The dry 2021 growing season will certainly put pressure on the risk for herbicide carryover in 2022, and special attention should be paid to pulse crop planning in that respect.
“Growers should look to understand what active ingredients were applied and what crops may be sensitive to carryover,” says Sarah Anderson, agronomy manager at Saskatchewan Pulse Growers Association, in this Pulse School episode.
Anderson says that referring to herbicide labels is a great start, and on top of that, farmers should check with manufacturers for re-cropping warnings or restrictions.
Pulse crops are sensitive to herbicide carryover in general, depending on the herbicide, says Anderson, but there’s a strong case to be made that pulses are sensitive to a wide variety of active ingredients from different groups. It’s important to keep more than just Group 2s on the radar.
“Microbial degradation is one of the major pathways for herbicide breakdown and microbial activity of course is higher when soils are relatively warm or relatively moist,” says Anderson. Soil pH and organic matter will also influence how long a certain active ingredient will reside in the soil, so it’s not a black and white answer as to how long that active ingredient will persist. Some fields could present a risk of herbicide residue from 2020 or earlier, in areas with a drawn-out dry cycle.
Rains later in the fall are a good thing to move that breakdown process along, but it might not be enough says Anderson. The other part of that equation is how warm soils are in order for microbes to be active. Rainfall that did happen later in the season usually moves very quickly deeper into a soil profile, since the topsoil has been so dry for so long.
Testing for residual herbicides is possible, but on the flip side of getting the results, Anderson says it’s hard to generate a meaningful recommendation.
“Our lab testing diagnostic ability is quite good to be able to pick up these residues at even low amounts, but then understanding ‘is that herbicide active going to be biologically available to the plant and for how long and to what extent’ is much more difficult,” says Anderson.
Anderson cautions that understanding what conditions herbicides were applied in, what the environmental conditions were after the fact, and relying on record keeping and manufacturer recommendations, will be more prudent than relying on a herbicide residue soil extraction test.
Understanding all of these factors can help in evaluating risk levels between fields to inform field selection for 2022.
All of this echoes the importance of herbicide documentation and practicing good record keeping. The least amount of risk is desired for crop rotations and what that implies for the future rotation plans.