Four modes of competition were studied in relation to early development of common milkweed (Asclepias syriaca L.) alone and in competition with green foxtail [Setaria viridis (L.) Beauv], redroot pigweed (Amaranthus retroflexus L.), and sorghum [Sorghum bicolor (L.) Moench ‘RS-626’]. The three species were used to study the effect of no competition, light competition, soil competition, and light and soil (full) competition on common milkweed. Common milkweed height was significantly reduced by soil and full competition with the three species, while only green foxtail caused a significant reduction in height due to light competition. Shoot weight of common milkweed was significantly reduced by light, soil, and full competition by the three species. Reproduction percentage of common milkweed roots was most severely reduced by light and full competition from green foxtail and redroot pigweed. An environment conducive to the establishment of the perennial common milkweed from seed exists when annual weeds are removed by herbicides.

Presently accepted methods for calculating the expected response for combinations of agents such as herbicides or other growth regulators from the responses of individual agents can lead to unacceptable results. This may be true even if responses within the linear region of the individual response curves are used in such calculations. A solution to this problem is to assign parameters to response curves of individual agents and to define the expected response function for combinations of agents by the weighted algebraic means of these parameters.

The two isomers of diallate [S-2,3-dichloroallyl)diisopropylthiocarbamate] can be isolated by preparative scale gas chromatography using a Carbowax 20M liquid phase. on the basis of IR and NMR spectroscopic and other evidence the isomer with the shorter retention time is assigned the trans configuration and that with the longer retention time is assigned the cis configuration.

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.

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 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.

Screening studies initiated in 1969 to determine the effectiveness and selectivity of herbicides for herbaceous weed control during the first growing season in American sycamore (Plantanus occidentalis L.) plantations indicated that simazine [2-chloro-4,6-bis(ethylamino)-s-triazine], atrazine [2-chloro-4-(ethylamino)-6- (isopropylamino)-s-triazine], and dalapon (2,2-dichloropropionic acid) had the desired properties at economically feasible use rates. To further evaluate these herbicides and ametryne [2-(ethylamino)-4-(isopropylamino)-6-(methylthio)-s-triazine], permanent plots were established in a sycamore plantation in 1970. Weed control obtained from a single herbicide application in the spring after planting seedlings in the winter has resulted in increased height, diameter, and volume growth of the sycamore for at least three growing seasons. No serious phytotoxicity on the crop species was noted with application rates from 4.5 to 9.0 kg/ha of the s-triazines alone or from 5.6 kg/ha of dalapon in combination with the lower rate of simazine and atrazine.

Postemergence herbicide treatments that controlled tighthead sprangletop [Leptochloa panicoides (Presl) Hitchc.] and redstem (Ammannia auriculata Willd.), did not injure water-seeded rice (Oryza sativa L.), and substantially improved grain yields included: (a) a single application of propanil (3′,4′-dichloropropionanilide) at 4.5 kg/ha applied to weeds 8- to 10-cm tall; (b) two sequential treatments of propanil each at 3.4 kg/ha, with the first treatment applied to weeds 2- to 5-cm tall, and a second treatment applied 4 to 8 days later; (c) a tank mixture of propanil at 3.4 kg/ha and 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid] at 0.6 kg/ha, applied to weeds 8- to 10-cm tall; and (d) a tank mixture of propanil and molinate (S-ethyl hexahydro-1H-azepine-1-carbothioate), each at 2.2 kg/ha, applied to weeds 8- to 10-cm tall. Treatments that controlled tighthead sprangletop or redstem unsatisfactorily or inconsistently and did not improve grain yields as much as more effective treatments included: preplant or postemergence treatments of molinate, or preplant treatments of nitrofen (2,4-dichlorophenyl-p-nitrophenyl ether), fluorodifen (p-nitrophenyl α,α,α-trifluoro-2-nitro-p-tolyl ether), CNP (2,4,6-trichlorophenyl-p-nitrophenyl ether), or He-314 (p-nitrophenyl-m-tolyl ether).

We evaluated nine systems of weed control over a 3-year period for cotton (Gossypium hirsutum L.) tolerance and control of weeds on Tifton loamy sand soil. Cultivation with sweeps did not control weeds effectively, which allowed weed competition to reduce cotton yields. Five cultivations plus an annual average of 185 hr/ha hand-hoeing was required to keep cotton relatively free of weeds for 7 to 8 weeks after planting. The use of herbicides offered certain economic advantages over systems using only cultural practices in controlling weeds. A preplant application of trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) reduced annual hand-hoeing requirements by 66 hr/ha. The level of weed control at harvest time increased each year in all systems that used a herbicide program, but not where only cultivation or hand-hoeing was used. The continuous use of specific weed-control systems shifted weed populations. Systems that achieved 100% control by the end of the third year included all of the following: (a) a preplanting or at planting treatment of trifluralin or EPTC (S-ethyl dipropylthiocarbamate) plus (b) a preemergence application of fluometuron [1,1-dimethyl-3-(α,α,α-trifluoro-m-tolyl)urea], (c) two directed postemergence sprays with MSMA (monosodium methanearsonate), and (d) diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea]. Large crabgrass [Digitaria sanguinalis (L.) Scop.] and Florida pusley (Richardia scabra L.) were nearly eliminated where we used trifluralin. Systems that did not include a postemergence application of MSMA allowed common cocklebur (Xanthium pennsylvanicum Wallr.), Florida beggarweed [Desmodium tortuosum (Sw.) DC], and other weeds to emerge and remain in the crop row, reducing yield of lint cotton. Cotton yield appeared to be directly related to the level of early-season weed control. We did not detect biologically active soil residues at harvest time each year. Our weed-control systems did not affect cotton fiber length, strength, or color.

The effect of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] and alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] combinations on various physiological processes was studied in order to establish a basis for the synergistic effect of this mixture on Japanese millet [Echinochloa crus-galli (L.) Beauv. var. frumentacea L.]. In contrast to atrazine, alachlor had no effect on the Hill reaction activity of isolated Japanese millet chloroplasts. When used in combination with atrazine, alachlor had no influence on the inhibitory effect of atrazine on the Hill reaction. Atrazine and alachlor combinations reduced chloroplast protein and severely inhibited chloroplast protein synthesis relative to protein synthesis by other particulate fractions. This inhibition appears to be the basis for the synergistic effect of this herbicide mixture on Japanese millet. Although the mechanism of action of the herbicide mixture is not understood, it does not seem to involve inhibition of atrazine detoxification.

The addition of 1.5 ml water or 1.5 ml 0.1 N oxalic acid solution to 2 g soil samples containing 14C-labeled herbicides increased the 14C extraction efficiencies. The employment of a modified dioxane extracting-counting solution allowed the addition of 2 ml water without causing a liquid phase separation. Extraction of benzoic acid herbicides from soil was enhanced when 5 mg oxalic acid was added to 2 g soil or when soil was acidified to pH 4.1.

Weed population shifts occurred when simazine [2-chloro-4,6-bis(ethylamino)-s-triazine], diuron [3-(3,4-dichlorophenyl)-1,1-dimethyl-urea], terbacil (3-tert-butyl-5-chloro-6-methyluracil), and dichlobenil (2,6-dichlorobenzonitrile) were applied annually for 5 years to an apple orchard. The number of brambles (Rubus sp.) and Virginia clematis (Clematis virginiana L.) plants increased with terbacil. usage. Bramble populations increased with the high rate of dichlobenil and goldenrod (Solidago sp.) increased at the low rate. The major vegetation on plots receiving amitrole (3-amino-s-triazole) was not altered from that of a mowed check. However, amitrole reduced growth of shallow planted trees. Postemergence applications of paraquat (1,1′-dimethyl-4,4′-bipyridinium ion) four times during each growing season resulted in more bare soil than was in the mowed checks at the end of the fifth growing season. Yields of apples were higher in the simazine, terbacil, and dichlobenil-treated plots than in the mowed checks. Simazine and terbacil residues did not accumulate in the upper 60 cm of soil during the study; a slight accumulation of dichlobenil appeared to occur in the surface 0 to 15-cm.

Two, 4-dichloro-6-fluorophenyl-p-nitrophenyl ether (CFNP) is similar in herbicidal property and chemical structure to nitrofen (2,4-dichlorophenyl-p-nitrophenyl ether). CFNP inhibits the growth of the hypocotyl and young buds of germinating weeds. Henry Crabgrass (Digitaria adscendens H.B.K. Henr.) was controlled by CFNP at 0.5 kg/ha, however sunflower (Helianthus annuus L.) was tolerant to 16 kg/ha of CFNP. CFNP also effectively controlled largeseed dodder (Cuscuta indecora Choisy) which is a serious problem in sunflower. Although trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) did not control common dodder at 8 kg/ha, CFNP gave control at 0.5 kg/ha.

A dense bloom of the blue-green alga, Anabaena variabilis Kuetzing suddenly died in a pond heavily stocked with channel catfish (Ictalurus punctatus L.). Low light intensity caused the formation of gas vacuoles in A. variabilis. The buoyant filaments accumulated at the surface in large numbers during a period of calm, warm weather. Light injury to cells in the surface scum apparently triggered the mass mortality of phytoplankton. Decomposition of dead algae quickly resulted in depletion of dissolved oxygen. Concentrations of CO2 and ammonia increased to high levels and the pH declined following the phytoplankton die-off. A period of 7 days was required for re-establishment of a phytoplankton community and little improvement in water quality occurred until high densities of healthy phytoplankters were present. Massive mortality of fish was prevented by pumping oxygenated water into the pond at two sites and by mechanically agitating surface water in a limited area.

Early postemergence application of atrazine [2-chloro-4- (ethylamino)-6-(isopropylamino)-s-triazine] at 0.28 kg/ha in combination with alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] at 0.42 kg/ha plus a non-toxic oil-surfactant blend gave season-long weed control in sweet corn (Zea mays L. var. rugosa ‘Seneca Chief’). Bioassay with oats (Avena sativa L.) showed no detectable atrazine residue in the soil 3 months after herbicide application. In greenhouse studies, the response of Japanese millet (Echinochloa crus-galli L. Beauv. var. frumentacea) to early postemergence application of atrazine and alachlor combinations plus an oil-surfactant blend was synergistic. Soil uptake played a major role in the activity of the herbicides. Formulants used in commercial alachlor when added to atrazine spray, enhanced atrazine activity in a way similar to that of the non-toxic oil-surfactant.

Soil enrichment studies were conducted with trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) with and without exogenous carbon and nitrogen. From 72 bacterial isolates obtained, eight representative ones were tentatively identified as members of the genus Pseudomonas based on size, shape, Gram reaction, and reaction on fluorescent and motility agar. Two of these eight isolates exhibited a significant increase in numbers of viable cells when grown in a medium with trifluralin as the only exogenous carbon and nitrogen source. Chromatographic analysis of culture medium extracts revealed the presence of a degradation product which was not present in the herbicide control. The concentration of this unknown was greatest at the 50 mg/L level of trifluralin which also supported the greatest bacterial growth.

Preplanting applications of diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] with and without trifluralin (a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) were made for 5 years in irrigated cotton (Gossypium hirsutum L.). All treatments containing trifluralin temporarily stunted cotton. In two of the 5 years, trifluralin reduced diuron injury to cotton seedlings which resulted in better crop stands. The trifluralin-diuron combinations resulted in better weed control at harvest than either herbicide alone. The 5-year average yield of cotton was greater with trifluralin-diuron combinations than with the same rates of diuron alone.