Cover Crops Herbicides Research

Herbicide residues and cover crops

Amanda Green – Weed Management Program Lead – Horticulture, OMAFRA

As some vegetable crops are being harvested and June bearing strawberry harvest has wrapped up some growers may be considering planting a cover crop. Cover crops give the soil many benefits such as increasing organic matter, improving soil aggregation or fixing or scavenging nitrogen. One thing to consider when selecting your cover crop is the herbicides that you applied earlier this year and the previous year.

Some residual herbicides, mainly those that are soil applied pre-emergence (PRE), can have a negative impact on crop establishment and cause visible injury. PhD candidate María Angélica Rojas, from the University of Guelph under the supervision of Dr. Darren Robinson, has studied the effects of three PRE herbicides applied in the spring on the functionality of subsequent cover crops to benefit the soil. The herbicides studied were:

  • INTEGRITY (saflufenacil/dimethanamid-P), registered in field and sweet corn
  • PRIMEXTRA II MAGNUM (s-metolachlor/benoxacor/atrazine) + CALLISTO (mesotrione), registered in field, seed and sweet corn
  • Imazethapyr (PURSUIT or PHANTOM or NU-IMAGE), registered in soybeans, a number of different edible beans, Clearfield corn, Clearfield Canola, processing peas, snow peas and alfalfa

The fall planted cover crops studied were fall rye, hairy vetch, oilseed radish and fall oats. The spring planted cover crops were spring wheat, buckwheat, sorghum-sudangrass and annual rye. What Rojas found was that after a year an effect of all three herbicides showed injury as seen in a bioassay done using sugarbeets. Specifically, with imazethapyr (PURSUIT), all of the cover crops showed visible herbicide injury, with oilseed radish and buckwheat grass showing the greatest extent of injury. Imazethapyr even negatively affected annual and fall rye’s and hairy vetch’s ability to improve soil aggregation. Spring wheat showed little to no visible injury but there was a decrease of soil aggregation in the INTEGRITY and imazethapyr treatments. Out of the three herbicide combinations studied INTEGRITY showed the least harmful effects on the cover crops with fall rye and annual rye showing the least injury. These herbicides may not be in your regular herbicide rotation but it is important to consider herbicide carry over from other residual herbicides.

Figure 1: Imazethapyr residual injury in a cruciferous crop
Figure 1: Imazethapyr residual injury in a cruciferous crop

When selecting your cover crop make sure to check the labels of residual herbicides that you applied this year and the previous year for rotational intervals. Some examples of residual herbicides used in horticulture include CHÂTEAU (flumioxazin), GOAL 2XL (oxyfluorfen), SENCOR (metribuzin), TREFLAN (trifulrualin), SANDEA (halosulfuron) and SINBAR (terbacil). Labels can be found on the internet if you no longer have them. Some rotation intervals may not be accurate in years where it was unusually dry or cool. Unfortunately the label may not list your desired cover crop and in which case it recommends performing a bioassay or lab test.

A bioassay can be done in the field with the crop you want to plant or indoors. In the field you can plant a few strips of the crop in the field. If possible, plant a strip into soil that has not been sprayed with a residual herbicide in the last two years as a check. For preforming an assay indoors you can collect soil from 15-20 different spots in the field and mix it together and put it into pots and plant your desired crop. Also make sure to collect soil separately from headlands where more herbicide may have been applied and soil not exposed to residual herbicides. For the field or indoor sampling allow the plants to grow out enough to see potential injury. You may see stunting, deformed leafs, or lower discolored leaves

Knowing which cover crops are safe to plant after a residual herbicide has been applied to the previous crop will allow you to get the most benefit for your soil. If you have further questions about María Angélica Rojas research please feel to contact her at


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