Fighting the Weed

Source from:  tobaccojournal.com 05/10/2004

They are the nightmare of farmers, and not only of those who grow tobacco: weeds. Rather than exclusive use of herbicides, integrated weed management, combining crop rotation, cultivation and sound crop management with judicious use of herbicides, is the instrument of choice. Weeds are green plants growing in the wrong place at the wrong time. Weeds may grow on seedbeds, in fields being prepared for tobacco or amongst transplanted tobacco. They compete with tobacco plants for moisture and nutrients, inhibit crop growth and development and prevent the attainment of full potential leaf yield. Some weeds can be a serious source of foreign material in mechanically harvested tobacco. As with most other crops, tobacco is most susceptible to weed competition in the early stages of growth. Weeds have the least competitive effect on tobacco when the soil is dry because the deeper growing root system of tobacco can absorb moisture from greater depths in the soil profile. In wetter soils, water can be absorbed equally well by tobacco and shallower-rooted weeds from the surface layers of soil. Under these circumstances tobacco plants lose their competitive edge and succumb to faster growing weed species. Generally speaking the scale of weed growth is more important than weed number per unit area of soil surface. Tobacco yields may be severely affected by weed populations as low as five plants per square metre, while in other situations 20 times this number of weeds may be needed to produce the same effect. It depends on the stage of tobacco plant development, the stage of weed plant development, weed development in relation to that of the tobacco, type of tobacco, weed species and soil moisture level. The consequences of doing nothing about weeds are clear and well established. Flue-cured yield from non-weeded crops in North Carolina were recorded at 75 to 90 per cent less than crops receiving comprehensive weed control. Similar trials in New South Wales, Australia, reported 50 to 70 per cent more tobacco leaf from weed-free plots, compared with those where weed growth was uncontrolled. When calculated over a four-year period in Georgia, USA, the consequences of no weed control was a 26 per cent loss in yield, decreased leaf values and lower sugar content, together with higher leaf levels of alkaloid and nitrogen. In Zimbabwe delays in weeding tobacco fields by two, four and six weeks were sufficient to reduce yields by, respectively, 8, 13 and 50 per cent. Weed control timing is just as important, if not more so, as weeding itself, especially when using herbicides for chemical control. The ability of a herbicide to control a specific weed, while not affecting the growth and quality of tobacco, depends on weed stage in relation to age and development of the crop. Application timing is critically important. Nicotiana tabacum originated in the sub tropics but is now grown in countries stretching from the Equator into temperate regions of North America, Europe, Australia and elsewhere. As such tobacco is forced to compete with a huge number and variety of weeds too extensive to catalogue in this article. Weeds that typically infest tobacco are generally grouped as perennial grass, annual grass and broad-leaf (dicotyledonous) weeds. Each group has its own specific mode of competition. And within the broad-leaf weed category those belonging to the same plant family as tobacco (solanaceae) may compete very closely with the crop for the same range of nutrients. The south-eastern United States stretching from Virginia and Maryland in the north through the Carolinas and in to Florida at the southern tip is one of the world's most concentrated areas of tobacco production. Climatically it descends from temperate in the north with cold winters and hot summers typical of Virginia, to the classical sub-tropical climate of southern Florida. As a region it records a range of weeds fairly representative of those infesting tobacco throughout the world. These include perennial grass weeds like nut sedges (Cyperus sp) and Bermuda grass (Cynodon dactylon), annual grasses including Eleusine indica, Cenchrus incertus, Digitaria sp and Dactyloctenium aegyptium and broad-leaf weeds such as Amaranthus sp, Senna obtusifolia, Desmodium tortuosum and Ipomea hederacea. The latter, commonly called Morning Glories, can pose particular problems for harvested tobacco. The stems of Morning Glory vine and wrap themselves around tobacco leaves and stalks to interfere with harvest. They may even end up in the sheets of cured tobacco, which is clearly undesirable. Prevention better than weeding Integrated weed management combining crop rotation, cultivation and sound crop management, with judicious use of herbicides, is most successful for tobacco. Exclusive use of herbicides is an expensive, ineffective, and unsound strategy for weed management in this sensitive crop. Fast growing tobacco crops shade out weed growth from the beds and inter-row areas. Insect pest damage and disease, plant injury by fertiliser, or chemical damage from pesticides may aggravate weed problems by holding back rapid tobacco leaf growth and expansion. It is essential to follow practices like insect pest and disease control and correctly timed fertiliser application to promote well-formed root systems and fast production and expansion of tobacco leaves. Sound sanitation and strict adherence to crop rotation recommendations minimise weed seed dispersal from adjacent areas by air currents, or from weeds growing in the preceding crop. Indeed, many problem weeds in tobacco fields originate from seed produced during the preceding crop or that blown into the field or seedbeds from adjacent areas, such as fence rows or ditch banks. And preventing weed plants in these areas from setting and producing seed will undoubtedly help to minimise weed populations in the following tobacco crop. This is achieved by destroying all weeds around seedbeds and tobacco fields or erecting natural and artificial windbreaks to filter out wind-borne weed seeds. Fumigated seedbeds will lose their sterilised status if weed-contaminated soil on boots, vehicle tyres or equipment is re-introduced into the fumigated area. Fumigation kills almost all in its path but does not leave any residual activity. Weeds in surrounding areas will almost certainly be harbouring polyphagous insect pests like aphids, whiteflies, leafhoppers and thrips that have a huge host plant range, which includes many different weeds and crops including tobacco. For instance Bemisia tabaci (tobacco whitefly) is known to feed and breed on weeds belonging to many different plant families including Solanaceae, Malvaceae, Amaranthaceae, Cucurbitaceae and Convolvulaceae. Winged forms of these pests may move into tobacco often with devastating consequences if they are carrying and transmitting virus diseases of tobacco. More than sixty species of aphid, and notably Myzus persicae and Aphis gossypii, carry and transmit CMV (cucumber mosaic virus). Various thrips including Thrips tabaci and Franklienella occidentalis are vectors of the tomato spotted wilt virus (TSWV) and Bemisia tabaci are responsible for spread of 'Frenching' or leaf curl caused by the tobacco leaf curl virus (TLCV). Common ragweed (Ambrosia artemisifolia) infestations in North Carolina are known to increase the incidence of Granville Wilt in tobacco caused by the bacterium Pseudomonas solanacearum. Advantage offered by established crop rotations in reducing populations of weeds (as well as pests, diseases and nematodes) are well documented and understood. Large seeded weeds in particular (e.g. Senna obtusifolia) are more easily controlled in cereal crops like maize and sorghum. If rotations are planned so that one of these crops precedes tobacco then the weed seed bank in the soil and that available for germination in tobacco is significantly reduced. But tobacco is more sensitive to weed competition than most other field crops. Weeds that may not be a problem for cereals, cotton, potatoes, peanuts and other field crops could well pose problems for the following tobacco crop, unless they are controlled right through a three to four year rotation. Tobacco is more sensitive to herbicides than most other field crops, and it may not tolerate a soil herbicide residue level left from the preceding crop where it was not a problem. This increased sensitivity of tobacco to herbicides must be taken into account when choosing herbicides for crops that precede tobacco. Herbicide label recommendations for all crops in the rotation should be followed 'to the letter'. Cultivation and hand-hoeing A large amount of the world's tobacco in Africa and other developing nations is still weeded entirely by hand. Even where herbicide use is the norm, cultivation and the occasional hand-hoeing are still practised. Indeed, there are several weeds found in tobacco around the world for which hand-hoeing between plants is the only effective means of control. Cultivation is also a good crop management tool. Proper cultivation and building a high row ridge helps to ensure good drainage, which aids disease management and reduces the chance of drowning from flooding. It improves aeration and water permeation by reducing the extent of soil crusting, which benefits the tobacco root system and decreases the incidence of lodging. In addition, it makes the mechanical harvesting of leaves more efficient. However, excessive cultivation increases leaching of potassium and nitrogen, injures root systems, increases leaf scald in hot weather, spreads tobacco mosaic virus, and contributes to soil erosion. Use of a hand-hoe may seem like a primitive and easy task but nothing could be further from the truth. Skill is required to carry out an efficient job of weeding, without compromising the integrity of plants and in a reasonably short time. In parts of Africa it is customary to scrape the weeds and soil away from the crop into the centre of the row. But this draws soil away from the plants whereas tobacco benefits from the opposite effect, i.e. ridge formation. Another common fault seen in hand hoeing is going too deep into the ridge. This may not cause damage up to ten days after transplanting. But after this time the fibrous tobacco roots will have 'colonised' the sub-surface soil layers and are easily damaged by careless and poorly timed hoeing. Timing is just as crucial for hand-hoeing as it is for herbicide treatment. Hand-hoeing started too late after transplanting will allow weeds to get a head start and also set back tobacco growth by damaging the fibrous roots just beneath the soil surface. Herbicides and their application The number of different herbicides approved and cleared for use in tobacco is relatively small, mainly because the crop is particularly sensitive to chemical injury. The herbicides labelled for use on tobacco control weeds in three specific and different ways. They may restrict cell division during seed germination (napropamide, pebulate and pendimethalin), be absorbed by emerging roots and shoots and then inhibit photosynthesis (clomazone) or generally affect metabolism of the weed plant (sethoxydim). These herbicides with the exception of sethoxydim have a limited affect, if they are applied after the weeds have emerged. Soil incorporation plays an important part in the efficiency of most herbicides used in tobacco. It ensures the herbicide is in the zone where the weed seeds germinate, and in the case of light sensitive herbicides like pebulate prevents loss of activity. Dr Terry Mabett Exclusive: Dr Terry Mabett writes in TJI From this issue on, Dr Terry Mabbett will start an exclusive series of articles for TJI. A consultant and journalist in the production, storage and processing of tropical crop commodities, including tobacco, Dr Mabbett has wide experience of research and development throughout the tropics in West Africa, India, South East Asia and the West Indies. His particular areas of expertise are pest, disease and weed control, plant nutrition, post harvest storage and processing. Enditem