Providing evidence to improve practice

Action: Water: Exclude grazers Mediterranean Farmland

Key messages

Water use (0 studies)

Water availability (4 studies): Four studies (three replicated, randomized, and controlled) in grasslands and shrublands in the USA and Spain found less water in areas with cattle and sheep excluded, compared to grazed areas, in some or all comparisons.

Pathogens and pesticides (0 studies)

Nutrients (2 studies): Two replicated, randomized, controlled studies in wet grasslands in the USA found inconsistent differences in nitrogen, phosphorus, and pH in surface water in areas with cattle excluded, compared to grazed areas. One of these studies found more nitrate in stream water in ungrazed areas, compared to grazed areas, in one of two experiments.

Sediments (1 study): One replicated, randomized, controlled study in wet grasslands in the USA found no difference in surface water turbidity between areas with cattle excluded and grazed areas.

Supporting evidence from individual studies

1 

A replicated, randomized, controlled study in 1992–1996 in grazed wetlands in northern California, USA, found no differences between ungrazed and cattle-grazed plots in nitrate or pH levels in surface water. A separate three-year experiment (1999–2001) found higher nitrate levels in streams in ungrazed plots, compared to grazed plots. Nutrients: A five-year experiment found no differences between ungrazed and grazed plots in nitrate or pH levels in surface water (data not reported). A three-year experiment found higher nitrate levels in streams in ungrazed plots, compared to grazed plots (81–1,200 vs 23–100 micromoles). Methods: A five-year experiment from 1992–1996 was established in three meadows. Within each meadow, three watersheds were randomly assigned to one grazing intensity: cattle excluded, light grazing (leaving 800–1,000 pounds of residual dry matter at the end of the season), or moderate grazing (leaving 600–700 pounds). Samples were taken from the spring and along the creek in each watershed. The second experiment was in 1999–2002 in marshy areas in four meadows. Two plots were established in each meadow: one ungrazed and one with moderate grazing. Water samples were taken monthly.

 

2 

A replicated, randomized, controlled study in 2000–2003 in wet in alpine meadows in central California, USA (same study as (5)), found that pools in plots from which cattle were excluded were wet for less time than those with two of three grazing regimes, and they dried more frequently than those in plots with one of three grazing regimes. Water availability: The maximum time that pools were wet was lower in ungrazed plots, compared to grazed plots, for two of three grazing regimes (65 vs 78–115 days), but not compared to plots that were grazed in the wet season. During a particularly dry year, pools in ungrazed plots dried more frequently than those in continuously-grazed plots, but not seasonally-grazed plots (2 vs 1 drying episodes). Methods: Eighteen plots were established in 2000, each with three pools (70–1,130 m2) and nine times more dry land than pool. Areas were grazed continuously or seasonally (dry: October–November; wet: April–June). Before the experiment, the area had been grazed for at least 100 years.

 

3 

A replicated, randomized, controlled study in rangelands in central California, USA, found that temporary pools dried earlier in plots from which grazers were excluded, compared to cattle-grazed plots. Water availability: Temporary pools were wet for less time in ungrazed plots, compared to grazed plots (maximum of 65 vs 115 days). Methods: Thirty-six pools in 12 groups on a cattle ranch were studied, 18 of which (six groups) were fenced to exclude cattle. The rest of the ranch was grazed at a density of one cow-calf pair/ha. Pools were monitored each week in the rainy season.

 

4 

A replicated, randomized, controlled study in 2006–2010 in alpine meadows in central California, USA, found that most measures of water quality did not change in pools in meadows from which cattle were excluded, compared to pools in grazed meadows. Nutrients and Sediments: There was no change over time in ungrazed meadows, compared to grazed meadows, in total nitrogen concentration (0.4–1.5 ppm), nitrate-nitrogen (0.006–0.016 ppm), dissolved organic carbon (4.5–9.2 ppm), turbidity (reported as nephelometric turbidity units), or pH (6.1–6.8) in pools. Ammonium-nitrogen, soluble reactive phosphorus, and total phosphorus differed between meadows with different grazing regimes, in some years, but there was no clear pattern (details not reported). Methods: Nine meadows were studied, with cattle completely excluded from three meadows in 2006–2008, excluded from Yosemite toad Bufo canorus breeding habitat in three meadows, or not excluded (grazed over summer). All meadows were grazed for at least a decade before the study. Water quality was sampled each summer.

 

5 

A replicated, randomized, controlled study in 2002–2010 in central California, USA (same study as (2)), found that pools in plots from which cattle were excluded were shallower and wet for less time than those in grazed plots. Water availability: Pools were shallower in ungrazed plots, compared to grazed plots (8 vs 12 cm maximum depth), and were wet for fewer days each year (16–178 vs 41–192 wet days/year). Differences were more pronounced in drier years.  Methods: Eighteen plots were established in 2000, each with three pools (70–1,130 m2) and nine times more dry land than pool. Areas were grazed continuously or seasonally (dry: October–November; wet: April–June). Before the experiment, the area had been grazed for at least 100 years. Pools were monitored in 2002–2010.

 

6 

A replicated site comparison in 2008–2010 in shrubland in central Spain found less soil moisture in ungrazed plots, compared to sheep-and-cattle-grazed plots. Water availability: Less soil moisture was found in ungrazed plots, compared to grazed plots (4.4–6.5% vs 6.3–7.7%). Methods: Eight holm oak Quercus ilex trees were selected in each of two grazed and two ungrazed areas. Soils surrounding four trees in each area were tilled in April 2008. Soil moisture at 10 cm depth was measured nine times in July 2008–February 2010.

 

Referenced papers

Please cite as:

Shackelford, G. E., Kelsey, R., Robertson, R. J., Williams, D. R. & Dicks, L. V. (2017) Sustainable Agriculture in California and Mediterranean Climates: Evidence for the effects of selected interventions. Synopses of Conservation Evidence Series. University of Cambridge, Cambridge, UK.