Cropping Sequence

Table of contents

    Important tips for best management

    • Canola yields better when seeded into cereal or pulse stubble than into canola stubble.
    • Canola stubble degrades two times faster than cereal or forage stubble; therefore soil organic matter may decline over time in a heavy canola rotation.

    Impact of preceding crop on canola

    A seven-location Agriculture and Agri-Food Canada study led by research scientist John O’Donovan in Lacombe, Alta., compared yields for canola seeded into various stubbles: wheat, lentils, peas, fababeans and fababeans harvested as green manure.

    In the study, canola yielded best on fababean green manure, but the green manure did not provide a marketable crop in the previous year. Of the harvested crops, canola did best on peas, on average. O’Donovan’s study showed good results for canola on fababean and lentil stubble, and results for Indian Head, Sask., found canola performed best on lentil residue.

    O’Donovan launched the study to see if a pulse before canola could reduce dependence on inorganic nitrogen. In plots with no nitrogen fertilizer applied, canola yield on pulse stubble was often higher than canola yield on wheat stubble. That gap narrowed, in general, as nitrogen fertilizer rates increased.

    From a quality perspective, O’Donovan’s study shows that canola after peas may have higher protein and lower oil than canola on wheat stubble.

    Residue is another consideration when choosing a rotation sequence. Wheat may produce more tonnes of residue per acre, which may present a greater challenge for canola seed placement and stand establishment than canola in pea stubble. Peas also leave more bare ground that will warm up faster in the spring, which may be an advantage for canola. But pea vines, though lower in volume than wheat stubble, can bunch up in the drill.

    Herbicide carryover is always a consideration when choosing land for canola. This is discussed in the Herbicide Rotation and Residues Chapter.

    Effect of canola on subsequent crop

    Cereal root and foliar diseases are usually reduced after canola. The reduction in cereal diseases arises because canola tends not to host cereal diseases and thus their field populations tend to decline in the absence of a suitable host crop.

    Also, recent Australian research indicates that canola actually kills certain cereal pathogens such as take-all root rot due to release of inhibitory compounds such as isothiocyanates during decomposition. This direct disease reduction has been termed biofumigation.

    Commercial field data from provincial crop insurance departments confirm cereal yield advantages after canola. Manitoba crop insurance data from 1997 to 2007 show that wheat yields when seeded into canola stubble are 103% of average while wheat yields when seeded into wheat stubble are 90% of average.

    Negative Impact

    Phytotoxic effect of brassica residues. The phytotoxic effect of canola residues has been documented in Canada by several researchers and is attributed to toxins leached from residues as well as toxins produced during microbial decomposition.

    Phenolic compounds are considered to be the main class of phytotoxic compounds that reduce the growth rate of roots and shoots. Isothiocyanate, a breakdown product of glucosinolate, has been shown to reduce weed seed germination, especially with small weed seeds such as spiny sow thistle and flixweed. The phytotoxicity is greatest with fresh residues and young tissue. Mature canola residue does not normally cause subsequent crop germination and growth reductions except for nitrogen deficiency due to immobilization by the decomposing residue and colder soil temperature. This is likely due to leaching of the phytotoxins out of the residues by rain and snow before the next crop is seeded.

    Therefore, problems may occur in situations where thick volunteer canola is worked under in the spring before seeding flax or a cereal crop. Also, poor straw and chaff spreading with the combine may result in an N deficiency and toxicity problems in the chaff row for the next crop, especially if heavy volunteer canola growth occurs in the chaff row.

    Stress conditions such as an N deficiency and high temperatures can increase the allelopathic effect. The allelopathic nature of canola and other Brassica species have potential to be used in green manure systems for weed control.

    Arbuscular mycorrhizal (AM) fungi. AM fungi in the soil can greatly improve phosphorus uptake by flax and corn, in particular. These crops benefit from healthy AM population in the soil. AM do not colonize canola roots, thus their populations will decline after canola. (Summerfallowing and other extensive tillage practices can also reduce AM by breaking apart the long hyphae that connect to roots and help draw in phosphorus.) An AAFC study in Brandon found that colonization of flax roots by AM was reduced when flax was seeded after canola compared to colonization when flax was seeded after wheat.

    Less residue. An average canola crop provides less residue cover than an average cereal crop because it produces slightly less residue and breaks down more quickly. Crop residue losses during fallow were lentil>canola>rye>barley>wheat>flax. Lentil and canola residues often break down twice as fast as wheat. Thus, wind or water erosion risks increase with canola especially when it is followed by summerfallow. When land is frequently cropped to canola, there is a slight risk that soil structure may deteriorate because of the short residue life.

    Studies at AAFC in Melfort, Saskatchewan and Beaverlodge, Alberta have shown an advantage for a grain-forage rotation on degraded and Grey Wooded soils. Several years of other crops like cereals and forages will build up or maintain soil organic matter due to crop residues with slower decomposition rates. This will promote better soil structure and tilth, and reduce soil erosion when a trash cover is maintained.

    Weeds. Frequent canola production can increase weeds such as stinkweed, cleavers, wild mustard, kochia and Canada thistle. Frequent canola production will also increase the number of canola volunteers.

    Most B. napus varieties have little if any seed dormancy and will germinate within the first year after the crop is grown. Pre-seed or early in crop weed control of canola volunteers in a cereal crops will prevent canola volunteers from setting seed, therefore virtually eliminating volunteer canola as a weed in the second year after the crop. However, some B.napus cultivars will have longer dormancy or secondary dormancy and can last for years.

    Seeds of B. rapa varieties, especially if buried by deep tillage and exposed to certain temperature and moisture conditions, may develop dormancy similar to wild oats. The seed can stay dormant for many years then germinate, causing volunteer and contamination problems. One way to reduce this problem is to keep the seed on or near the soil surface to promote germination and mortality and prevent induced dormancy.

    Seed bank additions from harvest operations can be relatively large. If the harvest operation contributed 300 seeds per square foot to the seed bank, which is fairly typical, then one seed every 2 sq. ft. could still be present three years later.

    Under certain environmental conditions (predominately warm, dry conditions after the seed has taken up some water), canola seed may develop secondary dormancy resulting in prolonged soil persistence. Some canola varieties have a higher potential for secondary dormancy than others. Unfortunately, varieties are not normally evaluated for this characteristic nor do breeding programs attempt to screen out this trait.

    In a tight canola rotation, rotating between herbicide tolerant systems will reduce selection pressure for herbicide resistant weeds and reduce selection of weed populations that are more competitive within a particular system.

    John O’Donovan et al, 2011, “Legume Crops to Improve Soil Fertility for Enhanced Canola Production,” Agriculture and Agri-Food Canada.

    Wanniarachchi and Voroney, 2007, Phytotoxicity of canola residues: Release of water-soluble phytotoxins, Canadian Journal of Plant Science

    Jan Petersen et al, “Weed Suppression by Release of Isothiocyanates from Turnip-Rape Mulch, Agronomy Journal, Vol. 93 No. 1, p. 37-43

    Cynthia Grant, 2004, “Soil and fertilizer phosphorus: Effects on plant P supply and mycorrhizal development,” Canadian Journal of Plant Science.

    Blackshaw, R.E. and Lindwall, C.W. 1995 “Species, herbicide and tillage effecson suface crop residue cover during fallow.” Can J. Soil Sci., 79: 97-100

    Harker et al, 2006, “Persistence of Glyphosate-Resistant Canola in Western Canadian Cropping Systems,” Agronomy Journal

    Gulden, R.H. et al, 2003, “Harvest losses of canola (Brassica napus) cause large seedbank inputs,” Weed Science, 51:83–86.

    Eric Johnson et al, “Ecology and Management of volunteer canola,” AAFC fact sheet.