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Wild radish Beating herbicide resistance in wild radish by Toni Commens, CO-OPERATIVE RESEARCH CENTRE FOR WEED MANAGEMENT SYSTEMS F ast-developing resistance to herbicides by wild radish has forced producers to tackle the weed using a combination of control methods. This extremely competitive weed, which can reduce wheat yields by up to 20 per cent, already is resistant to the major weapon used to combat it — Group B herbicides. Researchers also fear wild radish’s resistance may be transferred to other weeds by outcrossing (breeding with other plant species). The solution to effective wild radish control lies in preventing seed production and reducing the existing soil seedbank. This can be done by adopting a variety of cultural, chemical and biological approaches. Distribution Wild radish is a major weed of winter grain crops and, to a lesser extent, pastures in southern Australia. It is is distributed in a broad band from south eastern Queensland, through New South Wales, Victoria, Tasmania and South Australia, and is widespread in the cereal cropping areas of Western Australia (see Figure 1 on page 39). In WA, wild radish is rated as one of the most troublesome weeds of broadacre cropping, particularly in the wheat-lupin rotation which dominates the northern wheatbelt. In NSW, wild radish infestations occur in the higher rainfall areas of the southern wheatbelt and along the western slopes of the Great Dividing Range, and is becoming increasingly problematic. In Victoria wild radish is a significant weed of crops in the north east and western district and in the cropping areas of the Gippsland, as well as the high rainfall and intensive cropping areas of SA. One of the reasons for the wide distribution of wild radish is its prolific seeding — it can produce up to 17,000 seeds per square metre. It also competes vigorously with crops. Densities of 10 wild radish plants per square metre can reduce yields in wheat by up to 20 per cent. While livestock readily eat the weed, losses attributed to wild radish poisoning are rare. The seeds are the most dangerous part of the plant. When deaths have occurred, postmortem examinations have revealed jaundice, anaemia and liver damage. Animals may become affected when confined to areas where wild radish is abundant. Symptoms include initial loss of appetite, weakness, stupor and paralysis. Some may become very excitable. Dung is covered with mucus and as the animal’s condition worsens watery blood-stained diarrhoea is 36 Wild radish plants are prolific seeders and very competitive. Wild radish can produce up to 17,000 seeds per square metre and densities of 10 wild radish plants per square metre can reduce yields in wheat by up to 20%. evident. The symptoms are often confused with copper poisoning. Reportedly, up to half affected animals die within three to five days while animals that survive will take up to 10 days to recover fully. Wild radish is very competitive during the early growth stages of crops. The initial flush of germination at the start of the season following cultivation is the most prominent. The remaining dormant seeds germinate sporadically after rain. ✔ Wild radish is a problem weed in cropping and pasture areas throughout Australia. ✔ Long-term wild radish control can be achieved by integrating cultural, chemical and biological methods to reduce the existing soil seedbank and seed production. ✔ Useful cultural practices include cultivation, spray-grazing and slashing, hygiene (cleaning machinery and sowing clean seed) and crop swathing. ✔ Thoughtful use of herbicides as part of an overall management plan is advised as wild radish already has developed resistance to Group B chemicals. The fibrous stems of wild radish make harvesting difficult by choking the header comb. The damaging effects of green plant material also can reduce stored grain seed quality, especially lupin or peas. Characteristics Characteristics that enable wild radish seeds to survive and remain as an agricultural weed are dormancy and longevity (persistence). Wild radish seeds are dormant at the time of production and, by the start of the cropping season, as much as 70% of the seeds are still in a state of dormancy due to the protective seed pod. Other factors influencing wild radish seed dormancy are geographical location and temperature. Seeds from WA’s warmer northern agricultural districts have lower dormancy levels than seeds from cooler southern districts. Wild radish seeds have a relatively long life span. Seeds buried to 100 millimetres may remain viable for up to six years. The majority of seeds will germinate during autumn and winter. Optimum temperature for germination is 20 degrees Celsius, but if soil moisture is adequate, germination can occur at any stage during the growing season. Temperature is the major factor controlling plant development to flowering, while day length and temperature will influence the duration of flowering. The appearance of first flowers and the duration of flowering depend on the time of seedling emergence. The primary means of seed dispersal is as a contaminant of agricultural produce, especially in seeds and hay. Seeds also are spread by wind, water, livestock and by adhering to machinery. As wild radish is an outcrossing plant, bees can carry pollen over long FARMING AHEAD No. 68 - August 1997 Wild radish... TABLE 1 Herbicides registered for use in major cereals, pulse crops and pastures Herbicide Crop Time of application Chemical group bromoxynil all Z12-15 C chlorsulfuron wheat barley, oats PPI, Z11+ Z12+ B* Comments CEREAL CROPS Mainly used in mixtures diflufenican all Z12+ F Used in mixtures diuron all Z13-16 C Used in mixtures diquat all ripening L Pre-harvest, dessication flumetsulam all Z12+ B* linuron all Z13-16 C Used in mixtures. MCPA all Z13-37 I Used in mixtures, for early application Used in mixtures methab’thiazuron all Z13-16 C metosulam all Z12+ B* metribuzin barley PSPE-Z13 C prometryn all Z13-16 C terbytryne all Z13-16 C Used in mixtures thifensulfuron wheat, barley Z12+ B* Used in mixtures with metsulfuron triasulfuron wheat barley, oats PPI, Z12+ Z12+ B* 2,4-D all Z15-37 atrazine lupin bentazone cyanazine diflufenican Used in mixtures I Late application PPI C Used with simazine faba beans 2-leaf stage C Some crop effects chickpea, faba beans, field pea, lentil PPI, 2-3 leaf stage (faba beans) C Safe but may have problems with effectiveness lupin, field peas pre-emergence, 2-4-leaf stage F Good control diuron lupin, field peas pre-emergence C May have problems with effectiveness flumetsulam field peas 2-4-leaf stage B* Some crop effects imazethapyr faba beans, field peas pre-emergence B* MCPA, MCPB field peas 2-4-leaf stage I Some crop effects metosulam lupin 8-leaf stage B* Some crop effects metribuzin chickpeas, faba beans, field peas, lupin pre-emergence, 2-leaf stage C prometryn field peas pre-emergence C May have problems with effectiveness simazine chickpeas, faba beans, lupin PPI, pre-emergence, 2-4-leaf stage C Herbicide of choice bromoxynil subclover, lucerne post-emergence diflufenican subclover, lucerne post-emergence F Used in mixtures diuron subclover, lucerne post-emergence C Used in mixtures diquat (+paraquat) lucerne post-emergence L Lucerne must be one year or older flumetsulam all post-emergence B* imazethypyr subclover, lucerne post-emergence B* MCPA subclover post-emergence I methab’thiazuron subclover post-emergence C terbytryne subclover post-emergence C terbacil lucerne post-emergence 2,4-D, 2,4-DB subclover post-emergence PULSE CROPS PASTURES C I Used in mixtures Spray-graze Spray-graze * Some wild radish populations have been shown to be resistant to all Group B herbicides. Source: David Bowran, Agriculture WA The Co-operative Research Centre for Weed Management Systems makes no warranty of any kind, expressed or implied, concerning the use of products or active constituents mentioned in this table and article. Consult labels and seek local advice before use. FARMING AHEAD No. 68 - August 1997 37 Wild radish... distances. Already herbicide resistance in wild radish is approaching dangerous levels and it is particularly worrying that this resistance could be transferred to other populations. To achieve long-term control of wild radish, seed production must be prevented and the existing seedbank in the soil reduced. As with other weeds, integrating cultural, chemical and biological controls is encouraged. FIGURE 1 Wild radish distribution Cultural management The biological characteristics of wild radish favour the integration of a number of cultural control practices for management in cropping and pasture systems. These include cultivation, spray-grazing and slashing, hygiene and crop swathing Cultivation: Shallow burial of wild radish seed to a depth of 10 to 50mm results in the greatest emergence of wild radish. One or two years of cultivation to achieve maximum germination, followed by conditions of minimum soil disturbance in subsequent years, will reduce wild radish seed reserves. It will be necessary to ensure that later cultivations do not bring buried seed back close to the surface where it could germinate. The use of a mouldboard plough to achieve deep burial of the seed in the first year, followed by direct drilling or shallow cultivation during years two to four also will reduce wild radish populations in crops significantly. But the use of a mouldboard plough over large areas will have limitations. Spray-grazing and slashing: Spray-grazing involves spraying the pasture infested with wild radish at the rosette stage, with sub-lethal rates (500 grams per litre at 350 millilitres to 1.4L per hectare) of 2,4-D or MCPA, waiting for seven to 10 days and grazing heavily with sheep. Subclover is the legume pasture most tolerant of these rates. These rates do not kill the weed outright, but improve palatability by mobilising sugars in the plant. The weed is then preferentially Chemical management Source: Parsons and Cuthbertson, 1992 grazed. If adequate stocking rates are used, the weed growth and seed production will be reduced severely. Take care not to overgraze desirable legume species. As wild radish produces flowering stems and seeds rapidly, slashing needs to be repeated frequently to reduce seed production. Hygiene: Hygiene is vital in preventing the spread of wild radish. Cleaning cultivation and harvesting equipment, sowing clean seed, quarantining stock likely to have consumed wild radish seeds recently and feeding weedfree fodder will reduce the risk of spreading wild radish. Toxic or noxious weed seeds are prohibited by state laws from being used in stockfeeds. The damaging effects of green plant material, especially in stored lupin or pea seed also should be an incentive to produce and purchase weed-free seed. Grain to be used for sowing should be obtained from wild radishfree paddocks or seed should be cleaned to remove as much of the weed seed as possible. Crop swathing: Where green wild radish is likely to be present at harvest, delays can be avoided by swathing or windrowing the crop. This protects the crop from shattering and wind Wild radish appearance This two-leaf wild radish seedling shows the distinct characteristics of the weed: classic heartshaped cotyledons, obvious veins on leaves and stiff hairs. The weed has a distinct radish taste. FARMING AHEAD No. 68 - August 1997 damage and allows the green wild radish to dry off with the crop, overcoming problems associated with harvesting crops containing green wild radish. Swathing is a useful management tool but it does not reduce wild radish seed carryover unless the seed is trapped and removed at harvest. Other measures such as hand weeding, hoeing or burning may work in situations where odd plants appear in previously wild radish-free areas. This will help to prevent any build-up of wild radish infestations. Wild radish is a member of the Brassicaceae or mustard family. It is an erect annual herb growing up to 1.5 metres high. It grows best in highly fertile soils, in moderately exposed, cool to warm sites up to subalpine levels. In cooler regions, wild radish may take two years to complete its lifecycle. Wild radish cotyledons are heart shaped and hairless. The first leaves are oval shaped, have a round apex and a toothed margin. These leaves are covered with short stiff hairs. Leaves at the base of the plant have a strong turniplike odour when crushed. The plant develops as a flat rosette and branches from near the base as it matures. The upper stem leaves are narrow, shorter and often undivided. Wild radish flowers are pale yellow, purple, white or cream with purple veins in the petals. Wild radish is an extremely competitive weed. Early control when it is most susceptible to herbicides, and before it competes strongly with the crop, will help to avoid major yield losses. Often the weed is sprayed when present at low densities and sometimes with multiple applications in the same season. Repeated applications of Group B herbicides (especially multiple use of herbicides such as chlorsulfuron) to control wild radish will result in wild radish and other weeds rapidly developing herbicide resistance to Group B chemicals. About 80% of herbicide resistance tests held by Agriculture WA on spray failures in the 1996 season showed resistance to Group B herbicides. Knockdown herbicides containing chemicals such as glyphosate, paraquat and diquat are registered for seed-bed establishment in a range of crops. A wide selection of herbicides is available for use in cereals. Treatments registered for early control of wild radish in most cases will prove effective in removing competition and providing yield increases. Applications for control of later emerging plants may not result in yield increases, but can reduce seed set, assist in harvesting and maintaining grain quality. Although a number of products is available for pre-emergence application in pulses, the range of herbicides for use in chickpea, faba beans and lentils at post-emergent timings is limited. The poor effectiveness of triazines on wild radish, in lupin crops in particular, usually is related to limited soil moisture at germination. No-till cropping and stubble retention will impact on the effectiveness of pre-emergent herbicides. There are currently no in-crop herbicides available for use in nontriazine-tolerant canola. Simazine, atrazine and possibly cyanazine and metribuzin, all Group C herbicides, are available for wild radish control in triazine-tolerant canola. Herbicides such as bromoxynil (Group C), MCPA (Group I) and bromoxynil+MCPA are available for use in linseed or linola crops. There is a range of herbicides registered for controlling wild radish in cereals, pulse crops, and pastures (see Table 1). Acknowledgements: Jim Dellow, NSW Agriculture; Aik Cheam, David Bowran, John Holmes, Agriculture WA; Ross Britton, PISA; John Matthews, CRC Weed Management Systems; National Agricultural Commodities Marketing Association. 39