Weed Watch: Overgrazing is an invitation to weeds

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Darrell Deneke Every pound of gain you put on a calf before weaning is currently worth well over $2.00. And, of course, the less you spend on feed to produce that pound, the more of that cash goes to your bottom line. This all means the forage resource on cow-calf and stocker operations is more valuable than ever, and weed infestations that limit forage production are more costly than ever.

South Dakota State University (SDSU) Extension integrated pest-management coordinator Darrell Deneke says even a light infestation of perennials such as Canada thistle or leafy spurge can reduce forage production by 20 percent. Heavier infestations can cut forage production, and the carrying capacity of the land, by 50 percent or more.

Pasture and rangeland weeds, whether native or invasive, are an ongoing challenge, and erratic weather conditions over the past few years have added to that challenge in many areas.

Some parts of the country, particularly in the Southwest and Pacific Coast, remain in severe drought. Many other areas have experienced drought within the past few years, resulting in stressed and possibly overgrazed pastures.

Deneke says the reduction in forage cover resulting from drought and overgrazing often provides a foothold for weed growth as soon as moisture returns. Many weed seeds can remain viable for years in dry soil, and reduced competition from grasses and desirable forages allows them to grow quickly. So, ranchers entering their first, second or even third post-drought growing season could see weeds emerging in new places and in larger numbers than usual this spring.

Hoary cress has infested this South Dakota pasture. Photo by Darrell Deneke Deneke stresses that the best strategy for weed control in pastures is to maintain a strong, healthy population of desirable forage plants. Grazing management before, during and after a drought can help maintain that healthy forage cover and minimize weed infestations.

A system using appropriate grazing periods and rest periods helps ensure that pastures are in good condition when drought hits. And once drought begins to set in, timely decisions to adjust grazing schedules to prevent overgrazing will speed pasture recovery once the rains return. Coming out of drought, Deneke advises ranchers to defer grazing stressed pastures where possible, allowing forage plants to recover and build root reserves.

Deneke also notes that weed growth in depleted pastures is not always a bad thing. Native broadleaf plants such as sagewort, goldenrod, aster and sunflower sometimes move into drought-stressed areas and provide valuable forage for cattle. Once conditions improve, grasses become more competitive and tend to crowd out the broadleaf plants, so efforts to control native forbs might not be necessary. Some other weed infestations, particularly those involving non-native or invasive weeds, require early intervention.



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