The phytotoxic monoterpenes, camphor, pulegone, and borneol, exhibit a marked synergistic action when used in combination. On a molar basis, these phytotoxic concentrations are enhanced from 40 to 100 times by using two compounds simultaneously. The toxicity of these compounds is about the same for radish (Raphanus sativus L.) and wheat (Triticum vulgare Vill.) but is less by an average of tenfold for oats (Avena sativa L.). When the compounds are used singly, they are from 3 to 12 times as effective in inhibiting root elongation as they are in inhibition of germination.

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.

Degradation of 2-chloro-4-ethylamino-6-isopropyl-s-triazine (atrazine) was studied in the 0 to 9, 14 to 24, and 36 to 48-inch depths of Sharpsburg silty clay loam and Keith silt loam soils. The effects of moisture, temperature, and microorganisms on degradation were observed. Atrazine was degraded two to three times faster in the topsoils than in the subsoils. Each 10 C temperature increase from 15 to 35 C caused the degradation rate to increase two to three times in these soils. Increasing the moisture content of the soils from 0.4 to 0.8 field capacity caused 14CO2 evolution from chain-labeled atrazine-14C treated soils to increase zero to six times. At 25 C and 0.8 field capacity, 100 to 194 atrazine molecules were detoxified for each molecule evolved as 14CO2 from chain-labeled atrazine. Solution culture studies showed cleavage of the side chain and ring of atrazine-14C except in the 14 to 24 and 36 to 48-inch depths of Keith silt loam.

Sugar beets (Beta vulgaris L., var. MSU 126 × 5460) were treated with 5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone (pyrazon) applied to the soil. The pyrazon was labeled with either 3H in the phenyl ring, or with 14C at the 4 and 5 positions of the pyridazinone ring. The disappearance of pyrazon and the appearance of metabolites in the soil and shoots were examined 1, 2, 4, 6, 8, 12, and 16 weeks after treatment. The total recoverable radioactivity decreased with time, but there was a greater loss of 3H-labeled compounds than 14C-labeled compounds from the soil. Thin-layer chromatography of the soil and shoot extracts revealed the presence of one metabolite in the soil and three metabolites in the plant. By co-chromatography with known pyrazon derivatives and by comparison of the labeling (14C, 3H, or both), the metabolites in the shoot were identified as N-(2-chloro-4-phenyl-3(2H)-pyridazinone)-glucosamine (hereinafter referred to as N-glucosyl pyrazon), 5-amino-4-chloro-3(2H)-pyridazinone (hereinafter referred to as ACP), and ACP-complex. The other moiety in the ACP-complex was not identified. The metabolite in the soil was identified as ACP, and was detectable 2 weeks prior to its appearance in the shoot, suggesting that it was absorbed from the soil.

Weed competition in this experiment had a more marked effect on sorghum [Sorghum bicolor (L.) Moench, var. RS 610] grain yields than did row spacings or sorghum populations. Sorghum grown in 20-inch, as compared to 40-inch, rows had higher grain and stover yields and lower weed yields and 1000-seed weight. Sorghum populations of 42,000 to 50,000 plants per acre produced higher grain and stover yields than populations of 26,000 to 28,000 plants per acre which had greater weed yields, heads per plant, seed weight per head, seed moisture at harvest, and 1000-seed weight. In hand weeded plots, sorghum yields were not affected by row spacing or sorghum population; but as weed competition increased, sorghum yields increased with narrower rows and higher plant populations. Sorghum yields were reduced 4, 12, and 18% by delaying weeding of plots 3, 4, and 5 weeks as compared with 2 weeks after planting. Weeds that did not emerge until 4 weeks after planting did not reduce sorghum yields.

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.

The effects of five herbicide and three in-the-row cultivation treatments on weed control and sugarbeet (Beta vulgaris L.) yield were studied at Mitchell and North Platte, Nebraska, during 1966 and 1967. Preplant soil incorporation treatments of 5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone (pyrazon) + N,N,dimethyltri-decylamine salt of 7-oxabicyclo[2.2.1]heptane-2,3-dicarboxylic acid (endothall), and S-ethyl N-ethylthiocyclohexanecarbamate (cycloate), and postemergence application of pyrazon + 2,2-dichloropropionic acid (dalapon) resulted in better weed control and more sucrose per acre than S-propyl butylethylthiocarbamate (pebulate) applied preplant and soil-incorporated, and the untreated check treatment. Herbicide-treated plots kept weed-free by handweeding did not differ from the handweed check in sucrose production. Rotary hoeing improved weed control and increased sucrose yields as compared to flextine harrowing and the no in-the-row cultivation treatment.

Several investigations were undertaken to study the metabolism of alpha,alpha,alpha-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) by intact peanut (Arachis hypogaea L.) and sweet potato (Ipomoea batatas L.) plants and crude extracts of these plants. Attempts also were made to determine the biological activity of three isolated degradative intermediates. Accumulated data indicated that trifluralin was degraded by both intact and crude extracts of these plants; the degradation pathway differing in each instance. In crude extracts of peanuts, the initial degradation product was an incomplete dealkylated derivative of the parent compound, alpha, alpha, alpha-trifluro-2,6-dinitro-N-propyl-p-toluidine. The initial degradation product formed in sweet potato crude extracts was a reduced derivative of trifluralin, N2,N2-di-n-propyl-3-nitro-5-trifluromethyl-o-phenylenediamine. All isolated degradation intermediates used in bioassay studies demonstrated root inhibition in cucumber (Cucumis sativus L.) at many of the concentration levels utilized. The greatest amount of root inhibition was observed in cucumber seeds germinated in alpha,alpha,alpha-trifluro-2,6-dinitro-N-(n-propyl)-p-toluidine.

Total petiolar fatty acid content of sicklepod (Cassia obtusifolia L.) treated with 0, 0.14, 0.28, 0.56, 1.12, 2.24, or 4.48 kg/ha S-ethyl dipropylthiocarbamate (EPTC) was measured by gas-liquid chromatography. Neither total petiole fatty acid content nor percentages of the various 53 identified constituents changed in relation to herbicide application. Isostearate (17.5%), stearate (5.7%), oleate (5.7%), linolate (9.8%), and arachidate (8.2%) accounted for 46.9% of the total petiole fatty acid content. Anteiso derivatives of C15 to C31 were identified and quantitated at concentrations of 0.25 to 2.00%. Petiole cuticle thickness decreased 35% as herbicide concentration increased to 4.48 kg/ha.

Grain sorghum (Sorghum vulgare Pers.) was grown in combination with varying densities and durations of yellow foxtail (Setaria glauca (L.) Beauv.) during 1963-1965 at Manhattan, Kansas. Weed yield and competitive influence were greatest during the year of above-average rainfall and supplemental N fertility. Reduction in sorghum grain yield was due principally to a reduction in number of seeds per head. A 16-inch weed band on the crop row reduced soil moisture more than any other density treatment, but lesser weed densities reduced soil moisture below that of weed-free plots.

Rates and magnitude for phytotoxic diminuation of 5-bromo-3-sec-butyl-6-methyluracil (bromacil) at 0.25, 0.5, and 1 ppm for oats (Avena sativa L.) and 1, 2, and 4 ppm for sorghum (Sorghum vulgare Pers.) plants were related to increased nitrogenous composition of organic and inorganic amendments with a Psammentic paleustalf, Eufaula sand. Soil isolates not inhibited by bromacil concentrations up to 2,000 ppm in defined broth media sans presynthesized growth factors included Aspergillus tamarii Kita, A. flavus Link ex Fries, A. oryzae (Ahlburg) Cohn, Penicillium funiculosum Thorn. P. brevicompactum Dierckx. Curvularia lunata (Wakker) Boedijn, Mucor pusillus Lindt, Trichoderma viride Persoon ex Fries, and Myrothecium verucaria Ditmar. A. niger van Tieghem was inhibited with bromacil levels less than 1500 ppm. This substituted uracil fungitoxicity was offset with increasing inorganic N levels to 500 ppm as NH4NO3 and/or 1,000 ppm of presynthesized growth factor additive as cyanocobalamin, yeast extract, and peptone. Bromacil levels to 100 ppm reduced nitrification temporarily for only 5 days in contrast to sustained inhibition of nitrification for 15 days by equivalent levels of 2-amino-4-chloro-6-methylpyrimidine (Toyo Koatsu AM).

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.

In a 2-year study, kochia [Kochia scoparia (L.) Schrad.] competed with sugarbeets (Beta vulgaris L.) for various periods after emergence. Kochia then was removed and plants that emerged subsequently were controlled by hand-weeding. Yield of sugarbeet roots was reduced when kochia competed for more than 5 or 6 weeks. Kochia that competed with sugarbeets for the entire season reduced yields more than 95%. In another study, kochia was controlled by frequent hand-weeding for different periods after sugarbeet emergence. When kochia was controlled initially for 3 to 4 weeks, yield of sugarbeets was not reduced. Some kochia emerged following the 3 to 4 weeks of hand-weeding. However, competition from sugarbeets reduced the weight of this kochia at harvest by 59 and 92%.

A survey has been made of the literature on biochemical characteristics of plants with particular emphasis on factors affecting photosynthesis. From these characteristics, plants have been divided into two groups, efficient and non-efficient. This division is justified on the basis of data published by numerous workers. A hypothesis has been developed that efficient plants often have been used in agriculture because of their high production, but efficient plants also are very competitive and the observation has been made that almost all weeds which have been tested are members of the efficient groups of plants. This hypothesis is discussed and some general conclusions have been advanced related to plant breeding, development of herbicides to inhibit specific biochemical reactions, ecology, and increasing food production.

Grain sorghum (Sorghum vulgare Pers.) was grown in combination with varying densities and durations of tall waterhemp (Acnida altissima Riddell) during 1964 and 1965 at Manhattan, Kansas. As waterhemp density and duration increased, sorghum fodder and grain yields were reduced. Yields of both species and competitive influence of waterhemp were highest during the year of high rainfall and supplemental N fertility. The net competitive influence of waterhemp on sorghum grain was accounted for principally by reduced seed size. Competition for soil moisture in 1965 was greatest when weeds remained full-season, and was most pronounced early in the season at soil depths below 20 inches.

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.

The herbicide 5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone (pyrazon) had a ten-fold greater inhibitory effect on the growth of common lambsquarters (Chenopodium album L.) than on sugar beets (Beta vulgaris L.), Production of dry matter was inhibited by accumulations of pyrazon in the tissues. When these accumulations reached a level approximately twice that which affected dry matter production, plants wilted, collapsed, and died. Pyrazon failed to affect the structural integrity of either chlorophyll or the chloroplast, but inhibited the Hill reaction of chloroplasts isolated from both sugar beets and common lambsquarters. The I50 value was 6 × 10−6 M pyrazon. There was no difference between sugar beets and common lambsquarters in the degree of inhibition in oxygen evolution by leaf discs placed in pyrazon. When plants were cultured in vivo with pyrazon, the rate of oxygen evolution was correlated with herbicide accumulations in the leaf tissues. Sugar beet leaves contained only one-third to one-fifth the level present in common lambsquarters. The rates of respiration of roots and leaf discs of both species were not affected by pyrazon treatments in vivo or in vitro.

The effect of 2,3,5-trichloro-4-pyridinol (pyriclor), as well as carbonyl cyanide m-chlorophenyl hydrazone, 2,4-dinitrophenol, and oligomycin, on oxygen uptake and oxidative phosphorylation was investigated with mitochondria isolated from hypocotyls of etiolated soybeans (Glycine max Merrill, var. Lee). Pyriclor inhibited both coupled and uncoupled reactions. This suggests that pyriclor either acts on a nonphosphorylating intermediate close to the electron carrier chain or with a component of the electron carrier chain, or both.

Both carrot (Daucus carota L.) and common ragweed (Ambrosia artemisiifolia L.) plants metabolized 3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea (linuron). Traces of all of the following derivatives of linuron were detected in both plants: 3-(3,4-dichlorophenyl)-1-methoxyurea, 3-(3,4-dichlorophenyl)-1-methylurea, 3-(3,4-dichlorohenyl)urea, and 3,4-dichloroaniline. Differences in absorption and concentration of these derivatives were observed between carrot and ragweed plants. All of the above derivatives were phytotoxic to common ragweed plants except for 3-(3,4-dichlorophenyl)urea and 3,4-dichloroaniline, while none of the derivatives were phytotoxic to carrot. In carrot, 87% of the applied linuron was metabolized to nonphytotoxic derivatives compared to only 13% in common ragweed plants. It appears that a combination of differences in absorption, metabolism, and phytotoxicity of several of the metabolite derivatives of linuron to carrot and common ragweed plants may be the primary reason for differences in sensitivity of these two plants to linuron.