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.

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.

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.

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.

EPTC (S-ethyl dipropylthiocarbamate) reduces hypocotyl growth of lettuce [Lactuca sativa (L.) ‘Grand Rapids’] seedlings in response to gibberellic acid (GA3) in the lettuce hypocotyl bioassay. Doses of 0.01, 0.1, and 1.0 mM EPTC were effective over a range of gibberellin concentrations. Significant reductions in hypocotyl growth in response to all doses of EPTC and GA3 at 7.68 μg/ml occurred between 12 and 24 h after initial treatment. In a 72-h experiment, pulse treatments of EPTC at 0.01, 0.1, and 1.0 mM plus GA3 at 7.68 μg/ml for only 1 or 2 h at the beginning reduced the subsequent response of the bioassay when plants were transferred to GA3 alone for the remaining 71 or 70 h, respectively. Final hypocotyl growth in GA3 at 7.68 μg/ml was reduced by exposure to EPTC at 0.01, 0.1 and 1.0 mM plus GA3 at any time during the entire 72 h test period. The interaction between EPTC and GA3 was not due to herbicide-induced precipitation nor decomposition of GA3 in the medium. This was demonstrated by separation of [14C]-GA3 in four thin-layer chromatography solvent systems after incubation in the presence or absence of 1.0 mM EPTC for 48 h. Apparently EPTC can reduce the growth response of sensitive species to exogenous gibberellins in addition to its effects on endogenous gibberellin-like substances or gibberellin biosynthesis.

Field experiments were conducted to study the effects of jimsonweed (Datura stramonium L.) densities and planting dates on the growth and yield of soybeans [Glycine max (L.) Merr. ‘Amsoy 71’]. Jimsonweed densities of 1.5 and 2 plants/m2 did not reduce the dry weight of soybean leaf, stem, root, or pod and seed tissues, leaf area index, plant height, pod number, or seed yield when soybeans and jimsonweed were planted at the same time. Weed densities ranging from 3 to 16 plants/m2 did reduce one or more of these soybean growth parameters. A significant density-by-year interaction was observed in which soybean growth reductions caused by high jimsonweed densities were less when competition for water was minimized by above-average precipitation in 1 of the 2 yr. No density of jimsonweed that emerged when soybeans were at the fourth trifoliolate-leaf stage reduced crop growth or yield.

In greenhouse studies, spurred anoda [Anoda cristata (L.) Schlecht.] control by Alternaria macrospora Zimm. was influenced by stage of growth, inoculum concentration, and the temperature and duration of the dew periods following inoculation. Seedlings inoculated at the cotyledon to first-leaf stage were most easily controlled. Optimum dew periods for killing seedlings were ≥ 24 h at 20 to 30 C. Spray mixtures containing 2.5 to 10 × 105 spores/ml gave the best control. In field studies, A. macrospora significantly reduced the number and the dry weight of spurred anoda plants.

The influence of manure and herbicide applications on weed-free fieldbeans (Phaseolus vulgaris L.) and root rot of fieldbeans was studied during 1978 and 1979 at Scottsbluff, Nebraska. Rates of cattle feedlot manure used were 0, 30000, 56000, 112000, or 168000 kg/ha. Preplant herbicide treatments used were alachlor [2-chloro-2′6′-diethyl-N-(methoxymethyl)acetanilide] at 3.4 kg/ha, EPTC (S-ethyl dipropylthiocarbamate) at 3.4 kg/ha, EPTC + trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) at 2.2 + 0.6 kg/ha, or dinoseb (2-sec-butyl-4,6-dinitrophenol) at 6.7 kg/ha. Manure rates of 56000 kg/ha or higher reduced fieldbean yields both years, but did not increase electrical conductivity or soil exchangeable sodium enough to explain these yield reductions. In 1978, but not in 1979, height and yield of weed-free fieldbeans were reduced by EPTC, EPTC + trifluralin, and alachlor treatments. A significant interaction between manure and herbicide treatments was not detected and none of the treatments increased the severity of root rot in either year.

Glyphosate [N-(phosphonomethyl)glycine] significantly decreased the chlorophyll content of field-grown soybeans (Glycine max L. Merr. ‘Williams’) within 48 h after a 2.24-kg/ha treatment. In laboratory studies, 0.0001, 0.001, 0.01, 0.1, and 1 mM glyphosate reduced chlorophyll content of 7-day-old etiolated barley (Hordeum vulgare L. ‘Barsoy’) shoots 8, 12, 25, 49, and 77%, respectively, following 8 h dark incubation and 24 h illumination. Reduction of chlorophyll accumulation ranged from 6% with 0.001 mM glyphosate to 82% with 1.0 mM glyphosate. In studies with 8-day-old etiolated corn (Zea mays L. ‘Pioneer Brand 3535’) shoots, 0.1, 1.0, and 10.0 mM glyphosate decreased chlorophyll content of corn shoots 24, 42, and 50%, respectively, after 12 h of illumination. The rate of chlorophyll accumulation in corn shoots was significantly reduced 64% by 1.0 mM glyphosate over a 15-h illumination period. These rapid and substantial effects on chlorophyll accumulation suggest that interference with greening may be important in the mechanism of action of glyphosate.