Non-lethal dosages of 2,4-dichlorophenoxyacetic acid (2,4-D), which stunted pigweed plants, markedly reduced transpiration. Lethal dosages of 2,4-D, 2,2-dichloropropionic acid (dalapon), or toxic oil reduced transpiration rapidly, but transpiration continued until treated plants were air dry. Large plants died slower than small plants after herbicide treatment; consequently, they transpired more water before death.

The initial and residual toxicities, after 13, 22, and 31 weeks, of dimethyl 2,3,5,6-tetrachloroterephthalate (DCPA), 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron), 3-(hexahydro-4,7-methanoindan-5-yl)-1,1-dimethylurea (norea), 2,4-bis(isopropylamino)-6-methylmercapto-s-triazine (prometryne), and α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) were determined in the greenhouse in four soil types by an oats bioassay. DCPA caused the least oat injury initially, and also after 13 weeks. None of the herbicides persisted in amounts toxic to oats for as long as 22 weeks when applied at concentrations approximately equivalent to field rates necessary for weed control. The residual toxicities of these same herbicides, after 22 weeks or longer, were also determined in Bosket silt loam collected from two depths in two preplanting subsurface and two preemergence surface field experiments by using oats, cotton and / or soybeans as bioassay indicators. Results from field applications at rates recommended for weed control in cotton confirmed that neither DCPA, diuron, norea, nor trifluralin persisted in amounts toxic to oats, soybeans, or cotton; however, prometryne residues were moderately toxic to oats and soybeans. Prometyrne residues from a 4 lb/A non-incorporated subsurface treatment were nearly twice as toxic to oats 25 weeks after application, as were residues from the same amount applied as a surface preemergence treatment.

In studies of the effect of root temperature on the toxicity of 2,4-dichlorophenoxyacetic acid (2,4-D) to rubber rabbitbrush (Chrysothamnus nauseosus (Pall.) Britton), 75 F resulted in the highest mortality following 2,4-D treatment. Root temperatures had no effect on leaf cuticle thickness. In similar studies with Canada thistle (Cirsium arvense (L) Scop.) and Russian knapweed (Centaurea repens L.), little injury resulted from 2,4-D treatment at 45 F whereas at 90 F in Canada thistle and at 60, 75, and 90 F in Russian knapweed, severe injury and/or death resulted.

Barnyardgrass (Echinochloa crusgalli (L.) Beauv.) emerged in fields as early as May 3. The shortest period between seeding and emergence and between emergence and tillering was during the warmest period of the growing season. The number of days from emergence to heading and maturity decreased progressively for later seedings. The height of plants was greatest when the plants emerged early in the growing season. The highest number of tiller and heads per tiller were produced by plants emerging in July. Plants which emerged in May and June produced the highest yields. Barnyardgrass seedlings grown under 11-hr photoperiod and 16-hr photoperiod produced mature seeds in 52 and 84 days, respectively. Barnyardgrass grew best under wet soil moisture conditions.

Ethyl N,N-dipropylthiolcarbamate [EPTC] inhibited the deposition of external foliage wax of cabbage (Brassica oleracea L. var. capitata, L.). Wax inhibition was correlated directly with the rate of application of EPTC. Differential inhibition of wax deposition showed that foliage waxes were important in inhibiting the penetration and absorption of the alkanolamine salts of 4,6-dinitro-o-sec-butylphenol [DNBP]. Several other thiolcarbamates also altered the susceptibility of treated plants to subsequently applied herbicides. When EPTC was applied as a spray, only leaves then in the bud were affected. Leaf primordia developed later were not affected. Granular applications of EPTC extended the period during which the formation of foliage wax was inhibited. The reduction of foliage wax resulted in increased spray retention and decreased contact angle of spray droplets. EPTC increased the transpiration rate proportionally to its rate of application.

The response of four growth stages of Setaria glauca (L.) Beauv. (yellow foxtail), Eleusine indica (L.) Gaertn. (goosegrass), and Dactyloctenium aegyptium (L.) Richt (crowfootgrass) to dimethylarsinic acid (cacodylic acid) applied at rates of 0.25, 0.5, and 1.0 lb/A was studied.

The four growth stages for chemical treatments were generally: (a) early tillering, (b) late tillering, (c) boot, and (d) post-heading. Seed production was reduced by as much as 96% in yellow foxtail, 80% in goosegrass, and 37% in crowfootgrass. Application of the chemical at the “boot” stage was most effective although earlier applications also were effective. Inflorescences often were malformed. Other growth effects suggested a growth regulator type activity.

Some factors affecting the absorption and translocation of radioactive 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in blackjack oak (Quercus marilandica Muench.) were studied. Absorption of 2,4,5-T was less in plants treated late in the growing season than in those treated earlier. Absorption and translocation of 2,4,5-T were inversely related to soil moisture stress. Increases in the rate of application from 10 to 50 μg of 2,4,5-T per plant caused a decrease in absorption and translocation, but decrease of the latter did not occur until the rate was increased above 25 μg/plant. Translocation was highest in June and was higher from the leaves of mature plants than from young regrowth.

The relationship between susceptibility of certain plant species to 4,6-dinitro-o-sec-butylphenol (DNBP) and their capacity to accumulate externally supplied P32, and to form ATP in leaf disks was studied. A correlation was found between susceptibility of the species to DNBP and their capacity for P32 accumulation and ATP synthesis. Therefore, it was postulated that susceptibility depends upon the level of ATP in the tissues.

Soil samples were taken from 32 commercial cotton fields of the mid-south which had been treated with one or more practical applications of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron)2 every year for the past 4 to 8 years. These samples showed no phytotoxic residues in laboratory and greenhouse studies conducted approximately one year after the last application. Cotton seed samples from eight of these fields were analyzed, and none contained a detectable residue of diuron or any of its breakdown products. Diuron degradation in soil under field conditions was demonstrated by isolation and identification of the intermediate breakdown products: 3-(3,4-dichlorophenyl)-1-methylurea, 1-(3,4-dichlorophenyl)urea, and 3,4-dichloroaniline.

Herbicides were evaluated for weed control in seedling perennial grass stands at six locations in eastern Washington. Most annual weeds commonly found in grass seed fields were controlled by 2-methoxy-3,6-dichlorobenzoic acid (dicamba), 2,3,6-trichlorobenzoic acid (2,3,6-TBA), and 1-butyl-3-(3,4-dichlorophenyl)-1-methylurea (neburon). Dicamba was the most effective herbicide for annual weed control but caused some injury to Kentucky blue-grass seedlings. Seedling grasses injured by dicamba recovered and became established in each trial. Grasses were injured about the same extent when treated with 2,3,6-TBA as when treated with dicamba, but 2,3,6-TBA did not control as many weed species. Neburon did not control as many species as dicamba and 2,3,6-TBA but was effective in killing a large number of species without measurable injury to grass seedlings.

Detoxication of 3,6-dichloro-o-anisic acid (dicamba) in incubated soils was greater in heavier soil types and soils with higher moisture contents and higher temperatures. Dicamba phytotoxicity was less at 21 C than at 27 or 32 C. Over a pH range of 4.1 to 9.4, dicamba had a net negative charge. Dicamba showed considerable adsorption onto a kaolinite clay but little adsorption onto other clays and soils studied. Dicamba-1-C14 mixed with autoclaved Sharpsburg silty clay loam soil and incubated at 0, 33, 75, and 100% relative humidity and 35 C sustained very little volatilization loss over an 8-week period. However, nearly one-half of the radioactivity of dicamba-1-C14 on bare planchets at 0 and 100% relative humidity and 35 C was lost during an 11-week incubation period.

In soil enrichment studies, the microbial degradation of 2,2-dichloropropionic acid (dalapon) was inhibited in the presence of 3-amino-1,2,4-triazole (amitrole), but dalapon had little or no effect on behavior of amitrole. Phytotoxic residues of the herbicides persisted longer in soils when the herbicides were applied in combination than when each was used individually. Although amitrole disappeared rapidly from all amitrole-treated soils, dalapon, when applied in combination with amitrole, disappeared more slowly than when used alone. Dalapon did not affect the behavior of amitrole in muck soil, but affected the availability of amitrole in Hagerstown silty clay loam treated with high rates of amitrole.

Cotton plants were treated with 2,4-D-2-C-14 (2,4-dichlorophenoxyacetic acid) applied with pentachlorophenol and arsenic acid. Seed kernels and regrowth leaves were extracted and various components of the extracts were applied to leaf surfaces and injected into the stems of cotton seedlings. Results indicated that 2,4-D itself and/or a metabolite of 2,4-D, were responsible for the persistence of the 2,4-D stimulus in cotton.

N-1-naphthylthalamic acid (NPA), 3-amino-2,5-dichlorobenzoic acid (amiben), and the dimethyl ester of 2,3,5,6-tetrachloroterephthalic acid (DCPA), in granular and liquid or wettable powder formulations, were applied to the soil and then activated by either overhead or sub-irrigation.

Granular formulations of the herbicides were as effective as liquid formulations for weed control, providing there was ample moisture for activation. High temperatures and dry soils were conducive to losses by volatilization of NPA. DCPA underwent less loss by volatilization at high temperatures. Both overhead and sub-irrigation were effective in activating these herbicides, but overhead generally was more effective. Under sub-irrigation, use of a granular carrier was more effective with a volatile herbicide. Formulation differences decreased in importance under overhead irrigation. Under conditions of high temperatures and soil moisture, soil persistence of the herbicides varied inversely with their volatility.

The compounds 3-(p-chlorophenyl-1,1-dimethylurea (monuron), 5-bromo-3-sec-butyl-6-methyluracil (bromacil), 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine), 2-chloro-4,6-bis (ethylamino)-s-triazine (simazine), 1,1′-dimethyl-4,4′-dipyridylium salt (paraquat), and 3-amino-1,2,4-triazole (amitrole) were added to a complete nutrient medium on which chlorotic Nicotiana tobacum was grown in the dark. Herbicide concentrations generally were 0, 10-4, 10-5, 10-6, and 10-7 molar. Callus growth was inhibited by molar concentrations greater than 10-7 for the triazines, 10-5 for monuron, 10-4 for bromacil, 10-6 for paraquat and amitrole. At these concentrations, interference with a mechanism other than photosynthesis was responsible for inhibition of growth.

Glasshouse and field experiments were conducted on fine sandy loam, sandy clay loam, and clay to study under high temperatures 2-chloroallyl diethyldithiocarbamate (CDEC) and ethyl N,N-dipropylthiolcarbamate (EPTC) incorporated with the surface ½, 1, 2, and 3 in of soil and unincorporated. Herbicides, sprayed on the soil surface, were incorporated either manually in the glasshouse or with a powered, rotary tiller in the field. The principal weed species were Palmer amaranth (Amaranthus palmeri S. Wats.), redroot pigweed (Amaranthus retroflexus L.), junglerice (Echinochloa colonum (L.) Link), and barnyardgrass (Echinochloa crusgalli (L.) Beauv.). Performances of herbicides were determined by counting the plants 2 weeks after treatment.

The herbicidal performance of CDEC was greater when the applications were incorporated with the surface ½ or 1 in whereas the performance of EPTC increased with increasing depths of incorporation up to 3 in. Soil temperature, soil moisture losses, and the distribution of herbicide in soil apparently contributed to differential herbicide performance.

Foliar absorption of carboxyl-labeled 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) by mesquite [Prosopis juliflora var. glandulosa (Torr.) Cockerel] seedlings was continuous throughout a 72-hr period. Approximately 50% of the 2,4,5-T applied to a single leaf was absorbed when 5 to 100 μg were applied. No significant differences were found in the amounts of 2,4,5-T absorbed at 70 and 85 F, but an increase occurred at 100 F after 72 hr. Only slight differences in absorption were found at different humidity levels. Translocation was primarily basipetal from the point of application at 70 F, both acropetal and basipetal at 85 F, and only a short distance acropetal at 100 F. The quantities of 2,4,5-T translocated into untreated tissues at 100 F were less than at 70 and 85 F. The highest concentrations of 2,4,5-T were found in tissues with highest soluble sugar concentrations. From 3 to 27% of the 2,4,5-T absorbed by mesquite leaves was subsequently detected in untreated stem, leaf, and root tissues.

Approximately 80% of the 2,4,5-T absorbed by mesquite leaves was metabolized after 24 hr. Moving the seedlings to a different environment at the time of treatment increased the rate of 2,4,5-T metabolism. Metabolism of 2,4,5-T was completely inhibited at 50 F and a lower rate of metabolism was noted at 100 than at 70 and 85 F. No important differences in metabolism were found at different humidity levels.

Gypsophila paniculata L., a dicotyledonous, herbaceous member of the Caryophyllaceae, was introduced into North America from Eurasia as a garden ornamental and has escaped to become troublesome in some areas. A life history study has revealed that this species is spread entirely by seed, its tumbleweed character being an important factor. Germination occurs mainly in the spring in Saskatchewan stands, perhaps as a result of no dormancy in the seeds. Although seedling development is characterized by a rapid penetration of the root system, mortality in natural stands is high. Under favorable conditions, at least 2 yr are required for the production of floral primordia. The mature plant is characterized by a root system which penetrates to depths of 4 meters or more. Shoots arise each year from a thick woody caudex.

When 2-(2,4,5-trichlorophenoxy)propionic acid-1-C14 (silvex) was applied to Opuntia polyacantha Haw. (prickly pear), pad absorption, root absorption, and translocation were low. The rate of translocation, as measured by acid fuchsin movement, was one cm per hour, about one tenth that found in soybean. Increasing the temperature from 21 to 32 C only slightly increased root absorption and upward translocation of silvex. Pre-treatment of a prickly pear pad, either with a 4% 2,2-dichloropropionic acid solution or mechanical removal of the cuticle, increased the absorption and translocation of subsequently applied silvex. The decarboxylation of silvex was 2 to 3 times greater in soybean than in prickly pear plants. Sixteen days after silvex was applied to prickly pear, four C14-metabolites were found in addition to unaltered silvex.

The loss of 3-(p-chlorophenyl)-1,1-dimethylurea (monuron) and 1-butyl-3-(3,4-dichlorophenyl)-1-methylurea (neburon) from an aquatic situation was studied. Appreciable loss of herbicides did not occur until 8 and 16 weeks after application of neburon and monuron, respectively. Periods of loss at an accelerated rate followed until approximately 50% of the herbicides disappeared. Further loss was observed but at a steadily decreasing rate. Approximately 15% of the monuron and 39% of the neburon remained after 128 weeks. More monuron was found in water than in soil during the first 32 weeks. After 32 weeks, most of the monuron was found in the soil and was largely in the top 4 in. The largest fraction of neburon was recovered from the soil and soil surface during all 128 weeks. Monuron moved down in the soil more readily than neburon.