Use of herbicides

One problem that has arisen with the use of herbicides in agriculture is spray or vapor drift. When fine spray droplets are released, especially if applied aerially, they may be deposited beyond the target area due to air movements to cause damage there. In the first place, this is a question of application technique. Herbicides, like other pesticides, should not be applied as sprays under windy conditions. In most... 

It has been estimated that without the use of herbicides in the United States the losses due to weeds in cotton, corn, peanuts, and soybeans would be 40, 25, 90, and 24% of the total crop value, respectively (12). An impact assessment study has shown that the loss of all herbicides would increase the consumers food bill by 8 billion annually (13). 

In 1949 herbicides were used on 9.3 million ha of agricultural land, on 21.5 million ha in 1959, and on 28.7 million ha in 1962 (14). Herbicides were used on 39.6 million ha in 1966 and on 79.7 million ha in 1976 (15). These figures show that the use of herbicides over the past three decades has increased nine fold. 

As a result of the long-term use of herbicides, the soil s natural capacity for recovery is decreasing irreversibly. 

Resistant weeds may appear and spread with the intensive use of herbicides these are plants whose root systems are found in the deep layers of the soil, and are damaged by herbicides to a lesser degree [3]. Thus, as a result of the intensive use of pesticides, the number of target species increases rather than decreases. 

One more consequence of pesticide use is that old target species expand to new territories. Because of the intensive use of herbicides, barnyard millet has not only survived, but has spread to fields of corn and other crops for thousands of hectares, entering agricultural environments where it had never grown before [11]. 

Pesticides in wastewaters come typically from point sources of contamination such as disposal sites and landfills where industrial or agricultural wastes are buried without any consideration, as well as discharges from industrial effluents from pesticide production plants. Furthermore, nonpoint sources derived from regular agricultural activities, especially in intensive agricultural areas, and accidental spills can also be significant. Urban use of pesticides is also possible in large cities where the use of herbicides and insecticides may result in runoff into the sewers. These sewers in turn may expel pesticides into wastewater treatment plants (WWTPs). 

Pasture land and railroad rights of ways in connection with sugar cane operations offer additional opportunities for the use of herbicides. On one property of 100,000 acres, over 300 miles of permanent railway are maintained. Roughly half of this right of way receives one or more applications of a 2,4-D and TCA mixture each year the rest is burned. A cheap, nonpoisonous soil sterilant is indicated. All Cuban plantations have their own rail systems with similar problems, but permanent rail transportation is less common in other areas. 

Potential use of herbicides in sugar cane might be placed at 8,000,000 to 10,000,000 pounds a year, while actual use currently is probably 15 to 20% of this amount. Doubling the use of herbicides in the near future would appear possible, making the probable consumption approach 3,000,000 pounds a year. 

Many changes are occurring in the cultivation of tropical crops, and it is difficult to make any predictions in a field which is undergoing rapid changes. However, the present trend in the use of herbicides appears to be away from the use of single chemicals. Usually the combination of chemicals involves 2,4-D with some other herbicide, although TCA combined with a contact herbicide has been widely used. Some of the newer chemicals, such as TCA and CMU, will probably be used more in the future, especially if the cost can be reduced. 

Most of today s practices are based on empirical data. Further investigation into the mechanism of herbicidal action is of fundamental importance in establishing the use of herbicides on a sound basis. More study into the synergistic effects of combining various chemicals is needed, as well as studies on methods of formulation, optimum quantities, times of application, and the influence of environmental conditions on the herbicidal action. Secondary effects, such as the reported tainting of coffee from 2,4-D deserve further inquiry. 

Table IV> Descriptions of Three Soil Biodegradation Studies Involving Use of Herbicide Orange...

Incorporating allelopathy into agricultural management may reduce the use of herbicides, cause less pollution, and diminish autotoxic hazards. Authentic inhibitors isolated from plant material have been subjects for examination in, vitro, but attempts to compare their effects in soils are limited. Soils contain a heterogeneous collection of organic matter of various origins. 

The technology can be used in agricultural applications, the manufacture or use of herbicides, munitions manufacturing, and in the manufacture and use of pesticides. 

Herbicides. The use of herbicides (qv) based on iodine compounds has its main market in Western Europe. In Canada and the United States these compounds are used only to a small extent. The only significant iodine-containing herbicide is ioxynil [1689-85 4] (4,-hydroxy-3,5,-diiodobenzoic acid). This compound, often used in combination with other herbicides, is formulated for controlling many annual broad-leaved weeds, especially black-bindweed, knotgrass, mayweeds, and com marigold post-emergence in wheat, barley, oats, rye, and triticale (142). Annual consumption of iodine in relation to ioxynils is considered to be about 300—500 t (66). 

Duke (1995) contended that resistant weeds have driven growers to develop and use integrated pest management (IPM) or integrated weed management (IWM) methods, such as biocontrol, cover crops, more crop and herbicide rotations, more selective use of herbicides, weed thresholds, etc. 

We are learning from experience and research that there are problems in crop production from repeated use of herbicides without mixing partners, or without alternating the use of herbicides with different modes of action. Not only do the resistant weeds become problems, but they lead to additional applications of the herbicides, adding further expense and further pressure to the development of resistant populations. 

There are three potential solutions for annual and perennial weed problems with drip irrigation. One is the use of preemergence herbicides that have residual weed control properties, particularly under continuous moisture. Another is the use of contact herbicides on the emerged weeds in the wet spot or persistent herbicides with contact activity. A third approach is the use of herbicides through the drip system when allowed by the product label (i.e., the use of herbicides capable of killing weeds as they germinate). With all three approaches it is essential that perennial weeds... 

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