Formulation and preparation of herbicidal chemicals for the selective control of weeds in mechanized crop production have become increasingly more difficult with the introduction of a wide variety of organic and inorganic herbicides during recent years. Commercial herbicides and experimental chemicals are usually applied in systems which contain not only the active ingredients but also various solvents, cosolvents, surfactants (surface-active agents), carriers, and other adjuvants. Formulations of herbicides are usually evaluated for their maximum effects on weeds and minimum effects on crop plants. The manner in which the inactive constituents of formulations influence the ultimate herbicidal activity of the active ingredients is not well understood and extensive research will be required to obtain the fundamental information needed.

Competition of downy brome (Bromus tectorum L.) with seeded perennial grasses is an important factor in the lack of success of many seedings on sagebrush conversion areas in the Great Basin. The seriousness of downy brome competition was emphasized by Stewart and Hull. Their work in Idaho indicated repeated failure in seeding of crested wheatgrass (Agropyron cristatum [L.] Gaertn.) on areas where downy brome was not eliminated or reduced. Hull and Pechanec pointed out that many studies indicate that reduction of dense stands of downy brome is essential for the early establishment of seeded species. They also note that in some cases seeding of perennials has been successful without reduction of this weedy annual grass.

On the rich alluvial soils of the Mississippi Delta it is not uncommon for Johnsongrass (Sorgum halepense (L.) Pers.) to spread over entire plantations. Eradication is not feasible, and adequate control in the Delta areas is accomplished only by the diligent use of control practices.

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.

Annual grasses continue to be a problem. Some of the most prevalent annual grass weeds are members of the foxtail (Setaria) genus. Although these species can be controlled chemically, farmers have often had failures. One possible cause of these failures could be variability within the species.

The compound, 3–amino–1,2,4–triazole (amitrole), has been used as an herbicide against different weed species with varying degrees of success. In the field, Canada thistle (Cirsium arvense (L.) Scop.) has been found relatively susceptible while field bindweed (Convolvulus arvensis L.) has been found relatively resistant to the action of amitrole. The reason(s) for this difference in sensitivity is not known. Because penetration of amitrole from the surface of the leaf to the interior is the first step in the chain of events leading to a response from a chemical by the treated plant, the nature of this process was examined.

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.

Johnsongrass (Sorgum halepense (L.) Pers.) is a serious problem in many parts of the United States but little is known about the carbohydrate metabolism of this weed. Rapp studied the seasonal variation of various carbohydrates in Johnsongrass grown in pots. These plants were placed in the greenhouse for several weeks. The purpose of the present investigation was to study carbohydrates in field-grown plants and to obtain information on the effects of herbicidal treatments on the carbohydrate balance.

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 behavior of highly volatile pre-emergence herbicides in soil and their relative availability to plants under various conditions is poorly understood and pertinent information is much sought after by workers in chemical weed control. Ashton and Sheets reported an inverse relationship between injury to oats and soil adsorption of ethyl–N,N-di–n–propylthiolcarbamate (EPTC) in various soil types. Later, Sheets demonstrated a close relation between the initial toxicity of EPTC and soil types. EPTC was inactivated about one-third as rapidly in an autoclaved soil as in a corresponding unautoclaved one, from which he suggested that microbial breakdown may be a major pathway of EPTC inactivation when the chemical is incorporated in the soil.

A number of the substituted symmetrical triazines have been shown to possess considerable herbicidal activity. Their high biological effectiveness against a wide spectrum of plants has aroused much interest in their use. While they have been found useful as soil sterilants, they are particularly valuable as selective herbicides on a number of crops. Two of the more promising triazines are simazine (2–chloro–4,6–bis(ethylamino)–s–triazine) and atrazine (2–chloro–4–ethylamino–6–isopropylamino–s–triazine). Both of these compounds have found extensive use in the control of weeds in corn. For this reason, it was of interest to determine the absorption, translocation and metabolism of these compounds by corn plants. The studies of this nature reported here were undertaken using C14 ring-labeled triazines. The labeling of the triazine was accomplished using C14 tagged cyanogen chloride in the synthesis, thus giving a random labeled ring.

Witchweed [Striga asiatica (L.) Kuntze] is an annual parasite which was discovered in North Carolina and South Carolina of the southern United States in 1956. This weed presents a hazard to the economic production of corn, sorghum, sugarcane, and other host plants.

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.

Pennisetum clandestinum Hochst. is a vigorous fast growing grass introduced into sub-tropical areas of the United States from tropical Africa in approximately 1920. It has been used for soil erosion control and turf to a limited extent in California. However, in many places it has become a serious weed.

Monuron [3–(p–chlorophenyl)–1,1–dimethylurea] is reported to be a potent inhibitor of the Hill reaction by Cooke and Wessels and van der Veen. It is reported to be 125 times as active as phenylurethan [ethyl–N–phenylcarbamate] in inhibiting the Hill reaction. Diuron [3–(3,4–dichlorophenyl)–1,1–dimethylurea] is 2500 times as active as phenylurethan as a Hill reaction inhibitor. Monuron and diuron have been used in photosynthetic studies. There are no reports in the literature indicating a difference in the mode of action of these compounds.

In coastal Oregon, mild climate, highly productive soil, and annual precipitation of as much as 130 inches combine to produce an exceptionally high rate of forest growth. When these forest stands are harvested, however, brush establishes rapidly in the openings, competes strongly with conifer reproduction, and eventually causes a reduction in timber yields. Such losses are serious in a region which depends largely on the timber resource.

2–chloro–4,6–bis(ethylamino)–s–triazine (simazine) is a highly selective herbicide. Corn (Zea mays) is very resistant to the toxic action of this compound while wheat, (Triticum spp) and oats (Avena spp) are susceptible.

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.

Many of the pathways by which weedkillers, such as 2,4–dichlorophenoxyacetic acid (2,4–D), affect plants are still obscure. Frequently the adverse effect seems to involve an alteration of metabolism, but the exact nature of this is sometimes not clear. Photosynthesis and respiration under different conditions may be either stimulated or inhibited. Other results, in which a decolorization of tissue with increasing amounts of 2,4–D was observed, suggest that the herbicide either interferes with the formation of chlorophyll, or causes increased destruction.

The growth promoting activity of substituted benzoic acids and 2,4-dichlorophenoxyacetic acid (2,4–D) was reported in 1942 by Zimmerman and Hitchcock. Interest in benzoic acids as herbicides was revived in 1950 when Bentley reported outstanding growth regulation with 2,3,6–trichlorobenzoic acid. Shortly thereafter several workers confirmed these results. Chlorinated benzoic acids have proven to be effective for eradication of field bindweed, and many other perennial weeds. At the present time these herbicides are labeled and sold for the eradication of field bindweed, leafy spurge, bur ragweed, Canada thistle, and Russian knapweed.