Other biodiversity: Use seasonal grazing

Supporting evidence from individual studies

1. A replicated, randomized, controlled study in 1989–1991 in grasslands in north-central California, USA, found higher survival of, and less damage to, oak seedlings in plots grazed in winter, compared to plots grazed in spring or summer. Plants: More blue oak Quercus douglasii seedlings survived in winter-grazed plots, compared to spring- or summer-grazed plots (46–50% vs 7–29% survival). Seedlings in winter-grazed plots were less likely to be damaged by browsing or trampling, compared to those in spring- or summer-grazed plots (12–52% vs 40–93% of seedlings damaged). Methods: In December 1989, oak seedlings were planted in three pastures, each containing nine plots grazed for a week each in winter (January–February), spring (April), or summer (June–July), at one of three grazing intensities. Each plot received 24 seedlings (720 in total), of which half had the area around them treated with glyphosate herbicide to reduce competition from grass.

   Study and other actions tested

   Referenced paper

   Hall L.M., George M.R., McCreary D.D. & Adams T.E. (1992) Effects of cattle grazing on blue oak seedling damage and survival. Journal of Range Management, 45, 503-506.

2. A replicated, randomized, controlled study in 1993–1997 in grassland in north-eastern Israel (same study as (8)) found that the number of plant species and community composition varied between plots grazed by cattle in different seasons. Plants: Fewer plant species were found in seasonally-grazed plots, compared to continuously-grazed plots, in three of four years (28–50 vs 49–66 species/plot). Fewer plant species were found in late-grazed plots, compared to early-grazed, at moderate, but not heavy, stocking rates (28–42 vs 42–49 species/plot). Cover of annual thistles was higher for three out of four years in continuously grazed and early-grazed, compared to late-grazed plots (continuous: 4–20% cover; early: 5–18%; late: 1–11%). This effect was stronger at heavy, compared to moderate, stocking rates. Cover of crucifers was lower in late-grazed, compared to continuously grazed plots at heavy stocking rates (6–11% vs 11–20%). Other contrasts and eight other functional groups did not differ between treatments. Methods: In 1993, eight plots were established. Half were grazed continuously (January–October), and half were grazed seasonally. In seasonal plots, half of the plot was grazed early (January–April/May), and half was grazed late (April/May–October). In addition, half of the plots were moderately grazed (0.55 or 1.1 cow-calf pairs/ha for continuous and seasonal, respectively) and were heavily grazed (1.1 or 2.2 cow-calf pairs/ha). Plants were surveyed every spring, every two steps along permanent transects.

   Study and other actions tested

   Referenced paper

   Sternberg M., Gutman M., Perevolotsky A., Ungar E.D. & Kigel J. (2000) Vegetation response to grazing management in a Mediterranean herbaceous community: a functional group approach. Journal of Applied Ecology, 37.

3. A replicated, randomized, controlled study in grassland in central California, USA, found no difference in purple needlegrass Nassella pulchra mortality in plots grazed by sheep in spring, compared to summer. Needlegrass density was higher in spring-grazed plots in some comparisons, and needlegrass size increased by more in spring-grazed plots, compared to summer-grazed plots. Plants: There was no difference in mortality between spring- and summer-grazed plots (0–15% annual mortality for all). Needlegrass density was greater in spring-grazed plots, compared to summer-grazed, when the plots were also burned (0.8–0.9 vs 0.3 plants/m²), but not when they were unburned (0.5–0.9 plants/m²). Average basal area of needlegrass plants increased by more in spring-grazed plots, compared to summer-grazed (86% increase vs no increase). Methods: In 1988–1995, eighteen 20 x 20 m plots were maintained. One-third were ungrazed, one-third were grazed by sheep in spring, and one-third were grazed in summer each year. Half of the plots were also burned in 1988, 1991, and 1994.

   Study and other actions tested

   Referenced paper

   Dyer A.R. (2003) Burning and grazing management in a California grassland: growth, mortality, and recruitment of Nassella pulchra. Restoration Ecology, 11, 291-296.

4. A replicated, randomized, controlled study in 1995–1998 in forested pastures in central California, USA, found no difference in plant cover on stream banks or in grassy areas in pastures grazed by cattle in the dry season, compared to the wet season. Plants: There was no difference in plant cover on stream banks or in the surrounding grassy areas between pastures grazed in the dry season, compared to the wet season (31–84%). Methods: Two pastures in each of three streams were assigned to either dry- or wet-season grazing (July–October and October/November–May, respectively). Half of each were grazed at moderate intensity, and half at high intensity (reducing stubble to 2–3 and <2 inches, respectively). Plant cover was measured in June, on 10 transects running across the streams.

   Study and other actions tested

   Referenced paper

   George M.R., Larsen R.E., McDougald N.K., Tate K.W., Gerlach J.J.D. & Fulgham K.O. (2004) Cattle grazing has varying impacts on stream-channel erosion in oak woodlands. California Agriculture, 58, 138-143.

5. A replicated, randomized, controlled before-and-after study in 2000–2003 in wet grassland in central California, USA, found that the number of aquatic invertebrate species and native plant species, and the cover of native and exotic plants, varied between plots grazed at different times, in some comparisons. Invertebrates: There were more aquatic invertebrate species in pools in continuously-grazed plots, compared to seasonally-grazed plots, in one of three years (14 vs 11–12 species). Plants: The number of native plant species in pool edges declined more in 2001–2003 in wet-season grazed, compared to continuously or dry-season grazed plots (pool edges: 0.5 fewer species vs 1–1.8 more species). Changes within pools and on the surrounding dry land did not differ between plots with different grazing timings. There was a higher relative cover of native species in pool edges and surrounding dry land in continuously grazed plots, compared to seasonally grazed plots (pool edges: 72% vs 53–58%; dry land: 31% vs 17–18%). There was no difference in relative coverage of native species within pools between grazing timings. There was lower cover of exotic grasses in continuously grazed plots compared to dry- and wet-season grazed plots in dry land but not in other habitats (52% vs 69–84%). 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-season: October–November; wet-season: April–June). Before the experiment, the area had been grazed for at least 100 years.

   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.

6. A replicated, randomized, controlled study in 1997–2006 in former farmland in central Spain found that the plant cover varied between plots grazed by sheep in the spring and autumn, and this effect varied with irrigation, for two of seven functional groups of plants. Plants: Spring-grazed plots had lower cover of spring annuals than autumn-grazed plots, for two of three irrigation treatments (100–105% vs 81–91% cover). Spring-grazed plots had lower cover of large-seeded spring annuals, for all irrigation treatments (30–62% vs 17–51% cover). There were no differences in cover for five of seven functional groups. Methods: Twelve plots were established in 1997 (162 m² plots), with a combination of either spring grazing (April) or autumn grazing (November) and one of three irrigation regimes (none, spring and autumn, or year-round). Plots were grazed for one week each year at 5.2 or 4.3 sheep/ha for spring and autumn, respectively. Plants were surveyed in six 50 x 50 cm quadrats in each plot in May and September each year.

   Study and other actions tested

   Referenced paper

   Pérez-Camacho L., Rebollo S., Hernández-Santana V., García-Salgado G., Pavón-García J. & Gómez-Sal A. (2012) Plant functional trait responses to interannual rainfall variability, summer drought and seasonal grazing in Mediterranean herbaceous communities. Functional Ecology, 26, 740-749.

7. A meta-analysis from 2013 of 15 studies in coastal and interior grasslands in California, USA, found that seasonal grazing increased the cover of native plants, but continuous grazing decreased the cover of some native plants, compared to no grazing. Wet-season grazing decreased the cover of exotic grasses, compared to dry-season or continuous grazing. Plants: Native grasses and native forbs had higher cover with wet-season grazing, compared to no grazing (data reported as the response ratio of grazed to ungrazed plants: 0.96 for grasses; 0.44 for forbs). Native forbs also had higher cover with dry-season grazing, compared to no grazing (response ratio: 1.24), but native grasses did not. Native forbs had lower cover with continuous grazing, compared to no grazing (response ratio: –0.31), but native grasses did not. Lower cover of exotic grasses was found with wet-season grazing, compared to dry-season or continuous grazing (data not reported), but no differences were found in the cover of exotic forbs. Methods: The Web of Knowledge and Google Scholar databases were searched for publications from 1923 to 2011, using the keywords, “California” and “grassland” or “prairie”, or “grazing” or “livestock”, and 15 replicated studies from 1997 to 2009 were meta-analysed.

   Study and other actions tested

   Referenced paper

   Stahlheber K.A. & D’Antonio C.M. (2013) Using livestock to manage plant composition: a meta-analysis of grazing in California Mediterranean grasslands. Biological Conservation, 157, 300-308.

8. A replicated, randomized, controlled study in 1993–2012 in grassland in north-eastern Israel (same study as (2)) found that plant diversity and cover varied between plots grazed by cattle in different seasons, for four of five plant functional groups. Plants: Plant diversity was higher under early or continuous grazing, compared to late grazing (data not reported). Cover of tall annual grasses was higher in late-grazed plots, compared to early-grazed (34% vs 14% cover­), but only with moderate grazing, rather than heavy. Cover of tall perennial grasses was higher with one of six timing-intensity combinations, but this difference disappeared by the end of the study. Cover of short grasses was higher in early-grazed plots, compared to late-grazed (2–41% vs 1–24%), but similar to continuously grazed plots (1–37%). Cover of annual forbs was higher in early-grazed plots, compared to late-grazed, with moderate grazing, but not with heavy grazing (19–60% vs 4–39%). Cover of perennial forbs did not vary between grazing timings. Methods: In 1993, eight plots were established. Half were grazed continuously (January–October) and half were grazed seasonally. In seasonal plots, half of the plot was grazed early (January–April/May) and half was grazed late (April/May–October). In addition, half of the plots were moderately grazed (0.55 or 1.1 cow-calf pairs/ha for continuous and seasonal, respectively) and half were heavily grazed (1.1 or 2.2 cow-calf pairs/ha). Plants were surveyed every spring, every two steps along permanent transects.

   Study and other actions tested

   Referenced paper

   Sternberg M., Golodets C., Gutman M., Perevolotsky A., Ungar E.D., Kigel J. & Henkin Z. (2015) Testing the limits of resistance: a 19-year study of Mediterranean grassland response to grazing regimes. Global Change Biology, 21, 1939-1950.