Brassica cover cropping for weed management: A review

Transcription

1 Renewble Agriculture nd Food Systems: 19(4); Brssic cover cropping for weed mngement: A review Erin R. Hrmoto nd Eric R. Gllndt* DOI: /RAFS Deprtment of Plnt, Soil nd Environmentl Sciences, University of Mine, Orono, ME , USA. *Corresponding uthor: gllndt@mine.edu Accepted 2 September 2004 Review Abstrct Cover crops offer mny benefits for frmers seeking to reduce their relince on externl inputs. These include mintining nd improving soil qulity, preventing erosion nd, in some cses, llelopthic weed control. Allelopthic potentil hs been well documented for cover crops such s cerel rye (Secle cerele L.), hiry vetch (Vici villos Roth) nd red clover (Trifolium prtense L.). Much less is known bout other potentilly llelopthic cover crops, including certin brssicceous species tht re normlly grown for their oilseeds, including cnol nd rpeseed (both Brssic npus L.) nd mustrds (e.g., Sinpis lb L., white nd yellow mustrd). Becuse of their potentil contribution to pest mngement, there is incresed interest in growing brssics, both s cover crops nd s seed crops hrvested for oil production. In this review, we first discuss unique ttributes of brssics tht mke them promising options for pest mngement, s well s generlly beneficil cover crops. Next, we review the literture from controlled settings on the effects of brssics, brssic extrcts nd isolted compounds contined therein on seed germintion, seedling emergence nd estblishment, nd seedling growth effects tht, combined or tken lone, could contribute to reducing the density nd vigor of weed communities in the field. Field studies exmining the detrimentl effects of brssics in rottion with other crops, s well s exmining the effects of brssic cover crops on weed dynmics in subsequent crops, lso re reviewed. Finlly, we review some importnt gronomic considertions bout the use of brssic cover crops. Key words: brssics, cover crop residues, green mnure, llelopthy Introduction One of the hllmrks of sustinble griculture systems is the use of multiple tctics to ddress short- nd long-term gols concerning pest nd soil mngement. In this context, single tctics tht provide multiple benefits re of vitl importnce. The prctice of cover cropping is such tctic, providing mny services to groecosystems, including improved soil qulity, incresed nutrient cycling nd pest mngement 1. Given the prominence of weeds s production-relted problem for orgnic nd diversified vegetble growers 2, cover crops re often chosen nd mnged for weed control. The weed suppressive effects of winter rye (Secle cerele L.), hiry vetch (Vici villos L.) nd red clover (Trifolium prtense L.) cover crops hve been well documented 3 8. Other cover crops, including certin brssic species, hve been exmined for weed control on more limited bsis. However, there is good reson to consider brssics s cover cropping option when weed mngement is priority. Members of the Brssiccee fmily contin glucosinoltes sulfur-contining molecules tht re hydrolyzed to form compounds toxic to vriety of soil-borne orgnisms, including weeds 9. Lbortory nd greenhouse studies suggest tht the hydrolysis products of glucosinoltes, s well s brssic residues nd extrcts contining these compounds, re toxic to weeds 10 16, insect pests 17,18 nd pthogens In ddition, limited number of field studies hve confirmed the bility of brssics in rottions to suppress weeds 22 24, nemtodes 25 nd fungl pthogens 26. These pest-mngement possibilities, in ddition to generl benefits of cover cropping, mke members of the Brssiccee promising cover crops for use in gronomic nd horticulturl systems. Severl recent reviews hve ddressed glucosinolte chemistry nd the potentil use of brssics for the mngement of insect pests nd plnt pthogens 9, Here we review the literture, with focus on the use of brssic cover crops for weed control in griculturl nd horticulturl production systems. # CAB Interntionl 2004

2 188 E.R. Hrmoto nd E.R. Gllndt Brssic Cover Crops: Unique Chrcteristics Members of the Brssiccee, including yellow nd white mustrds (e.g., Sinpis lb L.), cnol (Brssic npus L.) nd rpeseed (lso B. npus L.), re being used incresingly s cover crops in temperte regions of North Americ. These species re typiclly grown for their seeds, which re hrvested for oil production, s with cnol nd rpeseed, or used for condiment production, s with mustrd. However, ech of these species cn lso be grown s shortseson cover crop or, if climte llows, s n overwintering cover crop. Rpid growth nd nutrient cpture Both spring- nd fll-plnted brssic cover crops cn rpidly produce biomss. Winter vrieties of rpeseed nd cnol cn provide more thn 80% ground cover during the winter 30, n importnt considertion for erosion control. Fll-plnted rpeseed cover crops in estern Wshington nd southern Idho yielded kg dry biomss h -1 by the time they were incorported the following spring 22,23,30. In Mine, spring-plnted yellow mustrd produced kg dry biomss h months fter plnting (Hrmoto, unpublished dt). On the other hnd, Krishnn et l. 24 reported much lower biomss production ( kg dry biomss h -1 ) in Nebrsk for mustrds nd rpeseed. Brssics re lso cpble of cpturing excess nitrte remining fter crop hrvest, preventing nitrogen (N) loss from leching, with overwintering cover crops being more efficient thn those tht re frost-killed Overwintering rpeseed, forge turnip (Brssic npus L.), cnol nd white mustrd were equl to winter whet (Triticum estivum L.), winter brley (Hordeum vulgre L.), winter rye nd Itlin ryegrss (Lolium multiflorum Lm.) in their cpture of excess N in the fll 32,33. The N scvenging in these studies ws relted to biomss production. Cnol nd forge turnip cover crops sown in August nd incorported t the end of October were similr to brley in their cpcity to cpture excess N following sweet corn (Ze mys L.) hrvest 31. White mustrd plnted fter sweet corn hrvest ws ble to cpture more N thn sudngrss (Sorghum bicolor L.) cover crop 33. As mustrds re frostsensitive, the fte of this cptured N then depended on residue mngement fter the cover crop ws frost-killed. More N from the mustrd biomss ws returned to the soil over winter if the residues were incorported in the fll; wheres spring incorportion of residues led to less N loss. Thus, in ddition to providing physicl protection, residues left on the soil surfce over winter my reduce N leching. Others hve reported N immobiliztion following incorportion of brssics 34,35. Slower relese of plnt-vilble N my be beneficil in certin cropping systems s the N is vilble for the crop insted of weeds tht emerge erlier 36. However, N immobiliztion my not be desirble for crops with erly N demnds, mking it necessry to consider nutrient mngement benefits ginst potentil N immobiliztion. Biologiclly ctive secondry compounds: Biofumignts All members of the Brssiccee exmined to dte contin glucosinoltes 9. Glucosinoltes consist of glucose molecule, sulfur moiety, nd side chin, the composition of which determines their properties. Glucosinolte molecules re not toxic but re enzymticlly hydrolyzed to yield vriety of biologiclly ctive products, including isothiocyntes, ionic thiocyntes, nitriles, oxzolidinethiones, orgnic cyntes nd epithionitriles 9. Myrosinse, the enzyme responsible for glucosinolte brekdown, is seprted intercellulrly from the glucosinolte molecules. Thus, tissue disruption is necessry before biologiclly ctive brekdown products re relesed 9. In the field, this disruption cn be ccomplished by mowing, grzing, freezing, tillge or root deth. Mustrds, e.g., white nd yellow mustrds; Indin, brown nd Chinese mustrds (Brssic junce L.); nd blck mustrd (Brssic nigr L.), typiclly hve high seed glucosinolte content, s evidenced by their biting flvors. In contrst, cnol, which hs very low seed glucosinolte content, ws derived from cultivrs of rpeseed tht contined low concentrtions of glucosinoltes nd erucic cid in the seed; these re lso referred to s doublelow cultivrs 37,38. Cnol contins less thn 30 mmol of liphtic glucosinoltes g -1 of oil, nd the oil is less thn 2% erucic cid 37. If the oil contins more of these compounds, it is not clssified s cnol (Jim Dvis, personl communiction). Vegettive tissues of cnol my contin higher concentrtions of these compounds, though typiclly the glucosinolte content of vegettive tissues is 10% tht of the seed (Jim Dvis, personl communiction). The term single-low refers to rpeseed vrieties with low concentrtion of erucic cid, below 2% s with cnol. Becuse of erly concerns tht higher mounts of erucic cid could led to helth problems, low concentrtions of this compound determine the suitbility of the oil for humn consumption, nd both cnol oil nd oil from single-low rpeseed vrieties re edible 39,40. Seed-mel glucosinolte content determines its suitbility s livestock feed supplement, s higher mounts of glucosinoltes in feed cn led to goiter in livestock Becuse of its low glucosinolte content, cnol mel cn be used for livestock feed. If the glucosinolte content of rpeseed seed mel remins low, it too cn be used s feed supplement. Profiles of glucosinolte molecules vry between different brssic species 41 43, between individuls of the sme species or genotype 30,44,45, nd even within different plnt tissues of single individul 46,47. While different cover crops t prticulr life stges my be used to mnge specific pests 43, complictions my rise becuse the hydrolysis

3 Brssic cover cropping for weed mngement 189 products of different glucosinolte molecules vry in their toxicity to different orgnisms 9. Such trgeted pest mngement my lso be complicted by edphic fctors, which cn lter the exct profile of hydrolysis products relesed from glucosinoltes. For exmple, nitrile formtion ws fvored t cidic soil ph, lbeit t ph vlues lower thn those typiclly found in griculturl fields 48,49. Isothiocyntes nd the other glucosinolte derivtives hve reltively high vpor pressure nd re thoroughly dispersed throughout the surrounding soil, where they my ffect soil-borne fungl pthogens, insects nd nemtodes 50. Suppression of fungl pthogens by brssic tissues ws so promising tht one resercher coined the term biofumigtion, in reference to their effects 20, nd the use of incorported brssic residues hs been proposed s n lterntive to the use of methyl bromide 51. Brssic residues inhibit fungl growth nd reproduction 20,51,52 ; they my lso reduce disese pressure in the field 26,53. The effects of glucosinoltes on insect pests hve been vrible nd re complicted by species-specific interctions. Specilist pests my be ttrcted by glucosinoltes nd dmge from these specilists tends to increse with incresing glucosinolte content of the plnt 18,28,29. Growth rtes of generlist insect feeders 17 nd feeding by generlist invertebrtes nd vertebrtes 18,54 both declined when the nimls were exposed to incresing concentrtions of glucosinoltes. Incorportion of rpeseed residues before potto (Solnum tuberosum L.) cropping reduced Columbi root-knot nemtode (Meloidogyne chitwoodi Golden et l.) coloniztion in the zone of residue incorportion for 6 weeks, lthough nemtode dmge remined severe enough to cuse economic loss 25. Additions of brssic residues to soil with the root-knot nemtode (Meloidogyne jvnic Treub) reduced the nemtode numbers, lthough the reductions were not correlted with glucosinolte content 55. Brssic Residues nd Allelochemicls: Biossys in Controlled Environments Cover crop residues, whether incorported or left on the soil surfce, cn ffect weed dynmics by reducing or delying seed germintion, reducing estblishment nd suppressing individul plnt growth. Ech of these mechnisms cn contribute to overll declines in the density nd vigor of the weed community 56,57. Biossys of isolted glucosinolte hydrolysis products, extrcts of brssic residues nd brssic residues themselves demonstrte tht seed germintion, emergence nd growth re ech dversely ffected. Germintion In purified form, glucosinolte hydrolysis products generlly inhibit or dely seed germintion. Glucosinolte hydrolysis products likely ffect protein synthesis in germinting seeds nd emerging seedlings 10. Allyl isothiocynte ws most effective in suppressing seedling growth if pplied directly within 3 dys of germintion, suggesting tht the effect ws due to the reltively lrge mounts of isothiocyntes bsorbed compred to the seedling mss, or tht one of the first processes in seed germintion ws inhibited 10. Glucosinolte hydrolysis products cn inhibit germintion of dormnt nd non-dormnt seeds 12,15,58,59. After exposure to methyl isothiocynte, ungerminted seeds in one experiment were dissected nd found to be ded 12 ; however, in nother experiment, ungerminted seeds exposed to vrious isothiocyntes remined vible 59. Germintion of redroot pigweed (Amrnthus retroflexus L.) nd lrge crbgrss [Digitri snguinlis (L.) Scop.] seeds buried in mesh bgs in field plots ws completely inhibited by 2.7 mm nd 5.4 mm methyl isothiocynte 12. Vrious isothiocyntes t 1, 5 nd 10 mm inhibited germintion of soyben (Glycine mx L.), corn, whet, rpeseed, dndelion (Trxcum officinle G.H. Weber ex Wiggers), lflf (Medicgo stiv L.) nd cucumber (Cucumis stivus L.), with vrying toxicities of the isothiocyntes to the plnt species 42. For exmple, benzyl isothiocynte inhibited germintion of dndelion nd lflf seeds t lower concentrtions thn the remining biossy species. Applictions of other isothiocyntes to number of different species produced similr results (Tble 1). Typiclly, reduction in seed germintion rtes ws positively correlted with the concentrtion dded, but with high degree of vribility, even in these highly controlled experimentl conditions. Isothiocyntes my lso dely germintion, cusing n increse in the time to 50% germintion from 2.7 to 8.5 dys for lrge crbgrss exposed to 1 mm methyl isothiocynte 12. Higher concentrtions of methyl isothiocynte (2.7 nd 5.4 mm) pplied to field soil suppressed emergence of weeds for up to 7 weeks, while lower concentrtions suppressed emergence for shorter periods of time. In ddition to the effects of isolted isothiocyntes noted bove, voltile nd wter-soluble brssic extrcts lso my inhibit or dely germintion. In Petri dish ssys, voltile compounds relesed from blck nd brown mustrd cused 89% nd 100% inhibition of lettuce (Lctuc stiv L.) seed germintion, respectively 13. Voltiles from these residues, long with those from rpeseed, white mustrd nd kle (Brssic olerce L.) residues cused n 8 19% inhibition of brnyrd grss [Echinochlo crus-glli (L.) Beuv.] germintion, but hd no effect on whet. Wter-soluble extrcts from vrious rpeseed nd mustrd cultivrs filed to inhibit the germintion of whet seeds 11, lthough similr extrcts from rpeseed leves nd stems inhibited nd delyed germintion of lettuce seeds in nother study 60. Extrcts of wild rdish (Rphnus rphnistrum L.) residues inhibited germintion of pitted morning glory (Ipomoe lcunos L.), sicklepod (Senn obtusifoli L.), prickly sid (Sid spinos L.) nd yellow nutsedge (Cyperus esculentus L.) 61. Voltile nd wter-soluble extrcts from rpeseed mel lso inhibited the germintion of lettuce seeds 62.

4 190 E.R. Hrmoto nd E.R. Gllndt Tble 1. Effects of different isothiocyntes on seed germintion. The lower vlue in the concentrtion rnge represents either the lowest concentrtion tested or the mximum concentrtion t which no effect ws noted; the mximum vlue is the mximum concentrtion tested. Isothiocynte Concentrtion rnge (mm) Species tested 1 Effect on germintion Reference Allyl Whet 3 13% decrese 15 Methyl Whet No effect to 88% decrese 57 Methyl Lrge crbgrss No effect to 97% decrese 12 Methyl Dormnt lrge crbgrss Increse to 95% decrese 12 Methyl 1 10 Scentless myweed % decrese 57 Methyl 1 10 Smooth pigweed, brnyrd grss, No effect to complete inhibition 57 spiny sowthistle Methyl 1 10 Blckgrss No effect to 80% decrese 57 Benzyl Velvetlef No effect to complete inhibition 56 Benzyl Corn No effect t ny concentrtion 56 Benzyl Soyben No effect to 58% decrese 56 Benzyl, butyl, phenyl, Whet No effect t ny concentrtion 15 methyl, ethyl 2-phenethyl Whet 8 38% decrese 15 1 Species tested included: whet (Triticum estivum L.), lrge crbgrss [Digitri snguinlis (L.) Scop.], scentless myweed (Mtricri inodor L.), smooth pigweed (Amrnthus hybridus L.), brnyrd grss [Echinochlo crus-glli (L.) Beuv], spiny sowthistle [Sonchus sper (L.) Hill], blckgrss (Alopecurus myosuroides Huds.), velvetlef (Abutilon theophrsti Medic.), corn (Ze mys L.), nd soyben (Glycine mx L.). Estblishment Mixed with soil, brssic residues cn reduce seedling emergence, combintion of effects on the processes of germintion nd erly seedling growth. Rpeseed residues decresed the number of hiry nightshde (Solnum srrchoides Sendtner), longspine sndbur [Cenchrus longispinus (Hck.) Fern.] nd hemp sesbni [Sesbni exltt (Rf.) Rybd. Ex A.W. Hill] seedlings tht emerged in greenhouse pots 22,42, lthough velvetlef (Abutilon theophrsti Medicus) emergence ws not ffected 24. Similrly, white nd brown mustrd residues decresed the emergence of kochi [Kochi scopri (L.) Schrd.], shepherd s purse [Cpsell burs-pstoris (L.) Medicus.], green foxtil [Setri viridis (L.) Beuv.], redroot pigweed, hemp sesbni nd velvetlef 23,24,42. Turnip-rpe (Brssic npus L.) residue decresed the emergence of spiny sowthistle (Sonchus sper L.) by over 50%, but ctully incresed the emergence of smooth pigweed (Amrnthus hybridus L.) 59. Common chickweed [Stellri medi (L.) Vill.] emergence ws decresed s the mount of rpeseed residue dded to pots ws incresed 23, indicting tht incorporting more residue would led to improved weed control. Some of the observed between-species vrition in brssic residue effects on germintion nd/or estblishment my be due to differences in seed size. Smller seeds re, in generl, more susceptible to residue-medited stresses, including llelopthy 59, Petersen et l. 59, for exmple, noted tht the sme concentrtion of methyl isothiocynte cused 79% decrese in germintion for spiny sowthistle nd only 5% decrese for whet, n effect ttributed to differences in seed size (0.2 g thousnd -1 spiny sowthistle seeds compred to 45.3 g thousnd -1 whet seeds). However, germintion of smooth pigweed nd spiny sowthistle, which hve similr seed mss (0.5 nd 0.2 g thousnd -1 seeds, respectively), ws similrly ffected by isolted isothiocyntes, but ws ffected very differently by turnip-rpe residues, s noted bove. As this exmple suggests, efforts to estblish threshold seed size t which inhibition of germintion is likely to occur hs thus fr been unsuccessful 64,66. Fctors other thn seed size, including seed morphology 67,68 nd biochemicl composition 63 64, my lso determine seed s bility to withstnd these stresses. Growth of estblished seedlings In lbortory ssys, isothiocyntes lso inhibit growth of estblished seedlings. Benzyl isothiocynte, for exmple, t 0.4 mm, completely suppressed seedling growth nd prevented secondry root formtion of velvetlef; hlf this concentrtion cused root rot in developing seedlings, while their shoots remined unffected 58. Whet root length, on the other hnd, ws only slightly reduced by this compound 15. Similr concentrtions of other isothiocyntes, including 2-phenethyl-, llyl- nd methyl-isothiocynte, cused reductions in coleoptile nd root elongtion of whet seedlings 15. Wter-soluble extrcts from cnol root, stem nd lef tissues suppressed the growth of whet, brley nd corn seedlings, even fter only one dy of incubtion 16. Extrcts of cnol leves were the most phytotoxic to the seedlings, cusing decresed coleoptile nd rdicle elongtion compred to the control 16. Similrly, wter-soluble extrcts from ded blck mustrd stlks nd leves were effective

5 Brssic cover cropping for weed mngement 191 in decresing rdicle growth of rigid ryegrss (Bromus rigidus Roth.) 69. Voltile llyl isothiocynte produced from mcerted blck mustrd tissues lso reduced the growth of rigid ryegrss, but tended not to persist in the environment 69. Extrcts of wild rdish residues reduced cotton (Gossypium hirsutum L.) rdicle length by 75%; pitted morning glory rdicle length ws inhibited by 63% 61. Seedlings grown in soil with incorported rpeseed or mustrd residues re ffected in wys comprble to those exposed to residue extrcts. Severl similr experiments hve demonstrted tht kochi, shepherd s purse, green foxtil, hiry nightshde, redroot pigweed nd velvetlef seedling biomss ws decresed when grown with mustrd or rpeseed residues ( g residue g -1 soil) compred to those grown without For exmple, when compred to potto residue, rpeseed residue decresed hiry nightshde nd longspine sndbur biomss by 82 nd 83%, respectively fter 3 weeks 22. Likewise, fresh white mustrd residue reduced hiry nightshde nd green foxtil seedling biomss by 83% nd 70%, respectively, compred to seedlings grown without brssic residues 22. Despite these generlly comprble results, other studies suggest tht mustrd residues my be more effective in suppressing seedling growth thn rpeseed, especilly the growth of smller-seeded species such s kochi nd shepherd s purse 23. Unexpectedly, the growth of whet seedlings ws ctully incresed by similr mounts of mustrd nd rpeseed residues when compred to soil lone, result tht is potentilly ttributble to the lrger seedling size of whet 42. Effects of Brssic Rottion Crops on Weeds Despite the generlly promising lbortory, greenhouse nd field results discussed bove, the issue of whether brssic cover crops re llelopthic nd cn consistently provide weed control remins contentious. Differentition between multiple residue-medited effects of cover crop residues, whether incorported or left on the soil surfce, nd cler llelopthic mechnisms is difficult 70. These residuemedited effects my result from physicl, biologicl or chemicl chnges to the soil environment, ech of which my impct weeds 65. Physicl effects my result from chnges to soil temperture or light penetrtion to the soil surfce, ech of which my impct weed seed germintion 6. These effects my be prticulrly pronounced when cover crop residues remin on the soil surfce. An exmple of biologicl residue-medited effects includes cover crop residues providing hbitt for seed predtors or decy gents 57. In ddition to llelopthy, chemicl residuemedited effects my include nutrient sequestrtion or relese, prticulrly tht of nitrogen in the cse of leguminous cover crops 36. Regrdless of the exct mechnism, in severl nturl nd mnipulted ecosystems, brssics hve hd negtive impct on surrounding plnts, or following crops. For exmple, historiclly, dyer s wod (Istis tinctori L.) cultivtion ws restricted becuse of hrmful effects on following crops 71 ; others noted tht reduced grss nd clover yields were common on fields tht previously grew brssics 72. Yield loss in flx (Linum usittissimum L.) ws greter in crops contining brssicceous weeds thn in flx infested by other types of weeds 73. Annul grsslnds in southern Cliforni contin monospecific stnds of blck mustrd tht yerly perpetute themselves 69. Studies of this system reveled tht neither soil nor edphic fctors were responsible for the exclusion of grsses from these res; likewise, preferentil niml grzing nd competition were ruled out. However, wtersoluble compounds leched from the ded mustrd residues during the riny seson inhibited the germintion of rigid ryegrss nd reduced its growth, supporting the conclusion tht llelochemicls leched from the brssic tissues were responsible for excluding the grsses nd mintining the monospecific blck mustrd stnds 69. Erly in the culture of rpeseed s n oilseed crop, growers reported negtive effects of rpeseed residues on subsequent cerel crops 74. Hevy rpeseed-residue-contining stubble nd plnt debris, s would be left in combine windrow, decresed the dry biomss nd height of whet, brley nd ots (Aven stiv L.) compred to light rpeseed residue composed of stubble only 74. Density of the cerels ws similr in the two residue tretments, indicting tht the bundnt residues were stunting the growth of estblished plnts 74. Interestingly, severity of root rot in the cerels ws lower following the hevy residue, consistent with reports tht brssic residues my help mnge fungl pthogens 74. Rpeseed windrows lso contined vible seed, lost through the combining process, tht could germinte s volunteer plnts. Simulting this, Ver et l. 67 found tht incorported rpeseed nd mustrd seedlings reduced the estblishment, delyed the development nd reduced the seed yield of the subsequent crops. Oilseeds, including rpeseed nd flx, were more sensitive thn the cerels tested, including brley nd whet; ot growth ws not ffected t ll 67. Both mustrd nd rpeseed seedlings cused similr decreses in growth, despite presumble differences in glucosinolte content, which ws not mesured 67. In other exmples, nitrogen immobiliztion hs been suggested s the cuse of reduced plnt growth following brssics 68,75. Growth of lflf, legume, ws not ffected by the incorportion of whet or rpeseed residues, while brley nd bromegrss (Bromus inermis Leyss) showed signs of nitrogen deficiency 75. However, Bell nd Muller 69 found no differences in soil nitrogen content between res dominted by blck mustrd nd res dominted by nnul grsses. Similrly, ddition of nitrogen to whet grown fter different brssic species nd cultivrs did not chnge the effects of the residues on whet growth or yield 11.

6 192 E.R. Hrmoto nd E.R. Gllndt Effects of Brssic Cover Crops on Weeds Rottion effects of brssics hve led to investigtions on their use s cover crops, grown without hrvesting of the seed, for weed control. In south-centrl Wshington, where fll-plnted rpeseed will typiclly overwinter, the effects of rpeseed residue on weeds in subsequent potto crop ws compred to sudngrss residue nd fllow tretment 22. Fll-plnted rpeseed residues were incorported in the spring, while sudngrss residues were incorported the previous fll fter they were frost-killed; pottoes were then plnted into these residues. In the subsequent potto crops, mid-seson weed density following rpeseed ws decresed by 73 85% reltive to weed density following fllow nd sudngrss, respectively, in the 2 yers of the study; however, these mid-seson weed densities were extremely low (<1m -2 ). At the end of the seson, weed biomss ws decresed by 50 97% by rpeseed compred to fllow nd sudngrss, respectively. Potto yield ws greter following rpeseed thn following fllow in both yers, indicting tht the potto plnts were not hrmed by the residues. The effects of fll-plnted brssics on weeds in green pe (Pisum stivum L.) were lso exmined in northwestern Wshington 23. In this study, brssics included rpeseed nd white mustrd; rpeseed successfully overwintered in both yers, while white mustrd ws winter-killed in 1 yer. Weed density t 30 dys fter plnting ws lower following rpeseed nd white mustrd thn following whet, lthough weed biomss t pe hrvest ws similr following ll cover crops. Rpeseed residue lso consistently decresed pe density nd pe yield, effects tht were not observed following white mustrd. Other field studies including green pe reported reduction in estblishment following both rpeseed 76 nd white mustrd 26. Spring-plnted mustrds cn be incorported into rottions before csh crop, or plnted in succession in fllowed fields. Krishnn et l. 24, t two sites in estern Nebrsk, plnted soyben into residues of spring-plnted rpeseed, two vrieties of white mustrd, nd brown mustrd. Weed biomss, mesured t 4 nd 6 weeks fter soyben emergence, ws lower following these cover crops t one loction, but not t nother. The weed control from these brssic residues ws not gronomiclly sufficient, however, s both loctions suffered yield loss following ll cover crops if herbicides were not pplied. Soyben yields were similr for ll cover crops nd fllow tretment, suggesting tht the brssic residues did not hrm the soyben plnts. Field studies testing the effects of spring-plnted brssic residues on weed nd crop emergence nd growth were recently conducted in centrl Mine 66. Sixteen biossy species, including both crops nd weeds, were plnted into fllow plots nd plots tht grew nd received incorported residues of three brssic cover crops rpeseed, cnol nd yellow mustrd nd three non-brssic cover crops buckwhet (Fgopyrum esculentum Moench.), ots nd Crop nd weed emergence (%) Preceding cover crop Fllow Brssic Non-brssic b b Yer Figure 1. Effect of incorported cover crop residues on men emergence of 11 crop nd weed species in 2002 nd 15 in Bsed on nlysis of vrince of results, dt were verged over the crop nd weed species. Single degree of freedom contrsts ( =0.05) compred emergence following no cover crop (fllow, n = 44 in 2002 nd n = 60 in 2003), to emergence following incorportion of brssic (yellow mustrd, spring cnol nd winter rpeseed; n = 131 in 2002 nd n = 176 in 2003) nd non-brssic (buckwhet, ot nd crimson clover; n = 131 in 2002 nd n = 177 in 2003) cover crops 66. Within ech yer, mens with the sme lower-cse letter were not significntly different (P > 0.05). crimson clover (Trifolium incrntum L.). Averged over biossy species, ll cover crop residues decresed seedling emergence compred to fllow but, unexpectedly, there were no differences in emergence detected following the brssic cover crops nd non-brssic cover crops which lcked glucosinoltes, even for species with smller seeds (Fig. 1). In nother experiment, green ben (Phseolus vulgris L.), redroot pigweed nd mixture of the two species were plnted into fllow plots nd plots tht grew nd received incorported residues of cnol nd yellow mustrd. Although the biomss vlues of both green ben nd redroot pigweed were often lower following the brssic cover crops, nd lowest following the highglucosinolte mustrd, differences were not significnt (Fig. 2). Competition reduced green ben nd redroot pigweed height nd biomss, but interspecific interference ws unffected by the incorported brssic residues. Similrly, mrketble green ben yield ws lower in plots contining redroot pigweed thn in plots with green ben grown lone, but incorported brssic residues did not medite the competitive effect. These results suggest tht the observed weed suppression by brssic cover crops in the field 22 24, when compred to soil without residues, is likely relted to their effects on estblishment but not on b b

7 Brssic cover cropping for weed mngement 193 Above-ground dry biomss (g m 2 ) A. B. Preceding cover crop Fllow Cnol Mustrd the growth of estblished individuls. The similr effects of the brssic nd non-brssic residues on estblishment contrdict results from severl other field studies, which suggest tht the impcts of brssics on weed suppression re greter thn those provided by other cover crops 22,23. These discrepncies highlight the vrible nture of residue-medited effects nd emphsize the need to consider the complex reltionships mong mngement, species (cover crops, weeds nd crops) nd edphic conditions. Mngement Strtegies nd Considertions Effects of environment nd growth stge on glucosinolte production Some brssics, including cnol, re selected to hve low concentrtions of glucosinoltes in their seeds, nd 13 August 19 August 16 September Dte Figure 2. Effect of incorported low-glucosinolte (cnol) nd high-glucosinolte (mustrd) brssic cover crop residues on bove-ground dry biomss of green ben (A) nd redroot pigweed (B) subsequently grown in mixture t fixed densities in 2002; results were similr in Within ech hrvest dte, mens with the sme lower-cse letter were not significntly different (P > 0.05) 66. generlly hve lower concentrtions in the vegettive tissue s well 46,77,78. However, environmentl nd soil chrcteristics, s well s mngement prctices, my produce plnts with higher nd more vrible levels of glucosinoltes 45,46,79. A comprison of single-low nd double-low rpeseed cultivrs found tht stems of one double-low cultivr contined glucosinolte concentrtion similr to tht of the single-low cultivr over twice the concentrtion found in the other double-low cultivrs 46. Fertiliztion with nitrogen nd phosphorus increses brssic biomss production nd my lso increse glucosinolte production 79. Phosphorus fertiliztion (60 kg P 2 O 5 h -1 ) nd high levels of nitrogen (132 kg N h -1 ) incresed the glucosinolte content of vriety of forge brssics by n verge of 26%; the sme mount of nitrogen t lower phosphorus levels did not ffect glucosinolte production 79. Both fertiliztion nd drought conditions before or fter flowering cn led to lrge increses in the concentrtion of glucosinoltes in rpeseed seed 78. Plnt growth stge lso ffects glucosinolte content. Glucosinolte concentrtions of brssic tissues usully decrese s the seeds germinte nd seedlings grow 77. However, plnt biomss increses rpidly during this time, leding to mximl glucosinolte content (the product of concentrtion nd biomss) in the whole plnt before flowering 43,46. Not until glucosinolte concentrtions were expressed on per plnt bsis, rther thn per unit biomss, ws it relized tht glucosinoltes were being diluted by incresing biomss 71,80. Growers seeking to use brssic cover crops for weed suppression would be most interested in the glucosinolte content per unit re. Mximl glucosinolte content for the entire plnt typiclly occurs t mturity 46,77, but the plnts re generlly incorported prior to this stge, to void potentil problems with subsequent volunteer seedlings. Glucosinolte content of the vegettive tissues is mximl prior to flowering 43,46, lthough overll plnt biomss t this stge is still reltively low. Thus, growers fce trdeoff incorporte smller mounts of biomss with the highest glucosinolte content, or wit until flowering hs progressed to incorporte more biomss with less glucosinoltes. The prticulr gols of the frmer would id in mking this decision. Types of effective glucosinoltes Different brssics hve different glucosinolte profiles 30,41 43, which my contribute to the conflicting results of some studies, s different glucosinolte hydrolysis products my hve different effects on seed germintion nd seedling growth (Tble 1). In ddition, not ll brssics relese isothiocyntes; the predominte voltiles relesed from leves of certin turnip nd rpeseed vrieties were the ftty cid derivtives, cis-3-hexen-1-ol nd cis-3- hexen-1-yl cette 42. Other studies hve identified trnsb-ocimene s the dominnt voltile in certin Indin mustrds, nd verbenone s the dominnt voltile in blck

8 194 E.R. Hrmoto nd E.R. Gllndt mustrd 81. Effects of these secondry compounds on seedling germintion nd growth re lrgely unknown 81. Both voltile nd wter-soluble compounds hve been implicted in suppressing seed germintion 60, lthough hydrolyzing brssic tissues is sufficient to remove voltile compounds. Wter-soluble compounds from hydrolyzed lef nd stem tissue of rpeseed completely inhibited the germintion of lettuce seeds, while wter-soluble root extrcts from the sme hydrolyzed tissues cused only dely in germintion 60. Only voltiles from intct tissues ffected germintion, cusing dely, s well s reduction in lettuce seed germintion 60. Under field conditions, voltiles from brssic residues re extremely short-lived (see below) nd wter-soluble compounds leched from the residues would likely hve the lrgest impct on germinting seeds. Limited soil life of glucosinolte hydrolysis products Glucosinolte hydrolysis products tend to be very ephemerl nd do not persist in the soil environment 50,59,69. After incorportion of brssic residues, relese of isothiocyntes is rpid, with concentrtions dropping to less thn hlf of the mximum by 72 h 50. When isothiocyntes re pplied directly, s opposed to being pplied in glucosinolte form or in residues, the disppernce is even more rpid. For exmple, the hlf-life of 2-phenylethyl isothiocynte pplied to soil ws pproximtely 1 h; it ws completely undetectble fter 91 h 82. Morr nd Kirkegrd 50 mesured isothiocyntes in soil 24 h fter the incorportion of Indin mustrd or rpeseed residues. The Indin mustrd plots verged 1.2 nmol isothiocynte g -1 soil, while plots with incorported rpeseed residue verged 0.8 nmol isothiocynte g -1 soil, concentrtions similr to those found by Grdiner et l. 83 following incorportion of rpeseed residues. However, it is notble tht these concentrtions re more thn n order of mgnitude less thn those found to inhibit seed germintion (Tble 1) nd seedling growth in lbortory settings 12,15,58,59. The low concentrtions of isothiocyntes found in the soil following incorportion of brssic residues re prtilly due to the low relese efficiency of biologiclly ctive brekdown products from glucosinoltes. For exmple, soil concentrtion of isothiocyntes represented only % of the glucosinolte pool following incorportion of Indin mustrd nd rpeseed residues 50,84. Other studies hve shown similrly low conversion rtes of glucosinoltes to isothiocyntes nd other hydrolysis products 83,85,86. These studies suggest tht the limiting fctor in determining isothiocynte evolution into soil is not the glucosinolte content of the plnt, but relese efficiency of the hydrolysis products. Prctices tht further disrupt plnt tissue t, or before, incorportion my increse the relese of isothiocyntes. Morr nd Kirkegrd 50 found tht freezing residues cused more cellulr disruption nd led to greter relese efficiency of isothiocyntes; frozen lef discs of Indin mustrd relesed 39 nmol isothiocyntes g -1 soil in modertely dry soil. Relese of isothiocyntes from these frozen lef discs ws greter in wterlogged soils, with concentrtion of 75 nmol isothiocyntes g -1 soil. However, relese of isothiocyntes does not ensure biovilbility. Glucosinolte hydrolysis products relesed from mcerted tissue my rect with orgnic mtter in the tissue itself nd my not be relesed into the environment 87. Soil chrcteristics, prticulrly orgnic mtter nd cly content, lso ffect both the types of compounds relesed upon glucosinolte hydrolysis, but lso, perhps more importntly, on the dsorption of these compounds to soil prticles 87. Relese of glucosinolte hydrolysis products my occur rpidly following incorportion of residues; lterntively, they my be slowly exuded from roots of living plnts or leched from dropped leves on the soil surfce. Dyer s wod relesed indolyl glucosinolte compounds from its roots 71 ; cnol relesed 2-phenylethyl isothiocynte into its rhizosphere 82. Soil smpled from pots contining lowglucosinolte rpeseed cultivr contined 24 ng llyl isothiocynte g -1 soil, while soil in pots contining wild-type rpeseed contined 60 ng llyl isothiocynte g -1 soil 70. Becuse the soil ws smpled without incorported plnt residues, isothiocynte ws presumbly relesed from the growing roots. The slow nd stedy relese of isothiocyntes my keep soil concentrtions t levels effective for suppressing pthogens nd reducing seed germintion. Effects on beneficil orgnisms Nitrifying bcteri, rhizobcteri nd mycorrhize re often desirble in griculturl soils, especilly those in systems with reduced externl inputs. Plnt secondry compounds tht hrm fungl pthogens nd reduce seed germintion might lso be expected to hrm the orgnisms tht re beneficil, nd often essentil, to helthy griculturl soils. Dily pplictions of low concentrtions (< 4nM) of 2-phenylethyl isothiocynte were sufficient to chnge the ctive portion of the soil microbil community composed of bcteri nd eukryotes 82. These results indicte tht brssic residues could ffect criticl microbil processes, including nitrifiction, nitrogen fixtion nd mycorrhizl symbioses. Nitrogen minerliztion from brssic residues is slower thn predicted by their C:N rtio, suggesting tht glucosinolte hydrolysis products inhibit the ctions of soil microbes involved in nitrogen cycling 34. Popultions of both mmonium-oxidizing nd nitrite-oxidizing bcteri were dversely ffected by pplictions of isothiocyntes, with soil- nd isothiocynte-specific effects on growth rte nd popultion levels 88. Nitrifiction cpcity ws reduced by 35 65% following 10 mg pplictions of different isothiocyntes to 1 g of soil 88, while ddition of 0.5 mg of 2-propenyl isothiocynte g -1 dry soil ws sufficient to completely inhibit nitrifiction 89. This ltter concentrtion is similr to those tht inhibited seed germintion nd seedling growth (Tble 1), but still higher thn tht found in

9 Brssic cover cropping for weed mngement 195 the soil following residue incorportion 50. Isothiocyntes lso cted synergisticlly with voltile sulfur compounds (including dimethyl sulfide) to cuse greter inhibition of nitrifiction 88. Compounds hrmful to microbes my lso ffect beneficil rhizobcteri (Rhizobium spp.) tht form root nodules on legumes, ssimilting tmospheric nitrogen in the well-known symbiotic ssocition. For exmple, nodultion of green pes following incorported white mustrd residue ws reduced compred to those following fllow 26. As soil nitrogen concentrtions were not reported, these observtionl differences cnnot be ttributed solely to the presence of white mustrd residues. However, Scott nd Knudsen 76 found tht green pes grown in soil with incorported rpeseed residues hd similr numbers of nodules to those grown in soil with no residues, nd tht crbon utiliztion ws similr between rhizobcteri from these two groups. Rhizobcteri isolted from green pe plnts grown in soil with rpeseed residues were similr to those isolted from soil without residue 76. Adverse ffects on pe growth, including reduced germintion, poor root development nd shorter plnts, were noted in both of these experiments, s well s by other reserchers 90. Mycorrhize re very importnt for helthy plnt growth in groecosystems, lthough plnts in the Brssiccee do not form mycorrhizl ssocitions 91. Despite the potent nti-fungl properties of glucosinolte hydrolysis products, there does not seem to be consistent effect of brssics on mycorrhizl spore germintion, or the mycorrhizl infection of nerby plnts. Compred to those ner tomto (Lycopersicon esculentum Mill.) nd tobcco (Nicotin tbcum L.) roots, vesiculr rbusculr mycorrhizl (VAM) hyphe ner brssic roots hd fewer germ tube brnches nd fewer hyphl tufts, suggesting tht they my be hrmed by compounds leching from brssic roots 92. Onions (Allium cep L.) grown in the sme pot s swede (Brssic npus L.) hd smller percentge of root length infected by mycorrhize 93. This evidence conflicts with other studies tht hve demonstrted tht mycorrhizl plnts grown with brssics re not ffected 94. In ddition, there is typiclly no effect on mycorrhizl infection by brssics in rottion with mycorrhizl plnts 92, While isothiocyntes re cpble of killing certin species of VAM fungi, other VAM species my hve the cpcity to detoxify glucosinoltes, s these fungi differ widely in their response to nti-fungl compounds 92. Other nonbrssicceous plnts tht relese glucosinoltes from their roots, including ppy (Cric ppy L.) nd nsturtium cress (Tropeolum spp.), do form mycorrhizl ssocitions, suggesting tht the glucosinoltes themselves re not toxic to ll mycorrhize nd tht other mechnisms prevent brssics from forming this ssocition 91. Conclusions Brssic cover crops hve been suggested s possible solution to mny pest problems in griculturl nd horticulturl systems. Considerble evidence of the negtive effects of brssic residues on seed germintion nd growth supports the ide tht brssic cover crops cn be used s biofumignts. Suppression of weed density nd weed biomss in field experiments further bolsters this ide. However, mechnisms behind this observed effect on weed communities in different crops remin unknown, s our recent field studies in Mine filed to find support for hypotheses tht brssic residues would: (1) provide dditionl suppression of seedling recruitment beyond tht provided by other non-brssicceous cover crops; nd (2) reduce the growth of estblished plnts. Clerly, mny fctors influence the pest-mngement potentil of brssics, nd erlier suggestions tht brssic cover-cropping systems my be tilor-mde to specific pest problems my prove to be both overly optimistic nd too difficult to be prcticl to growers. Brssic cover crops do hve plce in low-input frming systems s substitutes for other, more conventionl cover crops. Their bility to suppress soilborne disese pthogens my prove extremely beneficil. Although their effects on beneficil mycorrhize, rhizobi nd free-living nitrogen-fixing bcteri must be considered, evidence suggests tht brssic residues will not cuse significnt hrm to these beneficil biot. As with most tctics utilized in low-input frming systems, multiple benefits my result from their use, but it remins importnt to use multiple tctics for strong progrm of weed mngement. Acknowledgements. We would like to thnk Jim Dvis, University of Idho Brssic Breeding nd Reserch Group, Dr Mrinne Srrntonio, University of Mine, nd Dr Tim Griffin, USDA-ARS New Englnd Plnt, Soil nd Wter Lbortory, for their criticl reviews of this work. References 1 Srrntonio, M. nd Gllndt, E The role of cover crops in North Americn cropping systems. Journl of Crop Production 8: Orgnic Frming Reserch Foundtion (OFRF) Third Biennil Ntionl Orgnic Frmers Survey. OFRF, Snt Cruz, CA. 3 Putnm, A.R Allelopthy: problems nd opportunities in weed mngement. In M.A. Altieri nd M. Liebmn (eds). Weed Mngement in Agroecosystems: Ecologicl Approches. CRC Press, Boc Rton, FL. p Tesdle, J.R. nd Dughtry, C Weed suppression by live nd dessicted hiry vetch (Vici villos). Weed Science 41: Putnm, A.R Phytotoxicity of plnt residues. In P.W. Unger (ed.). Mnging Agriculturl Residues. Lewis Publishers, Boc Rton, FL. p Tesdle, J.R Contribution of cover crops to weed mngement in sustinble systems. Journl of Production Agriculture 9: Ohno, T., Dooln, K., Zibilske, L., Liebmn, M., Gllndt, E., nd Berube, C Phytotoxic effects of red clover mended soils on wild mustrd seedling growth. Agriculture, Ecosystems nd Environment 78:

10 196 E.R. Hrmoto nd E.R. Gllndt 8 Conklin, A., Erich, M., Liebmn, M., Lmbert, D., Gllndt, E., nd Hltemn, W Effects of red clover (Trifolium prtense) green mnure nd compost soil mendments on wild mustrd (Brssic kber) growth nd incidence of disese. Plnt nd Soil 238: Brown, P.D. nd Morr, M.J Control of soil-borne plnt pests using glucosinolte-contining plnts. Advnces in Agronomy 61: Leblová-Svobodová, S. nd Koštír, J Action of isothiocyntes on germinting plnts. Experienti 18: Mson-Sedun, W., Jessop, R., nd Lovett, J Differentil phytotoxicity mong species nd cultivrs of the genus Brssic to whet. Plnt nd Soil 93: Tesdle, J.R. nd Tylorson, R.B Weed seed response to methyl isothiocynte nd methm. Weed Science 34: Oleszek, W Allelopthic effects of voltiles from some Crucifere species on lettuce, brnyrd grss nd whet growth. Plnt nd Soil 102: Mson-Sedun, W. nd Jessop, R Differentil phytotoxicity mong species nd cultivrs of the genus Brssic to whet. II. Activity nd persistence of wter-soluble phytotoxins from residues of the genus Brssic. Plnt nd Soil 107: Bily, Z., Oleszek, W., Lewis, J., nd Fenwick, G Allelopthic potentil of glucosinoltes (mustrd oil glycosides) nd their degrdtion products ginst whet. Plnt nd Soil 129: Wnnirchchi, S. nd Voroney, R Phytotoxicity of cnol residues: relese of wter-soluble phytotoxins. Cndin Journl of Soil Science 77: Blu, P.A., Feeny, P., Contrdo, L., nd Robson, D.S Allylglucosinolte nd herbivorous cterpillrs: contrst in toxicity nd tolernce. Science 200: Gimoustris, A. nd Mithen, R The effect of modifying the glucosinolte content of leves of oilseed rpe (Brssic npus ssp. oleifer) on its interction with specilist nd generlist pests. Annls of Applied Biology 126: Ppvizs, G.C. nd Lewis, J Effects of mendments nd fungicides on Aphnomyces root rot of pes. Phytopthology 61: Angus, J., Grdner, P., Kirkegrd, J., nd Desmrchelier, J Biofumigtion: isothiocyntes relesed from Brssic roots inhibit growth of the tke-ll fungus. Plnt nd Soil 162: Srwr, M., Kirkegrd, J., Wong, P., nd Desmrchelier, J Biofumigtion potentil of brssics. III. In vitro toxicity of isothiocyntes to soil-borne fungl pthogens. Plnt nd Soil 201: Boydston, R. nd Hng, A Rpeseed (Brssic npus) green mnure crop suppresses weeds in potto (Solnum tuberosum). Weed Technology 9: Al-Khtib, K., Libbey, C., nd Boydston, R Weed suppression with Brssic green mnure crops in green pe. Weed Science 45: Krishnn, G., Holshouser, D.L., nd Nissen, S.J Weed control in soyben (Glycine mx) with green mnure crops. Weed Technology 12: Mojthedi, H., Snto, G., Wilson, J., nd Hng, A.N Mnging Meloidogyne chitwoodi on potto with rpeseed s green mnure. Plnt Disese 77: Muehlchen, A., Rnd, R., nd Prke, J Evlution of crucifer green mnures for controlling phnomyces root rot of pes. Plnt Disese 74: Ros, E., Heney, R., Fenwick, G., nd Ports, C Glucosinoltes in crop plnts. In J. Jnick (ed.). Horticulturl Reviews, Vol. 19. John Wiley & Sons, New York. p Mithen, R Glucosinoltes biochemistry, genetics, nd biologicl ctivity. Plnt Growth Regultion 34: Mithen, R. 2001b. Glucosinoltes nd the degrdtion products. In J. Cllow (ed.). Advnces in Botnicl Reserch, Vol. 35. Acdemic Press, New York. p Eberlein, C.V., Morr, M.J., Guttieri, M.J., Brown, P.D., nd Brown, J Glucosinolte production by five fieldgrown Brssic npus cultivrs used s green mnures. Weed Technology 12: Isse, A., McKenzie, A., Stewrt, K., Cloutier, D., nd Smith, D Cover crops nd nutrient retention for subsequent sweet corn production. Agronomy Journl 91: Bggs, E., Wtson, C., nd Rees, R The fte of nitrogen from incorported cover crop nd green mnure residues. Nutrient Cycling in Agroecosystems 56: Weinert, T.L., Pn, W.L., Moneymker, M.R., Snto, G.S., nd Stevens, R.G Nitrogen recycling by nonleguminous winter cover crops to reduce leching in potto rottions. Agronomy Journl 94: Bending, G., Turner, M., nd Burns, I Fte of nitrogen from crop residues s ffected by biochemicl qulity nd the microbil biomss. Soil Biology nd Biochemistry 30: Trinsoutrot, I., Nicolrdot, B., Justes, E., nd Recous, S Decomposition in the field of residues of oilseed rpe grown t two levels of nitrogen fertiliztion. Effects on the dynmics of soil minerl nitrogen between successive crops. Nutrient Cycling in Agroecosystems 56: Dvis, A.S. nd Liebmn, M Nitrogen source influences wild mustrd growth nd competitive effect on sweet corn. Weed Science 49: Rymer, P Cnol: n emerging oilseed crop. In J. Jnick nd A. Whipkey (eds). Trends in New Crops nd New Uses. ASHS Press, Alexndri, VA. p Anonymous Cnol. Cnol Council of Cnd, Winnipeg, MB, Cnd. 39 Dupont, J., White, P.J., Johnston, K.M., Heggtveit, H.A., McDonld, B.E., Grundy, S.M., nd Bonnome, A Food sfety nd helth effects of cnol oil. Journl of the Americn College of Nutrition 8: Anonymous Cnol new oilseed from Cnd. Journl of the Americn Oil Chemists Society 58: Sng, J., Minchinton, I., Johnstone, P., nd Truscott, R Glucosinolte profiles in the seed, root nd lef tissue of cbbge, mustrd, rpeseed, rdish, nd swede. Cndin Journl of Plnt Science 64: Vughn, S.F. nd Boydston, R.A Voltile llelochemicls relesed by crucifer green mnures. Journl of Chemicl Ecology 23: Kirkegrd, J. nd Srwr, M Biofumigtion potentil of brssics. I. Vrition in glucosinolte profiles of diverse field-grown brssics. Plnt nd Soil 201: Chong, C., Ju, H.-Y., nd Bible, B.B Glucosinolte composition of turnip nd rutbg cultivrs. Cndin Journl of Plnt Science 62: