Experiments were conducted in growth chambers by growing oats (Avena sativa L. ‘Ansi’) in water culture and soil with admixed TCA (trichloroacetic acid), pyrazon [5-amino-4-chloro-2-phenyl-3-(2H)-pyridazinone], and two mixtures (pyrazon: less TCA; 1:25 and 1:50). The sigmoid log-dose relationship for dry-matter yield was fitted with a four-parameter logistic equation. Generally, the mixtures appeared to be less active than would be expected under an additive dose model. Around GR50 the departure of joint action of the mixtures from the reference model was almost similar in the two growth media. At higher dose levels the mixtures approached the reference model, and the bioactivity of one mixture (1:25) in the water culture experiment even showed a higher activity than would be expected from the reference model.

Witchweed [Striga asiatica (L.) Kuntze] seeds conditioned for 3 weeks in either water or 0.01% (w/v) sodium hypochlorite (NaOCl) had 83 to 95% germination after being treated with dl-strigol, kinetin, zeatin, or natural stimulants from corn (Zea mays L.) root exudates. None of these compounds, however, induced germination of seeds conditioned in 0.025 or 0.050% (w/v) NaOCl. Seeds conditioned in 0.075, 0.1, or 0.15% (w/v) NaOCl had 8, 56, and 76% germination, respectively, during the first week of conditioning and 10, 100, and 100% germination after 2 weeks of conditioning. Germination in these concentrations of NaOCl was termed “atypical”.

Differential tolerances to dalapon (2,2-dichloropropionic acid), amitrole (3-amino-s-triazole) plus ammonium thiocyanate, and glyphosate [N-(phosphonomethyl)glycine] of 3- to 10-week-old seedlings of creeping red fescue (Festuca rubra L. ‘Boreal’) and redtop (Agrostis alba L.), which are desirable ditchbank grasses, and reed canarygrass (Phalaris arundianacea L.), a troublesome ditchbank weed, were evaluated in the field. Within each of four treatment dates (growth stages), dalapon affected all species similarly; effects among treatment dates ranged from none to severe. Amitrole at 4.5 kg/ha plus ammonium thiocyanate at 4.1 kg/ha reduced the stand of 3-week-old seedlings of redtop, creeping red fescue, and reed canarygrass by 0, 42, and 94%, respectively, but had little effect on older seedlings. Glyphosate at 1.1 kg/ha did not significantly reduce the stand of 5- to 10-week-old creeping red fescue seedlings, but killed nearly all reed canarygrass seedlings. Redtop was intermediate in its tolerance to glyphosate. Applications of amitrole plus ammonium thiocyanate or glyphosate should be effective in converting ditchbank vegetation from an undesirable stand of reed canarygrass to a desirable stand of redtop and creeping red fescue, respectively.

Seed samples of triazine-resistant and susceptible pigweed were received from the four areas of the United States and Canada where resistant pigweed populations have appeared. The reported resistance or susceptibility of all biotypes was verified, and several plants from each seed source were grown to maturity for species identification. Pigweed samples from Maryland and Pennsylvania were smooth pigweed (Amaranthus hybridus L.). One resistant and one susceptible biotype from Washington were also smooth pigweed. All other samples from Washington and the sample from Ontario were Powell amaranth (A. powellii S. Wats.). None of the triazine-resistant seed samples were redroot pigweed (A. retroflexus L.), despite earlier reports of resistance in this species. Taxonomic characteristics important in the identification of smooth pigweed, Powell amaranth, and redroot pigweed are discussed.

To study rainfall effects, simulated rainfall was applied to velvetleaf (Abutilon theophrasti Medic.), common lambsquarters (Chenopodium album L.), wild mustard [Brassica kaber (DC.) L. C. Wheeler var. pinnatifida (Stokes) L. C. Wheeler], soybean [Glycine max (L.) Merr. ‘Hodgson’], and redroot pigweed (Amaranthus retroflexus L.) in greenhouse and field studies following foliar applications of the alkanolamine (AKA) salt or the butoxyethanol (BE) ester of 2,4-D [(2,4-dichlorophenoxy)acetic acid] at rates that induced equivalent levels of phytotoxicity. Simulated rainfall less than 1 min after herbicide treatment reduced the phytotoxicity of the AKA salt of 2,4-D to a much greater extent than that of the BE ester with effects ranging from elimination of all injury from the AKA salt to soybeans to no reduction in phytotoxicity of the BE ester to common lambsquarters. The quantity of simulated rainfall required to induce maximum reductions in phytotoxicity of the BE ester ranged from 1 mm on common lambsquarters to 15 mm on velvetleaf. The time interval from 2,4-D treatment until rainfall required to achieve a phytotoxic response level of 80% of that attained without rainfall varied greatly among plant species and herbicide formulations; ranging from less than 1 min for the BE ester on common lambsquarters to more than 24 h for the AKA salt on velvetleaf. The addition of an alkylarylpolyoxyethylene glycol surfactant to 2,4-D spray solutions reduced herbicide rates required to induce equivalent levels of phytotoxicity, increased losses in phytotoxicity of the BE ester caused by rainfall, and reduced the time interval from treatment to rainfall required to attain an equivalent level of phytotoxicity with the AKA salt.

The conditioning and germination processes of witchweed [Striga asiatica (L.) Kuntze] seeds were regulated by synthetic dl-strigol (ST) or natural stimulants (STM) present in a corn (Zea mays L.) root-exudate solution. Seeds conditioned in water for 7 and 14 days required 10−10 and 10−12M ST, respectively, to induce maximum germination (ca. 80%). Surprisingly, conditioning seeds in ST or STM had an adverse effect on the responsiveness of the seeds to the same compounds after conditioning. Compared to seeds conditioned in water, these ST- or STM-conditioned seeds required not only higher concentrations of ST or STM to induce maximum germination, but also longer immersion time to become conditioned. ST or STM inhibits conditioning and thus the regulation of germination by ST or STM depends mainly on the stage of conditioning of seeds when they are exposed to ST or STM. ST or STM stimulates germination only after a pre-germination threshold is reached during conditioning. Exposure of seeds to ST or STM before this critical level is reached apparently inhibits conditioning.

Penetration, translocation, and metabolism of acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} in soybean [Glycine max (L.) Merr. ‘Ransom’], common ragweed (Ambrosia artemisiifolia L.), and common cocklebur (Xanthium pensylvanicum Wallr.) were studied. Using liquid scintillation spectrometry and autoradiography, little movement of 14C-acifluorfen from the leaf surfaces of the two weed species could be detected in 24 h. After 48 h, less 14C was recovered from the leaf surface and more was found within the leaves of the two weed species. Autoradiographs of the weed showed limited acropetal movement of 14C from leaves 24 and 48 h after treatment. For soybean, most of the 14C still remained on the leaf surface after 48 h. Autoradiographs of soybean plants showed no movement from the treated leaflet. Studies using thin layer chromatography suggested that acifluorfen was metabolized within the plants. Rate of metabolism was inversely related to plant susceptibility (common ragweed and common cocklebur>soybean). The more rapid penetration and translocation, coupled with slower metabolism of acifluorfen by the weed species in comparison to soybean, may account for the difference in susceptibility of the weeds and soybean to acifluorfen.

EPTC (S-ethyl dipropylthiocarbamate) was applied to seed of alfalfa (Medicago sativa L.) that was coated with a porous material consisting principally of lime or gypsum. When the treated seed was planted in the field or greenhouse, grass was controlled selectively. When seeds, each carrying 0.1 mg EPTC, were seeded 1 cm apart in rows, the rate of EPTC would have been 2 kg/ha if this quantity of EPTC had been distributed in a band 5 cm wide. Such applications controlled grass completely in bands 5 to 6 cm wide. When seeds, each carrying 0.2 mg EPTC, were evenly distributed at seeding rates of 14 or 25 kg/ha, the broadcast rates of EPTC were 1.3 and 2.2 kg/ha. Both rates controlled grass completely. Storing EPTC-treated seed for 1 yr at 5 C did not affect germination of the seed or emergence and growth of the seedlings.

The influence of temperature and relative humidity on the activity of acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} on common cocklebur (Xanthium pensylvanicum Wallr.) and common ragweed (Ambrosia artemisiifolia L.) was studied in growth chambers. Treatment at high relative humidity (85 ± 5%) resulted in a significant increase in acifluorfen phytotoxicity and decrease in plant dry weight as compared to treatment at a lower (50 ± 5%) relative humidity (RH). Temperature (32 C day, 22 C night or 26 C day, 16 C night) was not as critical as relative humidity in influencing acifluorfen phytotoxicity. Addition of a non-ionic surfactant to the herbicide solution resulted in increased phytotoxicity. Penetration and translocation of acifluorfen in common cocklebur were increased when applications were made at high relative humidity.

Four field experiments conducted over 3 yr indicated that cultivation alone failed to prevent johnsongrass [Sorghum halepense (L.) Pers.] from reaching densities that severely reduced yields of cotton (Gossypium hirsutum L. ‘Acala SJ-2’). Density of johnsongrass in plots cultivated four times and hoed weekly for 8 weeks after emergence was reduced to 1 shoot/m2 at harvest compared to 74 shoots/m2 for plots that were only cultivated. In addition to a 60% average yield loss of seed cotton, yield losses ranging from 40 to 76%, ginning losses were also greater from cultivated than from hand-weeded plots. Compared to cultivated plots, supplementing cultivation with two postemergence applications of 3.0 kg/ha of DSMA (disodium methanearsonate) increased the average yield of cotton by 20% and reduced perennial johnsongrass densities by 64% at harvest. Although yields were improved by applying DSMA, they averaged 40% less than those of hand-weeded plots. The temporary weed control obtained with DSMA was profitable in terms of the additional lint and seed obtained, but insufficient cotton was produced to pay expenses for producing the crop by any of the methods of weed control. High labor costs for hoeing prevented this treatment from being profitable.

To assess velvetleaf (Abutilon theophrasti Medic.) seed populations and trends in continuous corn (Zea mays L.), six fields were selected and sampled in August and November of 1977 and 1978 at the 0- to 10- and 10- to 20-cm depths, and at the 20- to 30-cm depth in August of both years. The overall population mean was 51 million viable velvetleaf seeds/ha in the top 20 cm of soil. Means of individual fields ranged from 34 to 88 million seeds/ha. The population mean increased 13 million (28%) from 1977 to 1978. Individual fields had increases ranging from 8 million (14%) to 24 million (112%) seeds/ha. The seed population increased 16 million (37%) from August to November in these 2 yr, and 70% of the seeds were found in the top 10 cm of soil. The 20- to 30-cm depth contained 7% (3 million) of the seed extracted in the August sampling. A comparison of techniques for estimating the number of velvetleaf seeds in a soil sample showed that wet sieving was more efficient and accurate than were soaking and sieving, dry sieving, or direct incubation and germination. Seed immersion in water at 70 C for 5 min increased velvetleaf germination from 11% to 84%, compared to 52% for sulfuric acid, 48% for scarification, and 62% for seed-coat puncture. In the greenhouse, velvetleaf control with atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] and metribuzin [4-amino-6-ten-butyl-3-(methylthio)-as-triazin-5(4H)-one] declined as the seed population increased.

The experiment was conducted during 3 yr to measure the effect of corn cockle (Agrostemma githago L.) competition on winter wheat (Triticum aestivum L. ‘McDermid’) grain yield. In all years corn cockle was removed monthly in separate treatments from emergence (October) to harvest (July). Hand-weeded and weedy controls were compared to herbicidal control using treatments of bromoxynil (3,5-dibromo-4-hydroxybenzonitrile). Reductions in yield of wheat as affected by competition from corn cockle were not significant if weeds were removed before February of each year in winter wheat that had been seeded the previous October. Bromoxynil controlled more weed plants when applied on corn cockle that had less than four leaves (fall treatment). Corn cockle densities of 170 to 340 plants/m2 at the Pendleton, Oregon site lowered wheat yields an average of 18% when competition was eliminated by March. Competition that continued until wheat harvest (July) reduced yields 60%.