The 2013 planting season is 2-4 weeks behind the early start of 2012, depending on your location in the Corn Belt. While many farmers have been yearning to sink the planter in the ground, cold soil temperatures and recent heavy rains have waylaid the 2013 planting season another week or two.
For many farmers delays only mean reduced yield, but for others delays may avert reduced yields. How is that so?
The drought of 2012 created more challenges than just reduced yields. Its agronomic impact not only reduced nitrogen use and left more of that nutrient in the soil for use this year, but it also left more herbicides in the soil for use this year.
If you don’t rotate crops you might not notice the added benefit of increased herbicide concentration. But if you do rotate crops, you may notice the impact of your 2012 herbicide doing its best to attack that strange new crop in the field. Suddenly, the crop you planted may have some serious challenges.
Herbicide carryover will be one of the legacies of the 2012 drought that will be as hard to quantify as how much nitrogen may still be available from last year.
University of Illinois weed specialist Aaron Hager says, “Dry soil conditions, similar to what most of Illinois experienced during the 2012 growing season, often slow the rate of herbicide degradation and increase the potential for damage to rotational crops from herbicide carryover.” He reminded farmers of the impact of the 1988 drought on 1989 crops when herbicide carryover was a significant problem for the first time.
The reason was the lack of herbicide degradation in the soil due to low soil moisture, which inhibited the work of microorganisms in breaking down the complex herbicide molecules.
Hager said there are many unanswered questions whether herbicides remain from the inactivity of the microorganisms or whether late fall rains were able to bolster their populations so they could finish their job. And he said there are many factors that interact to determine how long an herbicide will remain active in the soil.
- Some herbicides will have very little soil activity over a short period of time and others may persist for a long period. That is even the case for chemicals in the same family, subsequently, chemical makers have to provide guidance on each product of what crop can follow its application and by how long of a waiting period. But he says, “Variations in certain soil physical and chemical properties can influence the persistence of a particular herbicide apart from its chemical composition.”
- Soil properties that impact the rate of degradation can either be microbial, chemical, or physical. Hager says one of the more important factors is soil pH. In a high pH soil the chemical may have only a weak attachment to clay particles and is more available for plant uptake. The opposite is the case for a low pH soil.
- The soil pH will also determine how rapidly the herbicide molecule is degraded in the presence of water. That occurs more rapidly in a high pH or acidic soil, and less rapidly in a lower pH soil.
- The physical make up of the soil will also vary the degradation of the herbicide. Hager says soils with higher amounts of clay and organic matter will increase the carryover potential. When they are bound to soil particles, they are not available for plant uptake, don’t move down in the soil profile, or are degraded by microbes.
- The rate of degradation by microbes is also a function of the environment and soil conditions. Any factor that affects the microorganisms such as high temperature and low moisture will impact the rate of degradation. Currently, the cool soil temperatures are a hindrance to degradation because they are slowing microbial activity and reproduction. Hager says, “Herbicide degradation by soil microorganisms usually occurs most rapidly when adequate soil moisture is present. Under extremely dry conditions the rate of herbicide degradation by soil microorganisms can slow enough to allow the herbicide to persist into the next growing season.”