Here is another free, industry supported, UF/IFAS Hillsborough/Polk County Extension online pesticide CEU available to keep your pesticide licenses current. The article Unintended Herbicide Effects discusses how herbicides can move off target and negatively impact the environment. If you'd like to get a CEU credit, read the article and then click on the link at the bottom to take the quiz for credit. CEUs available=1 PRVT, 1 L&O, 1 CL&O, 1 LCCLM, or 1 LCL&O
Unintended Herbicide Effects
by Juanita Popenoe
Herbicides are something that almost everyone uses. Why hand weed when you can just spray? If you select the right chemical, you may not have to come back to control weeds for quite a while. What could be easier? Herbicides kill target weeds by extreme phytotoxicity, a big word for injury to plants due to chemical exposure. Herbicides are designed to do this. There are many perhaps unexpected ways that herbicides can have unintended effects on non-target plants, whether through drift, leaching, runoff, persistence, or residue.
Knowledge of the chemicals being used and a thorough reading of the label are required to avoid contamination of our environment. Most pesticide applicators realize that spray drift is a very common, and easily controlled cause of non-target exposure. Drift occurs when small droplets of pesticide are carried off-site by air movement. By applying when wind speeds are low (under ten miles per hour) and using larger droplet size or drift-reduction agents an applicator should be able to avoid this type of unintended consequence. If the target plant is a large tree, shrub or vine in a tree, you should not be applying herbicide as a liquid spray up into the air. The chances of drift to non-target plants is too great for this type of application. An alternative method such as hack and squirt, cut stump, or basal application technique should be used.
Because pre-emergent herbicides are directed at the ground and are intended to cover the soil surface, stick to the surface and form a barrier layer. They are formulated to spread out with a small amount of irrigation, and not move down through the soil into our ground water. Too much water and they may be washed away. Also, if the surface is mulch or loose soil, applied herbicides may be adsorbed, or bound to the particles, and carried away on the mulch or soil particles washed away by heavy rains. This could be termed runoff, even though it is not the pesticide in the rain water itself being washed away to non-target areas, but the pesticide on a carrier particle. Pre-emergent herbicides are more efficiently applied beneath a mulch, although you may get weeds starting up in the mulch.
Other types of herbicides may be very soluble and not be adsorbed to soil particles. Herbicides formulated to be easily taken up by the roots of plants usually do not bind to the soil surface. These types of herbicides may very easily move into the ground water with heavy rains or irrigation. They may be easily washed down or off site and impact non-target organisms. Again, the label is your friend as well as the law. The label will tell you when to be aware of these types of hazards. You would think persistent herbicides would be a great way to save money on applications. Persistent means that the herbicide remains present and active in its original form for an extended time before breaking down. Chemistry that allows a pesticide to be persistent is not that common, so we have few really persistent herbicides, and that may be a good thing.
Landscapes and plants change with time as any living thing will, and the use, design or layout of the plants will also change with time. I have had a landscaper come to me with a problem caused by using a persistent herbicide in what was a landscaped area that the owner wanted to change over to a vegetable garden. The vegetables just would not grow and the answer was in the history of the site. Another example involves a nursery grower who thought to cut down on his labor by applying imazapyr, a very persistent herbicide, to the ground cloth around his nursery pots. This was a completely off-label use; in other words, illegal. The herbicide did not bind to the ground cloth and when a heavy rain event occurred, the herbicide was washed up into the pots and taken up by the plant roots, causing all the plants to be stunted. They never grew out. Read the label of the herbicide carefully and be sure to think of potential future impacts. Even right of way applicators don’t necessarily want a barren desert for years. Persistent herbicides bound to soil particles or mulch, or unattached to anything, can wash away to a non-target site and cause a catastrophe.
Most herbicides are not persistent and break down under normal environmental conditions, either through microbial, chemical or photo degradation within a few days. Glyphosate is one herbicide that does not last in the environment for very long, however once inside the plant it is another story. Glyphosate is a systemic herbicide that is taken up by the plant and translocated throughout the plant. We used to say that if it got on anything green, it would be taken up. More recent research indicates that it will go through thin or pigmented bark as well, especially if it is combined with a surfactant to increase penetration. This is a very effective type of herbicide because you do not have to get good coverage of the weed to have an effect. However, this characteristic also makes it very dangerous for drift accidents. Another contributing cause to accidents is that many landscape plants seem to be unaffected by low rates of glyphosate, and applicators often spray over “resistant” plants to get at the weeds around them. Research now shows that woody plants will take up sub-lethal doses and store glyphosate in the roots for years. It may take two years for the symptoms of the sub-lethal doses to appear. Symptoms of sub-lethal doses of glyphosate include: stunting, bark cracking or splitting, loss of apical dominance, individual dead limbs, chlorosis and death. This is a great example of unintended consequences of herbicide use and a reason to keep up with the latest in research.
Another example of off target herbicide effects is found with residue from some herbicides. Aminopyralid, clopyralid, fluroxypyr, picloram, and triclopyr are all in a class of herbicides registered for use on pasture, grain crops, nonresidential lawns, certain vegetables and fruits, and roadsides. They kill a range of broadleaf weeds including some that are toxic to livestock. Livestock can eat the grass from treated pastures with no effect and no effect to humans eating meat from these livestock. However, these herbicides pass through the livestock and remain as residue in the manure and urine. The residues also remain on the pasture, hay and cut grass. Compost made from the manure, hay or grass clippings also retains the residue, which may take years to break down, depending on conditions. It has been well documented that gardeners using this compost or manure in their gardens reported poor seed germination, death of young plants, twisted/deformed leaves, and reduced yields. Beans, peas, lettuce, tomatoes, carrots, peppers, potatoes and eggplants are all plants sensitive to the residue in the compost.
Applicators need to be aware of the possible consequences of herbicide treatments and the possible future impacts. Unintended herbicide effects on non-target plants can be caused by something as simple as drift to something as complicated as compost from manure from a cow that had eaten in a treated field. As a pesticide applicator you have a responsibility to ensure that you have as little impact on the environment as possible beyond the intended application. Careful reading of the label and application practices should limit the unintended consequences without impacting the desired results.
Click here to take the quiz for a pesticide CEU credit. http://tiny.cc/Un-herb-effects