Soil columns and soil thin-layer chromatography were used to evaluate the mobility of napropamide [2-(α-naphthoxy)-N,N,-diethylpropionamide] in various soils. The surface-applied herbicide did not move deeper than approximately 6 cm in a Teller sandy loam soil after a water application of 10.2 cm. The Rf values for napropamide and two reference herbicides were in the order of fluometuron [1,1-dimethyl-3-(α,α,α-trifluoro-m-tolyl)urea] > napropamide > terbutryn [2-(tert-butylamino)-4-(ethylamino)-6-(methylthio)-s-triazine]. The mobility of each herbicide was reduced with an increase in clay and organic matter content. Carbon-14 ring labeled napropamide was used to determine the adsorption and desorption characteristics of the herbicide in various soils. The Rf values obtained with napropamide and each soil agreed with the adsorptive characteristics. Small applications of a muck soil to a sand (2%, w/w) significantly increased herbicide adsorption and decreased herbicide desorption.

Triallate [S-(2,3,3-trichloroallyl)diisopropylthiocarbamate] mixed with soil (1.5 ppmw) stimulated coleoptile extension, reduced the length of the primary leaf, but had no effect on mesocotyl development of wild oat (Avena fatua L.) grown in soil in a growth chamber. Incubation of the triallate-treated soil for as long as 20 weeks at constant temperatures (10 C, 16 C, and 20 C) did not reduce phytotoxicity. Triallate phytotoxicity was similar at soil moisture levels of either 5% greater than wilting point or 5% less than field capacity. The phytotoxicity remained the same after a freezing cycle during an 8-week soil incubation. Phytotoxic effects of triallate were markedly reduced as soil organic matter increased.

We investigated germination enhancement of downy brome (Bromus tectorum L.) seeds (caryopses) recovered from herbicide fallows in degraded big sagebrush (Artemisia tridentata Nutt.) communities. Numbers of germinable downy brome seeds were increased most by adding 0.1 mmole potassium nitrate (KNO3) or combinations of 0.1 mmole KNO3 and 0.14 mmole gibberellic acid (GA3). Germinability was enhanced in the fall and spring but not in midwinter when dormancy was greatest. Improved downy brome control and better establishment of seeded forage species with a fallow technique in sagebrush-dominated communities look promising if these growth regulators can be effectively applied in the field. Without germination enhancement, many germinable seeds of downy brome are present in litter under shrub canopies at the end of the fallow period. With near-optimal microenvironment under shrub canopies for plant growth, severe competition between downy brome and the seeded forage species can result.

The growth of germinating wild oat seedlings (Avena fatua L.) in soil treated with a range of concentrations of the individual isomers of diallate [S-(2,3-dichloroallyl)diisopropylthiocarbamate] and mixtures thereof has been studied. on the basis of diallate concentration required to reduce height by 50%, the cis isomer is less effective than the trans isomer by about 65%. A mixture of 42% trans and 58% cis diallate is shown to exhibit an antagonistic effect of about 95%. The trans isomer is more effective than the 42/58 mixture by a factor of 2.6. It is shown that height vs concentration response curves can be represented by y = A {(x/C)B + 1}-1, where y = height and x = concentration. A computer program has been developed to find the best parameters A, B, and C from any set of individual response data. A new technique for calculating synergism or antagonism has been employed in the part dealing with mixtures of diallate.

A new urea compound, 1-(α,α-dimethylbenzyl)-3-p-tolylurea (hereinafter referred to as K-223) is characterized by its selective control of cyperaceous weeds. This compound does not show any Hill reaction inhibition or foliar contact toxicity. Its main effect is toxicity to roots. K-223 selectively controls slender spikerush [Eleocharis acicularis (L.) R. & S.], purple nutsedge (Cyperus rotundus L.) and annual cyperaceous weeds. It is practically nontoxic to most broadleaf weeds. As the selectivity between crops and grass weeds is high, K-223 is applicable with little phytotoxicity. Thus sowing or transplanting is possible even just after application of K-223.

Rates of trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) applied preplanting were combined with rates of diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] applied postemergence in cotton (Gossypium hirsutum L. ‘Deltapine 16’). In another experiment, two levels of these herbicides were applied in three populations of cotton. All herbicide combinations improved weed control, compared to that of cultivated checks. Applications of 0.28, 0.42, and 0.56 kg/ha of trifluralin combined with 0.56 kg/ha of diuron controlled annual weeds and produced the highest cotton yields. Herbicide treatments had more influence than crop spacing on weed control and crop yield. Weed competition on cultivated checks affected boll weight, seed per boll, and fiber fineness.

Canada thistle [Cirsium arvense (L.) Scop.] and field bindweed (Convolvulus arvensis L.) were controlled in asparagus (Asparagus officinalis L. ‘Mary Washington’) fields by repeated applications of dicamba (3,6-dichloro-o-anisic acid) at 0.6 kg/ha or 2,4-D [(2,4-dichlorophenoxy)acetic acid] + dicamba at 1.1 + 0.3 or 1.1 + 0.6 kg/ha. Applications of 2,4-D + dicamba at 1.1 + 0.6 kg/ha in early May and again in mid-June controlled 97% of the Canada thistle in asparagus fields. A third application about August 1 was required to give similar control of field bindweed. Rates of dicamba exceeding 0.6 kg/ha injured the asparagus.

Specially constructed runoff plots were used to study the effect of simulated rainfall intensity, antecedent soil moisture, and subsequent rainfall on prometryn [2,4-bis(isopropylamino)-6-methylthio-s-triazine] movement across and through a field soil with a 1% slope. The first cm (45.4 L) of runoff was collected and subdivided. The initial 3.8 L of runoff water generally contained a higher concentration of prometryn than did a composite from the next 41.6 L. The sediment contained a higher prometryn concentration than did the runoff water. However, due to the greater volume of water lost compared to sediment, over 90% of the prometryn lost was in the water fraction. When prometryn was applied to a dry soil and rainfall simulated, runoff losses of prometryn were 0.5% or less of the total amount initially applied. The first runoff producing simulated rainfall caused the largest prometryn losses, but prometryn could not be detected in the runoff 1 month subsequent to application. Prometryn was never detected at soil depths greater than 5 cm. Prometryn runoff was greater from plots in which the soil was wet at the time of application.

We investigated seasonal variation in germinability of seed reserves in a degraded big sagebrush (Artemisia tridentata Nutt.) community. A bioassay was employed to determine germinability. There were many more germinable seeds and seedling establishment was greater under the shrub canopies than in the interspace. Caryopses of downy brome (Bromus tectorum L.) were the most abundant of any species in the litter where most of the seeds were located. Seasonal germinability of downy brome caryopses depends on the environment of the seedbed when the samples were collected and on the incidence of environmentally-induced dormancy or on loss of dormancy.

We studied the early root growth of four Setaria taxa: giant foxtail (Setaria faberi Herrm.), giant green foxtail [Setaria viridis var. major (Gaud.) Posp.], robust white foxtail (Setaria viridis var. robusta-alba Schreiber), robust purple foxtail (Setaria viridis var. robusta-purpurea Schreiber). Growth studies in controlled environments showed significant differences in root elongation among the taxa at three photoperiods. Seminal root lengths after 4 days followed the order presented for selectivity and metabolism of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] and propazine [2-chloro-4,6-bis(isopropylamino)-s-triazine] (robust white foxtail > giant green foxtail = robust purple foxtail > giant foxtail). Giant foxtail had the greatest root diameter, resulting in the greatest surface area and volume when lengths were equated. The order of seminal root lengths or diameters changed little after 7 days. Robust white foxtail had the most and longest first order lateral roots. Diameter of first order laterals showed giant foxtail > giant green foxtail = robust purple foxtail > robust white foxtail.

Atrazine [2-chloro-4-(ethylamino)-6-(isopropyl-amino)-s-triazine] applied to the soil of potted tomatoes (Lycopersicon esculentum Mill. ‘John Baer’) reached its maximum concentration in the xylem exudate collected from the stumps of detopped plants in 6 to 8 hr but continued to accumulate in the petioles for 36 hr. An atrazine concentration of 3.8 μg/g fresh weight of petiole at 12 hr after treatment was approximately the same as the maximum concentration of atrazine in the xylem exudate indicating that atrazine had accumulated extensively in tissues outside of the xylem vessels.

Picloram (4-amino-3,5,6-trichloropicolinic acid) herbicidal activity on seedlings of guava (Psidium guajava L.), strawberry guava (Psidium cattleianum Sabine), and dwarf beans (Phaseolus vulgaris L. ‘Bountiful’) was increased by adding ammonium sulfate to the spray solution. Picloram activity tended to increase with increasing ammonium sulfate concentration up to 10% (w/v), but by the end of each experiment ammonium sulfate concentrations above 0.5% were generally equally effective. The absorption of 14C-picloram by strawberry guava leaves was increased about five-fold by 0.5% and 10% ammonium sulfate. There was about four times more 14C-picloram in the upper stem and attached leaves when ammonium sulfate was added than with picloram alone. Ammonium sulfate at 0.5% increased picloram absorption by four-fold in guava, but there was no increase in the amount of picloram translocated acropetally. Picloram absorption by dwarf beans, measured by difference between the amount of picloram applied and the amount recovered, was increased by 0.5% and 10% ammonium sulfate by a similar magnitude as on guava species.

Several methods of evaluating plant responses to combinations of herbicides are discussed. They include: a direct-plot of percent response vs toxic units; determining ID50 (dosage that causes a 50% reduction in growth of the treated plant) for each herbicide and for each combination; percent response vs log-dosage; double reciprocal procedures; probit analysis, and calculation of expected values. A simplified version of the isobole method of Tammes (23) is presented as a method which can be readily applied by weed scientists to determine the validity of data on plant responses to herbicide combinations. Definitions are proposed which classify plant responses to herbicide interactions as additive, synergistic, antagonistic, or enhancement. Four points are listed that will improve the quality of data obtained from experiments on combinations.

The interaction of 2,4-D [(2,4-dichlorophenoxy) acetic acid], substrate, and calcium on eurasian watermilfoil (Myriophyllum spicatum L.) was determined. The effectiveness of 2,4-D was twice as great on plants grown on a substrate containing soil as it was on plants grown on a sand substrate. The presence of calcium chloride in sand culture during the period of 2,4-D uptake caused comparable or greater increases than sand culture without calcium in the effectiveness of 2,4-D. Possible differences in effectiveness of field treatments were calculated to be 48% greater at the highest natural concentrations of calcium than at the lowest concentrations in water of the Tennessee Valley. Calcium concentration and substrate altered the effectiveness of 2,4-D on eurasian watermilfoil.

Selected herbicides were evaluated for control of large crabgrass [Digitaria sanguinalis (L.) Scop.] and goosegrass [Eleusine indica (L.) Gaertn.] in planted ‘Tifway’ bermudagrass (Cynodon sp.). The most effective treatment technique was sequential treatments of DCPA (dimethyl tetrachloroterephthalate) at planting followed with two applications of MSMA (monosodium methanearsonate) + 2,4-D [(2,4-dichlorophenoxy) acetic acid] which controlled 95% of the weeds and permitted bermudagrass to form a 96% ground cover in 22 weeks. Other treatments, listed in order of decreasing effectiveness were a single application of oxadiazon [2-tert-butyl-4-(2,4-dichloro-5-isopropoxyphenyl)-δ2-1,3,4-oxadiazolin-5-one], pronamide [3,5-dichloro-N-(1,1-dimethyl-2-propynyl)-benzamide] applied in two applications and MSMA + 2,4-D in two applications.

EPTC (S-ethyl dipropylthiocarbamate) (33 μM) and diallate [S-(2,3-dichloroallyl)diisopropylthiocarbamate] (90 μM) inhibited the incorporation of 6 mM acetate-2-14 C (Ac∗) by 80% and 65%, respectively, and the incorporation of 0.5 μM malonate-2-14C (Mal∗) by 32% and 26%, respectively, into the lipids of spinach (Spinacia oleracea L.) chloroplasts. The inhibition of Ac∗ or Mal∗ incorporation into lipids was not observed in the presence of excess Ac∗ or Mal∗, respectively. Incorporation of palmitate-1-14C and oleate-1-14C into chloroplast lipids was inhibited by EPTC and diallate. Mal∗ incorporation into dienoic fatty acids was inhibited by EPTC and diallate. The concentration of EPTC and diallate inhibiting lipid synthesis falls into the physiological range of these herbicides, explains some metabolic effects of these compounds, and fits as the mode of activity of these herbicides.

The technique of microautoradiography was used to study the localization of 14C-SAN 6706 [4-chloro-5-(dimethylamino)-2-(α,α,α-trifluoro-m-tolyl)-3(2H-pyridazinone] in cotton (Gossypium hirsutum L. ‘Coker 203’), soybean [Glycine max (L.) Merr. ‘Lee’], and corn (Zea mays L. ‘W.F. 9’). SAN 6706 was removed from the treatment solutions by absorption and also by adsorption onto the surfaces of roots. Absorbed SAN 6706 appeared to move primarily in the apoplast and to be translocated acropetally and laterally in the transpiration stream. The herbicide accumulated in striking concentrations in the lysigenous glands and trichomes of the cotton plant. This accumulation is postulated to be a major factor in lowering the effective concentration of SAN 6706 in the leaves of cotton as compared to the leaves of the more susceptible soybean and corn plants, and thus may be a significant factor in the tolerance of cotton to SAN 6706.

Cotyledons of 6-day old etiolated cucumber (Cucumis sativus L. ‘Alpha Green’) seedlings treated on the second day with EPTC (S-ethyl dipropylthiocarbamate) accumulated 35 to 45% more protochlorophyll and 30 to 40% more glycolipids than the controls. on illumination the treated cotyledons accumulated chlorophyll faster than the controls. Exposure of cucumber cotyledons to a variety of amide, carbamate, and other thiocarbamate herbicides did not produce a similar effect. EPTC, was found to cause an increase in chlorophyll accumulation.

Peanut (Arachis hypogaea L. ‘Tifspan’ or ‘Florunner’) yields were not reduced when the crop was maintained free of Florida beggarweed [Desmodium tortuosum (Sw.) DC.] or sicklepod (Cassia obtusifolia L.) for 4 weeks after crop emergence and when vigorous crop growth was maintained for the remainder of the season. Sometimes weed-free maintenance for only 2 weeks resulted in near-normal yields, which indicated that the canopy of peanut leaves effectively suppressed the weeds. Conversely, these weeds had to compete with peanuts for more than 10 weeks before crop yield was reduced. Weeds that overtopped the peanuts at harvest emerged during the first 6 weeks after planting.