Q: I want to plant cover crops after harvest. Should I be concerned about any herbicide residues that would make it harder to get them established?
A: Two important factors influence the potential for herbicide carryover that could negatively affect establishment of a desired cover crop:
- The sensitivity of the cover crop to herbicide residues.
- The persistence of the herbicide in the soil.
An increasing amount of research is being done across North America to look into the issue, with some very good Ontario work to draw from.
In particular, studies in Ontario and Arkansas found no concerns with establishing cereal crops (e.g. oats, barley, cereal rye) in soils where common corn, soybean and cereal herbicides had been applied earlier in the season. The exception would be the soybean herbicide “Command” (active ingredient: clomazone), which has rotational restrictions for cereal crops.
Otherwise, establishing a cereal cover crop after soybeans, edible beans, corn or winter wheat harvest should not be influenced by the herbicide that was applied to those crops.
Broadleaf cover crop species tend to be more sensitive to herbicide carryover. Imazethapyr (the active ingredient found in Pursuit, Assignment, Conquest LQ, Freestyle, Phantom and Nu-Image) negatively affected the establishment of fall-seeded oilseed radish and hairy vetch in an Ontario study. An Arkansas study found that atrazine (e.g. Aatrex 480, Converge 480) caused the greatest reduction in the biomass of hairy vetch (25 per cent), crimson clover (30 per cent), buckwheat (32 per cent) and berseem clover (40 per cent).
Several factors will influence the rate at which a herbicide dissipates in the soil, such as rainfall, soil texture, organic matter and soil pH. Thus, in a dry year and on a coarse soil with low organic matter and high pH, you could see unacceptable injury to a desired cover crop that might not be evident if you were planting into a finer textured soil with high organic matter, a neutral pH and plenty of rainfall throughout the season.
Last, checking the “rotational restrictions” section of the herbicide label may identify any known negative effects from herbicide carryover.
M.G. Palhano, J.K. Norsworthy, T. Barber (2018), Sensitivity and Likelihood of Residual Herbicide Carryover to Cover Crops, Weed Technology 32:236–243. doi: 10.1017/ wet.2018.7.
M.A. Rojas, L.L. Van Eerd, I.P. O’Halloran, P.H. Sikkema, D.E. Robinson (2016), Effect of herbicide residues on fall-seeded cover crops influence soil aggregate stability and mineral, Canadian Journal of Plant Science, 2017, 97(3): 411-423.