Action: Reduce grazing intensity on grassland (including seasonal removal of livestock)
- Of 27 individual studies (including 10 replicated, controlled trials, four reviews and one systematic review) from France, Germany, Ireland, the Netherlands and the UK, 15 (including three randomized, replicated, controlled trials) from four countries found benefits to birds, plants or invertebrates in response to reducing grazing intensity on permanent grassland (including seasonal removal of livestock).
- Of these 15 studies, six (including one randomized, replicated, controlled trial) found that reducing grazing intensity throughout the year increased the abundance and diversity of plants (Tallowin et al. 2005, Marriott et al. 2009), frequency of certain plant species, invertebrate diversity, usage by geese and the number of northern lapwing and common redshank. Six studies (including three replicated controlled trials of which two randomized) found that excluding or delaying summer grazing increased plant species diversity, invertebrate abundance and benefited breeding Eurasian skylark. A review found a study that showed that removing autumn grazing after a silage cut increased the winter abundance of seed-eating birds. A review and a replicated controlled study from the UK found that reduced grazing intensity or seasonal removal of livestock increased the number of invertebrates, plant seed heads and foraging skylark, and that some bird species preferred plots with seasonal removal of livestock.
- Three studies (including one randomized, replicated, controlled trial) from the Netherlands and the UK found no benefit to plants or invertebrates from reduced grazing intensity. One randomized, replicated controlled trial excluded grazing in autumn/winter and another study excluded grazing in the summer. A further study found that reducing grazing intensity throughout the year did not increase plant diversity.
- Nine studies from France, Germany and the UK reported mixed results for some or all species or wildlife groups considered (including one randomized, replicated, controlled trial and two reviews and a systematic review). Of these, eight studies found that reduced grazing intensity throughout the year benefited some species but not others, one found that the impact depended on the type of vegetation grazed, and one found benefits to bee and wasp abundance but not species richness. One study found that the response of birds to removal of summer grazing varied between functional groups and depended on time of year. A UK review found that reduced grazing benefited invertebrates, plants, rodents and some but not all birds.
- A systematic review of the effects of grazing intensity on meadow pasture concluded that intermediate levels of grazing are usually optimal for plants, invertebrates and birds but that trade-offs are likely to exist between the requirements of different taxa.
Studies summarized within this section tested whether reducing livestock grazing intensity on permanent grassland (including seasonal removal of livestock) increased the abundance or species richness of plants, invertebrates or other wildlife. Permanent grassland habitats studied include species-poor improved grass leys, species-rich grasslands and grazed marshes or fens. Studies tested the effectiveness of reducing grazing intensity throughout the year or seasonally removing grazing, for example during or after silage-making or by seasonally excluding grazing in fenced headlands.
See also ‘Maintain upland heathland/moorland’ for the effects of reduced grazing intensity on unenclosed upland grassland areas (including upland acid grasslands).
Supporting evidence from individual studies
A before-and-after study in Gloucestershire, England (Owen 1977) found that the proportion of geese on a grassland site using a specifically managed 130 ha area increased from 33% in the winter of 1970-1971 to 87% by 1975-1976, following a reduction in grazing intensity over this period. Starting in 1970, stock were sequentially removed from three sections of the area: the first was ungrazed from the 30th September, the second from the 31st October and the third from the 30th November. A fourth area was not grazed at all.
A study of a perennial rye grass Lolium perenne and white clover Trifolium repens ley grassland in Ireland (Purvis & Curry 1981) found that management-induced changes in grass height had immediate effects on arthropod abundance. Arthropod abundance was greatest (up to 10,351/m²) in taller/denser silage grass and lowest (as few as 394/m²) in short grass subject to periodic heavy sheep-grazing. The abundance of most groups, particularly larger insects, increased in areas left for silage, whilst numbers of mites (Acari), springtails (Collembola) and many other taxa decreased most under heavy grazing and after cutting. The cropping systems had no overall influence on the range of dominant taxa. The five treatments were intermittent grazing, silage cut only, silage cut followed by grazing, grazing and silage, and continuous grazing. Inorganic fertilizers were applied as appropriate. Plant-dwelling arthropods were sampled at random (ten samples of 0.1 m²) in each plot using a 'D-Vac' suction net monthly from May to September 1976 and in March 1977.
A controlled, replicated trial in 1985-1989 on grassland in Oxfordshire, England (Gibson et al. 1992) found that as sheep grazing intensity increased, the number of species of bugs (Heteroptera), herbivorous beetles (Coleoptera), leafhoppers (Auchenorrhyncha), leaf miners and spiders (Araneae) decreased. In the most intensively grazed treatments, the total number of species of bugs, herbivorous beetles, leafhoppers, leaf miners and spiders was 11, 16, 17, 16 and 17 respectively, compared to 19, 24, 20, 34 and 25 in the least grazed treatment. Grazing treatments began in 1985. Three treatments were replicated six times in 30 x 30 m paddocks (ungrazed control, short-period spring and short-period autumn grazing) and two treatments were applied in larger areas (spring-and-autumn grazing and long-period autumn grazing, not replicated). Plants were surveyed four times a year in 12 quadrats (1 m2) in each replicate.
A trial from 1986 to 1989 on a grassland in the Netherlands (Neuteboom et al. 1994) found no difference in the number of plant species between three different cattle stocking rates. No fertilizer was applied during the study. By 1990, only two herb species (cuckoo flower Cardamine pratensis and meadow buttercup Ranunculus acer now R. acris) were present in more than 5% of 100 vegetation samples. The authors suggest that plant diversity did not increase because of the dense growth of a small number of competitive grass species. Species that could have colonized were present on the field borders and ditches. The 6.6 ha grassland had been grazed at a rate of five steers/ha from April to July, then 3.5 steers/ha until October, from 1973 until 1985. From 1986 onwards it was divided into three 2.2 ha paddocks, each grazed at a single fixed stocking rate - either 2.3, 3.6 or 4.9 steers/ha from April to October.
A replicated study in 1992 and 1993 within the South Downs Environmentally Sensitive Area, Sussex, UK (Wakeham-Dawson 1995) found that breeding Eurasian skylarks Alauda arvensis avoided heavily grazed pasture. Grassland that was heavily grazed by sheep was generally not used by skylarks during the second brood period, whereas areas that were not grazed or mown until mid-July onwards were used for both first and second broods. Four arable, 10 mixed and three pastoral farms were studied. Skylarks were sampled by mapping breeding males during two counts along transects on 12-17 farms from April to June.
A small randomized study over one year of a neutral meadow grassland at a farm in England (Smith et al. 1996) found that only four plant species exhibited significant differences in seed number as a result of grazing intensity. More Yorkshire fog Holcus lanatus was recorded when there was no grazing, more yellow oat grass Trisetum flavescens with autumn grazing and more downy oat-grass Avenula pubescens and creeping buttercup Ranunculus repens with autumn and spring grazing. The three grazing treatments were randomly applied to three plots. Vegetation was cut at 3 cm in three randomly placed 0.06 m² quadrats and seed collected.
A randomized, replicated, controlled study in spring and summer 1995 and 1996 in Sussex, England (Wakeham-Dawson et al. 1998), found that the densities of Eurasian skylark Alauda arvensis, grass seed heads and invertebrates were significantly higher on fields grazed at lower intensities (4.4-14.3 skylarks/km2, 155 seed heads/m2 and 17-112 invertebrates/sample on six lightly-grazed fields vs 1.3-2.4 skylarks/km2, 9.6 seed heads/m2, 10-68 invertebrates/sample and 4-16 invertebrate taxa/sample on six intensively-grazed fields). The density of carrion crows Corvus corone and rooks C. frugilegus did not vary between grazing intensities, nor did the number of invertebrate taxa in 1996 (1995: 12-17 invertebrate taxa/sample on lightly grazed fields vs 6-11 on intensively-grazed; 1996: 6-16 on lightly-grazed fields vs 4-16 on intensively-grazed). In 1996 there were significantly more individuals and taxa of spiders (Araneae) in lightly-grazed fields, but no differences in beetles (Coleoptera), flies (Diptera), bees, wasps and ants (Hymenoptera) or larvae. Twelve reverted permanent grassland fields (each field 5 ha) sown with perennial rye grass Lolium perenne, cocksfoot Dactylis glomerata and white clover Trifolium repens were studied. Sheep were used to control the grass height in fields between April and July each year: intensively-grazed fields were managed to keep the grass under 10 cm long, less intensively managed fields had a grass height of 15-25 cm. Skylarks were counted and their locations recorded every 10 days from April-June 1995 and April-July 1996. Foraging rooks were surveyed in 1995 and 1996, carrion crows were surveyed in 1996. Invertebrates were sampled at five locations/field in 1995 and 1996 using a D-Vac suction trap, and in pitfall traps in 1996.
A randomized, replicated, controlled trial in 1997-1998 on permanent pasture at three sites in Dumfries and Galloway, Scotland, UK (Haysom et al. 2000) found that fencing of field headlands to prevent grazing during the summer, substantially increased the abundance of three key chick-food insect groups. After one year, headlands protected from summer grazing had 19-32 times more chick-food insects (609 true bugs (Hemiptera), 75 sawfly larvae (Symphyta) and 18 caterpillars (Lepidoptera), on average per 10 samples) than grazed headlands (19 true bugs, 2 sawfly larvae and 1 caterpillar). Treatments were carried out from spring 1997 in adjacent plots (10 x 50 m long) on the boundaries of seven pasture fields: unfenced unsprayed, unfenced sprayed, fenced (May-September) unsprayed, and fenced (May-September) sprayed. In sprayed plots, herbicide was applied in April 1997 to clear strips to trial a method for increasing foraging access for birds. Unfenced plots were grazed by cattle and sheep during summer, and all plots were intermittently grazed by sheep during winter. Insects were sweep net sampled in June and July in 1997 and 1998.
A 2000 literature review of grassland management practices in the UK (Wakeham-Dawson & Smith 2000) found one study that reported that grass maintained at a height of 15-25 cm supported twice the number of invertebrates compared with grass grazed by sheep to less than 10 cm. In particular, reduced grazing resulted in significantly more web-spinning spiders (Araneae) and the number of grass seed heads in July was 15 times that in heavily grazed fields (Wakeham-Dawson et al. 1998). Two studies reported that taller or ungrazed grassland supported greater densities of rodents and predatory birds (Dodds et al. 1995, Shaw 1995). In contrast, increased vegetation height following a reduction in grazing by livestock and or rabbits Oryctolagus cuniculus has been found to result in a decrease in the number of breeding Eurasian thick-knee (stone curlew) Burhinus oedicnemus, woodlark Lullula arborea and northern wheatear Oenanthe oenanthe; however the number of Eurasian curlew Numenius arquata increased (Dolman & Sutherland 1992, Green & Taylor 1995, Bealey et al. 1999).
Dolman P.M. & Sutherland W.J. (1992) The ecological changes of Breckland grass heaths and the consequences of management. Journal of Applied Ecology, 29, 402-413.
Dodds G.W., Appleby M.J. & Evans A.D. (1995) A Management Guide to Birds of Lowland Farmland. Sandy: Royal Society for the Protection of Birds.
Green R. E & Taylor C.R. (1995) Changes in stone-curlew Burhinus oedicnemus distribution and abundance and vegetation height on chalk grassland at Porton Down, Wiltshire. Bird Study 42: 177-181.
Shaw G. (1995) Habitat selection by short-eared Owls Asio flammeus in young coniferous forests. Bird Study, 42, 158-164.
Bealey C E., Green R.E., Robson R., Taylor C.R. & Winspear R. (1999) Factors affecting the numbers and breeding success of stone-curlews Burhinus oedicnemus at Porton Down, Wiltshire. Bird Study, 46, 145-156.
A replicated comparison of six intensively (5.5 cattle/ha) and six lightly (1.5 cattle/ha) cattle-grazed meadows with six ungrazed meadows in Germany (Kruess & Tscharntke 2002) found that meadows with light grazing had a greater number of individual cavity-nesting bees, wasps (Hymenoptera) and their brood parasites than meadows with intensive grazing. There was an average of 47 emerging individuals/lightly grazed site, compared to 27 emerging individuals/intensively grazed site. Reduced intensity of grazing did not significantly increase the number of bee and wasp species. Both abundance and total species richness of these insects were significantly higher on ungrazed grassland (11.5 species) than on intensively (4.7 species) or lightly (6.2 species) grazed pastures. These results were linked to an increase in vegetation height as grazing intensity is reduced.
A 2004 review of experiments on the effects of agri-environment measures on livestock farms in the UK (Buckingham et al. 2004) found one randomized, replicated trial in Scotland showing that grass field headlands left ungrazed in summer had more insects in groups known to be food for game birds (true bugs (Heteroptera), butterflies and moths (Lepidoptera) and sawflies (Symphyta)) than grazed plots (this study also reported in (Haysom et al. 2000)). Another experiment (no reference given in the review) showed seed-eating birds were more abundant in winter on uncut silage plots left ungrazed than on plots grazed from September. Two other experiments examined effects of reduced grazing intensity, but bird numbers using experimental plots were probably too low for analysis. The review assessed results from seven experiments (some incomplete at the time of the review) in the UK and Europe.
A before-and-after study of grazing marshes in eastern England (Smart & Coutts 2004) investigated the effect of reducing grazing intensity and improving footdrain management on breeding wading bird numbers from 1993 to 2003. Northern lapwing Vanellus vanellus numbers increased from 19 pairs in 1993 to 85 pairs in 2003 and common redshank Tringa totanus rose from four to 63 pairs. Numbers of winter wildfowl also increased over the period and changes in vegetation communities to those more tolerant of flooding occurred. Grazing intensity was reduced from 1.5-2 head of cattle to 0.7 head/ha and fertilizer inputs were stopped. In 1993, water levels were raised by 45 cm. From 1995, approximately 600 m of footdrains were opened/year; from 2000 onwards, approximately 2,000 m of footdrains were opened or added.
A controlled study from 2000 to 2004 in the UK (Tallowin et al. 2005) found that reduced grazing pressure maintained botanical diversity and abundance, enhanced invertebrate diversity and abundance, but increased pernicious weeds on species-rich grasslands. The cover of positive indicator species of grasslands of high nature conservation value remained stable under lenient grazing pressure (8-10% cover), but decreased under severe (from 9% to 5%) and decreased (from 9% to 5%) but then recovered (8%) under moderate grazing pressures. Competitive grass and legume species increased across all treatments. Abundance and diversity of bumblebee Bombus spp. species was higher under moderate (0.34 individuals counted/minute) and lenient grazing (0.38) than severe grazing pressure (0.15). Spider numbers were also significantly higher under lenient (118 individuals/m²) than severe or moderate grazing (68 individuals/m²). Percentage cover of spear thistle Cirsium vulgare and creeping thistle Cirsium arvense increased under all three grazing pressures, but did not differ between treatments (2000: 0.4-1.0% cover; 2004: 2.9-3.6). Species-rich grasslands were grazed over five years with severe (grass height 6 to 8 cm), moderate (8-10 cm) or lenient (10-12 cm) cattle grazing pressures. A fertilized improved pasture was maintained at 6-8 cm as a control.
A 2006 review of UK studies (Stockdale et al. 2006) on the impact of farm management practices on below-ground biodiversity and ecosystem function indicates that the impact of reduced grazing intensity on the diversity of soil organisms is likely to vary among taxa. Three studies found that increased grazing pressure had a negative impact, in terms of species richness or abundance, on ground beetles (Carabidae) (Ni Bhriain et al. 2002), spiders (Araneae) (Macaulay Institute 2006), and nematodes (Nematoda) in lowland habitats (Mulder et al. 2003). One study found that grazing pressure had no impact on the profile of soil fungal communities (Clegg 2006).
Ni Bhriain B., Skeffington M.S. & Gormally M. (2002). Conservation implications of land use practices on the plant and carabid beetle communities of two turloughs in Co. Galway, Ireland. Biological Conservation, 105, 81-92.
Mulder C., De Zwart D., Van Wijnen H. J., Schouten A.J. & Breure A.M. (2003) Observational and simulated evidence of ecological shifts within the soil nematode community of agroecosystems under conventional and organic farming. Functional Ecology, 17, 516-525.
Clegg C.D. (2006) Impact of cattle grazing and inorganic fertiliser additions to managed grasslands on the microbial community composition of soils. Applied Soil Ecology, 31, 73-82.
Macaulay Institute (2006) Grazing and upland birds. Available at www.macaulay.ac.uk/projects/projectdetails.php?302797 Accessed 13 March 2006.
A replicated, controlled study of five grassland headlands on four intensively managed pastoral farms across Scotland (Cole et al. 2007) investigated the effect of conservation headlands, with no grazing from April-August and no fertilizer or pesticide applications on the abundance of ground active invertebrates and found that aphids/leafhoppers/planthoppers (Homoptera) and true bugs (Heteroptera) were more abundant in conservation headlands (no fertilizers, pesticides or grazing April-August) than conventional headlands and open fields. Aphids/leafhoppers/planthoppers had higher activity densities in conservation headlands (2.1) and field edges (conventional: 2.0, conservation: 1.9) than in conventional headlands (0.8) and open fields (0.6). Roundback slugs (Arionidae) showed the same pattern (2.3 conservation headlands, 2.1 conventional field edges, 2.4 conservation field edges, 0.7 conventional headlands, 0.3 open fields). True bugs were more abundant in conservation headlands (0.7) and field edges (1.1-1.2) than in open fields (0.2). Keelback slug (Limacidae) activity density was greater in both headlands (conventional: 1.9, conservation: 2.8) and field edges (2.3-2.7) than in open fields (1.1). Butterfly/moth (Lepidoptera) and sawfly (Symphyta) larvae showed a similar trend, whereas ground beetle (Carabidae) abundance did not differ with treatment (3.5-3.6). Ground beetle activity density was highest in open fields (4.0). One headland in each field was divided into two areas of 6 x 100 m, a conventional and conservation headland. In each field, invertebrates were sampled with five pitfall transects of nine traps in: the conservation headland, conservation field edge, conventional headland, conventional field edge and open field. Traps were set for 3–4 weeks in May-June and July-August 2000-2003.
A randomized, replicated, controlled trial from 2003 to 2006 on four farms in southwest England (Defra 2007) (same study as (Woodcock et al. 2007, Potts et al. 2009)) found that 50 x 10 m plots of permanent pasture managed just like conventional silage but without autumn/winter grazing did not attract more invertebrates or foraging birds than control plots. Plots were fertilized and cut twice in May and July. Control plots were grazed in autumn/winter. There were twelve replicates of each management type, monitored over four years.
A five year trial on a species-rich mountain meadow in France (Dumont et al. 2007) found that biodiversity and structure of the grass layer (sward) were affected not only by extensification of grazing, but also by the way in which the reduced stocking rate was applied. Extending the grazing season after the period of active grass growth, or using a rotational grazing system that excluded part of the area during the main flowering period, benefited plant species diversity. At the same time, negative effects on livestock performance were limited.
A replicated, controlled study in a grazed fen area in northern Germany (Rasran et al. 2007) investigated the effects of reduced grazing, topsoil removal and hay transfer on plant species diversity and abundance and found that grazing had minimal effects, but did result in a significant increase of cumulative frequency of wet meadow plant species. Four blocks (12 x 24 m) were established that each combined three treatments: moderate grazing (yes/no), topsoil removal (yes/no; to a depth of 30 cm) and hay transfer from a species-rich fen meadow (yes/no; layer of 1-3 cm). Plant cover and species dominance was sampled in 16 permanent squares (1 m²) within each subplot in each combination of treatments in 2002-2005. Ten soil seed bank samples were taken from each plot in 2002.
A randomized, replicated trial from 2002 to 2004 in the UK, Germany and France (Scimone et al. 2007) (same study as (Wallis De Vries et al. 2007)) found that reduced grazing intensity led to a reduction in the number of plant species at a productive species-poor grassland site (UK), but not at the other two sites: species-rich, semi-natural grassland (France) and moderately species-rich ‘mesotrophic’ grasslands (Germany). At the UK site, moderately grazed plots had on average 11.4 plant species, while lightly grazed plots had 10.2 species and were dominated by grasses. Under reduced grazing, the structural diversity (patchiness) of vegetation decreased at the UK site but increased at the German site. Paddocks 0.4 to 3.6 ha in size were either moderately or leniently grazed with a commercial livestock breed, with treatments replicated three times. Actual grazing rates differed according to local conditions, but lenient grazing treatments were 0.3-0.4 fewer animals/ha than the moderate grazing rates. Sites were grazed continuously with cattle. Plants were monitored in ten fixed 1 m2 quadrats in each paddock in April-May, June-July and August-September from 2002 to 2004. Other plant species seen within 5 m of the quadrats were also recorded. An additional study site grazed by sheep in Italy was also included in the analysis but is not reported here because it falls outside the geographical range of this synopsis.
A replicated, controlled trial in 2005-2006 on upland improved grassland in the UK (Vale & Fraser 2007) found that the response of bird populations to seasonal removal of grazing (late May-July) for silage making varied between functional groups and depended on the time of year. Plots with seasonal removal of grazing, had the greatest number of birds of songbird species between May and July (126 birds compared to 60 in continuously grazed control plots), and between July and September (312 birds compared to 169 in control plots), but numbers were similar to those in continuously grazed plots between October and January (13 and 11, respectively). There were more invertebrate-feeding birds between July and September (105 birds, compared to 41 on control plots), but between October and January there were more invertebrate-feeding birds on continuously-grazed control plots (5,833 birds, compared to 1,458 on plots with seasonal removal of grazing). At all times of year, crows (Corvidae) were more abundant on control plots. There were 10 replicates. Bird numbers and species were recorded in plots with and without seasonal removal of grazing for silage making in May-July 2005, July-September 2005 and October 2005-January 2006.
A randomized, replicated study from 2002 to 2004 in the UK, Germany and France (Wallis De Vries et al. 2007) (same study as (Scimone et al. 2007)), found more butterflies (Lepidoptera) and grasshoppers (Orthoptera), and more butterfly and grasshopper species under lenient than moderate grazing. There were on average 42-47 vs 31 butterflies, and 10 vs 8 butterfly species/paddock/year under lenient compared to moderate grazing, and 56-60 vs 34 grasshoppers from 5 vs 4 grasshopper species/paddock/year under lenient compared to moderate grazing. Some groups of insects caught in pitfall traps were more abundant under lenient grazing at some, but not all, sites (for example, ground beetles (Carabidae) and rove beetles (Staphylinidae) at the UK site). Numbers of birds, bird species and European hares Lepus europaeus were not different between grazing treatments. Paddocks 0.4 to 3.6 ha in size were either moderately or leniently grazed with a commercial livestock breed, with treatments replicated three times. Actual grazing rates differed according to local conditions, but lenient grazing treatments were 0.3-0.4 fewer animals/ha than the moderate grazing rates. Sites were grazed continuously with cattle. Animals were monitored in 2002, 2003 and 2004. Butterflies were counted once a fortnight from May to September and grasshoppers sampled with a sweep net each month from June to October, on three 50 m-long transects. Birds and European hares were counted fortnightly in the early morning, from May to October, with a 7 minute observation period and a walking transect. Ground arthropods were sampled in twelve pitfall traps at each paddock, in spring, summer and early autumn. An additional study site grazed by sheep in Italy was also included in the analysis but is not reported here because it falls outside the geographical range of this synopsis.
A randomized, replicated, controlled trial on four farms in southwest England (Woodcock et al. 2007) (same study as (Defra 2007, Potts et al. 2009)) found that 50 x 10 m plots of permanent pasture managed without autumn/winter grazing had similar numbers of beetles (Coleoptera) and beetle species to control grazed plots. All plots were cut for silage in May and July. There were twelve replicates of each management type, monitored over three years (2003-2005).
A replicated trial of two intensities of cattle grazing on permanent improved grassland plots in the UK (Fraser et al. 2008) found no strong difference in numbers of butterflies (Lepidoptera) between partially grazed and permanently grazed plots. On average there were 7-11 butterfly species (42-156 individuals) on permanently grazed plots and 5-10 butterfly species (21-67 individuals) on partially grazed plots of improved pasture. Ten experimental plots of improved perennial rye grass Lolium perenne/white clover Trifolium repens were grazed throughout the year by livestock. Ten similar plots were grazed in spring and autumn, but had livestock excluded from May to September and one silage cut taken. No location is given for the experiment. Butterfly transect counts were conducted weekly between mid-April and mid-September in 2005, 2006 and 2007.
A 2008 systematic review (Stewart & Pullin 2008) identified several studies that suggest that intermediate grazing levels on neutral grassland (MG5 crested dog’s-tail Cynosurus cristata-lesser knapweed Centaurea nigra grassland under the UK National Vegetation Classification) benefit plants, invertebrates or birds, but noted that trade-offs probably exist between the grazing requirements of different taxa. Four studies suggested that intermediate grazing levels are most appropriate for plant conservation objectives. Where grazing levels are very low, or grazing is abandoned, ecological succession leads to a reduction in the number of plant species. High grazing intensity also reduces plant species richness, because a limited number of competitive species become dominant. Six studies found invertebrate abundance or species richness to be greatest at intermediate grazing (or cutting) intensity. Similarly, it is assumed that bird diversity peaks at intermediate grazing levels because of reliance on invertebrates as a food source (three studies), or because intensive grazing increases the risk of nest predation/trampling for ground-nesting birds (three studies). However, one study found that insect diversity was greatest on ungrazed sites, and another found no relationship between plant species richness at the local scale and bird diversity. The authors conclude that trade-offs may exist between the grazing requirements of different taxa, and that the limited evidence base necessitates a flexible, site-based approach. Forty-two studies were included in the review.
A randomized, replicated trial of different winter cutting regimes, designed to simulate grazing intensity on grasslands in Oxfordshire, England (Whittingham & Devereux 2008) found that different groups of birds prefer different treatments. Foraging song thrushes Turdus philomenus, common starlings Sturnus vulgaris, crows (Corvidae) and common kestrels Falco tinnunculus preferred mown (grazed) plots to unmown (ungrazed) plots. Grey heron Ardea cinerea and meadow pipit Anthus pratensis preferred unmown (ungrazed) plots to plots that were mown (grazed) once or twice. For gamebirds, wood pigeon Columba palumbus and hedgerow species, there was no significant difference in numbers between the different mowing regimes. Seventeen grass fields (average size 5 ha) were used in the experiment, with two treatments (mown once vs unmown) or all four treatments in each. Winter mowing was used to simulate the effects of grazing or cutting for silage. Grass height did not differ between the 14 replicate plots mown once in November/December, once in January or twice during winter, so one winter cut or grazing period was sufficient to create the habitat advantage for bird groups that prefer short grass.
A long-term randomized, replicated, controlled trial from 1990 to 2005 at two upland grassland sites in North Lanarkshire and Scottish Borders, Scotland, UK (Marriott et al. 2009) found that a reduction in the level of grazing on intensively grazed and fertilized grassland led to an increase in the number and diversity of plant species. However, these changes were very slow and, in some cases, still ongoing after 16 years, suggesting that recovery from intensive management can take many years. Four grazing regimes were applied from spring 1990: conventional management (grazed to maintain grass at 4 cm, with fertilizer), two reduced grazing treatments (grazed to maintain grass at 4 cm or 8 cm, with no fertilizer) and an ungrazed control (with no fertilizer). Each treatment was replicated twice at each site in roughly 0.5 ha plots. Plants were sampled annually in June-July from 1990 to 2005 using an inclined point quadrat (18 quadrats/plot), except in 1991, 1993, 1995, 2002 and 2004.
A randomized, replicated, controlled trial from 2003 to 2006 in southwest England (Potts et al. 2009) (same study as (Defra 2007, Woodcock et al. 2007)) found plots of permanent pasture managed without autumn/winter grazing did not attract more butterflies (Lepidoptera), butterfly larvae or common bumblebees Bombus spp. than control grazed plots. All plots were cut for silage in May and July. Grazed plots were grazed with cattle from September until the grass height was 5-7 cm. Experimental plots 50 x 10 m were established on permanent pastures (more than five years-old) on four farms. There were nine different management types, with three replicates/farm, monitored over four years. Bumblebees and butterflies were surveyed along a 50 m transect line in the centre of each experimental plot, once a month from June to September annually. Butterfly larvae were sampled on two 10 m transects using a sweep net in April and June-September annually.
A 2010 review of UK experiments on the effects of agri-environment measures on livestock farms in the UK (Buckingham et al. 2010) found two replicated controlled trials that reduced grazing pressure (fewer cattle, cattle removed from July onwards, or both) in pastures over two to four years. One of the studies also reduced fertilizer input from 150 to 50 kg N/ha. Lenient grazing (grass height 12-16 cm) significantly increased the numbers of insects (bugs (Hemiptera)), the production of seed heads in the grasslands and the number of foraging Eurasian skylark Alauda arvensis on trial fields in both studies. Birds that eat only seeds - European goldfinch Carduelis carduelis and Eurasian linnet C. cannabina, preferred plots with cattle removed in July. There were eight replicates in the study with additional low fertilizer input and 14 replicates in the other study. These studies formed part of a Defra-funded project (BD1454) for which no reference is given in the review.
A replicated controlled study in 2006-2009 on 13 semi-improved and 12 intensively managed fields in Devon, Herefordshire and Yorkshire, UK (Defra 2010) found that lenient grazing or seasonal removal of livestock had positive effects on numbers of invertebrates and birds. Invertebrate abundance (including important bird food invertebrates) on semi-improved grassland was 34-78% higher in July on leniently grazed compared to moderately grazed plots. Seasonal removal of livestock on semi-improved grassland also resulted in positive but smaller increases in invertebrate abundance (5-35%) compared to controls. Leniently grazed plots with grazing removal were used by Eurasian skylark Alauda arvensis significantly more than controls (3.6-6.6 times higher than controls). Yellowhammer Emberiza citrinella, cirl bunting E. cirlus and seed-eating birds with mixed diets, showed a preference for plots with seasonal livestock removal. Bird species that feed only on seeds used plots on intensive grassland with grazing removal more than controls but showed no clear preference on semi-improved grassland. During the breeding season, other bird species showed no clear preference or did not prefer plots with reduced grazing. The number of plant species decreased on plots with reduced grazing intensity on semi-improved grassland compared with controls. Plant seed heads were more abundant on plots with reduced grazing intensity or early closure than controls. Weed cover remained low across the treatments. The treatments in approximately 0.6-1 ha plots were grass height maintained at 10-16 cm (lenient) or 6-9 cm (moderate) by cattle; plots were grazed April to mid-July with livestock removal from mid-July to the following spring. Controls were moderately grazed April-October or until grass growth stopped. Semi-improved grassland had historically received inputs of 50 kg N/ha/year and intensively managed grassland 150 kg N/ha/year plots (reduced fertilizer input was also tested on intensively managed grassland).
A randomized, replicated, controlled trial in Berkshire, UK started in 2008 (Pywell et al. 2010) found that grassland plots sown with a seed mix containing legumes and other herbaceous species had more pollinators (bees (Apidae), butterflies (Lepidoptera) and hoverflies (Syrphidae)) and pollinator species in the first summer with no summer grazing (rested from May to September) than on plots continuously grazed from May to October. The type of grazing (continuous or with a summer rest period) did not affect the cover of sown plant species. There were four replicates of each treatment combination, and grazed plots were 25 x 50 m.
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