L.K. Baishya, I. Walling and D.J. Rajkhowa
ICAR RC for NEH Region, Nagaland Centre
Chemical weed control refers to any technique that involves the application of a chemical (herbicide) to weeds or soil to control the germination or growth of the weed species. In economic terms, chemical control of weeds is a very large industry and there are scores of examples of chemical weed control products. Common examples of chemicals used to control weeds in forages are 2,4-DB; EPTC; bromoxynil; paraquat. Knowledge of weed seed characteristics, morphology, ontogeny, nature of competition and degree of association with crops are pre-requisite for suggesting some efficient weed control measures. It makes the users/scientists quite acquainted with the nature and spectrum of weeds existing in the crop fields and accordingly guides them to adopt certain measures. Identification and naming of a particular weed based on its genus, species or certain biological characters may not be much useful to users since weed control usually, unless specific weed problem in certain area, aims at composite weed culture and not on individual species of weeds. Therefore, some common characteristics of the species, which are clearly visible and easily understandable by users, are to be exploited for making of their classes/groups and for recommending suitable control measures.
CHEMICAL METHOD OF WEED CONTROL
Chemicals that are used to kill plants or weeds are called herbicides. A proper technical know-how is a pre-requisite for successful adoption of chemical method of weed control so-called herbicide technology. One has to exercise a lot of caution while using the herbicide for uniform application as well as higher herbicide efficiency. Herbicide selectivity and its dose, time and method of application are of paramount importance/consideration before applying to a crop.
There are 5 types of herbicides:
- Broad spectrum – these work on a wide variety of weeds
- Selective – these work on a narrow range of weeds
- Contact – these destroy weed tissue at or near the point of contact (they do not spread around the weed), and require even coverage in their application
- Systemic – these move through the weed’s circulation system, and can be injected into the weed
- Residual – these can be applied to the soil and destroy by root uptake. They remain active in the ground for a certain length of time, and can control germinating seedlings.
Based on application methods herbicides can be classified as under:
- Soil application
- Soil surface application: Herbicides are usually applied to soil surface to form a uniform herbicide layer. Applied herbicides, due to their low solubility may penetrate only few centimetres into the soil. Weeds germinating in the top layers are killed due to incidental absorption of herbicides.eg. triazines, ureas and anilide
- Soil incorporation: Some herbicides are applied to soil surface and incorporated into the soil either by tillage or irrigation for their effectiveness.eg. volatile herbicides viz., aniline and carbamate
- Sub surface application: Perennial weeds Cyperus rotundus and Cynodon dactylon are controlled by injecting herbicides to the lower layers of the soil at several points.
- Band application: Herbicides are applied as narrow bands over or along the crop row. Weeds in between crop rows can be controlled by intercultivation or band application of herbicide. This method is useful where labour is expensive and intercultivation is possible. eg. weeds in maize can be controlled effectively by spraying atrazine on seed row at the time of sowing.
- B) Foliar application
- Blanket application: Application of herbicide over the entire leaf area. Selective herbicides are applied by this method.
- Directed application: Herbicide is applied directly to weeds between crop rows, avoiding the crop foliage. Care is taken to avoid spray fluid falling on the crop. eg. Late weeds in cotton can be controlled by spraying non selective herbicide by directed spray.
- Spot application: Herbicides are applied or poured on small patches of weeds, leaving the relatively weed free patches untreated. It minimizes the herbicide usage per unit area.
Benefits of chemical method:
- Herbicides can be applied for weed control in crop rows and where cultivation is impossible.
- Pre-emergence provide early season weed control.
- Cultivation & manual methods of weed control may injure the root system.
- Herbicides reduce the need for pre-planting tillage. They are extremely useful in minimal / zero tillage.
- Herbicides can control many perennial weed which cannot be controlled by other methods. Eg: Cyperus,eg: Rice ecosystem
Limitation of chemical method of weed control:
- According to World Health Organization (WHO) :”Any substance or mixture of substances in food for man or animals resulting from the use of a pesticide and includes any specified derivatives, such as degradation and conversion product, metabolites, reaction products, and impurities that are considered to be of toxicological significance” are defined as herbicide/pesticide residues.
- Following herbicides registered and used in India, classified as potential carcinogens by the US EPA: Alachlor (B2), Atrazine (C), Diclofop-Methyl (C), Metolachlor (C), Oxadiazon (C), Oxyflourfen (C), Trifluralin (C).
- Herbicides banned in India: 2,4,5-T,Nitrofen, Paraquat dimethyl sulphate, Maleic hydrazide
- Herbicides still under review :Atrazine, Butachlor, Pendimethalin, Mepiquat chloride, Linuron
- Herbicide residues in soil and plant parts at harvest
- Residues in various cropping system and agroclimatic condition under AICRP-WC
- Herbicide persistence in soil (days): Atrazine (45-90), Alachlor (60-80) , 2, 4-D (45-90), Butachlor (60-100), Fluzifop p-butyl (30-90) Isoproturon (90-120), Imazosulfuron (60-90), Metoxuron (>80), Metribuzin (20-100), Pendimethalin (60-200)
- Herbicide residues and human health implications :
- Increasing incidences of acute herbicide such as butachlor, fluchloralin, paraquat, 2, 4D, pendimethalin, glyphosate etc are emerging in India.
- Paraquat poisoning is an uncommon entity in India, and is associated with a high mortality rate.
- Acute respiratory distress syndrome because of paraquat usually appears 24–48 h after ingestion (Singh et al. 1999).
- Most frequent routes of exposure to herbicides are, either accidentally or intentionally or through direct skin contact.
- 20 to 50 μg/mL concentrations of fluchloralin resulted in a significant dependent increase in number of micronucleated cells of human.
- Sinha et al. (1995) demonstrated oxidative role of butachlor in intracellular ROS production & consequent mitochondrial dysfunction, oxidative DNA damage, and chromosomal breakage, which eventually triggers necrosis in human PBMN cells
In India herbicide contamination of soil, plants and natural waters occurs infrequently and at low levels. With few exceptions aquatic herbicides do not accumulate and persist in fish. Though some reports of herbicide poisoning are reported though data on the occurrence of herbicide-related illnesses among defined populations in human, the domain of herbicide illustrates a certain ambiguity in situations in which people are undergoing life-long exposure. No doubt herbicides are boon for efficient weed control in all situations, so cause obvious risk of residues and health hazards, if not used properly, we have to find a way to deal with weeds as well as residues