Background information and definitions
This involves growing more than one species of forage crop (grasses and legumes) in a pasture to control invertebrate or weed pests in pastoral farmland. The use of mixed pastures to suppress pests in arable crops is not included here but relevant to other actions, e.g. ‘Include plants that are repellent or suppressive to pests in crop rotations’ and ‘Grow cover crops that are repellent or suppressive to pests when the field is empty’ (for inclusion in a future synopsis).
Here we present evidence from seven of 10 studies testing this intervention.
Supporting evidence from individual studies
A replicated, controlled trial from 1982-1985 at two pasture sites in Montana, USA (Hewitt & Onsager 1988) found that an index of overall grasshopper (Orthoptera) grazing intensity and presence was higher in interseeded pastures (19-2,852 grasshopper days/m²) than control native pasture (7-1,377 days/m²) but annual rates of grasshopper increase were similar between treatments (5.35x vs. 5.92x annual increase). One dominant species, the migratory grasshopper Melanoplus sanguinipes increased more in interseeded plots (29-1,367 estimated cumulative grasshopper days/m²) than controls (23-501 days/m²) from 1983-1985. Grasshoppers caused 10% seedling mortality in one interseeded plot. Forage yield was higher in interseeded (454-1,290 kg/ha total herbaceous yield) than control pastures (240-739 kg/ha). Two pastures (one at each site) were seeded with dryland alfalfa Medicago falcata and cicer milkvetch Astragalus cicer (both at 2.2 kg/ha) in April-May 1982. One was treated with herbicide, the other cut mechanically to control sagebrush Artemesia tridentata (weed). Control pastures had a mix of unsown species. Interseeded pastures were grazed by 10 steers for 48, 40 and 20 days and control pastures by five steers for 90, 60 and 40 days in 1983-1985 respectively. The effects of interseeding and different grazing intensities could not be separated.Study and other actions tested
A randomised, replicated, controlled trial in 1984-1985 at two sites in West Virginia, USA (Mackun & Baker 1990) found that pest insect numbers varied between monoculture pasture and mixed pasture. Spittlebug (Cercopidae) nymphs were significantly more abundant in mixed pastures of bird's-foot trefoil Lotus corniculatus with either perennial ryegrass Lolium perenne or orchardgrass Dactylis glomerata than in a bird's-foot trefoil monoculture. Mirid (Miridae) nymphs were significantly less abundant on two types of mixed pasture than monoculture. The ryegrass and bird's-foot trefoil mix had the highest numbers of adult and nymph leafhoppers and planthoppers (Cicadellidae and Delphacidae), mirids and aphids (Aphididae) compared to other mixes and bird's-foot trefoil monoculture. Forage yields were not different between the different pasture types. There were five pasture mixtures: bird's-foot trefoil monoculture (15 kg/ha) or 10 kg/ha bird's-foot trefoil plus: orchardgrass (4 kg/ha), timothy Phleum pratense (4 kg/ha), perennial ryegrass (10 kg/ha) or tall fescue Festuca arundinacea (6 kg/ha). Plots (11 x 5 m) were established in 1983. Insects were sampled seven times in 1984 and eight times in 1985, with five sweepnet samples/plot.Study and other actions tested
A randomised, replicated, controlled study of mixtures of alfalfa Medicago sativa and meadow grasses in 1990-1991 at two sites in Michigan, USA (Roda et al. 1997) found 22-30% fewer adult potato leafhoppers Empoasca fabae in an alfalfa-smooth bromegrass Bromus inermis mix and 22-48% fewer leafhoppers in an alfalfa-orchardgrass Dactylis glomerata mix compared with alfalfa monocultures. Alfalfa mixed with timothy Phleum pratense showed both slight reductions (4-5%) and increases (1-5%) when seeded at 4.5 kg/ha. There were eight treatments: alfalfa-only at 18 or 14.6 kg/ha, alfalfa at 14.6 kg/ha with: smooth bromegrass at 5.6 or 2.8 kg/ha, orchardgrass at 1.1 or 0.6 kg/ha, timothy at 4.5 or 2.2 kg/ha. Plots (9.9 x 12.2 m) were established in 1989. There were 4-5 replications at both sites. Plots were harvested twice in 1989 and three times in the following years. Potato leafhoppers were sampled using a D-vac suction sampler after the first and second cuts.Study and other actions tested
A series of two laboratory experiments and one randomised, replicated field trial from 1990-1992 in Wyoming, USA (Shigaki et al. 1998) found no difference in mortality caused by the northern root knot nematode Meloidogyne hapla for plants in mixed pasture (sainfoin Onobrychis viciifolia and meadow brome Bromus riparius) compared to monoculture pasture. Sainfoin had 26.6-90.0% nematode-caused mortality in monoculture and 26.7-98.3% when intercropped. Meadow brome (the species used as intercrop) had 0-16.7% mortality in monoculture and 0% mortality in intercropping. Total sainfoin mortality was 70.8-98.3% in monoculture and 91.7-100% in intercropping. Sainfoin shoot and root biomass were consistently higher in monoculture (shoot: 0.04-0.51 g dry weight/plant; root: 0.02-1.8) than intercropping (shoot: 0.00-0.01; root: 0.00-0.01). Meadow brome had higher shoot and root biomass in intercropping (shoot: 1.0-2.97 g dry weight/plant; root: 1.6-5.83) than monoculture (shoot: 3.5-36.2; root: 5.9-76.6). In the field experiment, forage yields were higher in intercropping (34.27 and 37.01 t dry matter/ha) than monocropped sainfoin (13.08-15.39) but lower than monocropped meadow brome (44.28). Plants in the laboratory experiments were grown in pots of pasteurized soil inoculated with 5,000 nematode eggs/l soil in a glasshouse or growth chamber.Study and other actions tested
A randomised, replicated, controlled trial in 1994-1997 on plots of alfalfa Medicago sativa in California, USA (Putnam et al. 2001) found fewer Egyptian alfalfa weevil Hypera brunneipennis larvae in mixed alfalfa pastures (2.7-9.5 weevils/sweep) than in alfalfa-only plots (5.5-12.2 weevils/sweep) in all three years. Total forage yield for the first four harvests of each year was higher in 1994-1995 in alfalfa mixed with either berseem clover Trifolium alexandrinum or oats Avena sativa and similar between monoculture and mixed pasture plots in 1995-1996 and 1996-1997. The density of weeds was generally lower in mixed pastures than alfalfa-only pasture over all three years. The alfalfa plots had been established for 2-5 years. Plots were replicated six times in 1994-1995 (plots 3.6 x 9 m) and 1995-1996 (3.6 x 14 m) and three times in 1996-1997 (16 x 30 m). The plots were lightly harrowed and sown with either berseem clover, oats, orchardgrass Dactylis glomerata or red clover Trifolium pratense in October each year. Ten sweep net samples were taken in each plot.Study and other actions tested
A randomised, replicated trial in 1998-2000 in pasture land in Pennsylvania, USA (Tracy & Sanderson 2004) found that weed density was lower in plots with a higher diversity of pasture species. Total weight of weed plant material was generally lower in plots with six or more species. Pasture species yield was higher in plots with lower weed density. An additional randomised, replicated, controlled greenhouse trial in 1999 found that plant weight of the weed curly dock Rumex crispus was lower when grown with a mix of 10 pasture species (1-10 g/m²) than with one or five species (75-89 g/m²), except when the single species was turnip Brassica rapa which completely suppressed the weed (see "Plant species that compete with damaging weeds"). The field trial used 2.25 m² plots with eight pasture species mix treatments of 1-15 species, replicated 12 times. Each plot received 120 g of seed divided equally between the pasture species. The greenhouse trial used 30 litre pots, each of which received 100 curly dock seeds and 100 pasture species seeds divided equally between species in six species mix treatments with one, five or 10 species. Control pots received no pasture species seeds. Each treatment was replicated six times.Study and other actions tested
A randomised, replicated, controlled trial in 2004 at the Semiarid Prairie Agricultural Research Centre in Saskatchewan, Canada (Atul et al. 2008) found the total number of nematodes (Nematoda) and nematode diversity in the top soil layer was significantly higher in plots of mixed pasture than monoculture. There were 901 nematodes/100 g dry soil in mixed pasture compared to 681 in monoculture. Fungus-feeding (199 vs. 170 nematodes) and omnivorous (380 vs. 100) nematodes were significantly more abundant in mixed pasture than monoculture. The number of plant parasitic nematodes was not significantly different in mixed pasture than monoculture (48 vs. 30 nematodes/100 g). There were two treatments in four blocks: alfalfa Medicago sativa monoculture and mixed pasture of alfalfa and Russian wildrye Psathyostachys juncea. Plots were 1.8 x 6 m, established in 1997 and seeded at 25 seeds/30 cm in 30 cm-wide rows. Two 5 cm-diameter samples of the top 7.5 cm soil layer were taken in each plot, on three occasions (30 June, 1 and 30 September). Nematodes were extracted using the sieving-Baermann funnel technique.Study and other actions tested