Question of the Week
Answer: Thank you for your recent request for information from ATTRA, the National Sustainable Agriculture Information Service. I am pleased to provide you with information on preventing and controlling thistles in forage crops.
Weeds are signs that something is happening in the field that may weaken grasses and decrease productivity. As long as a grass stand is maintained intact through proper management, the grass community will thrive. However, when some kind of disturbance occurs, the grasses often have a hard time bouncing back without weed populations becoming established. Why does this happen? Weeds take advantage of a niche left open by grasses as they disappear. But why can’t a grass plant take that niche just as easily as a weed? The reason is that weeds have evolved to be very competitive for nutrients and have adapted to soil conditions that most grasses have not. If a soil is disturbed through compaction (the destruction of soil structure due to animal impact or machinery), or if it has become infertile due to intensive cropping and harvesting of nutrients, weed species will find a comfortable home. This is nature’s way of correcting imbalances, because deep-rooted weeds can scavenge non-available nutrients, translocate them to their leaves and stems, and return them to the soil when they die. This is, in effect, a way of soil building in degraded soils. After the soil has become porous due to the rooting of weeds, and nutrients have been returned to the soil, natural grassland plant succession can take over and the grasses will begin to return.
Preventing Thistles in Forage Crops
The presence of weeds in an established forage crop or pasture is usually a sign of a management problem. Fertility, proper planting procedure, and harvest management are the most effective ways to maintain dense, productive pastures. Ensure adequate soil fertility and optimum pH with nitrogen-fixing legumes and applications of lime as per soil test. When establishing new pastures, ensure that you use weed-free seed on a well prepared seedbed, or use a no-till drill at the appropriate time. Also, be sure to practice good harvest management, whether grazing or haying, by leaving enough forage standing after harvest to allow for regrowth. It is especially important to rest pasture plants after grazing to allow full regrowth, thereby ensuring plant health and productivity.
Consider the following for establishing a weed-free forage crop:
• Control weeds prior to planting with successive tillage. This allows weeds to germinate, which can be killed by the next tillage. Some growers use a broad spectrum herbicide to control weeds prior to planting.
• Apply lime and fertilizers according to soil test, and incorporate with tillage.
• Prepare a good seedbed, with no large soil clumps.
• Select weed-free certified seed.
• Plant with a drill, or broadcast at a higher seeding rate and drag the field with a harrow to obtain seed to soil contact. Good seed to soil contact ensures good germination and seedling growth.
• If weeds occur in the stand, mow them high. Grasses have their growing points at the base of the plant prior to flowering, and weeds like thistles have their growing points higher on the plants. Mowing creates an environment that favors grass growth over weed growth, given proper fertility management.
Controlling Thistles in Forages
If thistles become established in a forage stand, they can be controlled through mowing, grubbing the individual plants (for small infestations), or using an herbicide. Broad-spectrum herbicides like Roundup kill all plants, not only the target species. The same goes with low-toxicity herbicides like citric and acetic acids. If these tools are used, I recommend spot treatment for best results.
The frequency of mowing for the control of thistles depends on rainfall. A single summer mowing is usually beneficial after flowering but before the seeds set. However, additional clippings will be required if later summer rain results in significant lush weed regrowth. Mowing after flowering but before seed set will reduce weed seed production and decrease the amount of weed seeds in the soil for the following year. Perennial weeds, like thistles or dock, should be mowed successively throughout the grazing season to starve the plant and reduce seed production. Note that many plants, like thistles, will continue to set flowers after they have been mown, and will usually set them closer to the ground after mowing. To compensate for this, mow perennial weeds high to clip flowers to minimize seed production and prevent the plant from setting subsequent flowers low to the ground.
Non-Selective Herbicides for Weeds
For a natural herbicide recommendation, you might try a citrus oil, vinegar, and soap mixture. The citric acid and acetic acid works to desiccate the leaves, and the soap acts as a sticking agent. This herbicide is only a "burn down" chemical, and will not kill the whole plant. Repeated treatments will be necessary to use up the energy reserves in the roots as they resprout.
Low toxicity herbicides are available from several suppliers. Scythe, produced by Dow AgroSciences, is made from fatty acids. Scythe acts fast as a broad-spectrum herbicide, and results can often be seen in as little as five minutes. It is used as a post-emergent herbicide, sprayed directly on the foliage. It is primarily a burn-down herbicide, has no residual activity, and is not effective on non-green, woody portions of plants.
Vinegar is an ingredient in several new herbicides on the market today. Burnout and Bioganic are two available brands. Both of these are post-emergent burndown herbicides. They are sprayed onto the plant to burn off top growth—hence the concept "burndown." As for any root-killing activity with these two herbicides, I cannot say. The label on Burnout states that perennials like thistle may regenerate after a single application and require additional treatment.
Researchers in Maryland tested 5% and 10% acidity vinegar for effectiveness in weed control. They found that older plants required a higher concentration of vinegar to kill them. At the higher concentration, they got an 85 to 100% kill rate. A 5% solution burned off the top growth with 100% success. Household vinegar is about 5% acetic acid. Burnout is 23% acetic acid. Bioganic contains 10% acetic acid plus clove oil, thyme oil, and sodium lauryl sulfate. AllDown contains acetic acid, citric acid, garlic, and yucca extract. Matran 2 contains 50% clove oil. Vinegar is corrosive to metal sprayer parts—the higher the acidity, the more corrosive. Plastic equipment is recommended for applying vinegar.
According to a study conducted in California by the UC Statewide IPM Program comparing several non-synthetic herbicides with Roundup Pro, following herbicides might prove effective in controlling broadleaf weeds like thistle.
Eco-Exempt is a contact, non-selective, broad spectrum, foliar-applied herbicide that will only control actively growing green vegetation. The active ingredients are 2-phenethyl propionate and eugenol (clove oil). 2-phenethyl propionate is considered a minimum risk pesticide by the EPA, is exempt from EPA pesticide registration (as are the following products with the exception of Roundup Pro), and is in the same risk classification as cinnamon oil, citric acid, clove oil, and corn gluten meal. In a California study, Eco-Exempt was reported to have minimal effect on broadleaf weeds.
Matran 2, like Eco-Exempt, is a contact, non-selective, broad spectrum, foliar-applied herbicide that will only control actively growing green vegetation. The active ingredients are clove leaf oil and wintergreen oil. Matran 2 had a significant effect on broadleaf weeds in a California study.
AllDown is a non-selective herbicide composed of citric acid, garlic, acetic acid, and yucca extracts. In the California study, AllDown provided the best control of broadleaf weeds after Roundup Pro.
Burnout II is another non-selective herbicide composed of citric acid and clove oil. In the California study, Burnout II had the second best control of broadleaf weeds after AllDown and Roundup Pro.
Roundup Pro is a postemergent, broad-spectrum, systemic herbicide with no soil residual activity, with glyphosate as the active ingredient. Roundup Pro had the highest percent control of broadleaf weeds in the California study. Although the Roundup Pro label states that it has no residual activity in the soil, it is moderately persistent in soil, in that it is readily adsorbed to soil colloidal matter. But since it has no pre-emergent activity, crops can be planted directly following application with no harm to seedlings. Glyphosate is degraded in the environment primarily by soil microbiological activity, and studies have indicated that it has no significant effect on soil microorganisms.
The non-synthetic herbicides mentioned above can be purchased from the following dealer:
Peaceful Valley Farm and Garden Supply, Grass Valley, CA, 1-888-784-1722
(1) Scythe Pesticide Label
(2) Bioganic Weed and Grass Killer
(3) Eco-Exempt EC Pesticide Label
(4) Resource Guide for Organic Insect and Disease Management, Cornell University
(5) Evaluation of Least Toxic Herbicides, Cheryl Wilen and Phil Boise, UC Statewide IPM Program
(6) Matran 2 Pesticide Label
(7) AllDown Green Chemistry Herbicide Technical Specifications Factsheet
(8) Burnout II Pesticide Label
(9) Roundup Pro Pesticide Label*
(10) Environmental Fate of Glyphosate, Jeff Schuette, CA Department of Pesticide Regulation, 1998
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