Pasture Improvement: Weed Control
Excerpt from Publication 19, Pasture Production, Order this publication Table of Contents
IntroductionWeed control in pastures starts with selecting forage species adapted to the physical characteristics of the pasture and managing them so they stay vigourous and able to out-compete the weeds. Maintaining 10–25 cm of dense forage growth with a good pasture rotation reduces or eliminates the opportunity for weeds to grow. To control weeds determine the state of the desirable plants and the extent of the weed problem. If desirable legumes and grasses are not growing well, assess whether growing conditions or management practices are responsible for their poor growth. Corrections to pH, drainage, fertilizer, grazing management or in the forage species themselves can then be made to develop a thick, competitive pasture. Certain plant species, called indicator plants, grow in conditions most other plants will not (i.e. wild strawberries indicate low phosphate levels and moss grows on land with very low nitrogen levels. Weeds indicate a problem. Management problems usually consist of over- or undergrazing. Overgrazing occurs when animals graze the forage to the ground. The forage is slow to recover and weeds, with good root reserves, grow faster. The effects of overgrazing are usually seen by midsummer. The pastures have been grazed several times, forage root reserves are low and growth is further slowed by hot, dry weather. Perennial weeds with root reserves that have not been used will grow. Chicory is an example of this type of weed. Undergrazing allows the forage to overmature and become reproductive. As the forage flowers and sets seed, growth slows and weeds can outgrow the forage. With either over- or undergrazing, changing the rotation to keep the forage between 10–25 cm reduces the opportunity for weed growth. Types of Pasture WeedsRecognize the weeds present in the pasture and know their growth habit. Understanding the way a plant grows allows you to control its growth and spread. Annual WeedsAnnual weeds germinate from seed; grow, flower and set seed during one growing season. Usually they grow quickly in the spring and compete heavily with crops. Since most grow in a disturbed seedbed, they are more of a problem in establishing pastures — but are rarely a problem once the pasture is established. Use cover crops or herbicides to control weeds in the first weeks of an establishing pasture. Clip direct-seeded pastures once the forage is established. Most annuals will not regrow after clipping. Winter annual weeds can be difficult to control in pastures. They germinate late summer or fall, and grow leaves, usually a rosette, when there is little competition from the pasture. They overwinter as a living plant and resume growing actively as soon as temperatures allow, usually before the pasture starts to grow. Use a chemical herbicide in fall or clip weeds in spring or early summer to prevent seeds from developing. Biennial and perennial weedsBiennial and perennial weeds cause the most problems in pastures. They grow when pasture species are dormant and noncompetitive. Growth of these weeds may be slow the first year or for several years, until the plant establishes root reserves. In following years, the plants send up shoots during periods of low competition from the forage. Plants like dandelions establish a large fleshy storage root to provide energy for early spring growth. Perennials, such as thistles, can also spread by underground stems and roots. Methods of Weed ControlMost weeds can be removed by clipping. Annuals clipped at flowering are prevented from going to seed and generally will not regrow. Perennials may have to be clipped several times - often over several years. Repeated regrowth and clipping depletes the root reserves of the plants, eventually making them less competitive than the pasture species. Apply chemical controls at the appropriate growth stage and time of year. Consider:
You may need to apply a chemical more than once to kill established perennial weeds. Generally, legumes are damaged by chemical treatments. Most chemicals are applied in early spring or late fall when the pasture is dormant but weeds are actively growing. Spring application can be difficult if the pasture is wet and there is a possibility of runoff. Fall applications must be made after the pasture is dormant but while the weeds are still actively growing. For a full discussion of weed control refer to OMAFRA Publication 75, Guide to Weed Control. Effects of Weed ControlRemoving weeds by mechanical or chemical means is a short-term solution. Long-term control relies on removing the problem weed and managing the pasture to prevent re-establishment. Removal of weed species does not necessarily improve yield immediately, as weeds often add to total pasture production. If the majority of a pasture was weeds killed by a herbicide, you must reseed to prevent other weeds from becoming established on the bare ground. Long-term weed control consists of a combination of chemicals, fertility, grazing management and reseeding, if necessary. Weeds do not need to be controlled if they are not damaging the pasture. Base your decision to control by determining how much damage it is doing by lowering the yield. If the weed is not lowering overall pasture yield, it may be better to leave it and consider it a forage plant. Dandelions growing in a rough grass pasture may be providing early feed for animals. Changing your thinking about the weeds is adequate control. Non-Pasture AreasDo not limit weed control to the pasture. Fencerows and waste areas are good reservoirs of seed and give perennials an opportunity to grow underground roots out into pasture fields. Clipping these areas once or twice a year keeps most weeds under control. Areas left uncut for natural regrowth or wildlife habitat should be checked for weeds and individual plants controlled. Weed Control During Pasture RenovationWeed control during renovation differs from normal weed control. During renovation young species are poor competitors and susceptible to weed competition. Consider chemical and mechanical weed control until the pasture plants are well established and competitive. Watch your pasture closely to ensure weeds do not out-compete the pasture species. Alternatively, you could use animals to keep all existing plants grazed down, allowing the new species to establish. Poisonous WeedsNot all plants are benign. Some are poisonous to animals. The toxicity of plants varies with the amount of plant ingested, growing conditions and animal health. Symptoms of poisoning include restlessness, loss of appetite, chills, tremors, etc. See Common Weeds Poisonous to Grazing Livestock - Part A, (Order No. 87-017) and Common Weeds Poisonous to Grazing Livestock - Part B, (Order No. 87-018), for a description of these weeds, where they are generally located and the symptoms of poisoning. If you see any indication of poisoning consult a veterinarian. Poisonous plants are normally eaten when pasture conditions are poor and there is little else for animals to eat. Take special care if animals have access to wooded or swampy areas, where poisonous plants grow without competition. Not all poisonous weeds have to be eaten to harm animals — some cause dermatitis. Watch fencerows and waste areas. Horse owners should watch trails and riding areas for plants that cause rashes, inflammation and animal discomfort. Control all poisonous plants near animals. Related Links
For more information: Toll Free: 1-877-424-1300 Local: (519) 826-4047 E-mail: ag.info.omafra@ontario.ca
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