Controlled release, of agricultural chemical

Controlled release (CR) is a method by which active chemicals are provided to specific plant species at preset rates and times. Polymers are mainly used to control the delivery rates, mobilities, and periods of effectiveness of the chemicals. The main benefit of the CR method is that if fewer chemicals are used for the protected plants over the predetermined period, then the effect on the other plant species is less, while leaching, volatihzation, and degradation are reduced. The macromo-lecular character of polymers is the key to reduction of chemical loss throughout the production. 

Physical systems into which agricultural chemicals have been introduced include microcapsules, physical blends, and dispersions in plastics, laminates, hollow fibers, and membranes. Kinetic models for chemical release have been developed 

One of the most important applications of CR technology in agriculture is fertilization. Urea, a main nitrogen source, readily reacts with formaldehyde to form a polymer. The subsequent hydrolysis of polymer releases urea, so it is a simple and inexpensive CR system [72, 73]. 

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The indiscriminate use of such agricultural chemicals as pesticides, herbicides, and fertilizers is an important source of environmental pollution. A novel application of polymer-bound materials has been made in the controlled release of agricultural chemicals (Allan et al., 1973 Beasley and Collins, 1970 Shambu et al., 1976 Schacht et al., 1977, 1978 reviews of Neogi and Allan, 1974 Scher, 1977). When these chemicals are covalently bound to a polymer from which they can be slowly released into the environment, they not only check pollution but their duration of action is prolonged. The same effect can be obtained by encapsulation of the chemicals in polymeric beads from which they can be released slowly, e.g., 2,4-dichlorophenoxyacetic acid and 4-amino-3,5,6-trichloropicolinic acid have been used in polymer-bound form. 

Controlled release of a chemical substance can be effected by diffusion or erosion of a polymer matrix ( 1). This method has been used with considerable success in the delivery of bioactive compounds in agriculture and, to some extent, in dispensing drugs by absorption through tissue upon contact. 

At the time of writing, the applications of biodegradable polymers are confined mostly to the field of agriculture, where they are used in products with limited lifetimes, such as mulch films and pellets for the controlled release of herbicides. The synthetic polyesters used in medical applications, principally polylactide and poly(lactide-co-glycolide), while claimed to be biodegradable, are degraded in the body mainly, if not entirely, by chemical hydrolysis. There is little evidence that the hydrolysis of these polyesters of a-hydroxyacids can be catalyzed by hydrolase or depolymerase enzymes. 

The extremely active academical and industrial research focused on the use of biodegradable polymers for agricultural applications has induced the introduction of several products to the market. These products based on biodegradable materials are supposed to be applied as mulching films, green houses sheets, laminates, and containers in agricultural practices and for application such as soil structurization and controlled release of chemicals such as fertilizers and herbicides. 

Controlled Chemical Release Systems The controlled release of drugs or agricultural chemicals from appropriate polymer fonnulations is of interest to numerous researchers and phamiaceudcal compiuiies (1-2). The ability to program release profiles by the application of external stimuli enables the design of more effective and more efficient systems. 

Oxamide is produced commercially by Ube Industries, Ltd. (Japan) and a pilot process is being operated by Enichem (Italy). It is not produced domestically as a commercial fertilizer, although it was the subject of much research and development activity by the Tennessee Valley Authority s (TVA) National Fertilizer Research and Development Center. It is made in small quantities for industrial use by AUied Chemical, Hummel Chemical Co., and United Guardian, Inc. Oxamide has appHcation as a controlled release nitrogen source for the turf and specialty agricultural markets. 

Mirex is not currently registered for use in the United States so release of mirex to soil from pesticide applications is no longer of concern. However, use of mirex as a pesticide for fire ant control required the spraying of this chemical on soils of an estimated 132 million acres in the southern United States (IARC 1979c). An estimated 226,000 kg (498,000 lb) of mirex was used in 9 states from 1962-1976 as part of the fire ant eradication program conducted by the Department of Agriculture (IARC 1979c). 

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