then consider residual effect.
If you use a custom herbicide applicator service on your farm, make sure that you get complete records of the herbicide mixes and rates that are used. It is important to have a good conversation ahead of spraying time to let the custom applicator know what your planned rotation is, so they can also consider alternative herbicides due to rotation restrictions if needed.
In the long run there are numerous benefits obtained by using a crop rotation. This includes being able to intensively grow more forage on our farms and fit in more cover crops in our rotations, but this also takes a level of management and thinking ahead about herbicide rotation restrictions.
Land Grant University Agronomy Guides, including the Penn State Agronomy Guide, are often a good source of information about herbicide rotation restrictions.
Cereal rye, wheat, barley and oats are four common crops that are often listed on a product label. For guidance use Penn State’s Herbicide Carryover Chart or the herbicide label to determine which products have less than or equal to 4-month rotational restriction for covers like rye, field pea, clover, oats and sorghum/sudan.
In general for corn herbicides, atrazine at less than 1 pound per acre can allow cereal grain establishment and at less than 0.75 pounds per acre may allow for most legume cover crops, mustards and annual ryegrass. This should be similar for simazine.
In addition to atrazine and simazine, mesotrione (sold as Callisto/Tenacity) is problematic for legumes. Clopyralid (sold as Confront, Curtail, Lontrel, Millennium Weed and Feed, and Transline) could also affect these small-seeded broadleaves.
For soybean herbicides, chlorimuron (sold as Kloben), Pursuit or Extreme, and Prefix or Reflex could be a problem for fall-seeded legume or mustard cover crops, but for cereal grains these should be fine.
Note: Consult your chemical rep and herbicide label for complete information about residual times and risks to other crops.