Weeds are often a major problem of lucerne. They reduce the quality, yield and useful life of the stand. Early competition from weeds in young lucerne can thin out a lucerne stand. The weakened stand will then be unable to keep out weeds in later years.
There is a range of herbicides registered and available in Queensland (see Tables 5.1 and 5.2), however they all have their limitations and should not be used as a substitute for good management.
At establishment — a clean seedbed
Start with a seedbed as free as possible from weed seeds. Avoid areas known to be heavily infested with weed seeds, and allow a long fallow period before planting during which the weed burden can be reduced. Because lucerne is a long-term crop, preparation for new sowing should begin with good weed control practices in preceding crops.
Time of planting
Because weed infestation early in the life of lucerne stands is so limiting to yield and hay quality, growers need to know which weeds are to be expected in a new stand, and their likely severity. Summer weeds are usually a greater challenge than winter weeds to lucerne seedlings in the establishment phase. Therefore, choosing the optimum planting time is important.
Generally there is less weed competition in sowings made between April and August than from plantings at other times of the year. Serious persistent perennial weeds such as nutgrass, couch grass and Johnson grass can be controlled in the summer fallow preceding an autumn planting using appropriate cultivation and/or economical herbicide treatments such as glyphosate.
In established lucerne stands — cultural control
Weeds usually begin to colonise established stands as a result of mismanagement, e.g. overgrazing, too frequent or ill-timed cutting, poor irrigation scheduling, poor nutrition and a lack of insect and disease control.
Once weeds are flowering, the stand should be mown, regardless of its stage of growth. Set the mower no lower than 5 cm above the ground to avoid damage to the young lucerne. If an early cutting has been necessary to destroy weeds before they seed, the stand should not be mown again until new shoots begin to grow from the crown.
Cultivation of the established stand with fine-tined implements, either straight after a cut, or during the semi-dormant winter period, may help control annual broadleaf weeds in dryland lucerne. This is especially so in lighter soil types.
Proper crop nutrition results in lucerne stands that are better able to compete with weeds. Inoculate lucerne seed with Group AL inoculum immediately before planting. This is inexpensive and is available from seed suppliers. Soil testing and fertiliser test strips can help determine the best fertiliser regime.
Weed problems in established lucerne
The most common major weed problem of established lucerne stands are annual and perennial grasses. The annuals include barnyard grasses (Echinochloa spp.), feather-top Rhodes grass (Chloris virgata) and urochloa grass, (Urochloa panicoides) and the perennials include Rhodes grass (Chloris gayana), couch (Cynodon dactylon), paspalum (Paspalum dilatatum), green panic (Panicum maximum var trichoglume) and Johnson grass (Sorghum halepense). Nutgrass (Cyperus spp.) is often a major problem weed in Queensland’s better lucerne soils.
Grass invasion inevitably results in many stands being ploughed out prematurely. Efficient grass control herbicides starts with monitoring grass seedlings that germinate in the crop. Herbicides can then be applied to those seedlings at the recommended growth stages for both the seedlings and the crop.
Herbicides
Herbicides can help keep weeds in check, especially in the early stages. There is no chemical which controls all weeds found in lucerne. The choice depends not only on the weeds, but also on the growth stage of both the weeds and the lucerne.
Some details of the herbicides registered and available in Queensland in 2001-02 are given in Tables 5.1 5.2. This should be used as a guide only. It is not practical to list every weed that can be controlled by the many available herbicides. Check with a chemical supplier and seek the most up-to-date registrations and label claims. The pesticide database Infopest is available through DPI Call Centre, (Ph: 13 25 23). Many farm chemical retailers also have access to Infopest.
Always refer to the current label and apply the product in accordance with the manufacturer’s directions. Consider the withholding period stipulated on the label, i.e. the time interval required by law between application to the crop and cutting or grazing. Take care, because the tolerance to herbicides of newer lucerne varieties may not have been established.
Other sources of up-to-date information
- Primary producers and their chemical suppliers now have ready access to the lnfopest pest management information system. lnfopest is a comprehensive listing of registered Australian Agvet chemical products. It is available in CD-ROM format, costing $99.00 (incl. GST, postage and handling). It includes all 6400 Agvet chemical products registered through the National Registration Authority for Agricultural and Veterinary Chemicals (NRA). lnfopest contains images of 2600 registered Agvet chemical labels, which the user can view and print. It also contains pest management information, e.g. find out what herbicides are registered for control of particular weeds, pests and diseases in, say, lucerne. lnfopest can be ordered through the DPI Call Centre (Ph 13 25 23), or direct via the DPI website, www.dpi.q1d.gov.au/aphs/infopest. Infopest is updated every four months. Annual subscriptions cost $198.00 (incl. GST, postageand handling).
- Weeds: The ute guide: Northern grain belt edition, a DPI/GRDC publication containing 350 colour photos that cover over 170 of the major cropping weeds. Photos of both seedlings and mature plants are included for easier identification. To obtain a copy, contact DPI Information Centre, Ph: 4688 1415, Fax: 4688 1416, PO Box 102, Toowoomba, Q 4350. Cost $27.50 (incl. GST) plus $7.70 postage and handling.
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