Conservation Evidence strives to be as useful to conservationists as possible. Please take our survey to help the team improve our resource.

Providing evidence to improve practice

Action: Crop production: Grow cover crops in arable fields Mediterranean Farmland

Key messages

Read our guidance on Key messages before continuing

Crop yield (24 studies): Six replicated, controlled studies (five randomized) from Spain and the USA found lower cash crop yields in plots with winter cover crops, compared to plots without them, in some comparisons. Three replicated, randomized, controlled studies from Italy and the USA found higher cash crop yields in plots with winter cover crops, compared to plots without them, in some comparisons. Eight replicated, randomized, controlled studies from Italy and the USA found inconsistent differences in cash crop yields (sometimes higher, sometimes lower) between plots with or without cover crops. Seven controlled studies (six replicated, four randomized) from France, Israel, Spain, and the USA found no differences in cash crop yields between plots with or without cover crops. One replicated, randomized, controlled study from the USA found inconsistent differences in cash crop yields between plots with or without summer cover crops.

Crop quality (6 studies): Three replicated, controlled studies (two randomized) from Italy, Spain, and the USA found no differences in cash crop quality between plots with or without winter cover crops. Two controlled studies (one replicated and randomized) from the USA found some differences in tomato quality between plots with winter cover crops or fallows. One replicated, randomized, controlled study from the USA found inconsistent differences in cash crop quality between plots with or without winter cover crops.

Implementation options (9 studies): Eight studies from Italy, Spain, and the USA found higher cash crop yields in plots that had legumes as winter cover crops, compared to non-legumes. One study from the USA found higher cash crop yields in plots that had a mixture of legumes and grasses, compared to legumes alone.

 

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 similar lettuce yields in plots with or without winter cover crops. Crop yield: Similar lettuce yields were found in plots with cover crops or fallows (210–664 g fresh weight/head). Methods: There were six plots (10.7 x 1.1 m raised beds) for each of two cover crops (broad beans or rye), and there were 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 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). Head weight was measured in 25 plants (autumn 1987) or all plants in 3 x 3 m quadrats (other harvests) in each plot.

 

2 

A replicated, randomized, controlled study in 1989–1991 in an irrigated lettuce field in Salinas, California, USA, found similar lettuce yields in plots with winter cover crops or bare fallows. Crop yield: Similar lettuce yields were found in plots with cover crops or bare fallows (290–312 vs 252 g dry matter/m2). Methods: In 1989–1990, six winter cover crops (Raphanus sativus oilseed radish, Brassica hirta white senf mustard, Brassica alba white mustard, Lolium multiflorum annual ryegrass, Secale cereale Merced rye, and Phacelia tanacetifolia) were grown on three plots each (two 12 m rows/plot), and bare fallows were maintained (with herbicide and hand cultivation) on three plots. In 1990–1991, two winter cover crops (Secale cerale Merced rye and Phacelia tanacetifolia) were grown on six plots each (two 8 m rows/plot), and bare fallows were maintained on six plots. Cover crops were tilled into the plots (15–20 cm depth in March 1990, depth not reported in February 1991). Lettuce was sown in April 1990–1991. All plots were irrigated and fertilized (56–85 kg N/ha, before sowing lettuce). Data on lettuce yields were reported for the harvest in July 1991.

 

3 

A replicated, randomized, controlled study in 1992–1993 in an irrigated broccoli field in the Salinas Valley, California, USA, found higher broccoli yields in plots with winter cover crops, compared to bare fallows. Crop yield: Higher broccoli yields were found in plots with cover crops, compared to bare soil, in one of four comparisons (the first broccoli harvest, with phacelia as the winter cover crop: 957 vs 830 g dry weight/m2). 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). Broccoli biomass was measured in two 1 m2 areas/plot.

 

4 

A replicated, randomized, controlled study in 1991–1993 in an irrigated tomato field in the San Joaquin Valley, California, USA, found lower tomato yields in plots with winter cover crops, compared to winter fallows. Crop yield: Tomato yields were lower in plots with cover crops, compared to fallows, in one of two years (in 1991: 76–84 vs 97 t/ha). 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 1991–1992 and incorporated into the soil in March 1992–1993 (15–20 cm depth, rotary tiller). Tomato seeds were planted in April 1992–1993. All plots were fertilized (12 kg N/ha before planting the tomatoes), but only plots that had not been cover cropped with vetch were sidedressed (168 kg N/ha, when thinning the tomatoes). All plots were irrigated with saline water (at rates to replace evapotranspiration).

 

5 

A replicated, controlled study in 1996–1998 in an irrigated tomato field in the San Joaquin Valley, California, USA, found lower tomato yields in plots with winter cover crops (and no tillage in spring), compared to plots with winter fallows (and tillage in spring). Crop yield: Lower tomato yields were found in plots with cover crops, compared to fallows, in four of 16 comparisons (27–36 vs 39–42 tons/acre). Crop quality: Similar amounts of soluble solids were found in tomatoes in the treatment and control plots (data not reported). Implementation options: Higher tomato yields were found in plots that were cover cropped with grass-legume mixtures, compared to legumes, in two of eight comparisons (36–38 vs 27 tons/acre). 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 and harvested in August 1997 and September 1998. The tomatoes were irrigated (two inches/week) and fertilized (0, 100, or 200 lb N/acre).

 

6 

A replicated, randomized, controlled study in 1995–1998 in an irrigated tomato field in Davis, California, USA, found higher crop yields in plots with winter cover crops, compared to plots without cover crops, but summer cover crops had inconsistent effects on crop yields. Crop yield: Higher tomato yields were found in plots with winter cover crops, compared to plots without them, in one of four comparisons (1996–1997: 104 vs 94 t/ha). Higher tomato yields were found in plots with summer cover crops, compared to plots without them, in one of three comparisons (1996–1997: 108 vs 96 t/ha), but lower yields were found in one of three comparisons (1997–1998: 34 vs 45 t/ha). Methods: Cover crops were planted in different numbers of plots in different years (1995–1996: 16 plots with winter cover crops, eight plots with summer cover crops, 16 control plots without cover crops; 1996–1997: 12 winter, four summer, eight controls; 1997–1998: 28 summer and/or winter, four controls). Plots were 3–4 beds wide and 10 m long. Some summer cover crops were retained over winter, and some were mown and replaced with winter cover crops. Summer cover crops were mixtures of oats and legumes, planted in August–September. Winter cover crops were legumes (Vicia sativa common vetch), planted in November. In spring, cover crop residues were mown and either removed or evenly distributed among all plots and incorporated into the soil. Some plots were irrigated during the cover-cropping season. All plots were irrigated during the tomato-growing season. Herbicide was used on all plots, but no inorganic fertilizer was used.

 

7 

A replicated, randomized, controlled study in 1998–2000 in an irrigated vegetable field in the Salinas Valley, California, USA, found lower lettuce yields in plots with winter cover crops, compared to plots without cover crops, but cover crops had inconsistent effects on crop quality. Crop yield: Lower lettuce yields were found in plots with cover crops, in one of four comparisons (281 vs 313 g dry weight/m2). No differences in broccoli yields were found in plots with or without cover crops (625–644 vs 606–633 g dry weight/m2). Crop quality: Larger lettuces were found in plots with cover crops, in two of four comparisons (1,080–1,140 vs 1,030–1,100 g fresh weight/plant), but smaller lettuces were found in one of four comparisons (750 vs 790). Larger broccoli plants were found in plots with cover crops (240–270 vs 210–220 g fresh weight/plant). 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). Crops were collected in two 2 m2 areas/plot. 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 lower grain yields in plots that were cover cropped with rye, compared to clover. Implementation options: Lower grain yields were found in plots that were cover cropped with rye, compared to clover, in three of seven years (data not reported), but there were no differences in grain yields 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.

 

9 

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 lower tomato yields in plots with cover crops, compared to dry fallows, but found inconsistent differences in tomato yields between plots with cover crops and wet fallows. Crop yield: Lower tomato yields were found in plots with non-nematode-resistant cover crops, compared to plots with dry fallows, in some comparisons (e.g., in Experiment 1: 40,000 vs 61,000 kg/ha). Inconsistent differences in yields (sometimes higher, sometimes lower) were found between plots with cover crops or wet fallows (e.g., in Experiment 3, in 2 of 9 comparisons: 21,000 vs 59,000–69,000 kg/ha; in 1 of 9 comparisons: 143,000 vs 110,000). 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.

 

10 

A replicated, randomized, controlled study in 1999–2003 in an irrigated tomato-cotton field in the San Joaquin Valley, California, USA (same study as (24)), found lower crop yields in plots with winter cover crops, compared to plots without winter cover crops. Crop yield: Lower tomato yields were found in plots with cover crops, compared to plots without cover crops, in one of two comparisons (with reduced tillage: 52 vs 58 t/ha). Methods: Sixteen plots (9 x 82 m) had six raised beds each. Rainfed winter cover crops (Triticosecale triticale, Secale cereale Merced rye, and Vicia sativa common vetch) were planted on eight plots, in October 1999–2002, and crop residues were chopped in March. Reduced tillage or conventional tillage was used on half of the plots, in 1999–2003. 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). Tomato seedlings were transplanted in April 2000–2003. Fertilizer and herbicide were used on all plots, and the tomatoes were irrigated. Tomatoes were grown in rotation with cotton.

 

11 

A replicated, randomized, controlled study in 2005–2007 in an irrigated tomato-maize field in Davis, California, USA, found lower tomato yields, but higher maize yields, in plots with winter cover crops, compared to bare fallows in winter. Crop yield: Lower tomato yields were found in plots with cover crops, compared to fallows (27–47 vs 68 Mg/ha). Higher maize yields were found in plots with cover crops, compared to fallows, for two of three mixtures of cover crops (mixtures with legumes: 30–31 vs 16 Mg/ha). Implementation options: Higher maize yields were found in plots that were cover cropped with legumes, compared to plots that were cover cropped with grains only (30–31 vs 16 Mg/ha). Similar tomato yields were found in plots with different mixtures of cover crops (27–47 Mg/ha). Methods: Three mixtures of winter cover crops (legumes only, legumes and grains, or grains only) were grown on five plots each, and five control plots were bare fallows on which weeds were controlled by burning (111 m2 plots; six raised beds/plot). Tomatoes were grown in 2006, and maize was grown in 2007, without fertilizer.

 

12 

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 crop yields, which varied with the species of cover crop. Crop yield: Higher tomato yields were found in plots with mulched cover crops, compared to plots without cover crops or mulch, for three of four cover crops (86–100 vs 78 t/ha), but lower tomato yields were found for one of four cover crops (oats: 66 vs 78 t/ha). Crop quality: Tomato quality was similar with or without cover crops and mulch (pH 5.1–5.6, 4.3–4.9 cm diameter tomatoes). Implementation options: The highest yields were found in plots that were cover cropped and mulched with hairy vetch (100 t/ha, 181 fruits/m2) and the lowest yields were found in plots that were cover cropped and mulched with oats (66 t/ha, 129 fruits/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 and used as mulch (6 cm depth, 80 cm width). 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 harvested in August. It was not clear whether these results were a direct effect of growing cover crops or mulching.

 

13 

A replicated, randomized, controlled study in 2006–2008 in an irrigated maize field in the Ebro river valley, Spain, found lower maize yields in plots with winter cover crops, compared to bare soils. Crop yield: Lower maize yields were found in plots with cover crops, compared to bare soils, in four of five comparisons (barley or winter rape as the cover crops: 14 vs 16–17 Mg/ha). Crop quality: Similar grain quality was found in plots with cover crops or bare soils (530–640 grains/ear; 0.25–0.29 g/kernel). Implementation options: Lower maize yields were found in plots that were cover cropped with non-legumes (barley or winter rape), compared to legumes (common vetch) (14 vs 18 Mg/ha). Methods: There were three plots (5.2 m2) for each of three winter cover crops (Hordeum vulgare barley, Brassica rapa winter rape, or Vicia sativa common vetch, sown in October 2006–2007), and three control plots (bare soil in winter). Similar amounts of nitrogen were added to all plots (300 kg N/ha), but less of it came from mineral fertilizer in plots with cover crops, to compensate for the organic nitrogen that was added to these plots when the cover crop residues were tilled into the soil. All plots were tilled in spring (March 2007–2008) and autumn (October 2006–2007). All plots were irrigated twice/week (drip irrigation, based on evapotranspiration). Maize was planted in April and harvested in October 2007–2008.

 

14 

A replicated, controlled study in 2004–2008 in an irrigated maize field in the Garonne River corridor, southern France, found similar maize yields in plots with or without winter cover crops. Crop yield: Similar maize yields were found in plots with cover crops or bare soil (11–13 vs 10–13 kg grain/ha). Methods: Winter cover crops (2006–2007: white mustard; 2004–2006 and 2007–2008: oats) were grown on six plots, and bare soil was maintained on six plots. The plots were 20 x 50 m. Maize was sown in April–May 2005–2008 and harvested in October 2005–2008. A centre-pivot sprinkler was used for irrigation (857–943 mm water/year, irrigation plus rainfall).

 

15 

A replicated, randomized, controlled study in 2005–2006 in an irrigated, organic tomato field in Yolo County, California, USA, found lower tomato yields in plots with winter cover crops, compared to winter fallows. Crop yield: Lower tomato yields were found in plots with cover crops, compared to fallows (162 vs 234 g harvestable fruit/m2). Crop quality: Tomatoes were not as red in plots with cover crops, compared to fallows (39.8 vs 38.9 L*a*b colour values), but firmer tomatoes were found in plots with cover crops (77% vs 75%). Tomato weight, soluble solids, pH, and titratable acidity did not differ between plots with cover crops or fallows (data not reported). 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). Tomatoes were transplanted on 18–19 May 2006 and harvested on 7–8 September 2006.

 

16 

A replicated, randomized, controlled study in 2006–2009 in an irrigated maize field in the Tajo river basin, near Madrid, Spain, found similar crop yields in plots with winter cover crops or bare fallows. Crop yield: Similar maize yields were found in plots with cover crops or fallows (9,800–14,900 vs 8,400–14,400 kg grain/ha, dry weight). Methods: There were four plots (12 x 12 m plots) for each of two treatments (barley or vetch, as winter cover crops) and there were two control plots (bare fallow). Cover crops were sown in October 2006–2009 and maize was sown in April and harvested in October 2007–2009. The maize was irrigated (sprinklers) and fertilized (210 kg N/ha, split into two applications, 120 kg P/ha, and 120 kg K/ha).

 

17 

A replicated, controlled study in 2007–2009 in an irrigated tomato field in Davis, California, USA, found similar crop yields in plots with winter cover crops or winter fallows. Crop yield: Similar tomato yields were found in plots with cover crops or fallows (28–34 vs 31–33 marketable t/acre). Methods: Conventional tillage or reduced tillage was used on four plots each (90 x 220 feet). Broadcast disking, subsoiling, land planing, and rebedding were used for conventional tillage. A Wilcox Performer was used for reduced tillage (two passes; beds were conserved). Winter cover crops (Triticosecale triticale) were grown on half of each plot, and the other half was fallow in winter. Sprinklers, furrow irrigation, and drip-tape (in furrows) were used to irrigate the tomatoes. All plots were fertilized.

 

18 

A controlled study in 2005–2006 in an irrigated tomato field in the Sacramento Valley, California, USA, found some differences in tomato quality between the parts of the field that were cover cropped or fallow over winter. Crop yield: Similar tomato yields were found in each part of the field (55–67 Mg undamaged tomatoes/ha, fresh weight). Crop quality: More pink or split tomatoes were found in the cover-cropped part, compared to the fallow part (pink: 13 vs 11 Mg/ha; split: 6.8 vs 6.5), but similar numbers of green (22 vs 15 Mg/ha), sunburned (19 vs 20 Mg/ha), and mouldy or rotten (36 vs 27 Mg/ha) tomatoes were found in each part of the field (fresh weights). 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).

 

19 

A replicated, randomized, controlled study in 2009–2011 in two irrigated pepper fields in central Italy found that cover crops had inconsistent effects on crop yields. Crop yield: Higher pepper yields were found in plots with cover crops, compared to plots without cover crops, in 15 of 27 comparisons (9–41 vs 2–15 t/ha, fresh weight), but lower pepper yields were found in one of 27 comparisons (8 vs 15). Implementation options: Higher pepper yields were found in plots with hairy vetch as the winter cover crop (9–41 t/ha, fresh weight), compared to canola (3–26 t/ha) or oats (4–21 t/ha). Higher pepper yields were found in plots with canola as the winter cover crop, compared to oats, in two of nine comparisons (25–26 vs 8–10 t/ha, fresh weight), but lower yields were found in one of nine comparisons (14 vs 18). 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 rows in May, and fruits were harvested twice/year in August–October 2010–2011. All plots were fertilized before the cover crops, but not after. All plots were irrigated.

 

20 

A replicated, randomized, controlled study in 2009–2012 in two irrigated vegetable fields in central Italy found higher crop yields in plots with winter cover crops, compared to plots with bare soil in winter. Crop yield: Higher crop yields were found in plots with cover crops, compared to bare soil, in one of three comparisons (in plots with hairy vetch as the winter cover crop: 17 vs 7 t/ha endive; 15 vs 4 t/ha savoy cabbage). Implementation options: Higher crop yields were found in plots with hairy vetch as the winter cover crop, compared to oats (endive: 17–23 vs 5–6 t/ha; cabbage: 15–23 vs 2–6 t/ha; fresh weights), or compared to oilseed rape, in five of six comparisons (endive: 17–23 vs 5–11; cabbage: 15–23 vs 2–6). Higher yields were found in plots with oilseed rape as the winter cover crop, compared to oats, in one of six comparisons (11 vs 4 t/ha endive, fresh weight). Methods: There were nine plots (6 x 4 m plots) for each of three treatments (hairy vetch, oats, or oilseed rape) and one control (bare soil, maintained with herbicide). Cover crops were sown in September 2009–2010 and suppressed in May 2010–2011 (chopped and incorporated into the soil with a mouldboard plough, 30 cm depth). Pepper seedlings were transplanted into these plots in May 2010–2011 and were last harvested in October 2010 and September 2011. After the pepper harvest, endive and savoy cabbage seedlings were transplanted into these plots, and they were harvested in December 2010 and November 2011 (endive) or March 2011 and February 2012 (cabbage). No fertilizer was added while the crops were growing, but the plots were irrigated.

 

21 

A replicated, randomized, controlled study in 2011–2014 in irrigated potato fields in Israel found similar crop yields in plots with or without cover crops. Crop yield: Similar potato yields were found in plots with or without cover crops (oats: 4.4–8.0 vs 4.9–8.0 kg/m2; data not presented for other cover crops). 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). Herbicide and fertilizer were used on all plots. Potatoes were planted under mown cover crops. Potato yields were sampled in 5 m2/plot.

 

22 

A replicated, randomized, controlled study in 2011–2013 in two irrigated tomato fields in central Italy (same study as (23)) found that cover crops had inconsistent effects on tomato yield. Crop yield: Higher tomato yields were found in plots that were cover cropped and mulched with hairy vetch, compared to control plots (5.4–5.9 vs 3.6–5.2 kg fresh weight/m2), but lower yields were found in plots that were cover cropped and mulched with lacy phacelia or white mustard (2.4–4.3 kg). Methods: Three species of winter cover crops (Vicia villosa hairy vetch, Phacelia tanacetifolia lacy phacelia, or Sinapis alba white mustard) were sown on three plots each, in September, and winter weeds were controlled with herbicide on three control plots (18 x 6 m plots). The cover crops were mown and mulched (strips, 80 cm width) in May, and the control plots were tilled (depth not reported). Tomato seedlings were transplanted in May (transplanted into the mulch) and harvested in August. All plots were tilled (30 cm depth) and fertilized (100 kg P2O5/ha­, harrowed to 10 cm depth) in September. Some plots were also fertilized (100 kg N/ha) in June–July. It was not clear whether these results were a direct effect of cover cropping, mulching, herbicide, or tillage.

 

23 

A replicated, randomized, controlled study in 2012–2013 in two irrigated tomato fields in central Italy (same study as (22)) found that cover crops had inconsistent effects on tomato yields. Crop yield: Higher tomato yields were found in plots that had been cover cropped and mulched with hairy vetch, compared to plots that had not (6.4–7 vs 3.2–5.3 kg/m2). Lower tomato yields were found in plots that had been cover cropped and mulched with lacy phacelia (in one of two comparisons: 4.2 kg/m2) or white mustard (2.1–3.5), compared to plots that had not (3.2–5.3). Implementation options: The highest tomato yields were found in plots with hairy vetch (6.4–7 kg/m2) and the lowest were found in plots with white mustard (2.1 kg/m2). Methods: Three species of winter cover crops (Vicia villosa hairy vetch, Phacelia tanacetifolia lacy phacelia, or Sinapis alba white mustard) were sown on three plots each, but not on three control plots (plot size not reported), in September. The cover crops were mulched in May, and the control plots were tilled (depth not reported). Tomato seedlings were transplanted in May (transplanted into the mulch) and harvested in August. All plots were tilled in September. It was not clear whether these results were a direct effect of cover cropping, mulching, or tillage.

 

24 

A replicated, randomized, controlled study in 1999–2009 in an irrigated tomato-cotton field in the San Joaquin Valley, California, USA (same study as (10)), found that winter cover crops had inconsistent effects on crop yields. Crop yield: Lower tomato yields were found in plots with cover crops, compared to plots without cover crops, in four of 10 years (95–118 vs 109–128 t/ha), but higher yields were found in one of 10 years (with conventional tillage: 142 vs 132 t/ha). Methods: Rainfed winter cover crops (Triticosecale triticale, Secale cereale Merced rye, and Vicia sativa common vetch) were planted on eight treatment plots, but not on eight control plots, in October 1999–2008. Crop residues were chopped in March. Reduced tillage or conventional tillage was used on half of these plots, in 1999–2009. 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). Tomato seedlings were transplanted in April 2000–2009. Fertilizer and herbicide were used on all plots, and the tomatoes were irrigated. Tomatoes were grown in rotation with cotton.

 

25 

A replicated, randomized, controlled study in 2009–2011 in an irrigated eggplant field in central Italy found that winter cover crops had inconsistent effects on crop yield. Crop yield: Higher eggplant yields were found in plots with winter cover crops, compared to plots with bare soil in winter, in six of nine comparisons (18–38 vs 11–21 Mg/ha fresh weight), but lower yields were found in two of nine comparisons (7–14 vs 18–21). Implementation options: Higher eggplant yields were found in plots with hairy vetch as the winter cover crop (32–38 Mg/ha fresh weight), compared to oats (7–18 Mg/ha) or oilseed rape (18–25 Mg/ha). Higher eggplant yields were found in plots with oilseed rape as the winter cover crop, compared to oats, in two of three comparisons (20–25 vs 7–14 Mg/ha fresh weight). Methods: Three species of winter cover crops (Vicia villosa hairy vetch, Brassica napus oilseed rape, or Avena sativa oats) were sown on three plots each (6 x 12 m plots) in September 2009–2010, and no cover crops were sown on three plots (weeded, bare soil). The cover crops were mown and used as mulch (50 cm wide) in eggplant rows, in May 2010–2011. Eggplant seedlings were transplanted into the plots in May, and fruits were harvested four times/year in July–September 2010–2011. All plots were fertilized before the cover crops were grown, but not after. All plots were irrigated.

 

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.