Change season/timing of livestock grazing: freshwater marshes

Overall effectiveness category Likely to be beneficial
Number of studies: 3
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How is the evidence assessed?

Effectiveness

50%
Certainty

42%
Harms

0%

Calculating overall effectiveness category

Background information and definitions

Grazing could have different effects on wetland vegetation depending on the time of year at which it is done. For example, it might be beneficial to avoid grazing when certain plants are young/flowering so that they can grow/reproduce and contribute to the wetland vegetation. Additionally, the effects of trampling may vary by season, being lowest in summer when a seasonal wetland might be dry or winter when the soil may be frozen. Seasonal variation in the value of the wetland plants as food for livestock could also contribute to the decision of when to allow grazing.

To be summarized as evidence for this action, studies should have compared grazing in different seasons (e.g. summer vs winter) or in different temporal patterns (e.g. 0.5 cows/ha every summer vs 1 cow/ha every other summer). The overall grazing intensity and type of livestock must have been similar under each treatment.

Study locations

Key messages

Three studies evaluated the effects, on vegetation, of grazing freshwater marshes in different seasons or at different times. Two studies were in the USA and one was in Canada. In all three studies, the livestock were cattle.

VEGETATION COMMUNITY

Community composition (1 study): One replicated, randomized, paired, controlled, before-and-after study of freshwater marshes and wet meadows in the USA reported that plots grazed in the summer and autumn experienced similar changes in overall plant community composition over a year.
Relative abundance (1 study): One replicated, randomized, paired, controlled study of ephemeral pools in the USAfound that pools grazed in the dry or wet seasons had similar cover of grasses relative to forbs over three years.
Overall richness/diversity (1 study): One replicated, site comparison study of freshwater marshes in Canada found that in summer, marshes grazed in the summer/autumn contained more plant genera than marshes grazed in the spring/summer.
Native/non-target richness/diversity (1 study): One replicated, randomized, paired, controlled study of ephemeral pools in the USA found that pools grazed in the dry and wet seasons experienced similar changes in native plant richness over three years.

VEGETATION ABUNDANCE

Overall abundance (1 study): One replicated, randomized, paired, controlled, before-and-after study of freshwater marshes and wet meadows the USA found that, in three of four habitat types, summer- and autumn-grazed plots experienced similar changes in live vegetation biomass over one year.

VEGETATION STRUCTURE

Height (1 study): One replicated, site comparison study of freshwater marshes in Canada found that in summer, marshes grazed in the summer/autumn contained taller emergent vegetation than marshes grazed in the spring/summer.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

A replicated, randomized, paired, controlled study in 2000–2003 of ephemeral pools in a grassland in California, USA (Marty 2005) found that dry season and wet season grazing had similar effects on relative cover of grasses and native plants, and on native plant richness. In three of three years, pools had similar cover of grasses relative to forbs regardless of the grazing season (dry-season-grazed: grass cover 46–55% of forb cover; wet-season-grazed: grass cover 48–54% of forb cover) and had similar cover of native plants relative to non-natives (data not reported). Over three years, pools experienced statistically similar changes in native plant richness regardless of the grazing season (dry-season-grazed: 0.5–1.0 more species/0.25 m²; wet-season-grazed: 0.6 fewer to 1.2 more species/0.25 m²). Methods: In 2000, six pairs of plots were established on a ranch grazed October–June for >100 years. Between 2000 and 2003, one plot/pair was grazed in the dry season (October–November and April–June) and one plot/pair was grazed in the wet season (December–April). All plots were grazed by 1 cow-calf pair/2.4 ha. Cattle access was controlled by electric fences. Each spring between 2001 and 2003, vegetation was surveyed in three pools/plot and in adjacent upland. Pools were 70–1,130 m² and dry when surveyed.
Study and other actions tested
Referenced paper
Marty J.T. (2005) Effects of cattle grazing on diversity in ephemeral wetlands. Conservation Biology, 19, 1626-1632.
A replicated, randomized, paired, controlled, before-and-after study in 1998–1999 in six fields containing a range of freshwater marsh and wet meadow habitats in Idaho, USA (Austin et al. 2007) found that summer and autumn grazing typically had similar effects on plant communities and on vegetation biomass. Over one year including a period of grazing, summer- and autumn-grazed plots experienced similar changes plant community composition (if any) in all four freshwater habitat types (data presented as graphical analyses; statistical significance of differences not assessed). In three of four freshwater habitat types, summer- and autumn-grazed plots experienced similar changes in live above-ground biomass (summer-grazed: decrease of 290 g/m²to increase of 60 g/m²; autumn-grazed: decrease, but not always significant, of 5–140 g/m²). For the other vegetation type, plant biomass declined in summer-grazed plots (by 350 g/m²) but did not change in autumn-grazed plots (non-significant increase of 20 g/m²). Methods: The study used three pairs of fields around a lake. Each field contained a range of freshwater habitats, from permanently flooded marshes to ephemeral wet meadows. All fields had been historically grazed and cut, but were undisturbed from 1996. In each pair, one random field was grazed July–August 1998 and the other was grazed September–October 1998 (both by cattle, at 2.3–2.5 animal unit months/ha; one AUM is the amount of feed required to sustain a 1,000-lb cow and her calf for one month). Vegetation was surveyed in June–July before (1998) and after (1999) one season of grazing.
Study and other actions tested
Referenced paper
Austin J.E., Keough J.R. & Pyle W.H. (2007) Effects of habitat management treatments on plant community composition and biomass in a montane wetland. Wetlands, 27, 570-587.
A replicated, site comparison study in 2008 of 13 ephemeral prairie pothole marshes in two pastures in Alberta, Canada (Silver & Vamosi 2012) found that summer/autumn-grazed marshes contained more wetland plant genera, and taller vegetation, than spring/summer-grazed marshes. The average number of wetland plant genera was significantly greater in the summer/autumn-grazed than the spring/summer-grazed marshes (data not reported). Three genera were only ever found in summer/autumn-grazed marshes: pondweeds Potamogeton spp., sedges Carex spp. and buttercups Caltha spp. Emergent vegetation was also significantly taller, on average, in summer/autumn-grazed (59 cm) than spring/summer-grazed marshes (32 cm). Methods: In July 2008, vegetation was surveyed in 13 ephemeral marshes: seven in one pasture grazed May–June and six in one adjacent pasture grazed August–October. Both pastures had been grazed by cattle (density not reported) under these regimes since 1994. All plant genera were identified in each marsh. For emergents, five random plants/genus/marsh were measured.
Study and other actions tested
Referenced paper
Silver C.A. & Vamosi S.M. (2012) Macroinvertebrate community composition of temporary prairie wetlands: a preliminary test of the effect of rotational grazing. Wetlands, 32, 185-197.

Please cite as:

Taylor N.G., Grillas P., Smith R.K. & Sutherland W.J. (2021) Marsh and Swamp Conservation: Global Evidence for the Effects of Interventions to Conserve Marsh and Swamp Vegetation. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

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This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

Related Actions

Actions	Effectiveness	Studies	Category
Exclude or remove livestock from historically grazed freshwater marshes Action Link	Likely to be beneficial	10	Synopsis Link
Reduce intensity of livestock grazing: freshwater swamps Action Link	No evidence found (no assessment)	0	Synopsis Link
Change season/timing of livestock grazing: brackish/saline swamps Action Link	No evidence found (no assessment)	0	Synopsis Link