Benefits of increasing grazing height on weed suppression in management-intensive rotational grazing systems in Wisconsin

Management intensive rotational grazing has many benefits to livestock production, but weeds can infest fields and cause reductions in forage quantity, quality and animal performance.  Common burdock (Arctium minus), Canada thistle (Cirsium arvense) and plumeless thistle (Carduus acanthoides), are common pasture weeds in Wisconsin that require management to avoid these adverse impacts. Experiments were established at two sites to determine if increasing grazing heights from fall through spring would reduce emergence and survival of burdock, Canada thistle, and plumeless thistle from seed. Canada and plumeless thistle seedling density was higher in treatments that were grazed to 5 cm in the fall and early spring compared to other treatments at one site.  In September, weed seedling survival was minimal, which prevented detections of differences between treatments.  Regression analysis demonstrated that this reduction was related to increased intercepted photosynthetic active radiation by forage in April.  Total forage yield was reduced when grazing heights were increased from the 5 cm to the 25 cm treatment.  However, no differences in total biomass were observed in the treatments with varying residual heights compared to the 15 cm treatment.  Increasing grazing heights in the fall and spring did reduce biomass in November at one site and in May at both sites, but resulted in increased biomass in June compared to other treatments. Forage quality was also reduced by increasing grazing heights, but differences were lower than desired values only in June.  Results suggest that increasing grazing heights can reduce emergence of Canada thistle and plumeless thistle, but can also result in reduction in forage quantity and quality at certain grazing events during the year.

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