Peanut (Arachis hypogaea L.) yields were not influenced by weeds that were removed within 3 weeks after planting and each week thereafter, but a yield reduction did occur when weeds were not removed until 4 to 8 weeks after planting. Peanuts kept weed free at least 6 weeks after planting showed no yield loss due to competition from weeds emerging later. As weed competition increases, peanut seed yield, forage yield, and soil moisture content decreases.

Dissipation of 2-methoxy-3,6-dichlorobenzoic acid (dicamba) was greater in Sharpsburg silty clay loam soil than in Anselmo sandy loam soil and was faster in the topsoil than in the subsoil. Breakdown increased with increasing soil incubation temperatures. Phytotoxicity of dicamba and 3-amino-2,5-dichlorobenzoic acid (amiben) in aqueous solutions decreased upon exposure to sunlight. Amiben was more susceptible to photodecomposition than was dicamba. Aqueous dicamba solutions exposed to sunlight for 16 days decreased growth of cucumber (Cucumis sativus L.) seedlings, while amiben solutions caused no growth reduction after 2 days' sunlight exposure. Dicamba solutions exposed to 60 C showed increased phytotoxicity while amiben solutions were unaffected. Dicamba was most phytotoxic to corn (Zea mays L.) seedlings at pH 4 and to cucumber seedlings at pH 5. Dicamba solutions were taken up by both shoots and roots of corn and cucumber seedlings; however, root uptake resulted in greater growth reductions.

Grain sorghum (Sorghum vulgare Pers.) was sprayed in 1966 and 1967 with 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine), propylene glycol butyl ether ester of 2,4-dichlorophenoxyacetic acid (2,4-D), alkanolamine salts of 2,4-D, and nonphytotoxic petroleum oil (crop oil) or their combinations when plant heights were 2 to 4, 4 to 6, and 6 to 8 inches for grass weeds, broadleaf weeds, and sorghum, respectively. Pollen mother cells from all herbicide-treated sorghum revealed chromosomal aberrations, mostly aneuploidy and polyploidy, and aberration frequencies increased when herbicides were combined. Added chromosomes were not always of the basic number. Atrazine-treated plots produced highest grain yields, kernel numbers were reflected in grain yield (r = .96), and untreated plants flowered first.

Common Kentucky bluegrass (Poa pratensis L.) sod-plugs were grown in the greenhouse in soil freshly treated with 11 herbicides and in soil collected from the 0 to 2 and 2 to 4-inch depths of field plots treated 10 months earlier with the same herbicides. Fresh herbicide treatments reduced significantly the root regrowth of the bluegrass plants. The 12-lb/A rate of the fresh application of 1-(2-methylcyclohexyl)-3-phenylurea (siduron) caused the least chemical inhibition of root regrowth. The most phytotoxic fresh applications were 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine) and 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine), both killing the bluegrass plants at the 4 lb /A rate. Root regrowth of bluegrass grown in the herbicide residue samples was significantly reduced for all treatments. Herbicides which tended to persist in the 0 to 2-inch soil level and were most phytotoxic to root regrowth were simazine, polychlorodicyclopentadiene isomers (hereinafter referred to as bandane), N-(2-mercaptoethyl)-benzenesulfonamide S-(O,O-diisopropyl phosphorodithioate) (bensulide), and 2,6-di-tert-butyl-p-tolyl-methylcarbamate (terbutol). Those herbicides which tended to leach into the 2 to 4-inch soil level and cause the greatest reduction of root regrowth were N-butyl-N-ethyl-alpha, alpha, alpha-trifluoro-2,6-dinitro-p-toluidine (benefin), α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin), atrazine, and O-(2,4-dichlorophenyl) O-methyl isopropylphosphoramidothioate (DMPA). Herbicides which tended to leach uniformly throughout the 0 to 2 and 2 to 4-inch soil layers were siduron, an equal mixture of 1,1-dimethyl-4,6-diisopropyl-5-indanyl ethyl ketone and 1,1-dimethyl-4,6-diisopropyl-7-indanyl ethyl ketone (hereinafter referred to as sin-done), and dimethyl 2,3,5,6-tetra-chloroterephthalate (DCPA).

A root bioassay was used to compare the adsorption of herbicides by activated carbon with that of muck soil, bentonite clay, a cation exchange resin, and an anion exchange resin. The effectiveness of different adsorbents was determined by comparing the concentrations of herbicide required to give 50% root inhibition of the test plant. Of eight herbicides tested, six were more strongly adsorbed by activated carbon than by any of the other adsorbents. The relative amount of adsorption by activated carbon as measured by the reduction in biological activity was as follows: isopropyl N-(3-chlorophenyl)-carbamate (CIPC) > α,α,α,trifluro-2,6-dinitro-N, N-dipropyl-p-toluidine (trifluralin) > 2,4-dichlorophenoxyacetic acid (2,4-D) > N,N-dimethyl-2,2-diphenylacetamide (diphenamid) > dimethyl 2,3,5,6-tetrachloroterephthalate (DCPA) > 4,6-dinitro-o-sec-butylphenol (DNBP) > 3-amino-2,5-dichlorobenzoic acid (amiben). The biological activity of 1,1'-dimethyl-4,4'-bipyridinium salt (paraquat), a cationic herbicide, was not reduced by activated carbon, but was reduced by bentonite clay and the cation exchange resin. DNBP was more strongly adsorbed by the anion exchange resin than by activated carbon. Desorption from activated carbon varied greatly for the herbicides tested. The most readily desorbed herbicide was 2,4-D while CIPC and DNBP showed little or no desorption.

We investigated the periodicity of germination, longevity of caryopses viability, and reproductive capacity of downy brome (Bromus tectorum L.) populations in relation to chemical and mechanical fallow and fall tillage weed control treatments. The important factors in the dynamics of downy brome populations are: (a) large numbers of viable caryopses are carried from 1 year to the next in the litter and soil; (b) downy brome caryopsis production is density dependent; and (c) the simultaneous germination characteristics of freshly harvested downy brome caryopses can be conditioned environmentally to continuous germination.

Twelve locations in the United States and Puerto Rico were the sites for determining the loss of 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine) and 2,3,6-trichlorophenylacetic acid (fenac) from soil. The herbicides were contained in tubes (1.88 by 6 inches) placed at depths of 3, 9, and 15 inches in the field. The samples were placed horizontally to minimize losses due to vertical movement of water from the tubes. After at least 3 months in the soil, the samples were returned to Beltsville and analyzed. Average recoveries showed 61% more atrazine and 41% more fenac from the 15-inch depth than from the 3-inch depth. Five northern samples contained more than twice as much atrazine and fenac residue as four southern samples. A positive correlation existed between fenac retention and soil organic matter content. Increasing soil organic matter and depth of placement, and decreasing temperature, tended to make the herbicides more persistent. However, the data were quite variable and the variations were often unexplainable.

Seedlings of 22 plant species were evaluated in aqueous suspensions of N-(2-mercaptoethyl)benzenesulfonamide S-(O,O-diisopropyl phosphorodithioate) (bensulide), dimethyl 2,3,5,6-tetrachloroterephthalate (DCPA), and 1-(2-methylcyclohexyl)-3-phenylurea (siduron) to determine their value as indicator plants for bioassays. The species showing the greatest sensitivity by reductions in root or shoot lengths were evaluated in soil mixtures to determine the level of sensitivity under soil conditions. These indicator plants, in most cases, proved reliable in detecting the presence and movement of bensulide, DCPA, and siduron in soils from field-treated plots. There appeared to be no accumulation in soil treated with four annual applications of bensulide, DCPA, or siduron.

Plots were planted to corn (Zea mays L., var. Mo. 880) and treated with 0, 2.5, 5.0, and 10 lb/A of 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine) or 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) in 1964 and in 1965. Soybean (Glycine max L., var. Clark 63) forage yields from soil samples collected in the spring, 1 year after application, were not affected by treatments of 5.0 lb/A or less of atrazine. Yields were reduced on soils collected in the fall (6 months) after application. Following 2 years of treatment, substantial reductions in yields of soybean forage occurred. Soybeans planted in the field following 2 consecutive years of herbicide treatment, yielded similarly to the control plots except when rates of 10 lb/A had been applied.

The influence of timing and method of application of sodium or disodium salts of methanearsonic acid (MAA) on cotton (Gossypium hirsutum L.) response, arsenic content of seed, and arsenic content of soils, was studied in field experiments in California, Arizona, and Mississippi. Methods of application consisted of directed and topical sprays. Directed sprays were applied to cover the small weeds in the drill row with a minimum of contact on cotton leaves more than 2 to 3 inches above the ground. Topical sprays were applied from nozzles positioned over the cotton in such a way that the entire cotton plant was subject to coverage. Rates of application were either 2 or 3 lb/A of the salts, and either two or three applications were made to treated plots. Cotton appeared highly tolerant to directed applications. A topical application to cotton 2 to 4 inches slightly reduced yields, but topical applications made at later stages of growth caused progressively severe reductions in yields and delayed maturity. The arsenic content of cottonseed from treated plots was not significantly higher than in that from untreated checks providing that applications were restricted to prebloom stages. Either directed or topical applications made during the blooming stage resulted in detectable increases of arsenic in the cottonseed. The effect of the treatments on arsenic content of soils was relatively small in comparison with natural levels, but the effect was detectable at one of the three locations.

Naturally-established individual specimens of 12 woody plant species occurring in North Carolina were treated with foliar, dormant stem, and basal applications of the propylene glycol butyl ether ester formulation of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), the propylene glycol butyl ether ester of 2,4-dichlorophenoxyacetic acid (2,4-D), and the potassium salt of 4-amino-3,5,6-trichloropicolinic acid (picloram). The triethanolamine salt of 2,4,5-T also was applied as a foliar treatment. Three rates of each herbicide were used. Responses measured were percent control of original shoots, percent regrowth, shoot height, and number of live stems per plant. No differences were observed between the amine and ester formulations of 2,4,5-T applied as foliar sprays except on rhododendron (Rhododendron maximum L.) where the ester produced 86% control compared to 28% for the amine salt. Foliar sprays of picloram were effective in controlling all species except white ash (Fraxinus americana L.), sweet gum (Liquidambar styraciflua L.), sweet bay (Magnolia virginiana L.), and rhododendron. Control of these species averaged only 45% compared to 96% on all other species studied. No species was effectively controlled by dormant stem or basal applications of picloram. All species considered, the propylene glycol butyl ether ester of 2,4,5-T applied as a dormant stem or basal application provided the most consistent results, giving 89% and 91% control, respectively.

During the planting of alfalfa (Medicago sativa L.), ethyl N,N-dipropylthiocarbamate (EPTC) was applied in subsurface bands beneath the legume seed and each of three triazine herbicides or EPTC was placed in subsurface bands between the rows. Triazines used were 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine), 2-chloro-4-ethylamino-6-isopropylamino-5-triazine (atrazine), and 2-isopropylamino-4-methoxyethylamino-6-methylmercapto-s-triazine (hereinafter referred to as G-36393). EPTC applied between and within the rows, as well as simazine between the rows and EPTC within the rows, gave superior weed control with little chronic effect on alfalfa. Alfalfa yields following both treatments were increased. Other treatments were unsatisfactory because of poor weed control or because of serious damage to alfalfa.

Light from sunlight, incandescent, and fluorescent sources was filtered through fresh tobacco (Nicotiana tabaccum L.), corn (Zea mays L.), and soybean (Glycine max (L.) Merrill) leaves. The spectral quality of the leaf-filtered light showed that much more of the incident red energy was absorbed than the far-red. The effect of the leaf-filtered light on the phytochrome-controlled germination of six weed species was generally to inhibit germination of seeds given a stimulatory pre-irradiation of red light from a standard source. Germination of seeds with no pre-irradiation was either not promoted or promoted to various degrees. Unfiltered light, at intensities equivalent to those under the leaf filters, caused no comparable effects. These results indicate an effect of altered spectral quality of the leaf-filtered light on the ratio of inactive/active phytochrome (Pr/Pfr) in the underlying seeds. We suggest that such phenomena could influence the germination of weed seeds in the field.

Phytotoxicity of five substituted urea herbicides 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron), 3-(p-chlorophenyl)-1,1-dimethylurea (monuron), 3-phenyl-1,1-dimethylurea (fenuron), 3-hexahydro-4,7-methanoindan-5-yl) −1,1-dimethylurea (norea), and 3-(m-trifluoromethylphenyl)-1,1-dimethylurea (fluometuron) at 0, 10, 100, and 1000 ppm were determined in factorial combination at four urea nitrogen levels of 0, 45, 450, and 900 ppm with three Aspergilli: A. niger, A. sydowi, and A. tamarii. Response interactions were apparent, with all three fungi most tolerant for fenuron and least for diuron. Apparent tolerance order of the three intermediates were: A. niger, norea > fluometuron > monuron; A. sydowi, fluometuron > monuron > norea; and A. tamarii, fluometuron > norea > monuron. Oat (Avena sativa L.) bioassay for residual herbicide toxicity indicated significant differences in herbicide degradation rates between these three fungi at 5, 10, and 20 ppm in Eufaula sand. Diuron was more rapidly degraded than monuron at these levels with fluometuron and norea somewhat intermediate. A. niger was most effective in degradation of these herbicides with A. tamarii greater than A. sydowi. High nitrogen levels in soil organic matter amendment generally favored increased rates of urea herbicide degradation.

Simazine (2-chloro-4,6-bis(ethylamino)-s-triazine) at 0.12 to 1.0 ppmw in nutrient cultures of common duckweed (Lemna minor L.), elodea (Elodea canadensis Michx.), and parrotfeather (Myriophyllum brasiliense Camb.) inhibited oxygen evolution within 24 hr. Of the plants studied, the submersed form of parrotfeather exhibited the greatest reduction in apparent photosynthesis as measured by dissolved oxygen in the water. Simazine in nutrient culture without plants remained relatively stable during the treatment period; a slight, but not significant, diminution of chemical was detected after 4 days. Each species, elodea or emersed parrotfeather, reduced the concentration of simazine in solution within 48 hr after treatment.

The range weed, Scotch thistle (Onopordum acanthium L.), is an increasing problem in the western United States. It competes with desirable forage plants and the flower stalks are a barrier to livestock movement. Prolific production of viable but dormant achenes makes this species a persistent pest. Applications of 4-amino-3,5,6-trichloropicolinic acid (picloram) in the spring or fall, at rates as low as 0.03 lb/A, gave control of Scotch thistle.

The longevity of brown and black forms of seed of halogeton (Halogeton glomeratus (M. Bieb.) C. A. Mey.), buried at four depths, was determined for a 10-year period in Nevada, Idaho, Utah, and Washington. The black form of seed had almost 100% field germination at all depths by the end of the first year and had completely germinated or was nonviable by the end of the second. Brown seed on the soil surface began field germination the first year at all locations and had germinated or become nonviable at the end of 6 years. Burial at 1, 3, and 6 inches delayed the peak of viability and tended to increase longevity of brown seed. Germination was less than 0.5% at the end of 10 years, and viability was near zero under both field and laboratory conditions. Seed germinated and lost viability most rapidly in Washington, the location where most favorable winter conditions for germination occurred.

We applied propylene glycol butyl ether esters of 2-(2,4,5-trichlorophenoxy)propionic acid (silvex) and 2,4-dichlorophenoxyacetic acid (2,4-D) to floating alligatorweed (Alternanthera philoxeroides (Mart.) Griseb.) and determined the level of carbohydrates in the underwater stems. The chemicals were applied at 4 and 8 lb/A on five application dates during a growing season at two sites. One month after initial application, the readily acid-hydrolyzable carbohydrates had been depleted by an average of 23.8% in a tidal area and 14.5% in a non-flowering area. Throughout the growing season, levels of carbohydrates were higher in a non-flowing area. The alligatorweed in the tidal area was more susceptible to herbicides. Regrowth from underwater nodes resulted in replenishment of the carbohydrates during the second month following treatment with 2,4-D. Carbohydrate levels remained low for 2 months following treatment with silvex, which resulted in more effective control of alligatorweed. Silvex at 8 lb/A, with retreatment after 2 months, provided the most effective control of floating alligatorweed.

Rates of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) up to 2.4 lb/A were applied as layby weed control treatments for cotton (Gossypium hirsutum L., var. Coker 100-A) in association with 0.0 or 1.0 lb/A of diuron as a preemergence treatment. Soybeans (Glycine max L. Merr., var. Lee) were grown as an alternate year rotational crop or as a rotational crop after cotton had been grown 3 years under diuron treatment. Similar rotational systems were evaluated in which corn (Zea mays L., var. Coker 71), tobacco (Nicotiana tabacum L., var. NC-95 or NC-2326), wheat (Triticum sativum Lam., var. Wakeland), cotton, and peanuts (Arachis hypogaea L., var. NC-2) served as the rotational crops in place of soybeans. In this mixed crop rotation, herbicidal treatments were 1.0 lb/A of diuron applied as a preemergence treatment plus 0.6 or 1.2 lb/A of diuron or 1.2 lb/A of 3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea (linuron) applied as a layby treatment. Two soil types near Rocky Mount, North Carolina were used. Herbicidal residues were determined by bioassay analyses in the fourth year of the experiment and by evaluating crops grown in the years when no herbicides were applied. Diuron residues were greater on Duplin soil than on Norfolk soil, but residues on Duplin soil were less phytotoxic to crops than those on Norfolk soil. The higher organic matter content of Duplin soil (1.2 vs 0.6%) is cited to account for both of these observations. When diuron was applied to Norfolk soil as a preemergence treatment at 1.0 lb/A plus 0.6 lb/A as a layby treatment, the residues frequently injured wheat, peanuts, and tobacco grown subsequently. Soybeans sometimes were injured by this treatment, but corn and cotton tolerated it even when the layby rate was increased to 1.2 lb/A. At equal layby rates, linuron produced residues of less consequence than diuron. At the higher application rates, greater residues occurred following 3 years of application than where application had been on an alternate year basis.

Applications of 2-methoxy-3,6-dichlorobenzoic acid (dicamba) and 2,4-dichlorophenoxyacetic acid (2,4-D) controlled top growth the year of treatment, but western ironweed (Vernonia baldwini Torr.) recovered in the following year. One-half lb/A or more of 4-amino-3,5,6-trichloropicolinic acid (picloram) effectively controlled top growth and destroyed roots and rhizomes. Treatment with 2 lb/A of picloram resulted in ironweed-free plots by the fourth season after treatment.