Analysis of soil in which corn (Zea mays L.) has been grown annually for 9 yr with or without yearly applications of 3.4 kg/ha atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] indicated that long-term atrazine use had little effect upon soil microbes or biochemical processes. With the exception of a transient inhibition of bacterial growth during the first week after field application, repeated applications did not affect the numbers of viable bacteria or fungi, relative abundance of bacteria producing hydrolytic enzymes, or soil enzyme levels.

The effect of picloram (4-amino-3,5,6-trichloropicolinic acid) on regeneration of detached leaves of Echeveria elegans Bgr. was studied. Picloram stimulated callus production at all concentrations. Low concentrations of 1 to 15 ppmw picloram did not affect shoot organogenesis and the same, however, stimulated rhizogenesis. Inhibition of both root- and shoot-organogenesis was noticeable at concentrations below 20 ppmw and above. Roots increased in number at all picloram concentrations below 40 ppmw, the maximum number being at concentration of 15 to 20 ppmw of the herbicide. Observations indicated that root elongation was considerably suppressed. Picloram-induced callus contained mature parenchyma cells devoid of contents and considerable amount of tissue disruption was noticed. Irregular vascular differentiation and obliteration of tissues particularly in the region of tracheary elements, were obvious. Although normal apical organization was lacking in root primordia and in arrested short roots, these elongated normally into mature roots when they were transferred to water, thus indicating that apical meristems were not severely affected by picloram.

Analysis of entire barley (Hordeum vulgare ‘Moravian’) seedlings exposed to triallate [S-(2,3,3-trichloroallyl)-diisopropylthiocarbamate] vapor for 5 days after a 5-day germination period resulted in a linear rate of accumulation of triallate, while wild oat (Avena fatua L.) seedlings accumulated triallate for 1/2 to 2 days after exposure with no net accumulation thereafter. Similar accumulation patterns were shown for a similar exposure period when barley and wild oat were seeded 1.3 cm below a triallate-treated soil layer in the greenhouse. More triallate accumulated in wild oat and barley when they were planted in a triallate-treated soil layer than below. Since barley accumulated greater amounts of triallate without injury than wild oat, differential uptake of triallate was not the reason for selectivity.

Cotton (Gossypium hirsutum L. ‘Stoneville 7A’) grown for 3 yr in competition with various weed densities and nitrogen levels showed annual weeds could reduce cotton heights as much as 75% and cotton yields as much as 88%. Cotton yields were increased by 18 to 30% by adding nitrogen to plots with no weeds. In 1973, cotton supplied with 224 kg/ha of nitrogen could tolerate up to 50% weeds without loss of yield. Weeds left to grow and reseed with no control resulted in increasingly greater competition with each succeeding year.

Benthiocarb S-[(4-chlorophenyl) methyl] diethylcarboamothioate which was applied postemergence, altered the growth and development of shoots of barnyardgrass [Echinochloa crus-galli (L.) Beauv.] and bearded sprangletop [Leptochloa fascicularis (Lam.) Gray] seedlings, two gramineous annual weeds commonly found in rice fields. The herbicide did not affect root development of either species. Benthiocarb retarded the growth of new leaves within the extending leaves and caused swelling of cells. The cells of the shoot apices of treated seedlings were often vacuolated. Tubular-like leaves were induced by benthiocarb, but their formation differed from that caused by phenoxy herbicides.

Foliar penetration of 14 C-difenzoquat (1,2-dimethyl-3,5-diphenyl-1-H-pyrazolium), in the presence of a surfactant, into both wild oat (Avena fatua L.) and barley (Hordeum vulgare L. ‘Conquest’), was rapid and continued at a nearly constant rate for 72 hr, the maximum time period studied; 93 and 84% of the applied dose was absorbed by the leaves of wild oat and barley, respectively. Added surfactants at 0.01% to 0.3% and high relative. humidity (RH) greatly enhanced the penetration of the herbicide into wild oat. An increase in temperature from 10 to 30 C resulted in a large increase in penetration. Commercial formulations of bromoxynil (3,5-dibromo-4-hydroxy-benzonitrile) + MCPA {[(4-chloro-O-tolyl)oxy] acetic acid} (1:1, w/w), 2,4-D[(2,4-dichlorophenoxy)acetic acid] ester and barban (4-chloro-2-butynyl m-chlorocarbanilate) markedly increased the penetration of 14C-difenzoquat into wild oat. Amine formulation of 2,4-D, on the other hand, had no effect on penetration. Foliarly-applied 14 C-difenzoquat rapidly translocated mainly in the acropetal direction. Most of the absorbed 14C, however, was accumulated in the treated area and a few centimeters above the treated area. There was some basipetal movement, and 24 hr after treatment 14C was detectable in the roots. Root uptake of 14C-difenzoquat was very rapid, but upward movement of 14C was slight, and even 24 hr after treatment most of it was retained by the roots. Chromatographic analysis of plant extracts up to 15 days after treatment with 14C-difenzoquat revealed no evidence of difenzoquat metabolism by wild oat, barley, and wheat (Triticum vulgare L. ‘Thatcher’). The selective action of difenzoquat in wild oat and barley cannot be accounted for by differences in its foliar penetration, translocation, or metabolism in these species.

Metribuzin [4-amino-6-tert-butyl-3-(methylthio-as-triazin-5(4H)one] degradation by soil microorganisms decreased as the soil pH decreased. Metribuzin and atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] mobility increased with increasing concentration and soil pH. 14C-metribuzin had greater mobility on soil thin-layer plates than 14C-atrazine because of differences in water solubility and basicity. Adsorption increased with decreasing soil pH. The pKa for metribuzin was 0.99 ± 0.08. Adsorption increased gradually as the soil pH decreased toward the pKa of metribuzin. Protonation of the amine group with subsequent adsorption to soil colloids was the postulated reason for the reduction in phytotoxicity, microbial degradation, and mobility of metribuzin at low soil pH levels.

Field and greenhouse studies of 2,4-D [(2,4-dichlorophenoxy)acetic acid] and glyphosate [N-(phosphonomethyl) glycine] absorption, translocation, and metabolism were initiated to explain field observations which indicated susceptibility of common milkweed (Asclepias syriaca L.) but not hemp dogbane (Apocynum cannabinum L.) to glyphosate and the reverse response to 2,4-D. Glyphosate was absorbed less than 2,4-D in both species with absorption of both herbicides being greater in common milkweed. Greater herbicide absorption by common milkweed was attributed to less epicuticular wax, less cuticle, lower contact angle of the herbicide spray, and the presence of stomata and trichomes on the adaxial leaf surface. No major translocation differences of the herbicides were noted between species. Translocation of glyphosate was more rapid than that of 2,4-D. More glyphosate than 2,4-D accumulated in areas of high meristematic and metabolic activity. Rapid 2,4-D metabolism occurred in common milkweed leaves above treated leaves. There was no detectable 2,4-D metabolism in hemp dogbane roots after 20 days, while 60% of the 2,4-D in common milkweed roots was metabolized. Limited absorption of glyphosate but not 2,4-D by hemp dogbane and metabolism of 2,4-D but not glyphosate by common milkweed were considered the primary factors involved in explaining observed susceptibility differences.

The intraspecific (competition between same species) and interspecific (competition between different species) competitiveness of tall morningglory [Ipomoea purpurea (L.) Roth] in soybeans [Glycine max (L.) Merr.] was evaluated for 3 yr at Fayetteville, Arkansas. Tall morningglory was planted at densities of one plant per 61, 30, and 15 cm of row, and competition duration ranged from 2 weeks to full-season. Soybeans were more competitive than tall morningglory for the first 6 to 8 weeks after emergence. The competitiveness of tall morningglory was dependent upon a rapid increase in photosynthetic area which occurred 4 to 6 weeks after emergence. Reductions in leaf area index (LAI) or plant dry weight and crop growth rate (CGR) of soybeans were closely correlated to percent soybean yield reduction. Tall morningglory could remain in soybeans for 12, 10, and 6 weeks without significantly reducing soybean yield at one plant per 61, 30, and 15 cm of row, respectively. Tall morningglory was three to four times more competitive during the soybean reproductive stage than during the vegetative stage.

The occurrence, abundance, and nature of weed communities were reviewed in relation to man's manipulation of the natural environment. Manipulations of the environment necessary for agricultural production have favored secondary successional species. That many of these seral species have opportunistic genotypes has led to the development of weeds. Weeds conform to the concepts of ecologic succession, but agricultural ecosystems are very dynamic with sudden and recurrent changes. The acceleration of successional patterns has provided environments conducive to the selection of competitive genotypes among numerous weeds. Specialized breeding systems provide rapid responses to changing conditions. Weeds increase the diversity of agricultural ecosystems by utilizing environmental potential concentrated for crop production. Weeds have been the shadow of history in that they have mirrored man's activities. The introduction of weed species to new environments may be one of the greatest manipulations of the natural environment, the total consequences of which will be determined in the future.

The effectiveness of several preemergence and postemergence weed control systems for control of common cocklebur (Xanthium pensylvanicum Wallr.) in soybeans [Glycine max (L.) Merr.] was determined in field research over a 3-yr period. Sequential applications of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5(4H)one] at 1.1 kg/ha applied preemergence, and dinoseb (2-sec-butyl-4,6-dinitrophenol) at 1.7 kg/ha, 2,4-DB [4-(2,4-dichlorophenoxy)butyric acid] at 0.2 kg/ha, or bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-(4)3H-one 2,2-dioxide] at 1.1 kg/ha applied postemergence, provided at least 90% control of common cocklebur and increased soybean yields. Use of single herbicide applications was less effective than sequential applications, but metribuzin applied preemergence was more effective in controlling common cocklebur and increasing soybean yields than single applications of any herbicide applied postemergence. Effective control of common cocklebur with sequential herbicide treatments increased soybean seed grades by reducing discounts caused by several grade lowering components including moisture content and foreign material in harvested seed. Use of all herbicide practices increased returns above specified costs, but greatest returns followed use of sequential treatments.

The divisions and subjects of weed biology were identified. Twenty-eight representatives from the herbicide industry ranked the priority of the identified subjects. They also gave their thoughts and impressions on specific questions dealing with weed biology. The divisions which received a moderate to high priority rating are: Identification and Classification of Weeds, Harmful Aspects of Weeds, Weed Growth and Development, and Weed Control. A review of the papers published in Weed Science indicates an increase in the number of papers being published on the subject of weed biology and a decrease in the number of papers dealing with chemical control. The proceedings of the regional weed societies in the U.S.A. had a high percent of the papers devoted to chemical control, with the lowest percentage devoted to weed biology. Comments are made as to why weed biology is important and how research data on weed biology would be used by industry. A comparison of the rating that industry assigned to the various subjects of weed biology to the amount of published literature indicates insufficient work has been done in many areas. It also appears that there is an imbalance in research. More work is being done on certain subjects that industry considers of lesser value than subjects of greater importance.

Bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-(4) 3H-one 2, 2-dioxide] was applied postemergence to common cocklebur (Xanthium pensylvanicum Wallr.) and velvetleaf (Abutilon theophrasti Medic.) at various times of the day in growth chamber and field studies and to soybeans [Glycine max (L.) Merr.] in field studies. Soybeans were tolerant of bentazon regardless of time of day when treated. Results with common cocklebur and velvetleaf suggest that the time of day when bentazon is applied can be of practical importance. Poor control might occur following application in late evening, night, or early morning.

Field experiments were conducted on clay and clay loam soils at the International Rice Research Institute to investigate the effects of competition between upland rice (Oryza sativa L.) and varying populations of purple nutsedge (Cyperus rotundus L.) for nitrogen, moisture, and light on the grain yield of upland rice. ‘IR5’ was used in 1972, while the experimental line ‘IR442-2-58’ was used in 1973. Grain yields of ‘IR5’ and ‘IR442-2-58’ increased with 60 kg/ha of nitrogen, and decreased with increased purple nutsedge population. The application of nitrogen to weedy upland rice fields benefited the purple nutsedge more than the rice. Purple nutsedge and upland rice competed extensively for moisture and the competition was much more serious with increased nitrogen fertilization. Purple nutsedge competition reduced the light transmission ratio (LTR). The reduction was proportional to the increase in purple nutsedge population density. Reductions in the LTR were greater at higher nitrogen levels during the three seasons.

Effects of glyphosate [N-(phosphonomethyl)glycine] and paraquat (1,1′-dimethyl-4,4′-bipyridinium ion) on turfgrass seed germination were evaluated in the greenhouse. Glyphosate caused little effect on germination of Kentucky bluegrass (Poa pratensis L.), red fescue (Festuca rubra L.), and tall fescue (F. arundinacea Schreb.) when applied to the soil immediately before seeding or when applied directly over the seed on the soil surface. Paraquat sprayed directly over the seed on the soil surface prevented germination of most of the seeds of the three grass species. Covering seeds on the soil surface with clippings from grass turf sprayed with paraquat greatly reduced germination. About half of those that germinated either died later or were severely chlorotic. Covering seeds on the surface of the soil with clippings from turf that had been sprayed with glyphosate did not significantly reduce the number of seedlings established.

Neither glyphosate [N-(phosphonomethyl)glycine] nor the soil metabolite aminomethylphosphonic acid were detected in the first flow of water through two canals following application of glyphosate at 5.6 kg/ha to ditchbanks when the canals were dry. Soil samples collected the day before canals were filled (about 23 weeks after treatment) contained about 0.35 ppm glyphosate and 0.78 ppm aminomethylphosphonic acid in the 0 to 10-cm layer. When glyphosate was metered into the water at a rate calculated to provide 150 ppb in the canal water at a single site on two flowing canals, about 70% of the glyphosate was accounted for 1.6 km downstream from the application site. Thereafter, the rate of disappearance diminished, and about 58% of the applied glyphosate was present at the end of the canals 8 or 14.4 km downstream from the introduction sites.

The effect of light on the response of green beans (Phaseolus vulgaris L. ‘Tendercrop’) and soybeans [Glycine max (L.) Merr. ‘Merit’] to fluorodifen (p-nitrophenyl α,α,α-trifluoro-2-nitro-p-tolyl ether) was evaluated with respect to light quantity, quality and time of exposure. Herbicide-induced injury was intensified with exposure of plants to reduced levels of light. Comparisons of light quality indicated that this herbicide interaction with light was greatest at longer wavelengths of visible light (yellow and red). The most critical time for light exposure to result in increased herbicide injury was at the time of emergence of seedlings.

Effectiveness of bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-(4)3H-one 2,2-dioxide] for control of common cocklebur (Xanthium pensylvanicum Wallr.) in soybeans [Glycine max (L.) Merr.] was evaluated in field research over a 3-yr period. Bentazon, applied over-the-top at 0.6 to 1.1 kg/ha, controlled common cocklebur more effectively than over-the-top or directed applications of dinoseb (2-sec-butyl-4,6-dinitrophenol) at 0.8 to 1.7 kg/ha or 2,4-DB [4-(2,4-dichlorophenoxy)butyric acid] at 0.2 kg/ha. Single applications of bentazon were highly effective for control of common cocklebur, but multiple applications usually provided the best control and highest soybean yields. A preemergence treatment of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5(4H)one] at 1.1 kg/ha and a postemergence application of bentazon at 1.1 kg/ha provided 92 to 99% control of common cocklebur. Control of common cocklebur with bentazon reduced discounts in harvested soybean seed caused by excessive moisture content and foreign material. The reduction in discounts improved soybean seed grades.

Emergence of tall morningglory [Ipomoea purpurea (L.) Roth] seedlings was delayed and reduced as planting depths increased from 1.3 to 5.0 cm. Development of seedlings emerging from 3.8 cm or greater depth was delayed for up to 4 weeks as indicated by dry weight accumulation. In both greenhouse and field studies, chemical control of tall morningglory was generally greater when seedlings emerged from planting depths of 2.5 cm or greater. The competitive ability of tall morningglory, as indicated by yield reduction of cowpea [Vigna sinensis (L.) Endl], was dependent upon both planting depth and duration of infestation. Cowpea yields were reduced more by the tall morningglory plants that emerged from 1.3 cm than by those that emerged from greater planting depths for a given infestation period.

Experiments involving the application of nitralin [4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline] at 0.56, 1.12, or 1.68 kg/ha on each of three successive plantings of pickling cucumbers (Cucumis sativus L. ‘Explorer’) were conducted during a 2-yr period. Residues of nitralin in soil increased with rate and number of applications. Nitralin residues accumulated to higher concentrations in a soil with 2.9% organic matter than in a soil with 0.7% organic matter. However, cucumber plants were injured less in the soil with the highest concentrations of herbicide. Oats (Avena sativa L. ‘Carolee’) planted following cucumbers were injured by nitralin residues resulting from the highest application rates. Bioassays and chemical analyses suggested that no appreciable soil residues of nitralin remained during the succeeding year.