Germination studies on camphorweed [Heterotheca subaxillaris (Lam.) Britt. & Rusby] revealed that freshly harvested disk achenes germinated best at 17.5 C (88%) while ray achenes were dormant. Camphorweed seed from disk achenes also germinated at temperatures as low as 3 C. Seedlings grown under long-day conditions at 23 C day and 8 C night temperatures for 144 days elongated at the rate of 0.18 cm/day. Plants grown under short-day conditions at the same temperature regime elongated at the rate of 0.06 cm/day. Total leaf surface area, fresh weight, and dry weight of shoots of plants grown under long days were at least 1.5 times greater than plants grown under short day conditions. Camphorweed, regardless of size and age, survived a 2-h exposure at −5 C. All plants in the rosette stage survived at −15 C in the freezer and an overnight temperature of −11.7 C in the field, whereas larger plants were killed at these temperatures. Stage of growth was an important factor in the herbicidal control of camphorweed. Plants in the rosette stage were generally more susceptible to herbicides than older plants. Simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] at 3.4 kg/ha, paraquat (1,1′-dimethyl-4,4′-bypyridinium ion) at 0.6 kg/ha, methazole [2-(3,4-dichlorophenyl)-4-methyl-1,2,4-oxadiazolidine-3,5-dione] at 5.0 kg/ha, and a formulated mix of diuron [3–3,4-dichlorophenyl)-1,1-dimethylurea] and terbacil (3-tert-butyl-5-chloro-6-methyluracil) at 4.5 kg/ha provided adequate control of camphorweed in the rosette stage. Asulam (methyl sulfanilylcarbamate) at 2.2 or 4.5 kg/ha applied alone did not control camphorweed in the rosette form but was more effective on older plants. Various combinations of these herbicides were generally effective at both stages of growth.

The occurrence of asymbiotic nitrogen fixing bacteria in the rhizosphere of twenty weed species has been investigated. Using the roll tube method root-rhizosphere soil samples were diluted and added to test tubes containing low N media. Tubes were flushed with nitrogen, stoppered, and rolled to form a thin film of media on the inner wall. After 7 days incubation the nitrogen fixation rates were estimated using the acetylene reduction technique. Rates ranged from 38 to 783 nmoles acetylene reduced per hour. Of twenty species tested, three showed high rhizosphere populations of asymbiotic nitrogen fixing bacteria.

The effects of Pennsylvania smartweed (Polygonum pensylvanicum L.) interference in soybeans [Glycine max (L.) Merr. ‘Pickett 71’] were studied in the field under a naturally occurring weed population. Soybean seed yield was reduced an average of 13% by a density of eight Pennsylvania smartweed plants per 10 m of row. Greater yield reductions of 21, 37, and 62% resulted from full-season interference by densities of 16, 32, and 240 weeds per 10 m of row, respectively. Fewer than five weeds per 10 m of row did not significantly reduce crop yield. A natural population of 240 weeds per 10 m of row did not reduce soybean yield if the period of interference was limited to 6 weeks or less after crop emergence. However, more than 6 weeks of weed interference resulted in significant yield reduction. Conversely, if the crop was kept weed-free for a period of 4 weeks or more after emergence, seed yield was not reduced. No allelopathic interaction between the two species was observed in greenhouse studies using a recirculating nutrient solution and alternate pots of soybeans and Pennsylvania smartweed.

Germination of fruit of common lambsquarters (Chenopodium album L.) from three out of four sources was enhanced by 9 to 41% by removal of the fruit coat, and by 21% by washing the fruit for 70 h. Brown-black seed germinated more rapidly than the black and brown seed after washing for 70 h. The washing treatment appeared to remove some unknown inhibitors. After washing, seed germination was found to be positively correlated with seed size.

A crop rotation of sorghum [Sorghum bicolor (L.) Moench] and soybeans [Glycine max (L.) Merr.] with various weed control treatments was conducted from 1968 to 1975 at Lincoln, Nebraska, in order to improve weed control in these two crops. Poor weed control during one growing season increased weed growth and decreased yields of succeeding crops. Trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) at 1.1 kg/ha on soybeans showed greater soil carryover toxicity to sorghum planted 12 months later than did atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] at 3.4 kg/ha on sorghum to subsequently planted soybeans. Under this crop rotation green foxtail [Setaria viridis (L.) Beauv.], tall waterhemp [Amaranthus tuberculatos (Moq.) J. Sauer], and large crabgrass [Digitaria sanguinalis (L.) Scop.] decreased; whereas, velvetleaf (Abutilon theophrasti Medic), sunflower (Helianthus annuus L.), and Pennsylvania smartweed (Polygonum pensylvanicum L.) increased. Germinating weed seedlings from soil samples were greater in weedy check plots during the final years of the crop rotation due to a buildup of seed from those species that increased. Handweeding, herbicides, and plowing reduced weed populations, weed competition, and increased sorghum and soybean yields.

Field experiments were conducted from 1974 to 1977 at Redfield and Centerville, South Dakota, to evaluate the tolerance of soybeans [Glycine max (L.) Merr.] at different growth stages and five varieties of soybeans to dicamba (3,6-dichloro-o-anisic acid) and to determine dicamba residue in the foliage. Yield reduction occurred from applications when soybeans were flowering. Furthermore, germination was reduced by dicamba application at pod-fill. Dicamba residue was detected in foliage 7 days but not 18 days after application. Extent of dicamba use and drift occurrence was determined by a telephone survey of 159 farmers. Thirty-one percent of the farmers surveyed used dicamba in 1976.

Mature plants of tomato (Lycopersicon esculentum Mill. ‘Super Sioux’), cucumber (Cucumis sativas L. ‘Early pickling’), and pinto bean (Phaseolus vulgaris L.), in small field plots, were exposed once to sprinkler or furrow irrigation water containing 0.11 kg/ha or 1.12 kg/ha MSMA (monosodium methanearsonate). Concentrations of MSMA in water were about 0.22 ppmw and 2.2 ppmw for the two rates, respectively. Samples of crops and soil were subsequently analyzed for elemental arsenic content by colorimetric and atomic absorption methods. Arsenic residues were higher, compared to controls, only in crops exposed to MSMA applied in sprinkler irrigation. Average arsenic levels were highest in tomatoes exposed to 2.2 ppmw MSMA by sprinkler irrigation, and after 30 and 45 days were 0.26 ppm and 0.35 ppm, respectively. Average arsenic levels in sprinkler-irrigated crops ranged from double to triple the arsenic levels in control (untreated) crops. Atomic absorption was more sensitive than the colorimetric method and could detect smaller differences in arsenic content of samples. The average arsenic residues in soil varied from 1 to 3 ppm, but the differences could not be attributed to treatment with MSMA.

The persistence of isopropalin (2,6-dinitro-N,N-dipropylcumidine), nitralin [4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline] and trifluralin (a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) applied at recommended (1X) and high (2X) rates was studies in the field on two soils. A sorghum [Sorghum bicolor (L.) Moench ‘RS 610’] root bioassay showed that the recommended rates of the herbicides caused 20% or less root inhibition at the end of the growing season in 1972. Both nitralin and trifluralin were more persistent in 1974 than in 1972 as shown by the root bioassay and growth of fall-sown rye (Secale cereale L.). The yield of sweet corn (Zea mays L. var. rugosa Bonaf. ‘Gold Cup’) was not significantly reduced when grown 1 yr after the initial application of the herbicides.

The effects of diluent volume and calcium concentration on glyphosate [N-(phosphonomethyl] glycine] phytotoxicity were evaluated by adding 0.0, 0.0025, 0.005, 0.01, 0.02, and 0.04 M CaCl2 to 1.68 kg/ha glyphosate and applying at 130, 190, 375, and 750 L/ha to tall morningglory [Ipomoea purpurea (L.) Roth] plants that were 10 to 12 cm tall. Thirty days after treatment, glyphosate phytotoxicity was reduced by calcium at diluent volumes of 375 and 750 L/ha. Addition of a sulfonine red dye to the spray solution showed that at 130 L/ha, runoff from tall morningglory leaves was negligible. At 375 and 750 L/ha different volumes, ½ to ¾ of the spray solution, as measured by dye retention, ran off the leaf surface. At 750 L/ha, the water and dye collected at the leaf margins and the spray pattern on the leaf surface was not uniform.

Before I try to paint a broad canvas as a background to this evening's discussion I think we should be clear what we mean by ‘discipline of weed science’. The word ‘discipline’ according to the Oxford English Dictionary means ‘a department of knowledge’ or ‘the art of war’. This latter 15th Century definition might seem appropriate to our struggle against weeds! What, though, is meant by ‘weed science’?

Methods of assessing and describing the joint action of materials as herbicides are reviewed, and the shortcomings of the terms synergism and antagonism are discussed. Some of the lack of agreement on their meaning and evaluation is attributed to failure to define or recognize appropriate models to represent absence of synergism (and antagonism). General analytical and graphical procedures are presented, and for two simple models, the additive-dose model (ADM) and the multiplicative-survival model (MSM), the dose-response curves and isobols are compared. Three examples from recent literature are reworked.

Six herbicides, simazine [2-chloro-4,6-bis(ethylamino)-s-triazine], propham (isopropyl carbanilate), terbacil (3-tert-butyl-5-chloro-6-methyluracil), carbetamide [D-N-ethyllactamide carbanilate (ester)], pronamide [3,5-dichloro(N-1,1-dimethyl-2-propynyl)benzamide], and secbumeton [N-ethyl-6-methoxy-N′(1-methylpropyl)-1,3,5-triazine-2,4-diamine] were applied annually for four years to establish forage alfalfa (Medicago sativa L. ‘Washoe’). Downy brome (Bromus tectorum L.) control ranged from 80 to 100%. The average control of broadleaf weeds was 90 to 100% with simazine, terbacil, and secbumeton, compared to 30 to 40% with propham, carbetamide, and pronamide. Only simazine and terbacil were phytotoxic to the alfalfa. Based on weed control and crop tolerance, secbumeton performed best on the coarsetextured soil in this experiment.

Experiments were conducted in controlled environmental chambers to determine the influence of temperature on the phytotoxicity of seven soil-applied herbicides. Diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid} soil incorporated or surface applied, was more toxic to wild oat (Avena fatua L.) shoots at 10 and 17 C than at 24 C. Efficacy of diclofop was enhanced with soil incorporation. Diclofop toxicity to wild oat roots was not influenced by a change in temperature. EPTC (S-ethyl dipropylthiocarbamate) stimulated sugarbeet (Beta vulgaris L. ‘American Crystal Hybrid B’) shoot dry weight production at 10 C and caused dry weight reduction at 24 C. Atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] toxicity to barley (Hordeum vulgare L. ‘Larker’) and alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] toxicity to oats (Avena sativa L. ‘Chief’) increased with increased temperature from 10 to 17 C. Temperatures within the range of 10 to 24 C did not affect trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) toxicity to barley or BAY-5653 [N-(2-benzothiazolyl)-N-methylurea] or chloamben (3-amino-2,5-dichlorobenzoic acid) toxicity to oats.

Seven dinitroaniline herbicides were tested in the field for 3 yr at rates up to twice that normally used for annual grass control. Trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) and profluralin [N-(cyclopropylmethyl)-α,α,α-trifluoro-2,6-dinitro-N-propyl-p-toluidine] gave 95 and 94% control of johnsongrass [Sorghum halepense (L.) Pers.] from rhizomes, respectively, after the third year at the double rate. These two herbicides controlled johnsongrass from rhizomes better than nitralin [4-methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline], butralin [4-(1,1-dimethylethyl)-N-(1-methylpropyl)-2,6-dinitrobenzenamine] and pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] at comparable rates. After 3 yr, seed cotton (Gossypium hirsutum L. ‘Stoneville 213’) yields were higher with double rates of trifluralin, profluralin, fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)aniline], and pendimethalin, than with nitralin, dinitramine (N4,N4-diethyl-α,α,α-trifluoro-3,5-dinitrotoluene-2,4-diamine), and butralin. In greenhouse studies, reduction of cotton raproot length by the dinitroaniline herbicides incorporated at the normal rate of use followed the order: butralin ≤ pendimethalin ≤ fluchloralin ≤ trifluralin < profluralin < dinitramine. All herbicides except pendimethalin at 0.28 and 0.56 kg/ha reduced lateral root production of cotton. The dinitroaniline herbicides at normal rates of use reduced fresh weight of cotton seedlings from 12 to 34%. The mean vapor toxicity of the dinitroaniline herbicides to Japanese millet [Echinochloa crus-galli (Roxb.) Wight ‘frumentacea’] shoots averaged from no injury with oryzalin (3,5-dinitro-N4,N4-dipropylsulfanilamide) and nitralin to 66% injury with dinitramine and trifluralin.

Our purpose was to investigate the influence of growth reserves of perennial range grasses on their tolerance to applications of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine]. Competing annual grasses were removed with atrazine treatment at 1.1 kg/ha. The carbohydrate reserves of the perennial grasses were reduced by mowing the previous season. Crested wheatgrass [Agropyron desertorum (Fisch. ex Link) Schult.] plants with reduced carbohydrate reserves, as indicated by reduced etiolated growth, were less tolerant to atrazine than their nonmowed counterparts. Without reduced carbohydrate reserves, squirreltail [Sitanion hystrix (Nutt.) J. G. Smith] was much more susceptible to atrazine than crested wheatgrass. Thus, the weed control treatment did not necessarily increase the density of desirable perennial grasses.

Decomposition of 11 dinitroaniline herbicides was determined following 7 days exposure to unfiltered solar radiation in July and compared with that in the dark. Radiolabeled herbicides were applied at 1 kg/ha to dry soil thin layer plates. Decomposition of all dinitroanilines was higher when exposed to sunlight than if held in the dark under otherwise similar conditions. Thermal decomposition was noted for all dinitroanilines except profluralin [N-(cyclopropylmethyl)-α,α,α-trifluoro-2,6-dinitro-N-propyl-p-toluidine] and dinitramine (N4,N4-diethyl-α,α,α-trifluoro-3,5-dinitrotoluene-2,4-diamine). Photodecomposition for each herbicide was: 7.8% AC 92390 [N-(1-ethylpropyl)-2,6-dinitro-3,4-dinitro-3,4-xylidine], 8.2% isopropalin (2,6-dinitro-N,N-dipropylcumidine), 9.0% butralin [4-(1,1-dimethylethyl)-N-(1-methylpropyl)-2,6-dinitrobenzenamine], 9.9% pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine], 17.1% benefin (N-butyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine), 18.4% trifluralin (α,α,α-trifluoro-2,6-dinitro-N-N-dipropyl-p-toluidine), 26.6% oryzalin (3,5-dinitro-N4,N4-dipropylsulfanilamide), 30.4% fluchoralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)aniline], 40.6% nitralin [4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline], 47.6% profluralin, and 72.3% dinitramine.

When seed coats of scarlet morningglory (Ipomoea coccinea L.) were pricked with a needle or scarified by H2SO4, seed germination was improved. The highest percent germination was obtained with seeds pricked with a needle or exposed to H2SO4 for 180 min prior to incubation in moist paper towels. Seed planted in a potting mixture of sandy clay loam, ground sphagnum moss, and sand had the highest germination after 75 min exposure to acid. Scarlet morningglory seed germination was highest (74%) between 15 and 30 C after 144 h incubation.

The translocation and effects of glyphosate [N-(phosphonomethyl)glycine] were determined in greenhouse-grown peach [Prunus persica (L.) Batsch] seedlings obtained from different seed parents. Two lower branches of 10-week-old seedlings of Rutgers Red Leaf, Bailey, Kalamazoo, Siberian C, and Harrow Blood were sprayed with 0.9 to 18 mg of glyphosate per plant. Seedlings of Harrow Blood, Bailey, and Kalamazoo were more susceptible to injury than were those of Siberian C and Rutgers Red Leaf. Glyphosate at 6, 9, and 18 mg produced necrotic areas on the stems of the former seedlings and at the higher rates tree death resulted. The Siberian C and Rutgers Red Leaf seedlings showed only chlorosis and malformation of apical leaves although slight stem injury occurred 1 month after treatment. Measurements of plant weight, height, and stem diameter indicated that seedlings of Harrow Blood, Bailey, and Kalamazoo tolerated only 3 mg of glyphosate without growth reduction. This was one-half and one-third of the amount which seedlings of Siberian C and Rutgers Red Leaf tolerated, respectively. The concentration of glyphosate in the treated lower branches and the main stem of the seedlings was determined by thin-layer chromatography. The concentrations in the bark were generally similar among all seedlings. Differential tolerance, therefore, was not based on differences in overall glyphosate concentration in tissues but on protective or resistance mechanisms present only in seedlings of Siberian C and Rutgers Red Leaf.

Tall morningglory [Ipomoea purpurea (L.) Roth.], pitted morningglory (I. lacunosa L.), ivyleaf morningglory [I. hederacea (L.) Jacq.] and entireleaf morningglory (I. hederacea var. integruiscula Gray) were compared for their competitive ability with cotton (Gossypium hirsutum L.) at weed densities of 4, 8, 16 and 32 weeds/15 m of row. Tall morningglory was the most competitive with yield reductions of as much as 88% at 32 weeds/15 m. The higher populations of tall morningglory also significantly reduced cotton populations, mechanical harvesting efficiency and lint percentage below that in the controls. Pitted morningglory densities twice that of tall morningglory were required to produce cotton responses similar to those induced by tall morningglory. Ivyleaf morningglory and entireleaf morningglory were almost identical in their competitive potentials and were less competitive than the other species evaluated. Their competitive ability was greatly reduced by disease. Competition from morningglories did not consistently affect cotton fiber micronaire, fiber length or strength.

The absorption and distribution of 14C terbutryn [2-(tert-butylamino)-4-(ethylamino)-6-(methylthio)-s-triazine] and 14C-fluometuron [1,1-dimethyl-3-(α,α,α-trifluoro-m-tolyl)urea] in cotton (Gossypium hirsutum L. ‘SJ-1’) and snapbean (Phaseolus vulgaris L. ‘Tenderette’) seeds during germination was studied. Seeds of the susceptible snapbean absorbed larger amounts of both herbicides from soil than the resistant cotton seeds. The herbicides accumulated mainly in the seed coat of cotton which shed following emergence, with no translocation to the seedling leaves. In snapbean seeds the herbicides accumulated in the cotyledons, which served as a source for acropetal flow to the developing foliage of the young seedling.