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What can you tell me about preventing and controlling thistles in forage crops?

Answer: 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 CropsThe 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 ForagesIf 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, spot treatment will provide the 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.For more information, see the ATTRA publication Thistle Control Alternatives, available at This publication focuses on the cultural, biological, organic, and least-toxic methods available for their control.