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Wednesday, Jan. 28, 2015

Keeping weeds under control

Thursday, December 13, 2012

An effective weed control plan for pasture and range involves several management practices. The grazing scheme takes into account the type of livestock as well as the grass and forb species available. Grazing intensity influences the relative abundance of undesirable forbs and grasses. Weeds that are unpalatable when mature may provide acceptable grazing for certain classes of livestock when weeds are young. Grazing schedules are a good IPM practice for weed management. Cultural or mechanical weed management includes mowing or clipping, hand digging, prescribed burning, or cultivation. Other IPM tactics include biological control, especially for noxious weeds. Herbicides are an aid to control unwanted weeds.

Mowing or clipping temporarily removes weed top-growth but also removes top-growth from grass. This system stops seed production but has different effects on the weeds. Annual forbs can be controlled by cutting below the lowest leaf early in the growing season. Undesirable annual grasses should be mowed after the seed stalk has elongated but prior to seed formation. Usually mowing perennial weeds one time reduces seed production; repeated mowing reduces vigor and slows spread. Clipping perennials like Canada thistle or leafy spurge in the spring works well as a set up for fall herbicides when moisture encourages new growth. Digging or chopping works well for scattered biennial thistle. Musk thistle rosettes can be stopped when the root is cut several inches below ground level. This requires more labor and is limited to small patches or scattered plants.

Burning is a valuable tool for managing weeds and grasses in range. Most annual broadleaf weeds and grasses and many undesirable perennial broadleaves and trees (Eastern Red Cedar) can be controlled with fire. Forb response to fire depends on the timing of the burn. Burning in late spring when the plants are actively growing is the best time to control most perennial forbs. Biennial weeds that are in the rosette stage are not controlled by fire. Burning requires extreme caution and prescribed burn experts must be consulted when using this method of weed control.

Biological control is another weed control tool, especially for noxious weeds. Biological control utilizes natural enemies as a means of weakening or killing the host plant. Insects have been a common approach to biological control in some locations. Noxious weeds that have approved biological control agents (insects) in several states include leafy spurge, musk thistle, Canada thistle, toadflax, St.Johnswort, and biennial knapweeds. For example, South Dakota currently has a collection and release program for leafy spurge flea beetles (Aphthona species), coordinated by the South Dakota Department of Agriculture. In many states, the county weed and pest board is the local contact point for landowners and managers considering the use of flea beetles on leafy spurge.

Herbicide options are available to control many of the undesirable plants found in pasture and range. Many of these treatments, especially those targeting broadleaf weeds, will also remove all or many of the desirable forbs or legumes. Reducing or eliminating beneficial forbs can reduce livestock gains and alter the forage mix. This can greatly influence plant diversity and have a detrimental effect on pollinator insect populations and the habitat necessary for their survival.

Herbicides perform best if conditions are favorable for plant growth. Careful and selective use of herbicides, combined with proper grazing management and other control tactics hasten recovery of weed infested pastures or range. Use herbicides that are labeled for the target weed and registered for use on pasture and range. Follow all grazing and haying intervals and environmental restrictions.

Deferred grazing gives the grasses an opportunity to build up root reserves, develop more top-growth and produce more herbage. In some pastures, desirable native species no longer abundant will become re-established during the rest period. Deferred grazing can be used in conjunction with other improvement practices to speed recovery.


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