|Herbicide Accumulation in Recycled Irrigation Water|
IntroductionSome growers I’ve spoken with have concerns over the potential for herbicide contamination of their irrigation water, especially if they recycle water captured from their container beds. This article will discuss pathways for herbicides to contaminate ponds, and best management practices (BMPs) that minimize the potential for herbicide accumulation in retention ponds. This information is targeted towards container growers.
Herbicides are one of the primary methods of weed control used in container crops. Most herbicides are applied in granular formulations, some nurseries apply spray herbicides. Herbicides are typically broadcast applied over the entire container bed. Due to labor costs, herbicides are not applied individually to each container. Broadcast-applied herbicides either fall in the containers or between them. We will examine individually what happens to herbicides that either fall in or between containers.
In a nutshell, herbicides commonly used by nursery growers move very little in containers. When applied at the recommended rate and incorporated with irrigation, (as instructed by the label) most herbicides remain in the top 1 inch of the media, with some moving as far as 1.5 inches below the container surface. Herbicides rarely (if ever) leach through the entire container and out the drain holes. While this may be a logical conclusion based on the chemical properties of herbicides, low solubility and high adsorption to organic matter, it has also been verified by several scientific experiments.
In a peat-based media, 96% of applied isoxaben remained in the top 2 inches,
with no detectable isoxaben below 4 inches (Rouchaud et al.,
1999). Over 99% of oxadiazon applied to a 3:1 pinebark:peat or 7:1 pinebark:sand
media remained in the top 1 inch of the container, with no oxadiazon detectable
below 1.25 inches (Wehtje and Gilliam, 1993). Oryzalin is only slightly
more mobile, with 99% remaining in the top 1.5 inches of the pinebark:sand media
and 99% in the top 0.75 inch of the
pinebark:peat media. Decreased movement in the pinebark:peat media is likely
due the higher organic matter content and increased cation exchange capacity
(CEC) in that mix (due to incorporation of peat instead of sand). Oxyfluorfen
also has a low probability of leaching from containers, and similar to oryzalin,
it’s been shown that peat is more absorptive of oxyfluorfen than redwood
bark alone (Horowitz and Elmore, 1991).
Fate of herbicide that falls between containers
Sequence of application timing
Vegetative filter strips
Vegetative filter strips are areas with dense vegetation (usually grass) through
which runoff must flow before entering a retention pond. Vegetative filter
strips can remove pesticides, nutrients, and sediment from flowing water. Because
herbicides used in containers have low solubility and high coefficients of
adsorption (Koc, the affinity for a chemical to bind to soil), it is likely
move while attached to some type of sediment.
Research at Mississippi State University has demonstrated that soil organic
matter increased by more than two-fold when covered with vegetation, compared
to bare soil. Increased organic matter increases the adsorptive properties
of soil, thus attracting and tying up herbicides and preventing their movement
into ponds. Higher organic matter also increases microbial populations
and activity (thus increasing herbicide breakdown). In their work, they
also found that herbicide half life on ground covered with vegetation was 12
days, compared to 100 days on bare soil.
So what’s the threat?To what extent do herbicides accumulate in retention ponds? It will, of course, depend on many factors, although there are two studies that have evaluated herbicide accumulation from container nurseries. One study found the highest concentrations for oryzalin and oxyfluorfen (from Rout) peaked at 0.15 ppm (parts per million) through the first day after application, and declined thereafter (Keese et al., 1994). By two weeks after application, the herbicides were nearly undetectable, and by 4 weeks herbicides were not detectable (below 1 ppb).
A separate study monitored herbicide levels in a retention pond on a 50 acre container nursery (Camper et al., 1994). Samples were collected over two years, and were analyzed for pendimethalin, oryzalin, and oxyfluorfen (from Rout and OH2). Pendimethalin levels peaked at 4 ppb, oryzalin at approximately 0.15 ppm (similar to the above mentioned study) and oxyfluorfen at 9 ppb. To put this into perspective, oxyfluorfen (when applied as a spray herbicide) is typically applied at about 6000 ppm, which is roughly 600,000 times more concentrated than what was found in the above experiment.
In both studies, herbicide concentrations declined shortly after peaking, indicating that degradative processes (microbial, chemical, and photo-degradation) prevent accumulation from repeated herbicide applications.
I have heard anecdotal reports of herbicide injury to crops resulting from contaminated ponds. It is not my intent to argue with growers who have reported this, however, I find herbicide contamination unlikely when products are used according to label instructions, and under normal nursery conditions.
SummaryHerbicide accumulation in ponds, to the point that it will cause injury to crops when reapplied through irrigation, is uncommon. However, when it occurs, it likely results from a combination of: excessively high herbicide rates, large areas treated with herbicides that drain into a small pond, and applying herbicides broadcast to containers that are spaced far apart. To minimize herbicide accumulation in ponds, consider the following BMPs:
Disclaimer: This article is for educational purposes only. Mention of a specific product should not be interpreted as an endorsement, nor should failure to mention a product be considered a criticism.