Change season/timing of livestock grazing: brackish/salt marshes

Overall effectiveness category Likely to be beneficial
Number of studies: 2
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How is the evidence assessed?

Effectiveness

40%
Certainty

40%
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

Two studies evaluated the effects, on vegetation, of grazing brackish/salt marshes in different seasons or at different times. One study was in the USA and one was in the Netherlands. In both studies, the focal livestock were cattle.

VEGETATION COMMUNITY

Community composition (1 study): One replicated, randomized, paired, controlled, before-and-after study on a salt marsh in the Netherlands found that plots grazed annually by 0.5 cattle/ha and plots grazed biennially by 1.0 cattle/ha experienced a similar turnover of plant species over six years, and had a similar overall plant community composition after six years.
Overall richness/diversity (1 study): The same study found that plots grazed annually by 0.5 cattle/ha and plots grazed biennially by 1.0 cattle/ha experienced similar increases in plant species richness over six years, and had similar species richness after six years.

VEGETATION ABUNDANCE

Overall abundance (1 study): One replicated, randomized, paired, controlled, before-and-after study in alkali marshes in the USA found that summer- and autumn-grazed plots experienced similar changes in live vegetation biomass, over one year.
Individual species abundance (1 study): One replicated, randomized, paired, controlled, before-and-after study on a salt marsh in the Netherlands found that grazing annually with 0.5 cattle/ha stimulated greater increases in cover of sea aster Aster tripolium than grazing biennially with 1.0 cattle/ha. There was no significant difference between the grazing regimes for cover of sea couch grass Elytrigia atheria. Vegetation was monitored over six years.

VEGETATION STRUCTURE

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, before-and-after study in 1998–1999 in six fields containing ephemeral alkali marshes in Idaho, USA (Austin et al. 2007) found that summer and autumn grazing had similar effects on vegetation biomass. Over one year including a period of grazing, changes in live above-ground plant biomass were statistically similar in summer-grazed alkali marshes (non-significant decrease of 30 g/m²) and autumn-grazed alkali marshes (non-significant increase of 30 g/m²). Methods: The study used three pairs of fields around a lake. Each field contained a range of wetland habitats, including alkali flats (seasonally flooded; developed salt crust in summer). 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, randomized, paired, controlled, before-and-after study in 2009–2013 on a salt marsh in the Netherlands (Lagendijk et al. 2017) found that annual low-intensity and biennial high-intensity cattle grazing had statistically similar effects on plant community composition and plant species richness, but that annual low-intensity grazing increased cover of one of two focal herb species. After six years, plots grazed under each regime contained a similar overall plant community (data not reported) and plant species richness (annual: 8.4 species/16 m²; biennial: 7.8 species/16 m²). Over six years, plots grazed under each regime experienced a similar turnover of plant species (data reported as a turnover index), similar increases in plant species richness (annual: gain of 1.9–2.9 species/16 m²; biennial: gain of 0.5–2.3 species/16 m²) and a similar lack of change in sea couch grass Elytrigia atheria cover (annual: 2% change; biennial: 3% change). However, sea aster Aster tripolium cover increased by 27% in annual low-intensity plots, but only 8% in biennial high-intensity plots. Methods: In 2009, two pairs of 11-ha plots were established on a coastal salt marsh. From 2010, one random plot/pair was grazed by 0.5 cattle/ha every summer, whilst one random plot/pair was grazed by 1 cattle/ha every other summer. Plant species and their cover were recorded in August/September 2009 (after a summer of intense grazing to standardize plots) and 2010–2015 (during cattle grazing treatments). Surveys were carried out in eight 16-m² quadrats/plot/year.
Study and other actions tested
Referenced paper
Lagendijk D.D.G., Howison R.A., Esselink P., Ubels R. & Smit C. (2017) Rotation grazing as a conservation management tool: vegetation changes after six years of application in a salt marsh ecosystem. Agriculture, Ecosystems & Environment, 246, 361-366.

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
Reduce intensity of livestock grazing: freshwater marshes Action Link	Likely to be beneficial	3	Synopsis Link
Change season/timing of livestock grazing: freshwater marshes Action Link	Likely to be beneficial	3	Synopsis Link
Reduce intensity of livestock grazing: brackish/salt marshes Action Link	Likely to be beneficial	9	Synopsis Link