Use grazing to control problematic plants: freshwater marshes
Overall effectiveness category Awaiting assessment
Number of studies: 3
Background information and definitions
This action involves using herbivores such as sheep, cows, horses or fish to control problematic plants. Herbivores remove shoots or flowers, limiting plant growth and/or reproduction. They might selectively graze certain plant groups or species (Grant et al. 1987), creating space for other species to grow.
Grazing may be a useful option for control of problematic plants over large areas, or areas that are not easily accessible to equipment. To ensure grazing actually occurs in wetlands within the larger landscape, the breed or population of herbivores should be carefully chosen. Herbivores adapted to upland areas may avoid wetland habitats altogether. Caution: Trampling, erosion and nutrient enrichment from herbivores can have negative impacts on vegetation, especially if the density of herbivores is high.
Bear in mind that the effects of grazing might be highly dependent on how it is carried out (e.g. species, intensity, timing, frequency and duration) and site conditions (e.g. nutrient availability, water levels, presence/density of wild herbivores) (Rinella & Hileman 2009).
For this action, “vegetation” refers to overall or non-target vegetation. Studies that only report responses of target problematic plants have not been summarized.
Related actions: interventions to address domestic livestock as a threat, e.g. Exclude livestock from historically ungrazed sites; Use grazing to maintain or restore disturbance.
Grant S.A., Suckling S.A., Smith H.K., Torvell L., Forbes T.D.A. & Hodgson J. (1987). Comparative studies of diet selection by sheep and cattle: blanket bog and heather moor. Journal of Ecology, 75, 947–960.
Rinella M.J. & Hileman B.J. (2009) Efficacy of prescribed grazing depends on timing intensity and frequency. Journal of Applied Ecology, 46, 796–803.
Supporting evidence from individual studies
A replicated, paired, controlled, before-and-after study in 2005–2007 in four freshwater marshes invaded by reed canarygrass Phalaris arundinacea in Nebraska, USA (Hillhouse et al. 2010) found that grazing had no significant effect on plant species richness, overall vegetation cover, or the abundance of reed canarygrass (both absolute and relative). Over two years, grazed and ungrazed plots experienced statistically similar changes in plant species richness (data not reported) and overall vegetation cover (grazed: decline from 8% to 3%; ungrazed: decline from 8% to <1%). The same was true for reed canarygrass absolute cover (grazed: decline from 8% to 2%; ungrazed: decline from 8% to <1%) and relative abundance (grazed: decline from 93% to 68% of recorded plants; ungrazed: decline from 96% to 68% of recorded plants). The study also reported increases in bare ground cover and decreases in litter cover in grazed plots – whereas the opposite was true in ungrazed plots (see original paper for data). Methods: Three 3–8 ha plots were established in each of four depressional marshes, in dense stands of reed canarygrass. Eight plots (two plots/marsh) were grazed in both 2006 and 2007 (at some point between April and August; 20–40 animal units for 10–49 days/year). The other four plots (one plot/marsh) were left ungrazed. Plant species and vegetation cover were recorded at points along transects (number of points not clearly reported) before grazing (2005) and after 1–2 years of grazing (July–August 2006 and 2007).Study and other actions tested
A replicated, paired, controlled, before-and-after study in 2008 in a wet meadow invaded by purple loosestrife Lythrum salicaria and reed canarygrass Phalaris arundinacea in New York State, USA (Kleppel & LaBarge 2011) found that grazed paddocks had higher plant species richness and greater cover of non-invasive plants than ungrazed paddocks. After two months, grazed paddocks contained more plant species in total (grazed: 25; ungrazed: 20 species/20 m2) and per quadrat (grazed: 4.0; ungrazed: 2.6 species/0.25 m2). Grazed paddocks had lower cover than ungrazed paddocks of the key invasive species: purple loosestrife (grazed: 20%; ungrazed: 65%) and reed canarygrass (grazed: 20%; ungrazed: 50%). Accordingly, grazed paddocks had higher cover of other grass-like plants (40%) than ungrazed paddocks (20%). Before intervention, cover of these plant groups was statistically similar in paddocks destined for each treatment (loosestrife: 50%; canarygrass: 43–45%; other grass-like plants: 20–30%). Methods: Four pairs of 200-m2 paddocks were established in an invaded wet meadow. Between 16 June and 3 August 2008, one plot/pair was rotationally grazed by sheep (two ewes/paddock for 2–3 days every two weeks). Detailed vegetation surveys were carried out after intervention (mid-August 2008; 20 quadrats/paddock). Cover was also surveyed before intervention (early June 2008).Study and other actions tested
A replicated, randomized, paired, controlled study in 2007–2008 in an ephemeral freshwater marsh in Costa Rica (Osland et al. 2011) found that amongst plots in which invasive southern cattail Typha domingensis was damaged, cattle grazing typically had no significant effect on the overall plant community composition, diversity or richness. Over 15 months, grazed and ungrazed plots had a statistically similar overall plant community composition (five of five comparisons; data not reported) and plant diversity (five of five comparisons; data reported as a diversity index). Plant species richness did not significantly differ between treatments in three of five comparisons (grazed: 5–10; ungrazed: 6–11 species/3 m2) but was lower in grazed plots in the other two (grazed: 4–7; ungrazed: 6–8 species/3 m2). After both three and 15 months, cattail properties did not significantly differ between grazed and ungrazed plots. This was true in terms of height (grazed: 7–74; ungrazed: 21–73 cm), density (grazed: 1–4; ungrazed: 1–4 shoots/m2) and dry above-ground biomass (grazed: 0–135; ungrazed: 5–95 g/m2). Methods: In February 2007, cattail-dominated vegetation was damaged (by driving over it in a tractor with large paddle wheels) in 15 pairs of 20-m2 plots. Cattle were then allowed to graze one plot in each pair. The other plots were fenced to exclude cattle. After 2–16 months, vegetation was surveyed in three 1-m2 quadrats/plot.Study and other actions tested