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Action: Pest regulation: Grow cover crops in arable fields Mediterranean Farmland

Key messages

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Pest regulation (1 study): One replicated, randomized, controlled study from the USA found that fewer aphids were parasitized in plots with cover crops (living mulches) between broccoli plants, compared to plots without cover crops, in some comparisons.

Crop damage (6 studies): Three controlled studies (two replicated and randomized) from the USA found similar numbers of diseased broccoli seedlings or tomato plants in plots with or without winter cover crops. Two replicated, randomized, controlled studies from the USA found less-severely diseased lettuces in plots with winter cover crops, compared to winter fallows, in some comparisons. One replicated, randomized, controlled study from the USA found inconsistent differences in tomato damage between plots with cover crops or fallows.

Ratio of natural enemies to pests (0 studies)

Pest numbers (14 studies)

  • Weeds (8 studies): Four replicated, randomized, controlled studies from Israel and Italy found fewer weeds in plots with cover crops, compared to plots without them, in some or all comparisons. One replicated, randomized, controlled study from the USA found more weeds in plots with winter cover crops, compared to plots without them, in some comparisons. Two replicated, controlled studies (one randomized) from Italy and the USA found that winter cover crops had inconsistent effects on weeds (sometimes more, sometimes fewer, compared to plots without winter cover crops). One controlled study from the USA found similar amounts of weeds in plots with winter cover crops or fallows.
  • Weed species (2 studies): One replicated, randomized, controlled study from Italy found fewer weed species in plots with winter cover crops, compared to plots without them, in one of three comparisons. One replicated, randomized, controlled study from the USA found different weed communities in plots with or without winter cover crops.
  • Other pests (6 studies): Two replicated, randomized, controlled studies from the USA found fewer aphids in plots with cover crops (living mulches) between broccoli plants, compared to plots without cover crops, in some comparisons. One replicated, randomized, controlled study from the USA found more mites (in some comparisons), but similar numbers of centipedes and springtails, in plots with winter cover crops, compared to plots without them. One replicated, randomized, controlled study from the USA found similar numbers of leafminers in plots with or without winter cover crops. One replicated, randomized, controlled study from the USA found similar amounts of fungus in soils with or without winter cover crops. One replicated, randomized, controlled study from the USA found inconsistent differences in nematode numbers between soils with cover crops or fallows.

Natural enemy numbers (0 studies)

Implementation options (13 studies): Nine studies from Israel, Italy, and the USA found that different cover crops had different effects on crop damage or pest numbers. Two studies from the USA found that different cover crops (living mulches) did not have different effects on pest regulation or pest numbers. Two studies from the USA found that different methods of seeding cover crops had different effects on pest numbers.

Supporting evidence from individual studies

1 

A replicated, randomized, controlled study in 1986–1988 in an irrigated lettuce field in the Salinas Valley, California, USA, found less-severely diseased lettuces in plots with winter cover crops, compared to winter fallows. Crop damage: Less-severely diseased lettuces were found in plots with cover crops, compared to fallows, in one of four harvests (autumn 1988: data reported as disease scores, based on taproot damage by corky root disease). Implementation options: Less-severely diseased lettuces were found in plots that were cover cropped with Secale cereale rye, compared to Vicia faba broad beans (data reported as disease scores). Methods: There were six plots (10.7 x 1.1 m raised beds) for each of two winter cover crops (broad beans or rye) and six control plots (bare fallow, maintained with herbicide). The cover crops were seeded in November 1986–1987, irrigated until emergence, and chopped, disked, and chisel ploughed in spring (25–30 cm depth). Lettuces were planted in May and July 1987 and March and August 1988, and they were harvested in July and October 1987 and June and October 1988. The lettuces were irrigated (1–2 cm every 2–3 days until emergence, then 2 cm/week). The severity of corky root disease was measured in 10 roots/plot at harvest.

 

2 

A replicated, randomized, controlled study in 1991 in a broccoli field in the Salinas Valley, California, USA (same study as (3)), found fewer pests and less parasitism of pests in plots with cover crops (living mulches) between broccoli plants, compared to bare soil. Pest regulation: Fewer aphids were parasitized in plots with cover crops, compared to bare soils, in 9 of 12 comparisons (0–7% vs 10–18%). Pest numbers: Fewer aphids were found in plots with cover crops, compared to bare soil, in 43 of 48 comparisons (0.01–0.52 vs 0.2–1.8 aphids/leaf). Implementation options: Similar numbers of aphids were parasitized in plots with different mixtures of cover crops (0–13%). Methods: Broccoli plants were transplanted into cover crops or bare soil (four replicates each, 10 x 10 m plots). The cover crops were white clover Trifolium repens, strawberry clover Trifolium fragiferum, or a mixture of birdsfoot trefoil Lotus corniculatus and red clover Trifolium praetense. Cabbage aphids Brevicoryne brassicae and green peach aphids Myzus persicae were sampled by taking 50 broccoli leaves from 50 plants in each plot (1990: every 2 weeks; 1991: every 10 days).

 

3 

A replicated, randomized, controlled study in 1991 in a broccoli field in the Salinas Valley, California, USA (same study as (2)), found fewer pests in plots with cover crops (living mulches) between broccoli plants, compared to bare soil. Pest numbers: Fewer aphids were found in plots with cover crops, compared to bare soil (pan traps: 0.2–2 vs 1–10, in four of 10 comparisons; broccoli leaves: 0.03–0.24 vs 0.25–0.5, in three of 10 comparisons). Implementation options: Similar numbers of aphids were found in plots with different mixtures of cover crops (pan traps: 0.04–0.79; broccoli leaves: 0.03–0.63). Methods: Plots had cover crops or bare soil (four replicates each). The cover crops were white clover Trifolium repens, strawberry clover Trifolium fragiferum, or a mixture of birdsfoot trefoil Lotus corniculatus and red clover Trifolium praetense. Broccoli plants were transplanted into these plots on 18 May 1991. Cabbage aphids Brevicoryne brassicae and green peach aphids Myzus persicae, were sampled in each plot with two yellow and black pan traps (12 x 8 x 8 cm), on 12, 22, 32, 42, and 52 days after transplanting the broccoli. Pests were also sampled by heat extraction on 22, 32, 42, 52, and 62 days after transplanting.

 

4 

A replicated, randomized, controlled study in 1992–1993 in an irrigated broccoli field in the Salinas Valley, California, USA, found more mites, but similar numbers of other pests and diseases, in soils with winter cover crops, compared to bare soils. Crop damage: Similar numbers of diseased seedlings were found in plots with or without cover crops (numbers of seedlings not reported). Pest numbers: More mites were found in plots with cover crops, compared to bare soils, in four of 28 comparisons (90–220 vs 30–150 mites/sample), but similar numbers of centipedes and springtails were found (0.3–15 springtails/sample; numbers of centipedes not reported). Similar amounts of disease-causing fungus were found in soils with or without cover crops (numbers of Sclerotina minor sclerotia not reported). Methods: There were three plots for winter cover crops (half Phacelia tanacetifolia and half Secale cereale Merced rye, sown in November 1992 and mown in March 1993) and three control plots with bare soil in winter. All plots (252 x 24 m) were tilled in March 1993 (15 cm depth), and the cover crops were incorporated into the soil. Two broccoli crops were grown on raised beds (first crop: April–August 1993; second crop: August–November 1993). All plots were irrigated (440–450 mm/crop, subsurface drip irrigation) and fertilized (41–42 g N/m2/crop). Pests were measured in soil samples (0–15 cm depth, 14 samples in March–November 1993). Broccoli diseases were measured in ten 2 m2 areas/plot.

 

5 

A replicated, randomized, controlled study in 1991–1994 in an irrigated tomato field in the San Joaquin Valley, California, USA, found similar amounts of fungus in soils with winter cover crops or winter fallows. Pest numbers: Similar amounts of Rhizoctonia solani fungus were found in soils with cover crops or fallows (0.3–1.7 vs 1.3 colony forming units/100 g dry soil). Methods: There were four plots (93 x 7 m plots) for each of three winter cover crops and one control (winter fallow). The cover crops were Hordeum vulgare barley, Vicia dasycarpa Lana woollypod vetch, or a barley-vetch mixture, seeded in October 1991–1993 and incorporated into the soil in March 1992–1994 (15–20 cm depth, rotary tiller). Fungus colonies were measured in soil samples, collected in spring 1994 (0–15 cm depth).

 

6 

A replicated, controlled study in 1996–1998 in an irrigated tomato field in the San Joaquin Valley, California, USA, found more weeds in plots with winter cover crops (and no tillage in spring), compared to plots with winter fallows (and tillage in spring), when herbicide was used on the fallows. When herbicide was not used, differences were inconsistent. Pest numbers: More weeds were found in plots with cover crops, compared to fallows, in some comparisons (in 9 of 12 comparisons with herbicide-use on fallows, in 1998: 4–12% vs 0–3% weed cover; in two of 12 comparisons without herbicide-use on fallows, in 1998: 5–6% vs 2%), but fewer weeds were found in two of 12 comparisons without herbicide-use on fallows, in 1998 (4–5% vs 11%). In 1997, similar weed cover was found in plots with or without cover crops (1–4%). Implementation options: Fewer weeds were found in plots that were cover cropped with grass-legume mixtures, compared to legumes, in two of six comparisons in 1998 (in May: 4–5% vs 11–12% weed cover). Methods: There were 12 plots (4.5 x 27.5 m plots) for each of four treatments (two grass-legume mixtures, or two legumes without grasses, as winter cover crops, sown in October 1996–1997, killed and retained as mulch, with no tillage, in March 1997–1998) and each of two controls (bare-soil fallows in winter, with or without herbicide, and conventional tillage in spring). Tomato seedlings were transplanted in April 1997–1998. The tomatoes were irrigated (two inches/week) and fertilized (0, 100, or 200 lb N/acre). All plots were hand weeded in May, June, and July, and control plots were also cultivated in May and June. Weed cover was estimated before cultivation (July 1997 and May, June, and July 1998) or after cultivation (May and June 1997), in three quadrats/plot (1.8 m2 quadrats).

 

7 

A replicated, randomized, controlled study in 1998–2000 in an irrigated vegetable field in the Salinas Valley, California, USA, found less corky root disease in plots with winter cover crops, compared to plots without cover crops. Crop damage: Less corky root disease was found in plots with cover crops, in one of four comparisons (2.2 vs 2.9 disease severity, on a scale from 1 to 12, on which 12 is the highest severity). Similar amounts of Sclerotina minor disease and big vein disease were found in plots with or without cover crops (S. minor: 0.3–1.9 vs 0.3–1.7% of plants had symptoms; big vein: 3.0–3.6 vs 2.7–3.4% of plants had symptoms). Pest numbers: Similar numbers of Liriomyza huidobrensis pea leafminers were found in plots with or without cover crops (10–81 vs 8–98 insects/sticky card). Methods: There were four plots (0.52 ha), for each of four treatments (reduced tillage or conventional tillage, with or without added organic matter). In plots with added organic matter, compost was added two times/year, and a cover crop (Secale cereale Merced rye) was grown every autumn or winter. Lettuce or broccoli crops were grown on raised beds. Sprinklers and drip irrigation were used in all plots. Soils were disturbed to different depths (conventional tillage: disking to 50 cm depth, cultivating, sub-soiling, bed re-making, and bed-shaping; reduced tillage: cultivating to 20 cm depth, rolling, and bed-shaping). It was not clear whether these results were a direct effect of adding compost or growing cover crops.

 

8 

A replicated, randomized, controlled study in 1993–2001 in a rainfed cereal field in central Italy found fewer weeds and weed species in plots with winter cover crops, compared to plots without cover crops. Pest numbers: Fewer weed species were found in plots with cover crops, compared to plots without cover crops, for one of three species of cover crop (rye: 16 vs 18 weed species). Fewer weeds were found in plots with cover crops, compared to plots without cover crops, for one of three species of cover crop (rye, in plots with conventional tillage: 7,000 vs 9,000 weed seedlings/m2; subterranean clover, in plots with no tillage: 32,000 vs 40,000). Implementation options: Fewer weed species were found in plots that were cover cropped with rye, compared to crimson clover (16 vs 18 species), but no differences were found in two of three comparisons between species of cover crops. Methods: Winter cover crops (Secale cereale rye, Trifolium subterraneum subterranean clover, or T. incarnatum crimson clover) were grown on 72 treatment plots, but not on 24 control plots on which cereal crop residues were retained over winter (21 x 11 m sub-sub-plots, in a split-split-plot experimental design). In spring, the cover crops were flailed, half of the plots were tilled (30 cm depth), and half were not. Herbicide and fertilizer were used on all plots. Weed seeds were sampled in soil cores in February 2001 (27 cores/plot, 0–15 cm depth, 3.5 cm diameter) and identified after germination in a greenhouse.

 

9 

A replicated, randomized study in 2001–2003 in an irrigated lettuce field in the Salinas Valley, California, USA, found different numbers of weeds in plots with different species of cover crops. Implementation options: Fewer weeds were found in plots that were cover cropped with mustard, compared to oats, in two of six comparisons (December 2001 and January 2002: 18–21 vs 65–110 g weeds/m2), and also compared to a legume-oat mixture, in three of six comparisons (December 2001, January 2002, and January 2003: 11–21 vs 121–188 g weeds/m2). Fewer weeds were found in plots that were cover cropped with oats, compared to the legume-oat mixture, in two of six comparisons (January 2002 and 2003: 37–65 vs 170–188 g weeds/m2). Fewer weed seeds (Urtica urens burning nettle) were found in plots that were cover cropped with mustard (0–1,300 viable seeds/m2), compared to oats (1,900–6,000) or the mixture (4,300–13,600), but the difference between plots with oats or the mixture was not significant. After the cover crops were incorporated into the soil, fewer weeds were found in plots that were cover cropped with mustard (113 weed seedlings/m2), compared to oats (246/m2) or the mixture (377/m2), in one of two years (2002), but the difference between plots with oats or the mixture was not significant. Methods: One of three cover crops (Avena sativa oats; Brassica hirta and B. juncea mustard; or Vicia faba, Pisum sativum, Vicia sativa, Vicia villosa, and A. sativa legume-oat mixture) was planted in October (2001: three 2.2 x 30 m plots each; 2002: four 3 x 30 m plots each). Weed biomass was sampled in two 30 x 30 cm quadrats/plot in 2001–2002, and in one 100 x 100 cm quadrat/plot and one 30 x 30 cm quadrat/plot in 2002–2003, in December, January, and February. Weed seeds were collected in January, in vacuum samples (30 x 30 cm/plot). Cover crops were mown and incorporated into the soil (rototilled, 15 cm depth) in March 2002 and February 2003. The soil was then watered (5–10 cm water), and weeds were counted in eight 50 x 50 cm quadrats and five 30 x 30 cm quadrats (2002: 36 days after incorporation; 2003: 48 days).

 

10 

A replicated, randomized, controlled study in 1997–2001 in irrigated tomato fields at two sites in the Coachella and San Joaquin Valleys, California, USA, found more root-knot nematodes Meloidogyne spp. and tomato roots with more galling (caused by nematodes) in soils with cover crops, compared to dry fallows, but cover crops had inconsistent effects on nematodes and galling, compared to wet fallows. Crop damage: More root galling was found in plots with cover crops, compared to dry fallows (e.g., in Experiment 1: 0.9–2.7 vs 3.2–7.6 root gall index), but inconsistent differences were found between plots with cover crops or wet fallows (sometimes more, sometimes less). Pest numbers: More nematodes were found in soils with cover crops, compared to fallows, in most comparisons (e.g., for dry fallows, in Experiment 2: 4–1,005 vs 1). Implementation options: For cover crops that were not resistant to nematodes, more nematodes were found in soils with cover crops, compared to fallows, in most comparisons (e.g., in Experiment 1: 9,148–9,803 vs 19–599). However, for cover crops that were resistant to nematodes, fewer nematodes were found in soils with cover crops, compared to wet fallows, in some comparisons (e.g., in Experiment 1, in four of 10 comparisons: 3–72 vs 19–599), and more nematodes were found in other comparisons (e.g., in Experiment 4, without incorporation: 26–35 vs 1). Methods: Six experiments compared plots with cover crops (cowpeas Vigna unguiculata: several nematode-resistant cultivars and one susceptible cultivar, sometimes incorporated into the soil, and sometimes not) to plots with fallows (dry or wet) between 1997 and 2001 (4–6 replicate plots/treatment/experiment). Some herbicide, but no fertilizer, was used. In the Coachella Valley, cover crops were sown in late July or early August and suppressed after 70–84 days. The following year, tomatoes were planted in late January or early March and harvested in June. In the Central Valley, cover crops were sown in May and suppressed after 83 days. The following year, tomatoes were planted in April and harvested in August. Nematode juveniles and eggs were counted in soils samples (0–30 cm depth). Root galling was measured at harvest (21 tomato root systems/plot).

 

11 

A replicated, randomized, controlled study in 2003–2005 on an irrigated vegetable farm in Salinas, California, USA (partly the same study as (12)), found fewer weeds in plots that were sown with more cover crop seeds, compared to fewer, and in plots that were planted in a grid of perpendicular rows, compared to parallel rows. Implementation options: Fewer weeds were found in plots that were sown with more cover crop seeds, in five of six harvests (270 kg seeds/ha: 0–10 kg weeds/ha; 180 kg seeds/ha: 1–22 kg weeds/ha; 90 kg seeds/ha: 6–47 weeds/ha). Fewer weeds were found in plots that were sown in a grid (two passes of the seed drill, in perpendicular rows, with half as many seeds/pass as conventional passes), compared to conventionally (one pass, in parallel rows), in one of six harvests (1 vs 6 kg weeds/ha). Weeds emerged at similar times in plots planted with different amounts of seed, and in plots planted in a grid or conventionally (data not reported). Methods: Twenty-four plots were planted with winter cover crops (Secale cereale Merced rye), with 90, 180, or 270 kg seeds/ha, in October 2003–2004 (12 x 12 m plots). Half of these plots were planted in grid, and half were planted conventionally. Weed biomass was measured 18 days after planting (two quadrats/plot, 50 x 50 cm quadrats).

 

12 

A replicated, randomized, controlled study in 2003–2005 in two irrigated fields in the Salinas and Hollister Valleys, California, USA (partly the same study as (11)), found fewer weeds in plots that were sown with more cover crop seeds, compared to fewer. Implementation options: Fewer weeds were found in plots that were sown with more cover crop seeds, compared to fewer, in seven of 12 comparisons (336 kg seeds/ha: 0–2% of dry matter was weeds; 112 kg seeds/ha: 0–10%). Similar numbers of weeds were found in plots that were sown in a grid (two passes of the seed drill, in perpendicular rows, with half as many seeds/pass as conventional passes), compared to conventionally (one pass, in parallel rows) (data not reported). Methods: In Hollister, there were twenty-four 12 x 12 m plots. Half were sown in a grid, and half were sown conventionally. In Salinas, there were nine 12 x 15 m plots. All plots were sown with cover crops in November 2003–2004 (112, 224, or 336 kg seeds/ha). The seeds were a mixture of oats and legumes (beans, peas, and vetch). Biomass was measured four times/year in December–April 2004–2005 (one quadrat/plot, 100 x 50 or 50 x 50 cm).

 

13 

A replicated, randomized, controlled study in 1999–2001 in two irrigated tomato fields in central Italy found that winter cover crops had inconsistent effects on weeds. Pest numbers: In spring, fewer weeds were found in plots with winter cover crops, compared to bare soil in winter, in six of 16 comparisons (10–55 vs 62–82 weeds/m2; 2–10 vs 12–50 g weeds/m2), but more weeds were found in one of sixteen comparisons (70 vs 50 g weeds/m2). Implementation options: The fewest weeds were found in plots that had been cover cropped with Avena sativa oats (10–21 weeds/m2; 2–10 g weeds/m2), and the most were found in plots that had been cover cropped with Vicia villosa hairy vetch (57–73 weeds/m2; 13–70 g weeds/m2). Methods: In September–May, cover crops were grown on 12 treatment plots, but not on three control plots, which were weeded with a disk cultivator (6 x 9 m plots). Cover crops were mown in May. All plots were irrigated and fertilized (100 kg P2O5/ha in September, 0–100 kg N/ha in June–July). Tomato seedlings were transplanted in May, and weeds were sampled 15 and 30 days later, between the tomato rows.

 

14 

A replicated, randomized, controlled study in 2005–2006 in an irrigated, organic tomato field in Yolo County, California, USA, found that similar numbers of tomato plants were lost to disease in plots with winter cover crops or winter fallows. Crop damage: Similar numbers of tomato plants were lost to Southern blight Sclerotium rolfsii in plots with cover crops or fallows (83% vs 89% survival). Methods: The field was levelled and fertilized (17 Mg compost/ha). Eight plots had winter cover crops (mustard Brassica nigra, planted on 3 November 2005) and eight plots had winter fallows. Each plot was 16 x 9 m. Cover crops were mown on 26 April 2006, sprinkler irrigated, and tilled into the soil (10 cm depth) after 19 days, when fallow plots were also tilled. Plots were weeded and sulfur was used against mites and diseases. Tomatoes were furrow irrigated (approximately every 11 days: 88 mm/event). Plants were assessed for Southern blight, 74 days after planting and at harvest (7–8 September 2006).

 

15 

A replicated, randomized, controlled study in 1993–2008 in a rainfed wheat-maize-wheat-sunflower field in central Italy found fewer weeds in plots with winter cover crops, compared to plots without cover crops. Pest numbers: Fewer weeds were found in plots with cover crops (7–18 vs 28 Mg/ha). Implementation options: Fewer weeds were found in plots with non-legume cover crops, compared to legumes (14–18 Mg/ha). Fewer weeds were found in plots with high-nitrogen-supply legumes, compared to low-nitrogen supply legumes (14 vs 18 Mg/ha). Methods: There were 32 plots (21 x 11 m sub-sub-plots) for each of three treatments (non-legumes, low-nitrogen-supply legumes, or high-nitrogen-supply legumes as winter cover crops) and there were 32 control plots (no cover crops: crop residues and weeds over winter). Different species of cover crops were used in different years. Half of the plots were tilled, and half were not tilled (but pre-emergence herbicide was used). Post-emergence herbicide and fertilizer were used on all plots. Weeds were collected when the crops were harvested or the cover crops were suppressed (2–4 m2 quadrats), in 1994–2008.

 

16 

A controlled study in 2005–2006 in an irrigated tomato field in the Sacramento Valley, California, USA, found no differences in crop damage or weed biomass between the parts of the field that were cover cropped or fallow over winter. Crop damage: Similar numbers of tomatoes were damaged by insects in each part of the field (5–11 Mg fresh weight/ha), and similar numbers had blossom end rot (4 vs 2 Mg fresh weight/ha). Pest numbers: Similar amounts of weed biomass were found in each part of the field (2 Mg dry weight/ha). Methods: A field was divided into two parts: one part with a winter cover crop (mustard Brassica nigra, planted in autumn 2005, and disked into the soil in spring 2006), and one part fallow. Tomatoes were planted in both parts of the field in spring 2006. Tomatoes were sampled on 393 m transects (1 x 3 m quadrats every 30 m).

 

17 

A replicated, randomized, controlled study in 2009–2011 in two irrigated pepper fields in central Italy, found fewer weeds in plots with winter cover crops, compared to plots without cover crops, and oat was a better cover crop than hairy vetch or canola for controlling weeds. Pest numbers: Fewer weeds were found in plots with cover crops, compared to plots without cover crops, in 16 of 18 comparisons (0–117 vs 48–152 plants/m2). Implementation options: Fewer weeds were found in plots with oats as the winter cover crop, compared to hairy vetch, in five of six comparisons (0–7 plants/m2), and compared to canola, in all comparisons (0–10 vs 38–117). Fewer weeds were found in plots with hairy vetch as the cover crop, compared to canola, in three of six comparisons (26–94 vs 38–117 plants/m2). Methods: Three species of winter cover crops (Vicia villosa hairy vetch, Brassica napus canola, or Avena sativa oats) were sown on nine plots each (6 x 12 m plots) in September 2009–2010, and no cover crops were sown on nine plots (weeded, bare soil). The cover crops were mown and used as mulch (50 cm wide) in some plots, or were chopped and tilled into the soil in other plots, in May 2010–2011. Pepper seedlings were transplanted into these plots in May, and fruits were harvested twice/year in August–October 2010–2011. Weeds were sampled 30 days after transplanting (six samples/plot). All plots were fertilized before the cover crops, but not after. All plots were irrigated.

 

18 

A replicated, randomized, controlled study in 2011–2014 in irrigated potato fields in Israel found fewer weeds in plots with cover crops, compared to bare soil, both in the potato-growing season and also in the winter. Pest numbers: During the potato-growing season, fewer weeds were found in plots with cover crops, compared to bare soil, for one of five cover crops (oats and vetch, 60 days after planting potatoes, in 2011–2012: 11 vs 44 weeds/m2; data not reported for other cover crops or other years). During the cover-cropping season, fewer weeds were found in plots with cover crops, compared to bare soil, for all mixtures of cover crops (2–26 vs 36–82 weeds/m2; data not reported for other years). Less weed biomass was found in plots with cover crops, compared to bare soil (2013–2014: 5–20 vs 505 g/m2; data not reported for other years). Implementation options: During the potato-growing season, fewer weeds were found in plots that were cover cropped with oats and vetch, compared to canola (11 vs 43 weeds/m2), but difference between these and other cover crops were not significant. During the cover-cropping season, similar numbers of weeds were found in plots with different mixtures of cover crops (2–26 weeds/m2). Methods: Different plots were used in different years (2011–2012: 350 m2 plots, 20 plots with cover crops, eight plots without cover crops; 2012–2013: 695 m2 plots, 10 with, 10 without; 2013–2014: 1,800 m2 plots, four with, four without). Different mixtures of cover crops were used in different years, but oats were used in all years, and triticale was used in Years 1 and 2 (2011–2013). Plots without cover crops were weeded (tilled bare; some plots in all years) or weedy (not tilled; some plots in Year 1). Fertilizer and herbicide (after cover crops, before potato emergence) were used on all plots. Weeds were sampled in 0.25 m2 round quadrats (1–3 quadrats/plot).

 

19 

A replicated, randomized, controlled study in 1999–2011 in an irrigated tomato-cotton field in the San Joaquin Valley, USA, found more weeds and different weed species in plots with winter cover crops, compared to plots without winter cover crops. Pest numbers: More weeds were found in plots with cover crops, in six of 12 comparisons (28–121 vs 3–98 plants/m2). Different communities of weeds were found in plots with or without cover crops, in one of two comparisons (in plots with conventional tillage: data reported as distance in ordination space). Methods: Rainfed winter cover crops (triticale, rye, and vetch) were planted on 16 treatment plots, but not on 16 control plots, in October 1999–2010. Crop residues were chopped in March. Reduced tillage or conventional tillage was used on half of these plots, in 1999–2011. The plots (9 x 82 m) had six raised beds each. Different numbers of tillage practices were used for conventional tillage (19–23 tractor passes, including disk and chisel ploughing) and reduced tillage (11–12 tractor passes, not including disk and chisel ploughing). All plots were fertilized (conventional tillage: 89.2 kg/ha dry fertilizer, 111.5 kg/ha urea; reduced tillage: 124.9 kg/ha urea). Weeds were counted in January 2003 (1 m2 quadrats, four quadrats/plot), as well as March 2006 and June 2011 (0.25 m2 quadrats, two quadrats/plot). Soil cores were collected in June 2011 (8.25 cm diameter, 0–10 cm depth). Seeds from these soil cores were germinated, and weed species were counted.

 

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.