No water, no herbicide breakdown. So what do you do if you rotate back to either corn or soybeans? That’s the question tackled by Bob Hartzler and Mike Owen, Iowa State University agronomists. The two say the relative persistence (half-life) and the rate of herbicides used in the field are the key factors. The four herbicides with the highest risk of carryover are: atrazine, chlorimuron, imazaquin and simazine, they report..
“The products listed as ‘High Risk’ have the potential to damage rotational crops under ‘normal’ conditions,” they comment.. “Most farmers have learned what rates can be used on their soils safely, but this year’s drought will result in a high risk even with reduced rates. Both chlorimuron and atrazine are more persistent in high pH soils. Pre-emergence applications of chlorimuron will have a much higher risk of problems because these rates are much higher than when chlorimuron is applied post-emergence (Classic). In certain situations, the best option may be to alter rotation plans to avoid planting a susceptible crop.”
In preparation for next year, the two suggest growers should consider rates, application date, soil characteristics and label restrictions -- especially for those herbicides listed in the High Risk category. “Keep in mind that if rainfall returns to normal, this rain will have much less effect on herbicide degradation than had it occurred near the time of herbicide application,” they state. “Also, while tillage should dilute herbicide residues within the soil profile, past experience has shown that this practice does not consistently reduce crop injury from herbicide residues. The conditions a crop experiences during establishment greatly influence its ability to tolerate residual concentrations of herbicides. Using practices that minimize additional stresses to the crop seedling (planting date, seedbed conditions, etc.) can reduce problems associated with low concentrations of herbicides,” they conclude.
