CHEMICAL METHODS OF WEED CONTROL Harry Agamalian, Weed Science Farm Advisor Monterey County, u.c. Cooperative Extension Salinas, California The principles of chemical weed control involve those chemicals capable of killing plants, especially certain types without injury to other kinds. These chemicals are thus grouped as herbicides. The introduction of chemicals as a means of controlling weeds is obviously a major scientific contribution. Its humble beginnings is reported to be around 1896-1897 with the use of copper salt solutions, iron sulfate, and sulfuric acid. By 1900 solutions of sodium nitrate, ammonium sulfate and potassium salts were being used as selective herbicides in grainfields thoughout Europe and the British Isles. By 1908, Professor H. L. Bolley reported on 12 years of experimentation using common salt, iron sulfate, copper sulfate, and sodium arsenite in North Dakota. He stated in his report "When the farming public has accepted this method of attacking weeds as a regular farm operation, the gain to the country at large will be much greater in monetary consideration than that which has been afforded by any other single piece of investigation applied to field work in agriculture". In reviewing this statement in 1983, perhaps there could be some concern to its validity against time but the contribution of weed science to agricultural productivity has to be a major component of successful farming. The developments of chemical weed control were greatly accelerated following the introduction of 2,4-D. It followed the pattern of earlier post-emergence methods in using chemicals to control weeds selectively in cereals. Other experimentors were evaluating chemicals for "soil sterilization", especially to control perennial weeds. Thus, two basic chemical methods were established, post-emergence application for selective and non-selective and soil applications for non-selective use. Herbicide Classification During the late 1940's, scientists began to develop herbicides that could be applied to the soil at time of planting. Investigations ultimately led to the concept of post-plant selective preemergence herbicides. As more chemical compounds were synthesized, application techniques were developed and many scientific concepts were utilized to establish weed control as a science in itself. In the process of organizing chemical weed control, the herbicides were later classified as to their use and or their 80
method of application. included the following: An early classification of herbicides I. Selective Herbicides A. Foliage 1. Contact 2. Systemic B. Root Applications II. Non-Selective Herbicides A. Foliage 1. Contact 2. Systemic B. Root Applications In recent times this classification would be modified to include soil-applied herbicides, replkacing root applications. Chemical Methods The utilization of herbicides in crop production is greatly oriented towards soil-applied or pre-plant incorporated applications. The concept of applying a herbicide just prior to or at planting time is greatly accepted with most seeded crops. This concept essentially allows for the emergence of the crop in a "weed-free environment". The acceptance of this method of chemical control is in part related to the vast number of selective herbicides on the market. Thus, the establishment of a preplant and/or preemergence selective herbicide. The utilization of post-emergence herbicides for selective weed control represents alternative and/or sequential treatments for crop production. The concept of using post-emergence herbicides only is rapidly gaining acceptance with several cropping systems. For many years this system was the only alternative for weed control in cereals. Today the grower has access to both pre-plant and post-emergence weed control methods for many crops. The concept of utilizing both methods greatly enhances weed management systems by allowing greater management alternatives. In discussing chemical methods of weed control, one has to distinguish between the forms of selectivity. Certain herbicides possess inherent plant (family) tolerances; this is often referred to as "physoilogical tolerance". This type of selectivity is often related to soil-applied herbicides. The other contributing component of this mehod of weed control is the 81
crop. A well-established, deep-rooted, perennial crop is a illustration of this type of selectivity. Chemical methods of weed control can also use the crop's growth habit for selectivity. The dormancy characteristics of alfalfa, for example, allows for the use of non-selective contact and preemergence applied herbicides, when this crop is in its dormant period. The time differential between crop seedling emergence and weed seedling emergence is often used with crops such as peppers and celery. This allows for the control of the weeds post-emergence, when applying the herbicide preemergence to the crop. Application techniques have greatly enhanced methods of chemical weed control. By utilizing basic modes of herbicide activity, directed spray, shielded spray and rope-wick applicators have extended use periods of many herbicides. These applications are mainly used with non-selective preemergence, systemic, and contact type herbicides. Another form of maintaining crop selectivity with herbicides is using various formulation technology. The application of granular herbicides, post-emergence to some crops becomes a useful method to gain crop selectivity in effective weed control programs. This concept is often used in the ornamental landscaping profession. In recent times the concern for economies in farming has brought forth the techniques of minimum tillage. The utilization of herbicides on fallow land and/or their use in crop residue has greatly benefited this practice. Both systemic, contact, and residual herbicides have been utilized in this procedure. The development of irrigation equipment such as drip (trickle), linear and pivotal systems, has produced another means of herbicide application, thus using established chemical methods. This concept greatly facilitates the application of preemergence, soil-applied herbicides. Both selective and non-selective herbicides can utilize such a system. The benefits of this application technology is just beginning to be demonstrated. Although certain herbicide physical and chemical properties may limit their usage in these systems; it will undoubtedly benefit performance of many herbicides. Certainly cultural practices established with long season row crops have necessarily evolved into chemical weed control systems. The application of "lay by" herbicides in many crops such as cotton, corn, beans, garlic, potato and tomatoes is characteristic of such cultural practices. In this concept, the application of selective preemergence herbicides at this period of crop growth will maintain a relatively weed-free environment to those crops through the harvest period. Perhaps the ultimate concept in chemical methods for weed control is the utilization of soil fumigation. This chemical method using non-selective biocides will destroy many weed seeds 82
in the soil profile. A subsequent waiting period allows for the planting if crops, which then develop in essentially a weed-free environment. But even with this costly practice, certain "hard weed seeds" are apparently resistant to this procedure. The practice of chemical weed control represents a complexity of biological, chemical, soil and engineering technology. Although "chemical methods" are a vital component of weed management, its usage has to be integrated into the entire A complete system of using chemical methods is not always essential for any given crop. The important factor is to be aware of these methods and their benefits. The following scheme represents a system of chemical methods associated with any given Preplant Phase (Non-Selective): treatment; soil fumigation. stale seedbed/fallow bed Preplant (Selective): prior to or at seeding. Post-Plant (Selective): seeding. soil incorporated application just preemergence application at Post-Emergence Weeds (Non-Selective): emergence. preemergence to crop Post-Emergence (Selective): application to crop and weeds. Post-Emergence (Non-Selective): rope wick appication. directed spray, shielded, Lay-by (Selective): development. Preemergence to weeds during crop Pre-Harvest (Non-Selective): applications of herbicides to destroy vegetation prior to harvest. The successful adaptation of chemical methods for weed control in crop production rest upon a thorough integration of the many components of these systems. ------- REFERENCES Anderson, W.P. 1977. Weed Science Principles, chap. 2. West Publishing Co., New York. Anonymous. 1972. Weed Control Study G Robins, W.W., A.s. Crafts, and R.N. Raynor. 1952. Weed Control, chap. 8. McGraw Hill Book Company, Inc., New York. 83
in the soil profile. A subsequent waiting period allows for the planting if crops, which then develop in essentially a weed-free environment. But even with this costly practice, certain "hard weed seeds" are apparently resistant to this procedure. The practice of chemical weed control represents a complexity of biological, chemical, soil and engineering technology. Although "chemical methods" are a vital component of weed management, its usage has to be integrated into the entire A complete system of using chemical methods is not always essential for any given crop. The important factor is to be aware of these methods and their benefits. The following scheme represents a system of chemical methods associated with any given Preplant Phase (Non-Selective): treatment; soil fumigation. stale seedbed/fallow bed Preplant (Selective): prior to or at seeding. soil incorporated application just Post-Plant (Selective): preernergence application at seeding. Post-Emergence Weeds (Non-Selective): emergence. preemergence to crop Post-Emergence (Selective): application to crop and weeds. Post-Emergence (Non-Selective): rope wick appication. directed spray, shielded, Lay-by (Selective): development. Preernergence to weeds during crop Pre-Harvest (Non-Selective): applications of herbicides to destroy vegetation prior to harvest. The successful adaptation of chemical methods for weed control in crop production rest upon a thorough integration of the many components of these systems. REFERENCES Anderson, W.P. 1977. Weed Science Principles, chap. 2. West Publishing Co., New York. Anonymous. 1972. Weed Control Study Guide. University of California Agricultural Publications, Berkeley. Robins, W.W., A.S. Crafts, and R.N. Raynor. 1952. Weed Control, chap. 8. McGraw Hill Book Company, Inc., New York. 84