Last winter, in a Horticulture Nova Scotia Research and Advisory committee meeting, we were discussing the effect of residual herbicides on cover crop establishment. Very good tables are available on the residual effects of herbicides used in field crops on cover crop establishment, but many of the horticultural herbicides are not include in these tables. In order to answer the question: “Which cover crops are best suited to be planted after commonly used horticultural residual herbicides?” a funding proposal was put forward to the Nova Scotia Department of Agriculture, and the following demonstration was laid out in the summer of 2022.
The scenario simulated early vegetable production, where various horticultural crops are planted in the spring with the intention of early harvest in July, followed by cover crops planted after harvest, starting in mid-July. We wanted to make sure we picked up on any residual effects of the herbicides, so we kept the period of time between herbicide application and planting the cover crop relatively short. Herbicides were applied on June 22nd, and on July 14th, half the plots were tilled. All plots were sprayed with glyphosate and then cover crops were drilled using a double disc opener grain drill. The herbicides used and the rates are listed in Table 1.

Table 1. Herbicides applied on June 22, 2022.
There are several factors that influence residual herbicide break down. Factors like soil pH, moisture and temperature, soil texture and soil biology are just a few. Horticultural crops tend to use a lot of tillage in order to terminate cash crops. This can have varying results on a residual herbicide. In the case of a product like Chateau, soil disturbance will dramatically decrease the residual effect of the herbicide. Conversely products like Converge and Dual II Magnum can get incorporated into the root zone of the cover crop through tillage, and we would expect for there to be more damage compared to being left on the soil surface.
On July 27th and 28th, photos were taken of each plot and uploaded to Canopeo, tool that measures fractional green canopy cover. On the right side of Figure 1, there is an index for cover crop establishment. Closer to zero meant that there was no cover crop present. Closer to one meant that there was cover crop present. As different cover crops will naturally establish at different rates, the data was scaled for within each crop. This means that when looking at Figure 1, you should compare the colour coding within the same crop – so you can’t compare brown mustard establishment to pea establishment, you can only compare buckwheat to buckwheat.

Figure 1. Measure of cover crop establishment, two weeks after planting. Comparisons should be made within cover crop type, not across cover crop type. Residual effects of herbicides were measured on ground that was tilled (T) before cover crop seeding, and on ground where the cover crops were seeded without tillage (NT).
As a general rule, tillage almost always improved establishment, even in the control (only glyphosate), with the exception of Converge and Dual II Magnum. Two weeks after planting was a bit early to fully detect the effects of residual herbicides on ryegrass establishment, so the effects of herbicides on annual ryegrass, Italian ryegrass and perennial ryegrass should be taken with a grain of salt in Figure 1. In the case of a product like Sinbar, cover crop establishment was supressed whether there was tillage or not.
However, four to six weeks after planting, most of the cover crops were able to grow out of any damage from the residual herbicide (Figure 2), with the exception of Sinbar, which reduced growth across the board. Figure 3 is a photo of Sonny Murray, Berry Specialist, standing in the Sinbar strip at our August 23rd field day, which you can also see very clearly running through the plots in Figure 4.

Figure 2. The effect of residual herbicides on cover crop establishment 4-6 weeks after planting. Residual effects of herbicides were measured on ground that was tilled before cover crop seeding, and on ground where the cover crops were seeded without tillage (NT). Green means the growth of the cover crop was no different from the control, yellow indicates some reduction in growth compared to the control, and red indicates unacceptable damage.

Figure 3. Sonny Murray, Berry Specialist, standing in the Sinbar strip at our August 23rd field day.

Figure 4. Aerial view of the effect of residual herbicides on cover crop establiment. Note Sinbar strip running through center of plots.
For the best success, growers must keep in mind the residual affects of herbicides used in their systems in order to avoid cover crop establishment failures.
Funding for this initiative was made possible through the support of horticulture growers in Nova Scotia, Horticulture Nova Scotia, and the Nova Scotia Department of Agriculture.
Written by Sonny Murray and Rosalie Gillis-Madden