Weeds have plagued man in his agricultural pursuits since biblical times in spite of backbreaking efforts to control them by hand and mechanical methods. At this second meeting of the Weeds Journal of Weed Society of America it appears appropriate to review briefly some of the modern accomplishments in weed control and to take stock of future needs.

In studying herbicides in relation to a complex weed like nut grass (Cyperus rotundus L.), it soon becomes clear that high efficiency is attained only if the herbicide is able to penetrate and translocate readily to all parts of the plant, still retaining sufficient toxicity to kill every one of the vital organs.

The biological effectiveness of any systemic herbicide is vitally dependent upon the absorption of the chemical by the plant. The process of absorption is of particular interest in the use of herbicides and it is only as this process is understood and turned to advantage that maximum effectiveness of applied chemical can be achieved. In foliar absorption, with which this paper is concerned, the cuticle plays a very important role (2). The thickness and general state of the cuticle and the attendant stomata have been shown to vary markedly with growth conditions (10). The submicroscopic structure of cuticles and the accretion of wax deposit on leaf surfaces have been studied (7, 8). These factors are of significance in the consideration of the intimacy of the contact of a particle impinging on the leaf. The chemical nature of the cuticle (6) is undoubtedly of great importance in the partitioning of certain herbicides between a particle of the chemical and the interior of this leaf.

A relatively simple method for screening and evaluating chemicals for algicidal properties has been developed which embodies spray application of chemicals and greenhouse culture conditions. The method utilizes equipment and facilities common to the majority of industrial herbicide screening programs.

Flax in Uruguay is normally sown in July and August. The common weeds in flax, radish (Raphanus sativus L.) and Rapistrum rugosum L., in the Cruciferae, and Ammi majus L., in the Umbelliferae, are readily controlled by hormone-type weed killers. Experimental plots of flax may be sown from May to September or even later. The weed problem in plots is different from that in farm fields. Plots sown in May or June are usually relatively free of weeds; plots sown in July or later may be heavily infested by various weed species, both broad-leaved and grasses.

Cattails (Typha spp) are common and troublesome emergent aquatic weeds throughout the United States. They interfere with proper utilization and maintenance of irrigation and drainage canals. They further interfere with flow of water in channels where water velocity is low, and cause deposition of silt. In reservoirs, farm ponds, marshes, and lake margins, cattails waste large quantities of water, crowd out plants which produce food for wildlife and often interfere with fishing, boating, and other uses of these bodies of water.

The herbicides, 3–(p–chlorophenyl)–1,1–dimethylurea (monuron) and 3–(3,4–dichlorophenyl)–1,1–dimethylurea (diuron), have been widely accepted as selective soil applied herbicides; they are now used in a number of crops and under a wide range of soils and climatic conditions. Their extreme residual herbicidal activity contributes to their success as residual pre-emergence herbicides and as semipermanent soil sterilants.

Once it has been determined that the primary mode of action of a herbicide is dependent upon entry into the plant through its roots, it becomes important to know what factors will influence its movement in the soil. The findings of Ennis (3, 4) and Baldwin, Freed, and Fang (1) that certain cereals absorb carbamates by means of their roots rather than through their leaves stimulated experiments to study the effects of various soil factors upon the persistence and movement of carbamates in the soil. Blouch and Fults (2) found that the effect of various soil types on the effectiveness of pre-emergence applications of CIPC to several crops was much less than the effect of soil structure and high levels of organic matter. Data presented by Ogle and Warren (9) indicated that organic matter may have considerable effect on inhibiting the movement of CIPC in the soil. They also emphasized that the herbicidal behavior of a chemical in soil cannot be predicted from a study of one soil type, since there appears to be an interaction between soil type and movement of the herbicide. Smith and Ennis (12) reported similar results and also found that oil solutions of CIPC were less mobile than aqueous solutions.

Monuron, 3–(p–chlorophenyl)–1,1–dimethylurea, is used extensively for weed control in asparagus. Growers apply approximately 1.5 lb/A of monuron in the spring before harvest begins, and the same amount in late June at the end of the harvest season. Each application usually controls weeds for 6 to 8 weeks. Since this chemical controls weeds for such a long period, growers are concerned as to whether, after continued use on the same soil for a period of years, this material might accumulate in the soil to the point where asparagus is injured, or the soil might be made unfit for crops which might follow asparagus. Therefore, a study was made of the persistence and penetration of monuron in soils of asparagus experiments where this chemical had been applied each year for a number of years.

The design and use of a variable-dosage spraying machine for the experimental evaluation of herbicides has been reported by Pfeiffer et al. (1). This equipment sprays at constant volume while simultaneously diluting the spray mixture exponentially. The machine is composed of a series arrangement of diluent tank, concentrate tank, pump, and nozzles. The concentrate tank is equipped with a rapid mechanical agitator. The pump propels the spray liquid from the concentrate tank through the nozzles in the conventional manner. The liquid withdrawn from the concentrate tank is replaced by an equal volume of water or other diluent from the diluent tank. Thus the concentration of active ingredient in the spray decreases logarithmically with the volume of spray delivered. Since spray volume is constant with time, by moving the sprayer at constant speed, the rate of application of the active ingredient becomes a logarithmic function of distance along the plot. The machine is mounted on a four-wheel-drive light truck and draws power for the pump and agitator from the power take-off.

The low-volume technique of spray application, which has gained such wide acceptance since the development of the phenoxy-type herbicides, has created problems which did not exist with the highvolume spraying techniques used with contact-type materials. Frequently, when spray applications of systemic herbicides are made, there is only a very narrow margin between the amount of chemical required to kill the weeds and the amount which injures the crop plants. There is evidence that the physical characteristics of both the spray and the plant surface influence some of the responses of plants to herbicides (1, 4, 6, 7). Smith (9) has shown that a surprisingly small percentage of the spray discharged from a nozzle is intercepted by plant surfaces and that the size of the spray particles affects total amount of material retained by the plant. Droplet size is also important since susceptible plants are often injured by small quantities of herbicide which can drift for miles. Moreover, there is evidence that certain plants respond differently to various sizes of spray droplets (3, 5).

Successful use of selective herbicides is dependent upon differences in susceptibility between the crop and the weeds. Often the cause and magnitude of these differences is not completely known. Intelligent use of a chemical requires some knowledge of the margin of safety between the weed and the crop. Beneficial modifications of existing chemicals or methods of application could be most efficiently accomplished if the cause of the differences in susceptibility to the chemical were known. The alkanolamine salts of 4,6–dinitro o secondary butylphenol (hereinafter referred to as DNBP) have shown variability in their toxicity to various plant species. This investigation is concerned with the degree and possible causes of this variability.

In extensive experimental studies with peanuts in Georgia, satisfactory results have been obtained with the pre-emergence herbicide, sodium 2,4–dichlorophenoxyethyl sulfate (sesone) (3, 4). During the past 3 years, the use of sesone for weed control in peanuts has been generally satisfactory and its use as a general farm practice in Georgia has greatly increased. When emergence was poor and the sesone was being used for the first time, some growers were very ready to place the entire blame for poor stands on the herbicide, without considering that poor emergence could, as in the past, result from a variety of other causes.

In the field of weed control many complications are encountered as the program is carried out. In order to make progress, the combined efforts of research, extension, industry and the regulatory forces are necessary to provide the farmers and other people concerned with weed control with the information and material required by them to carry out an effective program. Research workers must provide us with their findings so that proven practices and techniques for controlling grassy and broadleaf weeds can be used.

Kansas has just completed the twentieth year of its organized program for the control of noxious weeds. The Kansas Noxious Weed Law was passed in 1937 by the State legislature. At that time, field bindweed, Vonvolvulus arvensis, was recognized as a menace to agriculture and the law was passed to provide for an organized fight against this weed. The law at that time contained provisions only for control and eradication measures of that one weed. In 1945, it was foreseen that a program which did not include “preventive” measures was inadequate. Provisions were then added to the law which made it unlawful to sell or offer for sale, unless processed to destroy the viability of the noxious weed seeds, grain and hay (livestock feed material), screenings, nursery stock, soil and sod containing seeds of noxious weeds.