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.

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.

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.

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.

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.

Seedling legumes are slow growing plants and poor competitors. During this period of slow growth, weeds have opportunity to outgrow them, and choke out or seriously damage the legume stand. Many of the important weeds are annual broad-leaved plants such as pigweed (Amaranthus retroflexus), lambs-quarters (Chenopodium album), mustards, and certain annual grasses such as the foxtails (Setaria spp.) and crabgrasses (Digitaria spp.). Perennial weeds, when present, are a serious problem, but this study was directed primarily at annuals. Since the advent of the combine, volunteer wheat and oats from shattered grain have also become a serious problem in summer legume seedings after small grain.

As early as 1946, a limited study conducted in Oregon (5) showed big sagebrush (Artemisia tridentata Nutt.) to be susceptible to the sodium salt of 2,4–D. Cornelius and Graham (3) stated that big sagebrush was highly susceptible or hypersensitive to 2,4–D. Kissinger, Hull and Vaughn (7), reporting on experiments conducted at the Beaver Rim area near Lander, Wyoming, found that the isopropyl and amyl ester of 2,4,5–T consistently gave higher kills for a given amount of chemical. One pound acid equivalent (hereafter referred to as pounds acid) of 2,4,5–T usually gave somewhat higher kills than two pounds acid equivalent of 2,4–D.

Centipede grass (Eremochloa ophiuroides) is a major pest to pasture lands throughout northwest Florida. It is a low growing perennial which spreads by stolons. In appearance it is intermediate between carpet grass and Bermuda grass; it has shorter nodes than the latter and makes a dense mat of creeping stems and leaves. Although it is a pest to pastures it is very desirable as a lawn grass. The fact that it is adapted to a wide range of soils and fertility levels provided sufficient moisture is present, makes it ideal for this purpose. In pastures it is undesirable because of its low feed value, and its dense aggressive growth which tends to crowd out desirable plants.