Weed control in Tobacco production Weed control in
Weed control in Tobacco production
Weed control in field Weeds reduce tobacco plant growth by competing with the crop for light, nutrients and water. Weeds also serve as a host for diseases and insects that attack tobacco. Especially, when the climate is ideal for weed growth, tobacco producers must use many tools to control and/or suppress weed infestations. Weed management includes knowledge of the pest, crop rotation and other control methods.
Knowledge of the Pest Before a control measure is implemented in a tobacco field, knowledge of the pest is required. Producers must be aware of the weed species that infest an area in order to decide upon the proper control measures. Correct identification of a weed species can be obtained through the county agent. Weed maps and other documentation that are gathered over several years will help in avoiding weeds that are impossible to control in tobacco.
Crop rotation practices will directly influence the weed species and pressure to expect. Rotations that include corn, cotton, sorghum or soybeans will help reduce the weed pressure in tobacco fields. Important thing: Herbicides that are used in row-crop production can sometimes persist and injure tobacco planted the next season. Growers should be aware of the crop rotational restrictions before using any herbicide in a crop preceding tobacco.
Control Methods The most effective weed control is obtained through the use of several methods: - TILLAGE AND CULTIVATION - CHEMICAL TREATMENT
TILLAGE AND CULTIVATION Tillage should be used prior to transplanting tobacco to remove existing weed growth. There are no herbicides available that will kill emerged weeds without severely injuring tobacco. After transplanting, cultivation can be used to remove small weeds as they emerge. Cultivation should be shallow and should not disturb the established plants.
TILLAGE AND CULTIVATION First and important tillage provided before transplanting. The most important period to provide the weed control actions is from 4 weeks after planting to 9 weeks after planting. The cultivation should be provided not late then 4 weeks after the planting. The third main cultivation should be provided after harvesting.
CHEMICAL TREATMENT Herbicides are an effective tool for controlling weeds in tobacco. However, there are no herbicides labeled for use in tobacco that will control all weeds encountered in Georgia. Herbicide(s) applied before transplanting should be incorporated to control weeds before they emerge. Other herbicides are available that can be applied after transplanting.
Herbicides used in tobacco production Herbicides are classified as either selective or nonselective in their activity. Selective herbicides are weed killers that attack specific plants, while leaving desirable plants unharmed or with minimal injury. These include herbicides such as Propaquizafop and Fluazifop-p-butyl which control grass weeds only. Non-selective herbicides, on the other hand, kill or damage all plants and are used in situations where complete plant control is required. For example Bentazone is commonly used as a non-selective herbicide on tobacco. These can be difficult to use in areas where weeds and desirable plants are growing side by side.
Herbicide classification by Time of Application Pre-emergent herbicides Post-emergent herbicides
Pre-emergent herbicides are applied before the germination of weeds and they prevent the germination of weed seeds by inhibiting key enzymes involved in important internal bio-synthesis processes. Pre-emergent herbicides should be applied on the ground within 3 -7 days of transplanting followed by immediate irrigation (10 -15 mm) to activate the chemicals. In circumstances where irrigation is not available, the herbicide should be applied just before it rains, to achieve the same goal. Most commonly used preemergent herbicides include Clomazone, SMetolachlor, Trifluralin, Sulfentrazone and Pendimethalin.
Clomazone is a carotenoid and chlorophyll inhibitor that causes whitening in susceptible plants by inhibition of the biosynthesis of photosynthetic pigments. It controls several common annual grasses such as rapoko grass (Eluesine indica), shamva grass (Rotboellia cochinensis), stinkblaar (Datura stramonium L) and fat hen (Chenopodium album L). The application rate depends on the soil type but as a guide 1, 5 L of product is recommended per hectare in field application and one litre per hectare for seedbed weed control.
S-Metolachlor targets enzymes which catalyse the biosynthesis of very long chain fatty acids thereby inhibiting root and shoot growth in susceptible species. It controls annual grasses (except Shamva grass) and has suppressive effects on yellow nutsedge (Cyperus esculentus). The herbicide should either be sprayed on the soil after ridging but before holing out or onto the surface of the soil, over the seedlings, within 4 -7 days of transplanting.
Sulfentrazone is registered for the control of yellow nutsedge and annual broadleaf weeds. It functions by inhibiting protopophyrinogen oxidase, an important enzyme in the biosynthesis of chlorophyll. Sulfentrazone may cause seedling injury early in the season due to incorrect application method and delay in application. Sulfentrazone injury manifests as leaf discoloration and distortion, retarded growth and in extreme cases, crop death. Spraying should be directed at the base of the plants so as to avoid contact with the heart of the tobacco plant. The rate of application is 425 – 475 ml or grammes per hectare, depending on product formulation (liquid or granular).
Post-emergence herbicides attack weeds after they have emerged above ground. The “post” part of this type of herbicide refers to the fact that it is used on already existing weeds. Extreme caution should be exercised when applying post emergent herbicides to prevent spray drift onto the crop. Examples include Fluazifop-p-butyl, a selective herbicide for the control of annual and perennial grasses recommended in the seedbed and field at application rates of 0, 7 L and 3 L/ ha, respectively. Halosulfuron is registered for post control of sedge and broadleaf weeds applied at a rate of 50 g/ ha.
Safe use of herbicides Herbicide sprays must be directed towards the base of the crop to avoid contact with the leaves. Damage occurring on plants as a result of incorrect application of pesticides or herbicides is known as phytotoxicity. To prevent such undesirable effects, shields can be used during application, to prevent drift onto the crop. Various herbicides cause different types of damage on tobacco.
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