Weed Control on Pasture
Weeds are a challenge in any cropping situation and can be a major problem in pastures. There are a number of different approaches to weed control on pastures and often a combination of practices is the most effective.
Identify the Problems
Identify the weeds that are your major problems, then the strategy that is the most effective in managing those species. In pasture fields we can find annual weeds such as lambs quarters and pigweed; biennial weeds such as Nodding, Bull and Scotch thistles, wild carrot and blueweed; and perennials such as Canada thistle, and buttercup. To complicate this further some of these weeds spread by seed, others by root systems, and some by both seeds and roots. A good source for weed identification is Ontario Weeds - Weed Gallery.
Maintain a Healthy Sward
A strong healthy sward of desirable forage species is the first step to minimize the weed pressure in a pasture. A grazing strategy that allows for optimum animal density and sufficient rest and recovery time for the grass is critical. If the animal density on a paddock is low, the livestock will leave the less palatable species and over- graze the desirable species. This over- grazing will result in the exposure of soil and provide the opportunity for weeds to germinate and grow. With the correct animal density, the animals will consume (or tramp) all the available forage including the weeds. It is also critical that the desirable forage species have sufficient rest and recovery time between grazings. This time will depend on the time of year and moisture conditions. Most pasture managers find that 3-4 weeks is sufficient rest time in the early part of the season (May). After mid-June, and later it takes 40 days or more to have optimum regrowth - provided there is adequate moisture. If we think of hay fields and the time between cuttings, these numbers match up quite well. The only difference between a hay field and a pasture field is the harvesting equipment!
Clipping weedy species as they start to flower but before they set seed can be effective in managing some of the weed pressure. Clipping pastures to remove seed heads on desirable grass plants will stimulate new leaf growth. Clipping is an extra cost but will help to reduce the weed pressure, especially on taller weeds such as thistles and improves the appearance of the pasture. If there are only a few weeds ( thistles in particular), using a spade to cut them off about 2-3 cm (1 inch) below the soil surface is an effective way to reduce the spread of these weeds.
Spraying is an effective weed control measure for the weeds that are there, but once you eliminate those weeds you need to manage the pasture in a way that will discourage them from returning. The phenoxy products (2,4-D, MCPA, dicamba) will control many of the weeds. For some of the very challenging perennials it may be necessary to use a product containing aminopyralid (that will also kill the legumes). Guidelines for spraying pastures are in the Guide to Weed Control, OMAFRA Publication 75 on pages 188-191. Timing is important, depending on the weed species that you want to focus on. Herbicide applications in June and September may be necessary for biennial and perennial weeds. Keep in mind that many of the herbicides will kill or at least reduce the legume stand in your pasture. When selecting a herbicide, note the grazing restrictions that are on the label.
Maintain a Healthy Pasture
The best weed control program is a thick pasture with a minimum of 40% legume content that is grazed for short periods with a long enough rest periods between grazings to allow the plants to fully recover and regrow. This will promote good growth of the desirable legume and grass species and minimize the amount of selection that the, livestock do in their grazing. This same system will help to insure that there is no bare ground for weeds to get established.
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