Cover cropping

Cover cropping resembles intercropping and exploits broadly similar ecological principles but involves mulching across the entire cropped area into which the cash crop itself may be inserted. The mulch may be mown or treated with a herbicide applied at reduced rates to diminish its competitive abilities prior to establishing the cash crop. Growers using the techniques of 'organic husbandry' have long recognized that non-tillage cover crops reduce pest populations and weed competition and improve soil quality by protecting the soil surface, decreasing soil, wind and water erosion, conserving soil moisture and reducing nutrient run-off. Non-tillage cover crops are sown in the autumn or very early spring and mown during late spring to form a mulch on the soil surface just before planting the main cash crop.

Cover crop mulch systems modify the microenvironment of the crop with subsequent effects on pest populations and crop yields. Plants in non-tillage systems tend to be smaller and paler than those with conventional tillage. This results from lower soil temperatures observed in non-tillage plots which decrease the rate of mineralization of soil nitrogen reducing its availability to

Brassica crops. If a legume, however, is included in the mulch, then nitrogen is added into the soil. Suitable plant species for use as cover crops are given in Table 6.8.

Indirect benefits obtained from cover crops

Some cover crops release chemicals that inhibit weed germination, a phenomenon termed allelopathy as initially defined by Molisch (1922) and discussed more recently by Rice (1984). Rye (S. cereale) residues release chemicals inhibitory to annual weeds, and suppression is greater where the rye residues are left on the soil surface. Hairy vetch (Vicia villosa) has weed-suppressive properties but to a lesser extent than rye. Cover crops may also affect the chemical constituents of soils. Cover crop residues may make nitrogen unavailable to vegetable crops due to assimilation or denitrification. Conversely, cover crops can also increase nitrogen available for brassicas either by the decomposition of legume residues or by preventing nitrogen losses during winter. Rye also increases the concentration of exchangeable potassium near the soil surface.

Key findings from a detailed study of cover cropping by Brands^ter (1996) and Brands^ter et al. (1998) in Norway identified the following effects.

  1. Frost-sensitive cover crops are of no value for weed control in the subsequent spring - only those killed off in spring by herbicides (glyphosate-N-(phosphonomethyl) glycine) provided improved weed control.
  2. Spring-sown cover crop (clover) provided better weed control when mown in early summer.
  3. Competition and the failure of spring-sown clover to supply added nitrogen decreased the yield of white cabbage when compared with using monocultures.
  4. No significant differences in yield of white cabbage between monoculture, late spring-sown subterranean clover, and rye and rye/vetch treatments were found.
  5. Autumn-sown subclover (Trifolium subterraneum) did not suppress weeds effectively in spring, and mowing off the weeds that germinated in the spring or early summer was necessary.
Table 6.8. Plant species suitable as cover crops.

Grasses and cereals

Legumes

Rye - Secale cereale

Hairy vetch - Vicia villosa

Barley - Hordeum vulgare

Austrian winter pea - Pisum sativum subsp. arvense

Wheat - Triticum aestivum

Crimson clover - Trifolium incarnatum

Ryegrass - Lolium multiflorum

  1. Live mulch systems suppressed weeds more effectively than plant residues.
  2. It is necessary to know how to manage the yield depression caused by competition, for example, by finding winter hardy annual legumes suited to local environmental conditions.
  3. A considerable increase in cabbage yield was achieved both in subterranean clover (cv. Geraldton) and in white clover (cv. Pertina) treatments by rototilling between the rows 6 weeks after transplanting.
  4. The cover crops did not reduce weed biomass or number of weeds early in the season compared with monoculture, but weed biomass in late summer became significantly lower in living mulch systems.
  5. Rototilling was more effective for weed suppression than mowing.
  6. Cover cropping combined with rototilling reduced weed biomass by 89% compared with untreated monoculture.
  7. Subterranean clover used as a living mulch gave the earliest and most extensive ground cover and lowest cabbage yield, but it also reduced insect damage.
  8. Both subterranean clover and white clover living mulches were associated with a greater number of marketable cabbage heads resulting from reduced insect damage.
  9. White clover intercropped with cabbage gave significantly higher oat yields in the subsequent year compared with monocropping.
  10. About half as many cabbage heads were marketable (rating 1 or 20) in monoculture plots (21%) as compared with either of the living mulch treatments (subterranean clover 42% and white clover 38%) based on caterpillar damage. This gave damage indices for cabbage heads of 72.2 for the monoculture, 63.5 for the white clover and 61.4 for the subterranean clover. The differences in the damage index were highly significant (P = 0.0001) for both clover species compared with the monoculture, but there were no significant differences (P = 0.2) between the clover species.
  11. The number of turnip root fly eggs oviposited in the monoculture plots was significantly higher (P = 0.05) than in the plots with live mulch during peak oviposition of the turnip root fly. This effect was slightly more pronounced in the plots with subterranean clover, but there were no significant differences between subterranean clover and plots of white clover.
  12. Overall there seems to be a reduction of insect damage, but also loss of yield, resulting from cover cropping. These two effects will balance out, especially if there is a saving in the amount of pesticide used. The effects of yield enhancement by rototilling at 6 weeks suggest that this reduces the regrowth of the legume and enhances the availability of nutrients and soil moisture to the crop.
  13. Intercropping systems seem to reduce pest damage and are valuable for their green manuring effects on subsequent crops. Early establishment of the cover crop is important, but this must be balanced against yield depression that occurs as a result of maturing cover crops.
  14. Trials of different overwintering legumes showed that the hardiest for Norwegian conditions was hairy vetch (V. villosa) since it provided the highest biomass production, the lowest regrowth ability after mowing and the best weed suppression.
  15. Reducing competition between cover and cash crops has focused on the use of chemical or mechanical suppression systems. Alternatively, growth of the cover and cash crops could be synchronized to achieve maximum growth for the cash crop. One way to achieve this is to sow the cover crop in the middle of the growing period of the cash crop. Alternatively, the cover crop is sown first as a weed-smothering crop in advance of the cash crop. Here use is made of rapidly growing plants.
  16. As a further alternative, use can be made of autumn-sown annual legumes such as subterranean clover or wild white clover which grow vegetatively during the autumn, become dormant or semi-dormant in winter and then resume growth in the spring. The most advantageous strategy would be to use species that flower, senesce and die in spring. This then allows the cash crop to be transplanted into the senescing mulch that provides additional soil moisture and nutrients as it decays. It is essential to use species or cultivars that have low canopy height and terminate their vegetative growth in late spring or early summer. An added valuable factor would be the ability to produce seed at senescence and then regrow in the autumn.

Nutritional benefits of cover cropping

Vegetable growers in the northeastern USA use cereal rye as a winter cover crop, but rye does not fix nitrogen. It ties up nitrogen after being incorporated into the soil (Schonbeck et al., 1993). As an example of specialist studies of cover cropping, the effects on yield of broccoli and cabbage by cover crops of hairy vetch, grown alone or in combination with rye, were compared with rye alone and a no-cover control at four locations in New England. The effects of applied nitrogen were evaluated at two sites. Cover crops were grown until flowering and were either incorporated or mown and left on the surface. Brassica seedlings were transplanted into the plots, grown to maturity and assessed for yield, components of yield and foliar nutrients. Soil moisture was measured at two sites and inorganic nitrogen at one site.

Cover crops of vetch and vetch with rye consistently produced higher broccoli and cabbage yields than rye alone or with no cover crop. Measurements of foliar nitrogen contents indicated that nitrogen contribution was a major factor in the yield response, although cover crops sometimes affected concentrations of other nutrients. When rye was used alone, it reduced yields, probably through locking up soil nitrogen. At one location, broccoli yield responded dramatically to applied nitrogen (112 kg/ha as ammonium nitrate) regardless of cover crop or management system. Combinations of leguminous cover crop (cowpea, Vigna unguiculata; soybean,

Glycine max; velvet bean, Mucuna pruriens) and rates of applied nitrogen used for autumn broccoli production in coastal South Carolina, USA are reported by Harrison et al. (2004). The cover crops tended to promote higher and earlier yields of broccoli as compared with untreated plots. Other leguminous cover crops such as red clover (Triflolium pratense), biennial sweetclover (Melilotus spp.) and lucerne (Medicago sativa) will add nitrogen to the soil but take the land out of Brassica production for 12 months. Winter annual legumes such as hairy vetch (V. villosa), crimson clover (Trifolium incarnatum) and Austrian winter field pea (Pisum sativum subsp. arvense) can add nitrogen and organic matter without interrupting production for an entire season. Legumes when grown with a cereal grain or other grass (e.g. hairy vetch with rye) may produce more organic matter, protect the soil and suppress weeds better than either when grown alone.

Benefits of reduced tillage associated with cover crops

Traditionally cover crops are tilled into the soil before planting brassicas. Increasingly, however, growers adopt no-tillage systems to conserve soil and reduce fuel, machinery and labour costs. No-tillage cover crops reduced cabbage yields in Oklahoma, USA, but they also curbed soil erosion and reduced the number of pesticide applications required. Such crops may be killed with paraquat (1,1'-dimethyl-4,4' bipyridinum ion) herbicide sprays or by mowing close to the ground once flowering has commenced.

Manipulating competition from the cover crop

The main objection to cover cropping is the yield depression due to competition with the living cover crop. This may be mitigated by the following:

  • Using less competitive cover crop cultivars or choosing a combination of cover crop and sowing times to reduce mutual competition. This may be achieved by use of winter annual legumes whereby weeds are suppressed in the critical period for their development early in the cover crop life history and which synchronizes with the onset of maximum cash crop growth. This demands a species of legume suited to the local climate of the crop.
  • Suppressing the live mulch to reduce interference with crop growth by mechanical cultivation systems. Alternatively, tilled strips may be used in a similar manner to intercropping for the main (cash) crop rows; or timely overseeding of the main crop into the cover crop and chemical methods for cover crop suppression.

There were reduced yields of spring cabbage (B. oleracea var. capitata) in non-tillage crops compared with conventional tillage (Moore and Seward, 1986). By contrast, yields of autumn cabbage (B. oleracea var. capitata) increased with non-tillage systems. Differences in planting date influenced this response. Using legumes in non-tillage systems is a logical means of adding nitrogen. Hairy vetch (V. villosa) is the most efficient legume in this respect, adding 90-100 kg N/ha annually. Many experiments fail to mitigate the effects of competition between the cash and cover crops and hence find the crop growth component is less in living mulch treatments compared with bare soil control treatments. Hence, the biomass of cauliflower (B. oleracea var. botrytis) was reduced by 62% when grown in a vetch living mulch compared with clean cultivation (Altieri et al., 1985).

The yields of cauliflower and Brussels sprout (B. oleracea var. gemmifera) were reduced by 42 and 61%, respectively, when overseeded with white clover (Dempster and Coaker, 19 74; O'Donnell and Coaker, 1975). These authors planted Brussels sprouts in to an established stand of white clover (T. repens) at differing levels of soil cover (25, 50, 75 and 100%) and yields were reduced by 38, 40, 56 and 81%, respectively.

Andow et al. (1986) found that compared with clean cultivation cabbage (B. oleracea var. capitata), head size was smaller in live mulch combinations consisting of cv. Idaho clover compared with the dwarf cv. Kent. Further, Ryan et al. (1980) found that when cabbage was interplanted with clover in every third row (33% cover), average head weight was 15% higher compared with the 'no cover' treatment. The use of sublethal doses of herbicide has been frequently attempted as a means of cover crop suppression; or the use of non-chemical suppression that has involved mowing or partial tillage of the cover crop. It was found that when a white clover cover crop was kept mown to 15 cm high, then the components of yield of broccoli (B. oleracea var. italica) were statistically equivalent between 'no cover' and 'living mulch' treatments. The combined effects of cropping, tillage and herbicide treatments are shown in Table 6.9.

There is evidence that water stress reduces the main (cash) crop yield in a living mulch system, hence there is need for added irrigation to adjust this deficiency. The amount of water required to produce a given cabbage yield increased when perennial ryegrass was added into the system as live mulch (Graham and Crab tree, 1987). Mechanical suppression of the ryegrass by mowing twice per season did not increase cabbage yield. Use of herbicides (fluazifop-p-butyl; (R)-2 [4-(5-trifluoromethyl-2-pyridyloxy) phenoxy] propanoic acid) decreased competition for water, and at high irrigation rates cabbage yields were similar to those of conventional husbandry systems. Cover cropping rests on the acceptability of the ability of the cover crop to suppress weeds and tolerating the resultant competition with the main cash crop.

Cover cropping is seen as a particularly appropriate technique for the integrated management of brassicas. This is because the fields are finely tilled before sowing or planting and the soil is frequently wet during harvesting, which may involve the use of mechanical or mechanized harvesting such as gantry systems causing serious damage to the soil structure (Stivers-Young,

Table 6.9. Yield of broccoli (calabrese, Brassica oleracea var. italica)

in relation to cover

cropping, tillage and herbicide treatment.

Head fresh weight

Plant fresh weight

Treatment (Mg/ha) Maturity datea

(Mg/ha)

Rye

Till - herbicide

7.61

10.8

22.11

Till + herbicide

7.16

9.9

27.34

Mow - herbicide

7.32

21.8

20.58

Mow + herbicide

7.19

21.5

24.04

Rye + vetch

Till - herbicide

8.28

3.6

28.15

Till + herbicide

9.27

1.7

33.55

Mow - herbicide

10.19

7.8

35.26

Mow + herbicide

8.37

8.4

33.32

No cover

Till - herbicide

7.78

6.3

24.69

Till + herbicide

7.92

2.5

25.74

Mow - herbicide

0.05

43.0

1.38

Mow + herbicide

8.05

19.5

20.12

Factorial T Testb

Cover (C)

**

**

**

Tillage (T)

*

**

**

Herbicide (H)

**

**

**

C X T

**

**

**

C X H

**

**

**

T X H

*

NS

**

C X T X H

**

**

**

After Mangan et al. (1995). aDays after 18 July.

NS, i.e. P > 0.05; *P < 0.05; ** P < 0.01.

Site = South Deerfield, Massachusetts, USA; 3 X 2 X 2 factorial experiment, cover crops = rye (Secale cereale) + vetch (Vicia villosa); rye alone; no cover crop; tillage = conventional = cover crop tilled in and no-till = cover crop mowed; herbicide = DCPA (dimethyl tetrachloroterephthalate).

1998). Many brassicas are relatively inefficient users of nutrients; the rates of fertilizer applied frequently exceed crop demand, and excess nutrients, especially nitrogen, are lost through leaching into the soil water and eventually into free-flowing water courses such as streams and rivers. Nitrogen loss may also take place through denitrification and volatilization. Brassica crops tend to return relatively little organic matter to the soil and leave little surface residue that will protect the soil from wind and water erosion.

Growers need cover crops that have a relatively short growing season, accumulate leachable nitrogen and suppress autumn-growing weeds and are killed during the winter or easily destroyed by use of herbicides such as glyphosate (N-(phosphonomethyl) glycine). For efficient, profitable Brassica husbandry, however, it is essential that the use of an autumn cover crop does not delay spring cultivation and subsequent seeding or planting. In the temperate northeastern USA, Brassica cover crops such as oilseed radish (Raphanus sativus), white senf mustard and yellow mustard (Sinapis alba) are seen as well suited to autumn sowing as cover crops. These grow quickly, accumulate significant amounts of biomass, deplete soil inorganic nitrogen concentrations and suppress weeds. Agricultural brassicas such as oilseed rape (B. napus) or yellow mustard (S. alba) have been advocated as cover crops for horticultural brassicas to exploit the weed-suppressive effect of their secondary products such as isothiocyanates, ionic cyanates and epinitriles. Results obtained by Haramoto and Gallandt (2005a, b), however, suggest that weed suppression may not be any greater than that achieved by other methods.

Cover crops tend to decrease water evaporation from soil and increase infiltration, resulting in greater moisture content. This can lower soil temperature and delay the growth of early season brassicas. If, however, a rye crop grows excessively in the early spring, then soil moisture will be depleted. It was concluded that autumn cabbage and broccoli could be produced effectively in non-tillage soil management systems. Hairy vetch (V. villosa) and Austrian winter pea (P. sativum subsp. arvense) are more efficient cover crops than rye (S. cereale). Possibly this results from these legumes releasing nitrogen through mineralization.

In studies of the integrated use of cover crops and herbicides, transplanted cabbage were grown in conventional tillage (100% cultivated) and strip tillage (25% cultivated and 75% residue). Cabbage (B. oleracea var. capitata) yielded similar head weights irrespective of tillage treatment. Herbicides tested were: propanil (3',4' dichloropropionanilide), napropamide ((RS)-N,N-diethyl-2-(1-naphthyloxy) propionamide), oxyfluorfen (2-chloro-a,a,a-trifluoro-p-tolyl 3-ethoxy-4 nitrophenyl ether) and napropamide + oxyfluorfen (Hoyt et al., 1996).

Cabbage produced on sloping land prone to erosion should be grown using some form of conservation tillage to reduce water and soil erosion and to maintain soil productivity. Weed management programmes for conservation tillage are critical for successful economic yields.

Reduced tillage practices offer advantages of soil and water conservation, reduced erosion potential, less energy and time (labour) spent producing the crop, and more efficient land use. Grass and cereal cover cropping with cabbages tends to depress yields in comparison with systems using legumes or where adequate nitrogen is applied artificially. A highly significant yield response was obtained from broccoli treated with cover crop, tillage and herbicide treatments. The vetch plus rye treatment gave higher yields than rye alone and non-cover crop. The vetch plus rye treatment was also associated with earlier harvesting dates for the broccoli crop. The lowest yielding treatment was the non-cover crop, non-tillage and no herbicide. Incorporation of each cover crop treatment and herbicide use in tilled treatments hastened the maturity of the broccoli crop.

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Responses

  • yonatan
    What cover crop can be seeded between red currants to smother weeds?
    7 years ago
  • CHRISTINE
    Can I grow winter cover crops?
    7 years ago

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