The present importance of the discipline of weed control is outlined. More acres in the United States are treated today for weed control than for the control of insects or plant diseases, despite disparity in personnel engaged; e.g., a total in the North Central region of 44 men in weed control, 176 in entomology and 177 in plant pathology.

In undrained soil or solution cultures of oats or barley, the effects of hours of exposure and concentration of 2,2-dichloropropionic acid (dalapon) on height and fresh weight were equivalent. Dalapon was shown to be a growth inhibitor in corn coleoptile and pea stem sections but was inactive in oat coleoptile sections. Dalapon was antagonistic to IAA induced growth in corn coleoptile sections and 2,4-D induced growth in pea stem and oat coleoptile sections. The antagonism was reversible in pea and oat tissue, but was competitive in pea stem sections. Dalapon was shown to be an anti-auxin.

Radioactive sodium salt of 2,2–dichloropropionic acid (dalapon) was applied to the leaves or to the roots of sugar beets, a species tolerant of the herbicide, and yellow foxtail (Setaria glauca), a susceptible species. Autoradiographs indicated that dalapon entered through the leaves and roots, and moved throughout all portions of both species within 48 hours. Accumulation of dalapon was greatest in the younger tissues of both species. Paper chromatographic separations of aqueous extracts of the plants indicated no metabolic derivatives of dalapon in either species.

Cluster tarweed (Madia glomerata Hook.) is an annual which occupies rangeland from Saskatchewan to N. Arizona at medium to high elevations. In the laboratory dried tarweed plants and leachates from them reduced the germination of intermediate wheatgrass (Agropyron intermedium (Host) Beauv.) and radish, and caused high proportions of abnormal seedlings. The inhibitory agent(s) was highly water soluble, dialyzable, stable to boiling, drying, and prolonged storage in solution, destroyed by ashing, and not affected by pH 4 to 7. In soil in the greenhouse, tarweed leachates and dried tarweed significantly reduced growth of wheatgrass seedlings. In the field, tarweed infestation reduced the establishment and growth of wheatgrass sown in it in proportion to the stand of tarweed.

For the control of wild garlic in Missouri in comparison with 2,4-dichlorophenoxyacetic acid (2,4-D) during 1954–1960, 3-amino-1,2,4-triazole (amitrole), sodium 2,2-dichloropropionate (dalapon), maleic hydrazide (MH), 2,3,6-trichlorophenylacetic acid (fenac) and 2,3,6-trichlorobenzoic acid (2,3,6-TBA) were effective. 2,3,6-TBA alone was more effective than 2,4-D. The toxicity of 2,3,6-TBA was increased by addition of a wetting agent. Heavy spot treatments of wild garlic with 2,3,6-TBA killed the established plants, but did not eradicate wild garlic because of sprouting of dormant bulbs.

The sodium salt of 2,2–dichloropropionic acid (dalapon) at 4 lb per acre was applied as a foliar spray to sugar beets, a species tolerant of the herbicide, and yellow foxtail (Setaria glauca), a susceptible species. Shoots of treated and untreated plants were collected during three weeks after treatment and assayed. Dalapon appeared to cause protein degradation to amino acids in the shoots of both species. Striking increases in the concentration of amides (primarily glutamine in sugar beets and asparagine in yellow foxtail) were found. The free pantothenic acid in the shoots of both species rose greatly following treatment. In sugar beets, observed recovery of treated plants was accompanied by return of the concentrations of the constituents studied to levels near those in untreated plants. Yellow foxtail did not recover after treatment and concentrations of the constituents studied generally remained at a high level compared to the untreated plants. The similarity in initial biochemical responses of the two species and the fact that sugar beets recovered may indicate that some mechanism capable of inactivating dalapon was operating in sugar beets.

A review, particularly of the responses to herbicides of photosynthetic processes at the molecular level, with some discussion of the effects of herbicides on cell membranes.

Yellow rocket (Barbarea vulgaris R. Br.) was studied for 5 years under field conditions. Yellow rocket seeds germinated throughout the growing season, but vernalization of the rosettes formed was necessary for flowering. There appeared to be a minimum size plant that could be vernalized. In the spring vernalized rosettes formed flower stalks. As the flower stalk elongated, basal rosette leaves died, giving the impression of an annual. When the flower stalk was cut or seed matured, new rosette leaves developed from buds on the crown. This new growth was capable of going through the subsequent winter. Marked plants survived for at least 3 years. The ability to flower again and set viable seed in the same year after cutting appeared to be associated with the stage of the original flower stalk when cut. Yellow rocket is perennial, but it may not survive after its second year under conditions of disease or strong competition.

Repeated applications of amine methylarsonate over a two year period resulted in significant reductions in nutsedge tuber numbers at 1.3, 2.6 and 3.8 ounces per 1000 sq. ft. Single annual applications of amine methylarsonate at the same rates were ineffective. Amine methylarsonate and disodium monomethylarsonate treatments gave moderate reductions in nutsedge tuber numbers in thin Bermudagrass turf after one season.

A review of recent work, especially, on the multitude of factors affecting the germination of weed seeds.

The toxicity indices to cotton of 12 soil-incorporated herbicides were highly and positively correlated with soil organic matter, cation exchange capacity, exchangeable calcium, moisture equivalent, free drainage value and total exchangeable bases. The characteristics of the responses indicate that adsorption of the 12 herbicides on organic matter occurs and that this greatly affects their toxicity. For equal toxicity approximately five times more herbicide was required at 20% organic matter than at 4% regardless of the herbicide involved.

Spot-treatments of young cotton with 2,2–dichloropropionic acid, simulating control of clumps of perennial grass, resulting in 10 to 25% reductions in cotton stands, did not cause proportional reductions in yields of seed cotton. Increased growth of cotton plants adjacent to skips compensated for reduced stands. The treatments did not affect boll components or fiber properties of cotton.

Mixed stands of big sagebrush (Artemisia tridentata Nutt.) and green rabbitbrush (Chrysothamnus viscidiflorus (Hook.) Nutt.) were sprayed with butyl ester of 2,4-dichlorophenoxyacetic acid (2,4-D) at 1, 2, 3, and 4 lb/A on six phenological dates in 1956, 1957, and 1958 at the Squaw Butte Experiment Station, Burns, Oregon. Data on soil moisture; soil temperature; growth development of herbaceous species; twig elongation of big sagebrush and green rabbitbrush; dry-matter, crude-protein, and total-carbohydrate contents of green rabbitbrush herbage; and crude-protein and total-carbohydrate contents of big sagebrush and green rabbitbrush roots were obtained at weekly to biweekly intervals in all 3 years to evaluate the ecological and physiological conditions that might indicate seasonal patterns of susceptibility to 2,4-D. Big sagebrush, a non-deciduous, non-sprouting plant, was easily killed with 2,4-D at 2 lb/A when soil temperatures and moisture contents were satisfactory for vigorous growth. The development of abundant leaf area on green rabbitbrush, a deciduous, sprouting plant, was important for spray interception and a photosynthetic rate sufficient to promote carbohydrate accumulation. Green rabbitbrush was controlled about 80% with a single application of 2,4-D butyl ester at 3 lb/A after the new twigs were 3 inches long and while soil moisture contents were sufficient for active growth. Differences in susceptibility between the two species are compared with morphological, phenological, and physiological differences, and seasonal patterns of susceptibility are further compared with environmental conditions.

Patterns of absorption and distribution of dalapon (2,2–dichloropropionic acid) were determined by radioautography and counting. Dalapon was absorbed through the cuticle and through the large open pores of hypostomatous leaves. Stomata were clearly the most expeditious routes of entry, but results were erratic when no surfactant was included. An anionic surfactant, sodium dioctylsulfosuccinate, enhanced both (a) cuticular and (b) stomatal plus cuticular penetration 3- to 4-fold during a 2-hr. absorption period. Studies confirmed the view that dalapon is translocated to regions of high metabolic activity and/or storage in association with and dependent upon the movement of food materials. Dalapon did not move appreciably out of albino leaves or green leaves previously depleted of food reserves, but was transported readily out of normal leaves in the light when a surfactant was used to enhance cuticular penetration.

Witchweed (Striga asiatica), a parasitic chlorophyll-bearing plant (Scrophuladiaceae) which attacks corn, sorghum, sugarcane, and more than 60 other important crop plants and weeds belonging to the grass family, was found in North Carolina and South Carolina in 1956. It presents a potential hazard to the production of grain crops with an annual value of more than 5 billion dollars in the United States. Continued extensive surveys indicate that witchweed is now present in 20 counties in North Carolina and 10 in South Carolina on a total of 9,566 farms containing 210,671 acres. Federal interstate and state intrastate quarantines (the first for a weed in the U. S.) were established to reduce its spread. Witchweed produces up to 500,000 microscopic longlived seeds per plant. These do not usually germinate until stimulated by a chemical stimulant secreted by the roots of certain plants. A stimulant has been isolated from corn but its exact chemical structure has not be determined. More than 300 chemicals have been evaluated in the laboratory in attempts to find a synthetic stimulant. Of these, 18 stimulated witchweed seed germination in the laboratory but they have not been evaluated for their effectiveness in controlling witchweed under field conditions. After 4 years of catch or trap crops, excellent control of witchweed has been obtained and highly profitable crops of corn have been produced on heavily infested land. Corn grown continuously with witchweed seed production prevented by 2,4-dichlorophenoxyacetic acid [2,4-D] has resulted in excellent witchweed control and high corn yields. Corn grown continuously receiving 2,3,6-trichlorophenylacetic acid [fenac] as a pre-planting soil-incorporated treatment and 2,4-D as a post-emergence spray as needed have proved effective full-season control of witchweed. Tobacco and certain other crops cannot be grown in rotation with corn for a period of 2 years after this treatment because of fenac residues in the soil. Other herbicides as pre-planting soil-incorporated or premergence treatments to control broadleaved and grass-type weeds plus 2,4-D to prevent witchweed seed production have been highly effective combinations for witchweed control. Quick eradication of witchweed will not be easy but effective control now and future eradication look favorable.

The effect of PBA, dalapon and amitrole applications on the osmotic pressure, electrical conductivity and pH of quackgrass rhizome sap and the total nitrogen and hygroscopic capacity of dried rhizomes, from May 17 through October 15, are reported. Characteristic seasonal curves were developed for all five specific factors studied. The herbicide treatments accentuated the variation in some of these factors but did not appreciably modify the seasonal curves.

A significant amount of amiben (3–amino–2,4–dichlorobenzoic acid) was leached from the 0 to 1-inch soil depth with 1 and 2 inches of applied water. Weed control could be obtained from amiben at 1 lb/A if rain came within 3 or 4 days after application. One-half inch of water applied immediately after application markedly increased the toxicity of 1 lb of amiben per acre to Brassica juncea (L.) Coss and Panicum miliaceum L.; a similar response from 2 lb/A was obtained with 1/4 inch of water. As the amiben rate was increased, less water was required for optimum weed control. Increased water and/or amiben applications were required with Brassica campestris L., which is somewhat resistant to amiben.

Chemical control of weeds in the Hawaiian sugar industry began in 1913 when experiments with sodium arsenite were first conducted on the Island of Hawaii. Sodium arsenite and some sodium chlorate were the main herbicides in use until a concentrated, activated diesel emulsion (CADE) was developed in 1944. From that time on the chlorophenoxyacetic acids and their compounds, sodium trichloroacetate, sodium 2,2-dichloropropionate, and the substituted ureas and triazines have, in turn, been incorporated into use on plantations for weed control. At present, over a half million acres are sprayed each year at a cost of nearly seven million dollars for labor and material.

Radioactive sucrose-C14 and serine-C14 were “fed” to excised leaves from red kidney bean plants at various times following atrazine treatment. The distribution of C14 in the various labeled compounds was determined by two-dimensional paper chromatography and subsequent elution and counting. The results indicated that other processes entirely independent of the photosynthetic block are also involved in the herbicidal action. Atrazine did not affect the rate of respiration of excised embryos of red kidney bean plants.

Toxicity of 3,6–endoxohexahydrophthalic acid (endothal) was measured by the inhibition of growth of germinating flax roots, compared to known standards. In the field, endothal applied at 0.3 to 10.0 ppm was not detectable after an average of 2½ days, and a maximum of 4 days. In aquaria, endothal at these rates was detectable much longer than this, and much longer when the aquaria contained only water than when they also contained mud and plant debris.