Chemical - not water - application is cause of pollution
By Don Comis
Environmental crop and herbicide-use history are more critical to herbicide efficacy and environmental safety than the timing and amount of irrigation water used, according to Agricultural Research Service (ARS) scientists.
Dale Shaner and Lori Wiles made this discovery from ongoing experiments on two irrigated fields at Colorado State University at Fort Collins, Colorado. In collaboration with Colorado State colleague Neil Hansen, Shaner and Wiles compared the behavior of the herbicide atrazine in conventionally tilled corn grown continuously year after year vs. corn grown in three different crop rotations. They tested various levels of tillage and irrigation, including no irrigation.
Irrigation Had No Impact
The amount of irrigation, including a total absence of irrigation, had no impact on the rate of degradation of atrazine by soil microbes in the top 1 foot of soil. The only factors that made a difference were prior herbicide use and the choice of crop sequences, with prior herbicide use the most important factor by far.
This recent fieldwork confirmed what earlier studies had found: Previous applications of atrazine can predispose soil to more quickly degrade later applications of the herbicide. But until now, it was not clear if other factors (such as cropping history and quantity of irrigation) played a role in the degradation.
There are two consequences of the more rapid dissipation of atrazine in the plots.
The first consequence is a loss in weed control. In the plots with the most rapid dissipation, weeds began to reinfest the plots within four weeks of treatment, while the plots with the slowest rate of dissipation remained weed-free through the growing season.
The second consequence is atrazine leached more deeply in the soil where it did not dissipate rapidly. However, that herbicide did not move below the top 3 inches of the soil where it was degraded rapidly.
Atrazine Degrades In Delta Soils
Related research in the Mississippi Delta has found that atrazine undergoes a more rapid degradation in certain Delta soils. ARS researchers in Stoneville, Mississippi, discovered that a microbial process, which occurs after a short exposure to this herbicide, may result in a loss of atrazine’s effectiveness.