Change season/timing of livestock grazing: brackish/salt marshes
Overall effectiveness category Awaiting assessment
Number of studies: 2
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.
Related actions: Exclude or remove livestock from historically grazed sites; Use grazing to maintain or restore disturbance; Use grazing to control problematic plants; Modify livestock farming practices in watershed.
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
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