Most of the evidence for the production of toxic substances, by one plant, that affect the growth and survival of another, is circumstantial, according to a review by Bonner (5). There is direct evidence that the leaves of Artemisia absinthium L. (3), Encelia farinosa A. Gray (6), and Thamnosma montana Torr. & Frem. (2), produce substances which are toxic to other plants. A substance has been isolated from the roots of guayule which is toxic to guayule but which is unimportant agriculturally because this substance remains only a short time in the soil (4). Also dead brome grass roots have been found to be toxic to brome seedlings (1); however the evidence here is indirect (5). Le Tourneau et al. (13) have shown also that water extracts from tissues of many crops and weeds inhibit germination of wheat.

Johnson grass (Sorghum halepense L., Pers.) is a native of the Mediterranean coastal countries and was introduced into the United States about 1830 by Governor Means of South Carolina (5). About ten years later some seed was planted in fertile river bottom soil near Selma, Alabama, by Colonel William Johnson. As Colonel Johnson was the first person to cultivate the grass extensively in the United States, it was named for him. Since that time, the grass has been intentionally cultivated and unintentionally disseminated in many areas of this country. A recent distribution map by Hitchcock (3) shows that the plant may be found in all but about a dozen northern states.

Johnson grass (Sorghum halepense L.) is one of the most troublesome weeds in the United States. Since its introduction into the Southeast in 1830, this weed has spread to a total of 38 states. The perennial habit of Johnson grass contributes to its widespread geographical distribution and enables it to survive moderately severe winters. In addition to fleshy rhizomes, Johnson grass is usually a prolific producer of viable seed which may give rise to reinfestations after the established plants are controlled.

The wild oat (Avena fatua L.) is considered to have spread originally from Asia; it is widely distributed throughout Europe and its advent in North America is attributed to introduction by immigrants from there (8, 13). The species presents a very serious weed problem in the prairie regions of Canada and northwestern United States (18). In Britain it has become prevalent in some regions as a result of intensive cropping to cereals (17). Up to 70 bushels of wild oat seeds per acre have been estimated to occur in the soil in badly infested fields in western Canada (12). This seed cannot all be stimulated to germinate by cultural practices and may remain viable in the soil for at least 3 years (4). The wild oat problem would be solved if the seed in the soil could be destroyed. There are two ways to achieve this: firstly, by stimulating the dormant seed to germinate so that the plants can be killed by cultivation or, secondly, by killing the dormant seed through the addition of toxic materials. Up to the present, attempts to do either of these have been unsuccessful; thus, investigations to determine the factors involved in dormancy are of primary importance.

Studies of effects of weed competition on crop yields, and of procedures used in screening herbicides as to effectiveness for weed eradication, often require the planting of weed seeds under field or greenhouse conditions. Because of the many factors that contribute to delayed germination, field and greenhouse stands of weeds obtained in this way are often irregular and slow in developing. Although methods for obtaining laboratory germination of weed seeds have been worked out for many species the methods are not always applicable for field planting, particularly where removal of seed coats or use of alternations of widely different temperatures are advocated.

Tartary buckwheat (Fagopyrum tataricum (L.) Gaertn) and wild buckwheat (Polygonum convolvulus L.) are annual weeds which are becoming increasingly prevalent on the Canadian prairie. Tartary buckwheat has become widespread in north central Alberta where over 2,500,000 acres are estimated to be infested, with over 500,000 acres heavily infested. The infested area is growing larger and in recent years has begun spreading into the neighboring province of Saskatchewan. Only one infestation, some 300 acres in size, has been reported in Manitoba. Because it is similar in size, the seed of tartary buckwheat is extremely difficult to remove from wheat, oats and barley and under the Canada Seeds Act, grains containing it may not be sold for seed. Commercial grain containing these seeds is severely degraded. As an impurity in wheat, the seed causes discolored flour. Consequently, the spread of this weed is being viewed with considerable alarm in western Canada.

The purpose of this study was to find more information on how and why certain herbicides act as they do on plants. The need for this kind of basic fact finding is often emphasized (1, 5, 6, 8). One approach to this problem is to study the effects of various environmental factors on herbicide behavior with plants. An effective way is to hold certain factors constant and alter the amounts or qualities of one or more other factors. Previous work with various colored fluorescent lights (4, 7) had shown marked differences in plant response to various portions of the visible light spectrum. Using essentially the same technique, this study was made of the influence of light quality in modifying the herbicidal effect of 2,4–dichlorophenoxyacetic acid (hereafter called 2,4–D) as various salts and esters. The combined herbicide–light effect was measured principally by dry weight variations from those of normal (control) plants.

A very useful technique in toxicological investigation is probit analysis. This method, which was developed by Bliss and other workers (notably Gaddum and Finney, but see reference 4, pp. 42–46) consists of plotting the log concentration of a toxicant against the percentage response on a probability scale, and fitting a weighted regression line to the data. Although the technique has found wide application in pharmacology, entomology, and plant pathology, it has been almost ignored by workers in herbicide research. It is the purpose of this paper to demonstrate that the concepts of probit analysis are applicable to herbicide research, and that the mere plotting of data on logarithmic probability paper may permit a very fruitful interpretation of experimental results. The technique is recommended at the screening level, for its results are very useful in the construction of field tests. It is in no sense a substitute for the carefully designed and executed field test, or for the analysis of test results by other statistical techniques.

It has been shown that with proper timing and the correct concentration, 2,4–D sprays can be used to control strawberry runner production (2). Other effects of this chemical upon the strawberry plants have been studied in a general way by Denisen (4) and Carlson (1). However, these effects have not been shown to be related to a specific influence of the chemicals on the morphological development of the strawberry plant. As the 2,4–D compounds used for weed control have exhibited growth regulating properties (3, 7, 10) it seemed likely that certain preparations might have a more unfavorable influence on plant growth than others. Also, since the molecular structure of the various preparations were different, it could be expected that the various preparations might differ in their influence on the developmental processes of the strawberry.

Each year increasing numbers of garden chrysanthemums (Chrysanthemum morifolium Ram.) are grown in the field by commercial nurseries. The cost of labor for weeding and the scarcity of efficient seasonal labor presents a serious problem. This study was undertaken to determine if weeds in chrysanthemum plantings could be safely controlled chemically.

Ethyl N,N-di-n-propylthiolcarbamate (EPTC) has shown promise as a pre-emergence herbicide in both greenhouse and field studies. Antognini reported that EPTC was quite volatile. In volatility studies in this laboratory the loss of unformulated technical EPTC from a free liquid surface was 57 micrograms/sq cm/hr at 30° C. It was difficult to understand how a compound as volatile as EPTC could be an effective pre-emergence herbicide. Preliminary tests, utilizing radioactive EPTC, demonstrated that it was less volatile from soil surfaces than from stainless steel or leaf surfaces, indicating that a soil adsorption phenomenon might be involved. Greenhouse studies showed that at a given concentration of EPTC in the soil the phytotoxicity to oats (Avena sativa L.) was not the same in the various soil types studied. The purpose of this study was to investigate the relationship between soil adsorption of EPTC and oats injury in several soil types.

In 1955, three British workers (2) described a variable dosage sprayer which was designed for large field plots. The original description did not attract much attention in the United States when it was first published, but the principles described therein are receiving more attention at the present time, both in this country and abroad (1).

The chemical nomenclature used in connection with herbicidal compounds is frequently difficult for individuals who have little training in chemistry. A large molecular model has been constructed as a visual aid to facilitate audience comprehension of chemical nomenclature and to demonstrate relationships between chemical structure and herbicidal activity. The present model is designed to illustrate four of the major classes of herbicides, but it can be further modified in design to illustrate other herbicide classes.

The high cost and scarcity of farm labor and the realization that the control of weeds with chemicals is possible and economical has resulted in greater demand for up-to-date information from all those concerned with weed control. For those charged with the responsibility of research in weed control chemicals and techniques, the additional duties of extension teaching and “trouble shooting” can be burdensome if not properly handled.

The value of 2,4–dichlorophenoxyacetic acid (2,4–D) for control of broadleaved weeds is well known. It is equally well known that these chemicals can cause serious damage to certain crops. After 10 years of general use in agriculture it is well to review the extent to which 2,4–D has been a source of trouble and to examine regulatory policy with regard to 2,4–D.