For most effective weed control 2–chloro–4,6–bis(ethylamino)–s–triazine (simazine) must be applied to the soil. As a foliar spray it is much less effective. Simazine penetrates into leaves of some plants, but apparently the cuticle is a barrier to foliar absorption of this herbicide. It is absorbed rapidly by roots and is transported upward to the aerial plant parts.

In the central Great Plains, late summer seedings of alfalfa have often been used in order to avoid the annual weed problem associated with spring seedings. Because moisture conditions are unreliable in the fall, development of techniques to reduce the hazard of annual weed competition in spring seedings offers considerable promise. The authors, in previous reports, indicated that materials that give adequate control of annual weedy grasses and broadleaved weeds are available. These tests were made on plantings of Ranger alfalfa.

Bamboos are perennial grasses which exhibit great variation in culm height and diameter and are among the most widespread and useful plants in the tropics. More than fifty bamboo genera have been recognized, and the total number of described species and varieties throughout the world is probably near 1000. In India, this plant is considered the poor man's timber, and it is an irreplaceable basic material for millions of people in the Orient and in South and Central America. Many species are extremely vigorous, however, and become pests when they invade cultivated areas such as coffee farms, lawns, pastures, and sugarcane fields. In some localities, bamboo growth is so rampant as to interfere with the reproduction or development of valuable tree species, and measures must be taken to keep it in check.

Control of big sagebrush (Artemisia tridentata Nutt.) with fire, chemicals, or mechanical means has long been recognized as an effective method for rangeland improvement. If this sagebrush is killed or weakened by some method of control, excellent stands of grass may be obtained if all conditions are favorable. Bohmont states that as the percentage of sagebrush control increases, the grass and grasslike species also increase.

It has been only in comparatively recent years that any appreciable work has been conducted on equipment for the application of chemicals in the soil. Most of the early work was devoted primarily to methods of applying anhydrous ammonia and liquid solutions as fertilizers. In most of this work the principal injection tool consisted of a chisel-type applicator that released the chemical at a single point in the soil. In recent years, investigators have attempted to devise efficient and uniform methods of subsurface distribution. Hedman and Turner reported on a chisel-type applicator equipped with a “fishtail” to give more lateral spread to anhydrous ammonia. Arya and Pickard studied the effect of high pressures and velocities on the application of anhydrous ammonia, which was also single-point injection. Certain problems associated with the application of fungicides and herbicides in the soil have been discussed by Owen and Becker and Costel.

Work conducted in recent years with volatile herbicides has shown that many of these materials give extremely variable results when sprayed on the soil surface but are fairly consistent when incorporated in the soil. Methods of determining distribution and placement of these materials in soil are generally tedious and often complicated due to a number of factors, including volatilization and leaching. Substitution of a fluorescent material for the herbicide in the application equipment offers a useful method of studying placement and distribution of sprays in the soil.

Basal stem sprays of an ester of 2,4,5-trichlorophenoxyacetic acid (2,4,5–T) mixed with a light oil such as kerosene or diesel oil have proved to be effective for controlling a number of species of woody plants. In general, the recommendation has been to apply the herbicidal solution in a band around the base of the stems and allow the solution to flow down the bark to the ground. However, different band widths have been suggested by workers in various parts of the country.

The absorption of chemicals by the leaf of a plant is a problem of major importance in chemical weed control. The presence of a waxy cuticle on the surface presents a barrier to absorption. Interspersed in the cuticle are the stomata which may serve as a portal of entry for chemicals but even the interior chambers of the stoma are lined with a cuticle. It may be presumed that the more intimate the contact between a chemical and the leaf surface the greater the possibility for absorption. The water carrier commonly used in application of these chemicals makes but a poor contact with the cuticle; however, this deficiency may be overcome by the addition of agents to lower the surface tension of the solution. While intimacy of contact of the chemical with the leaf surface is important in absorption, the state, i.e. liquid or solid, of the chemical may also influence the ease of absorption. Consideration of the physical processes involved shows that a chemical will be absorbed with relative ease from a liquid phase in contrast to the same chemical deposited as a crystal on the surface of the leaf. Either humid conditions following application or the addition of hygroscopic agents may help to maintain a liquid state thus facilitating absorption. Many surface active agents used in sprays are also fair hygroscopic agents.

The wild oat developed along with man's early agricultural activities, adapted to his artificial environments and accompanied him in his migration, to become a major problem in grain growing areas of Europe, Asia, Australia and North America. Manson's survey revealed approximately 61 million acres of cultivated land infested with wild oats in western Canada. At the present time, the weed infests most soils in the black and transition black-grey soil zones and medium- to fine-textured soils in the dark brown soil zone. In the brown soil zone, wild oats are usually confined to fine-textured soils or to depressional areas. “Moderate” infestations reduced barley yields by 15.5 percent and flax yields by 79.4 percent during a four-year period. Severe infestations may occasionally produce as many as 900 to 1,000 plants per sq m, in which case they are “turned under” since competition from such an infestation makes crop production impractical.

Investigations showing the effective use of simazine [2-chloro-4,6-bis (ethylamino)-s-triazine] as a selective herbicide in several crops particularly in corn has resulted in a demand for information about its degradation in plants. It is extremely important then to know if toxic residues remain in plants. Evidence to the contrary would be obtained if plants treated with simazine uniformly labeled with carbon-14 in the triazine ring were found to respire radioactive carbon dioxide.

The control of hardwood vegetation is an essential step in establishing pine on upland sites in North Louisiana. In some instances removal of all vegetation is necessary before pine seeding or planting can be done. This type of operation is usually accomplished by conventional land-clearing methods, which are often rather expensive. The use of chemicals as a means of hardwood control offers certain advantages not inherent in mechanical methods. In situations where species selectivity is not desired, the substituted urea herbicides might provide an economical means of vegetation control.

Canada thistle and perennial sowthistle have been given several names by various authors. However, it appears that the Canada thistle used in this study is generally called Cirsium arvense (L.) Scop. and the perennial sowthistle was a mixture of Sonchus arvensis L. and S. uliginosus Bieb. The two species of sowthistle are very similar. However, S. arvensis has “long glandular-hispid peduncles and involucres covered with lead colored glandular phyllaries” and a chromosome complement of 54, while S. uliginosus has “slenderly turbinate to cylindric pale involucres and glaborous white-margined phyllaries” and a chromosome complement of 36.

The materials known as herbicides represent many different chemical groups. Well-known herbicides include aromatic hydrocarbons, halo-aryloxy organic acids, substituted carbamates, substituted ureas, sulfamates, cyanates, substituted phenols, symmetrical triazines, and many others.

Treatment of soil with fumigants to kill dormant weed seeds is a valuable practice but control in the surface one or two inches is often poor. This is because the fumigant vapors diffuse rapidly out of the soil into the atmosphere before building up a lethal concentration near the surface. Improved weed control has been reported by proper confinement of fumigant vapors in the soil with impervious seals. Water seals have been advocated by some workers as a satisfactory substitute for impervious covers. However, only limited experimental data are available comparing soil fumigation results with different methods of sealing the soil surface.

Considerable attention has been given during recent years to the fate of herbicides sprayed onto soil surfaces. The effect of light upon herbicides also has been studied extensively. Payne and Fults by means of a biological test found that the potency of 2,4–D was increased by exposure to ultraviolet light. Hansen and Buchholtz found that when a solution containing 2,4–D and riboflavin was exposed to light the activity of 2,4–D was reduced. They tentatively identified the breakdown product as 2,4–dichlorophenol. Later Bell positively identified it.

Several herbicides have been tested for pre-emergence weed control in soybeans. PCP (sodium pentachlorophenate) has given good pre-emergence weed control in soybeans but relatively high rates of 15 to 20 lb/A were required. Although CIPC (isopropyl N–(3–chlorophenyl)carbamate) and NPA (N–1–naphthylphthalmic acid) have given promising results in some tests they have not been uniformly good; CDAA, on the other hand, has consistently controlled annual grasses but not broadleaf weeds.

Medusahead, Elymus caput-medusae L., is currently the worst range weed in Idaho. It has gained this distinction because of its rapid migration, its vigorous competitive nature, and its extremely low forage value. In the absence of effective control measures, the medusahead problem is becoming more acute each year. The threat that the weed poses to Idaho's livestock industry is sufficient to merit grave concern.

Investigations of the competitive effects of Setaria lutescens (Weigel) F. T. Hubb. (yellow foxtail) on corn have dealt primarily with the factors of crop and weed populations, nitrogen fertilizer applications, duration of the period of competition and the amount and distribution of seasonal rainfall. Differences in the competitive efficiencies of various crops and weeds have been demonstrated under various production conditions, but relatively less is known regarding competitive efficiencies among varieties of given crop species. Working under the dryland conditions of Western Canada, Pavlychenko and Harrington compared the competitive efficiencies of several small grains. They ranked barley as the most efficient competitor of weeds, followed by spring rye, wheat and oats in that order. The importance of fertilizer applications in increasing the competitive efficiency of crops has been investigated widely. Previous studies with corn and soybeans have demonstrated some aspects of the role of soil moisture in determining the nature and extent of weed competition in these crops.

During the past several years, pre-emergence herbicides for crabgrass control in turf have come into prominence. Some herbicides are relatively new; others, such as lead arsenate, calcium arsenate, and chlordane, have been used as herbicides for a number of years. Research workers have included the old as well as the newer herbicides in experiments under controlled conditions to determine their effectiveness as pre-emergence herbicides. However, information on the effect of these herbicides on the germination and growth of turfgrass species is limited. Application of lead arsenate as orchard sprays over the years had a toxic effect on the soil after the orchards had been removed and the areas planted to other crops due to arsenic accumulation. The poor condition of young alfalfa and barley in the affected fields was roughly proportional to the concentration of readily soluble arsenic in the surface soil.

Selective herbicides that will control certain broadleaved but not grassy species or vice versa have been in use for a number of years, but recently selective herbicides have been developed that are toxic to a greater number of weed species. Such an herbicide is 2–chloro–4,6–bis(ethylamino)–s–triazine (simazine) which selectively controls virtually all grassy and broadleaved annual weeds in corn and in some horticultural and woody species. This approaches the ultimate in selective weed control since a farmer usually wants to grow one plant species and eliminate all others.