RECURRING QUESTIONS ON GRASS CONTROL IN FORAGE PLOTS
Weed Scientist and Agronomist
Tifton, GA
One of the most useful herbicides to managers of forage plots for whitetail deer are the selective postemergence graminicides that control annual and perennial grasses. Farmers have an array of herbicides in this group, but the only herbicide generally available to managers of forage plots is sethoxydim (Arrest®, Vantage®, Poast®, Poast Plus®). By ‘generally available’, I mean a herbicide that is registered by the U. S. EPA for that particular use and commercially available in small quantities that most wildlife managers use. Why purchase a herbicide that is sold only in five-gallon case lots, when you need only one quart? Postemergence graminicides are a powerful tool for managers of food plots. They are simple to use and reasonably goof-proof, but several questions are commonly asked.
Will sethoxydim (Arrest® et. al) injure clover, alfalfa, and other desirable species? Generally, these herbicides will not significantly injure any broadleaf plant, including clover, alfalfa, rape (canola), soybean, and pea. Sethoxydim can also be used on an array of broadleaf bedding plants, ornamentals, ground covers, and wildflower plantings. These plants have been exhaustively tested in their tolerance to sethoxydim. However, many wild or obscure plants have not been studied.
Occasionally the crop oil concentrate additive used with sethoxydim will cause foliar burn. I have seen this type of injury in the southeastern U. S. when it is extremely hot and humid, but incidence is not overly common. Similarly, foliar burn is not likely to occur on clover, alfalfa, and etc. planted for wildlife forage. If it does occur, I would not be overly concerned. Mild foliar burn does not affect new growth or forage yield, and the forages should quickly recover.
If a naturally occurring species is present that you want to protect in your forage plots and there is no information available on tolerance to sethoxydim, the only practical way to determine tolerance of the desirable species is to experiment on a small area. If you observe injury, you will have to decide which is more important; effective control of grasses or protecting the desirable species.
Many managers plant several forage species in food plots, often including grasses such as ryegrass, small grain, corn, and milo inter-planted with clover. Sethoxydim will severely injure and probably kill these grasses. Depending on the situation, controlling these grasses may be beneficial or an unfortunate accident.
Will sethoxydim control broadleaf weeds? Sethoxydim will not control broadleaf weeds or sedges (nutsedge, nutgrass, etc.). In weed control studies, agronomists such as myself plant weed seed and painstakingly cultivate specific weeds for the sake of science. I keep sethoxydim in my bag of weed science research tricks to control grasses without affecting my ‘pet’ broadleaf weeds in research plots. That is how confident I can say that sethoxydim will not control broadleaf weeds and sedges.
What is the best sethoxydim rate? Always refer to the product label for specific rates, since the three sethoxydim products have different concentrations. For annual grasses that are at the ideal stage of growth (1 to 6 inches tall), Vantage® should be applied at 2¼ pt./A. For grasses from 6 to 12 inches tall, the rate is 3¾ pt./A. Perennial weeds such as johnsongrass and common bermudagrass are another matter. They require two applications for adequate control during the growing season. Always use the recommended crop oil concentrate additive, mixed to a 1% solution based on the total spray volume.
If mixing sethoxydim for applications using a hand sprayer or spot spraying intermittently in a large area, refer to charts on the herbicide label. Those rates are converted to amounts of herbicide per total spray volume, which is useful for small sprayers. In this example, Vantage® rates are 1.5 to 2.25% spray solution, by total spray volume.
When is the best time to apply sethoxydim? Sethoxydim is best applied to small grasses (about 1 to 6 inches tall) when they are actively growing and not environmentally stressed. The specific time of year varies according to geographical location and species of grass. Warm-season annual and perennial grasses will normally be at this stage of growth in the spring and early summer. Cool-season grasses will be at this stage in the late fall or early spring. However, there are always exceptions. Fall tillage may stimulate emergence of warm-season grasses, making a fall application of sethoxydim necessary, which is contradictory to what is normally seen. Small grasses that are stressed by heat, drought, or cold should not be treated until growing conditions improve. That is why I emphasize letting weed size and vigor dictate the time of application, not the calendar.
Should I mow before or after spraying sethoxydim? Spray first, then mow about two weeks later. Mowing can be an effective weed control practice, but it alters weeds and their growth. Mowing forces grasses to proliferate tillers - new shoots from the base of the grass plant. Each tiller has its own growing point, greatly increasing the survivability of the grass. Accordingly, grasses that have been mowed are much more difficult to control with herbicides.
It is important to wait about two weeks after spraying before mowing. Sethoxydim is systemic, which means it is translocated throughout the plant. Waiting two weeks between spraying and mowing allows the herbicide to translocate throughout the plant giving more effective control, particularly with large grasses and perennials.
How long before rain should I wait to spray sethoxydim? This group of herbicides is very “rain-fast”, which means they are somewhat immune to wash-off. The label states that sethoxydim is rain-fast one hour after application. That is an extraordinary short period of time for a herbicide to be rain-fast. I have personally sprayed sethoxydim during simulated rain and had excellent weed control.
What is the best way to apply sethoxydim? There are many types of tractor- or ATV-mounted sprayers that can properly apply herbicides, including sethoxydim. These are best suited to larger areas that are easily accessible. Small plots or isolated weed patches can be treated with backpack sprayers. A herbicide sprayer needs to be able to maintain a constant pressure, move at a regulated speed, and produce a uniform spray pattern across the swath. Regardless of the specific type of sprayer used, calibration is important in order to correctly and accurately apply herbicides. Sprayer calibration prevents wasteful over-applications of costly herbicides that may not be necessary for adequate weed control or injure the crop.
How long until the grasses die? A common remark from first-time users of sethoxydim is that the herbicide is slow acting and not working. However, after two anxious weeks their dismay turns to elation. Sethoxydim is quickly absorbed and translocated throughout the grass plant and growth ceases two or three days after treatment. Growth cessation often escapes notice. Obvious whole-plant symptoms take two or three weeks to be expressed. However, symptoms in the growing points develop before whole-plant symptoms and are used in early assessment of herbicide activity. Unfolded leaf blades can be easily extracted from the whorl about 10 days after treatment, showing a brown decayed growing point.
Will sethoxydim hurt deer browsing on forage? Sethoxydim can be applied to clover and alfalfa that is either grazed or cut for hay. Herbicides registered on forage crops have been exhaustively studied to determine if the herbicide or any metabolites harm the animal or accumulate in meat or milk. Given the wide uses of these herbicides in many forage systems, we can be assured that they will not harm animals grazing on treated forages, provided that the herbicide is correctly applied.
It is highly recommended that all herbicide users closely study the herbicide label. A copy is always packaged with the pesticide. Other copies are available from the manufacturer’s web site or the industry’s web site: http://www.cdms.net. This site is regularly updated with information on most pesticides, including herbicides. It is the responsibility of the herbicide user to follow all guidelines on the label. The herbicide label is the official legal reference on how to use herbicides correctly.