The tremendous losses brought about by the presence of weeds in agricultural crops have been much underrated in the past. Recently it has been estimated, probably conservatively, that the total losses to agricultural producers in the United States approaches four billion dollars annually (18). Unless the basic causes of these losses and the places in which they occur can be determined, their significance cannot be fully evaluated.

Canada thistle has been considered a serious weed to agriculture in the northern parts of this country since its introduction. The plant characteristics and growth habits are described by Detmers (3) and Rogers (5) who also point out the seriousness of this weed to agriculture.

These studies were an attempt to evaluate some of the factors affecting the detoxification of 2,2–dichloropropionic acid in soils. Residual toxicity is a factor that must be evaluated before effective use can be made of any herbicide. Stability is preferred for purposes of soil sterilization; rapid disappearance of the herbicide may be essential in preplanting treatments; and in foliar applications, rapid disappearance of the material that reaches the soil may be important. Detoxification is usually considered to be the result of one or more of the several processes, decomposition, leaching, adsorption, and volatilization.

The magnitude of the problem created by wild oats (Avena fatua L.) is one that warrants intensive study not only from the practical but also from the biological standpoint. A knowledge of the entities or taxa present in the wild oats population is a logical beginning to more extensive phases of investigation. The purpose of this paper is to report taxonomic and genetics studies carried out on the wild oats population of the Prairie Provinces. It is hoped that the information obtained may be of use to other investigators interested in the biology of wild oats.

During recent years the increasing use of herbicides for the control of weeds in croplands has stimulated interest in the effect of these chemicals upon the soil microflora. In 1953 Hoover and Colmer (4) showed that the application of 2,4–D to Sharkey sugarcane soil in concentrations comparable to that used in the field had little effect upon the bacteria, actinomycetes, and fungi of this soil. Colmer (1), using the Winogradsky soil plaque technique which can serve to gauge the effect of herbicides upon the nitrogen-fixing genus Azotobacter in their natural habitat, found that field application rates were not harmful to A. chroococcum, and it was only when prohibitive rates of application of 2,4–D (400–600 ppm) were used that growth was markedly affected.

In weed control, soil sterilization is that process whereby the soil is altered so that it will no longer sustain the higher forms of plant life. In most common use the term refers to the application of chemicals to the soil surface. The sterile effect obtained is not instantaneous but develops as the chemical is carried down through the soil with water or after mixing the chemical with the soil by tillage. The sterile effect obtained is not permanent as leaching, fixation, or decomposition by microbes reduce the toxic effect with time. The length of time chemicals will remain active as soil sterilants varies. Highly soluble materials will be removed much more rapidly than will relatively insoluble compounds.

The fast growing woody plant known as marabú, (Dichrostachys nutans), is native to Africa but it has found a favorable environment in Cuba. At present, dense stands of it, 20 feet and more in height, cover vast areas of the island, and a great deal of land has had to be abandoned on account of it. The ease with which it becomes established in turf or in neglected sugar cane lends exceptional interest to the development of methods of controlling, or, if possible of eradicating the plant.

Foliage applications of 2,4–dichlorophenoxyacetic acid (2,4–D) to marabú (Dichrostachys nutans), a woody weed found in Cuba (3), generally give complete defoliation with varying amounts of stem kill. The older, more basal areas are not killed and the roots are rarely damaged by single applications of 2,4–D. Presumably therefore, little if any of the herbicide moves downward after these applications.

One of the cultural practises used to control weeds in wheat fields is the application of 2,4-D sprays. The spray is applied as a post emergent selective after the wheat and the weeds have made some growth. The success of selective weed control in wheat depends upon the relative resistance of the wheat plants and the relative susceptibility of the weed plants to the 2,4-D spray. Major factors affecting successful weed control in wheat fields are the stage of growth of both wheat and weed plants, the quantity and formulation of 2,4-D applied, the solvent or carrier for the 2,4-D, the variety of wheat, and the species of weeds present.

The herbicidal properties of the sodium salt of 3,5–dinitro-orthocresol were recognized in the early 1930's. The dinitrophenols and dinitrocresols have since found wide use in controlling weeds in corn, peas, beans, potatoes, cotton, grains, forage legumes, and many other field and horticultural crops. The 4,6–dinitro-orthosecondarybutyl phenol (hereafter referred to as DNBP) and its salts are the most widely used compounds of this group in the United States.

Most farmers are not taking full advantage of the information available on herbicides and weed control methods. With few exceptions there is too long a delay between the development of a new weed control practice and its use on the average farm. This lag may be attributed to the following factors:

1. Failure of public and commercial extension workers to inform individual farmers.

2. Unsatisfactory experiences by farmers due to improper use of herbicides.

3. Grower unwillingness to try something new without visual proof.

4. General complexity of herbicide usage.

5. Unwillingness of a large number of farmers to be the first to accept a new practice.

The alkanolamine salts of 2,4–dinitro–o–secondary butyl phenol (DNBP) have been used frequently as a pre-emergence herbicide in cotton without causing injury. However, under some conditions the chemical has severely reduced the stand. This paper presents data that show the effects of rainfall, lime, soil type, and temperature on the phytotoxicity of dinitro to cotton seedlings.

Aquatic weed control in storage reservoirs and conveyance systems of certain domestic water supply systems is a challenging problem to the waterworks engineer. The problem concerns not merely the control or elimination of aquatic growths. It involves water quality factors which may be inconsequential in other phases of aquatic weed control, but are of major importance in drinking water supplies. Where adequate treatment facilities are available, water quality factors are subject to control. However, many domestic water supply systems lack treatment facilities, and the effect on water quality by the use of any materials in the supply must be subjected to critical review.

Since the discovery of the growth substance activity of the phenoxy acids by Zimmerman and Hitchcock (14) in 1942 and the announcement of 2,4–dichlorophenoxyacetic acid (2,4–D) and 2,4,5–trichlorophenoxyacetic acid (2,4,5–T) as herbicides by Hamner and Tukey (9) in 1944, a new era in chemical weed control has developed. In 1949 workers at the Texas Agricultural Experiment Station at Spur, and the Weed Investigations Section of the U.S.D.A. (6) reported that aerial application of 2,4,5–T offered much promise of effectively and economically controlling mesquite on rangeland. Since that time, approximately 1,500,000 acres of mesquite infested grassland have been treated with this compound by aerial application. Though widespread use of 2,4,5–T has been made, the factors that influence its effectiveness in killing mesquite are not well understood. This paper attempts to summarize the information that is available on some of these factors.

The mesquite tree (Prosopis spp.) has been invading southwestern ranges to an alarming degree during the last half century, and has become a serious competitor for available moisture and light with perennial grass—the principal forage of livestock in this area. Its distribution in Arizona, New Mexico, and Texas has been estimated at over 70 million acres. Of the three varieties of mesquite found in these States, velvet mesquite (P. juliflora var. velutina (Woot.) Sarg.) is essentially limited to Arizona, and is the most common variety found therein.

Extracts of many plants have been shown to contain germination and growth inhibitors (1, 2, 3, 4, 5). In some instances these inhibitors play a role in the distribution of species. Thus dried Encelia leaves contain a material which leaches slowly into the ground and inhibits the growth of species susceptible to it (2). This suggests that certain weeds may be aided in their competition with crop plants by similar inhibitors. Helgeson and Konzak (4), for example, found that aqueous extracts of field bindweed (Convolvulus arvensis) and Canada thistle (Cirsium arvense) inhibited the germination of seeds and growth of seedlings of certain crop species.

Weeds in pastures are eaten in varying amounts and when consumed probably contribute to the diet of grazing livestock. However, many are not taken by the grazing animals and thus are largely useless plants that are using nutrients, water and light that would grow plants of higher value. In addition to the low palatability of some weeds, many are annual plants that fluctuate more widely in production with adverse weather than the more desirable perennial plants.

The way in which 2,4–D (2,4–dichlorophenoxyacetic acid) is most effectively used for controlling chamise is markedly different from that normally employed on woody plants. Old plants are difficult to kill, but young regrowth following cutting or burning is especially sensitive. The purpose of the present paper is to describe what is currently known (as obtained from hand sprayed plots) about the chemical control of chamise, and some of the factors that appear to be involved. Aircraft results will be published in a separate report.

Selective postemergence spraying of cotton in the western irrigated areas for control of weeds has not been widely practiced because of the lack of adequate herbicides and little knowledge of the effect of varying rates of the more promising herbicides on the growth, yield and fiber quality of cotton.

The herbicide, isopropyl N–(3–chlorophenyl) carbamate (CIPC), has proved to be efficient in eliminating certain weeds from several crops. The toxic mechanism of this herbicide is still somewhat doubtful. In order to further determine the reaction of plants to this herbicide the carbohydrate content of corn plants sprayed with CIPC was investigated.