In 2020, a few farmers observed leaf injury in their soybean crops. The injury caused parts of the leaf to turn white, then yellow (see photo at top). This injury is typically caused by “bleaching” herbicides. It was odd, though, because the bleaching herbicide had been applied to the previous year’s corn crop, and it was surprising because such applications had been made other times without any issues.
What factors affect the breakdown rate of bleaching herbicide residues in the soil? Why did we see more herbicide carryover injury in 2020? Can anything be done to reduce the risk of crop injury from herbicide carryover?
What the herbicide label tells you
The herbicide label states how much time is needed after the herbicide has been applied before a crop can safely be planted. These guidelines are often listed under the heading of “rotational crops” or “re-cropping guidelines.” They are typically expressed in months and they usually err on the side of caution. They are based on trial data where twice the labelled herbicide rate was applied the previous year.
How herbicide residues are broken down
Soil microbes break down most herbicide residues. Soil temperature and moisture will affect how quickly this is done. When soil was cool and dry, a Chinese study found that it took more than 75 days longer for Callisto herbicide to break down1. And while microbial activity is higher in soils with high organic matter, high organic matter soils also bind more herbicide molecules, making them less available for microbial decomposition, thus slowing their rate of breakdown.
Soil pH affects how easily herbicide molecules are adsorbed by soil particles. When herbicide molecules “stick” more easily to soil particles, it takes longer for those to break off into the soil water where they can be decomposed by microbes or taken up by plants. For example, at lower soil pHs (6.5 and lower), bleaching herbicides are more strongly adsorbed by soil particles.
An Ontario study found significant differences in Callisto herbicide carryover that was explained by differences in soil pH and soil moisture. Less green bean injury and yield loss were observed when Callisto was applied to soil having a pH of 6.7 and with adequate rainfall throughout the season. Significantly higher green bean injury and yield loss were observed when the same herbicide was applied to soil with a pH of 6 and where rainfall was about 22 per cent lower (Table 1 below).
Chemical decomposition is another way that herbicides break down in the soil. It only applies to a few herbicides such as atrazine, Sencor and Classic. With chemical decomposition, soil pH will alter breakdown rates. For atrazine and Sencor herbicides, lower soil pHs (e.g. < 6.5) will increase the rate of breakdown. For Classic herbicide, higher soil pHs (e.g. > 7.5) will lower the rate of breakdown. Microbial activity is also important for the breakdown of these herbicide residues in the soil.
Why was there more carryover injury in 2020?
The spring of 2019 was cold and wet, delaying planting by several weeks. In some regions, dry conditions persisted throughout the growing season, slowing microbial activity and herbicide breakdown. The 2020 season began with good soil conditions and earlier planting.
If you had planted and sprayed your 2019 corn crop on June 15 and then planted your 2020 soybean crop on May 5, less than 11 months would have passed between activities. The minimal re-cropping interval for planting soybeans after applying Callisto herbicide is 11 months. You’re now in a position where there is some risk of crop injury from herbicide carryover. If field conditions were cool and dry during that period, microbial activity and herbicide breakdown would be slower. If soil pH was lower than 6.5, herbicide molecules would be more tightly bound to soil particles and would be less available for decomposition by microbes.
In this scenario, there is a higher probability that Callisto residues exist in the soil and will be taken up by the soybean crop. If that soybean crop is growing in an environment with little stress, you may not see any injury because the healthy soybean plants can rapidly metabolize the herbicide. However, if the soybean crop is under stress (from root diseases, soil compaction, low fertility, etc.), then herbicide metabolism will be slowed and injury is more likely to occur.
What can be done to avoid this type of injury in the future
- Ensure that labelled rotational restrictions are known and can be followed.
- Adjust soil pH so that it is above 6.5 and below 7.5.
- Monitor seasonal rainfall and crop heat units. If they are below normal, consider planting crops that have no restrictions on when they can be planted.
1: Bull Environ Contam Toxicol (2017) 98:212–217.
2: Weed Technology 2013 27:92—100