Bioassays using binary mixtures that included a cover crop with known allelopathic potential and a weed species were employed to determine the importance of allelopathy compared to resource competition as interference mechanisms. Responses of weed species germinated with cover crops in a petri dish were measured. Interference between weed and cover crop seedlings was determined in a greenhouse experiment using the additive design, which included partitions to reduce above- and below-ground competition and used capillary mat subirrigation to control moisture and fertilizer availability. Germinating sorghum reduced radicle length of weeds, whereas germinating rye tended to increase weed radicle length. Methods limited above-ground competition, so likely interference mechanisms were below-ground competition and allelopathy. Germination with a cover crop had little effect on germination and shoot length of weeds. Increased density of rye but not of sorghum reduced growth of barnyardgrass seedlings. Reduced number of barnyardgrass leaves in the presence of rye was likely due to allelopathy. Suppression of barnyardgrass dry weight attributed to allelopathic interference by rye was successfully separated and compared to the combined effects of competition and allelopathy.

Vineyard weed communities were examined under four dormant-season cover-crop systems representative of those used in the north-coastal grape-growing region of California: no-till annuals (ANoT) (rose clover, soft brome, zorro fescue), no-till perennials (PNoT) (blue wildrye, California brome, meadow barley, red fescue, yarrow), tilled annual (AT) (triticale), and a no-cover-crop tilled control (NoCT). Treatments were carried out for 3 yr in the interrows of a wine grape vineyard. Glyphosate was used to control weeds directly beneath the vines, in the intrarows. Treatments significantly impacted weed biomass, community structure, and species diversity in the interrows. Orthogonal contrasts showed that tillage, and not the presence of a cover crop, impacted interrow weed biomass. Distance-based redundancy analyses (db-RDA) revealed significant effects of the cover-crop systems and of tillage on weed community structure in the interrows. For scarlet pimpernel and spiny sowthistle, the combination of ANOVA and orthogonal contrasts confirmed their association with the tilled treatments, as revealed by db-RDA. This same approach identified the association between California burclover and the no-till treatments. Our findings of no significant effects of the cover-crop systems on weed biomass, community structure, or diversity in the intrarows demonstrate that the impacts the cover-crop management systems had on the interrows did not carry over to adjacent intrarows. In addition, the fact that the cover crops did not affect vine yield, growth, or nutrition relative to the no-cover-crop control suggests that cover crops are likely to minimize soil erosion from winter rains, which is the primary purpose of vineyard cover cropping in northern California, without adversely affecting vine health or weed control.

In greenhouse experiments, conidia of Colletotrichum truncatum applied in an invert emulsion formulation controlled hemp sesbania 100% in the absence of a dew treatment. In field experiments, hemp sesbania control averaged 95 and 97% in 1989 and 1990, respectively, when this formulation was applied to hemp sesbania seedlings using tractor-mounted, air-assist nozzles. This level of weed control was comparable to that achieved from the herbicide acifluorfen. These results indicate that C. truncatum has excellent potential as a mycoherbicide for controlling hemp sesbania and that this potential can be augmented by formulating the pathogen as an invert emulsion.

Laboratory and field studies were conducted in 1988 through 1990 to determine the effect of soil pH on imazaquin adsorption and persistence in a Crosby silt loam (1.6% organic matter) and a Hoytville clay (3.3% organic matter). Above a pH of 6, 6 to 7% of the total imazaquin present was adsorbed to both soils. Adsorption increased as pH decreased from about 7 to 4.5, with a maximum of 16.5 and 31% adsorbed on Crosby and Hoytville soils, respectively. Imazaquin was more persistent in the Hoytville clay than in the Crosby silt loam. Herbicide dissipated more slowly in both soils in 1988 than in 1989 due to dry soil conditions immediately prior to and following herbicide application in 1988. In both soils, imazaquin persistence increased as soil pH decreased, over a pH range from 6.5 to 4.5. Increased persistence with decreasing pH was attributed in part to increased adsorption and decreased availability for microbial degradation.

Pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] degradation in three soils did not follow first-order kinetics but could be described by a quadratic model. Degradation increased as soil temperature increased in the order 10, 20, 35, and 30C. The rate was the same as 75 and 100% field capacity but slower at 50%. At the same temperature and soil moisture the influence of soil type was small. Based on chemical analysis of soil from field and laboratory studies, the half-life was approximately 47 days. Based on biological analyses of soil, the half-life was 78 to 111 days.

The effect of tillage systems on depth of emergence and densities of giant and green foxtail under different environmental and cropping conditions were evaluated from 1985 to 1987 at Arlington, Hancock, and Janesville, WI. Mean emergence depths in no-till were the shallowest, followed by chisel plow and conventional tillage at each location. At least 40% of the giant and green foxtail plants emerged from the upper 1 cm of soil in no-till compared to about 25% in chisel plow and less than 15% in conventional tillage. As many as 25% of the plants emerged from greater than 4 cm in conventional tillage compared to about 10% in chisel plow and less than 5% in no-till. Seedlings emerged from greater depths in a loamy sand than in a silt loam soil regardless of tillage system. At Arlington, green foxtail was the dominant species in conventional tillage, while giant foxtail dominated in chisel plow and no-till. Foxtail densities were greater in chisel plow and no-till than in conventional tillage at all three locations.

Weed seed persistence in the soil seedbank is central to weed population dynamics; however, limited knowledge of mechanisms regulating seed survival in soil remains an obstacle to developing seed-bank management practices. Weed seeds are rich in carbon and nitrogen, and therefore may represent an important nutritional resource to soil microbes. The objective of this study was to test the hypothesis that weed seed mortality due to microbial predation is limited by soil inorganic N availability and soil C:N ratio. A factorial of N fertilizer rate (0, 14, and 28 mg N kg soil−1) and corn stover addition rate (0 and 3,000 mg stover kg soil−1) was applied to bioassay units containing Illinois field soil (silt loam, 3.8% organic carbon) and seeds of one of eight annual weed species common to Illinois field crops: giant foxtail, green foxtail, yellow foxtail, wooly cupgrass, giant ragweed, redroot pigweed, velvetleaf, and Venice mallow. Seeds were incubated for 2 mo, after which they were recovered from the soil and tested for viability. Only three of the eight species, velvetleaf, giant ragweed, and wooly cupgrass, responded to the experimental treatments. Velvetleaf seed mortality was 40% lower in the corn stover–amended treatment than in the unamended treatment. Both giant ragweed and wooly cupgrass showed a more complex interaction between N fertilizer and corn stover treatments. Path analysis supported the hypothesis that the influence of soil N on seed mortality in velvetleaf was because of the direct effect of soil N on microbial predation of velvetleaf seeds, whereas for giant ragweed and wooly cupgrass, the effect on seed mortality appeared to be mediated through soil N effects on germination. Mechanisms underlying soil N fertility effects on weed seed mortality appear to be species-specific. Future investigations of this phenomenon should include quantitative measures of seed coat composition and quality.

Relevant data on the sorption of organic compounds by isolated plant cuticles are reviewed and discussed in relation to the foliar penetration process. The chemical properties and structure of plant cuticles favor sorption of lipophilic compounds and play an important role in the penetration of biologically active substances. With organic acid auxins 2,4-D and NAA, and methylene blue as molecular probes, concentration, pH, temperature, and surfactants were important factors affecting sorption. The constituent waxes of the cuticle markedly inhibit sorption of a wide range of organic compounds. Octoxynol surfactants that have 5 or 7.5 polyoxyethylene groups interact with the epicuticular wax to enhance the sorption of NAA. At sorption equilibrium, the cuticle has a lower affinity for methylene blue in the region usually rich in cuticular waxes and in a narrow band in the cuticular pegs.

The purpose of this study was to investigate seed dispersal and germination of seeds in the soil as factors in establishment of big sagebrush seedlings. Seed dispersal began at seed maturity in December. Under the shrub canopy the dispersal of seeds was greater to the east. Between shrubs the dispersal of seeds tended to be uniform. For 6 months of each year there were no detectable germinable seeds in the litter and surface soil at the sites sampled. Germinable seeds in the soil increased rapidly during seed dispersal but were not detected by June of the next season. Enrichment of bioassay samples with GA3 or KNO3 did not enhance emergence of big sagebrush seedlings. Big sagebrush seedlings were found at very low densities in areas previously burned in wildfires. This suggests the presence of viable big sagebrush seeds in the soil at a very low density.

On the rich alluvial soils of the Mississippi Delta it is not uncommon for Johnsongrass (Sorgum halepense (L.) Pers.) to spread over entire plantations. Eradication is not feasible, and adequate control in the Delta areas is accomplished only by the diligent use of control practices.

The herbicide AC 252 214 {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl] 3-quinoline-carboxylic acid} was absorbed by the roots and foliage of soybean [Glycine max (L.) Merr. ‘Williams’], common cocklebur (Xanthium strumarium L. ♯ XANST), and velvetleaf (Abutilon theophrasti Medic. ♯ ABUTH), and then translocated in the xylem and phloem to meristematic regions. AC 252 214 was metabolized rapidly by soybean and velvetleaf but appeared to be metabolized slowly by cocklebur. The order of tolerance of these three species to AC 252 214 was soybeans > velvetleaf > cocklebur. This order of tolerance was directly correlated in young plants with the half-life of AC 252 214 within the tissue. Velvetleaf exhibited increased tolerance to AC 252 214 with age, which was attributed partially to greatly reduced absorption of the herbicide by older leaves and more rapid metabolism of the herbicide.

Several field studies were conducted during 1981 and 1982 to determine whether early preplant (EPP) applications of residual herbicides would prevent the establishment of vegetation before planting no-till soybeans [Glycine max (L.) Merr. ‘Williams’]. Early preplant applications of either cyanazine {2-[[4-chloro-6-(ethylamino)-s-triazin-2-yl] amino]-2-methylpropionitrile} or cyanazine plus oryzalin (3,5-dinitro-N4,N4-dipropylsulfanilamide) were applied in the fall and 3, 2, and 1 month(s) before planting no-till soybeans. In all studies, the treatments prevented vegetation from becoming established before planting, and season-long weed control was achieved with several different treatments. Early preplant cyanazine plus oryzalin provided greater than 90% control for the entire season where grass densities were low. Where grass densities were high (greater than 90% ground cover), EPP cyanazine plus a preemergence application of metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] plus metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] or a postemergence application of sethoxydim {2-[1-(ethoxyimino) butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} controlled 90% of the weeds, which was equal to or better than the standard preemergence treatments used (80 to 98% weed control).

Four experiments were conducted to develop controls for downy brome (Bromus tectorum L.) in winter wheat (Triticum aestivum L. ‘Gaines’). In three experiments downy brome was removed at monthly intervals from emergence to harvest. In the third and fourth experiments, hand weeded and weedy controls were compared to chemical control with R11913 (3′-hydroxypropionanilide isopropylcarbamate). Downy brome, which emerged with wheat, was most competitive during March of each year. For maximum effectiveness, R11913 should be applied when downy brome has less than one to three tillers or chemical control will be erratic. On a silt loam soil with an average annual precipitation of 42 cm, downy brome densities of 108 to 160 plants/m2 lowered wheat yields 6% when competition was eliminated by March. When competition was permitted until wheat harvest (July), yield was reduced 40%.

Field studies were conducted to determine the effect of varying densities of hairy nightshade and varying durations of interference on the biomass and seed yield of dry beans. As few as two hairy nightshade plants per meter of row reduced bean seed yield by an average of 13% over the 2 yr of the study. Increasing the density of hairy nightshade to 100 plants per meter resulted in bean yield losses of 77%. Hairy nightshade interference during the first 3 weeks after crop emergence was sufficient to reduce bean yields. Up to 9 weeks of hairy nightshade-free maintenance after crop emergence was required to prevent bean yield losses. Hairy nightshade was a prolific seed producer. At low infestation densities, over 45 000 seeds per plant were produced. Hairy nightshade seed production peaked at over 300 000 seeds m–1 at about 30 plants per m of row. Depending on the length of the growing season, 6 to 9 weeks of weed-free conditions after crop emergence were required to prevent hairy nightshade from producing viable seeds before the first killing frost in the fall. Results are discussed in terms of timing and longevity of control required to reduce hairy nightshade interference in dry beans.

Germination of seeds of 10 grass and 33 broadleaved weed species was examined for response to ethylene. Germination was promoted in nine species, inhibited in two, and not affected in the remainder. Of the species promoted, common purslane (Portulaca oleracea L.), common lambsquarters (Chenopodium album L.), and several Amaranths, including redroot pigweed (Amaranthus retroflexus L.), were affected most. Transformation of phytochrome to the active form (Pfr) gave interactions that ranged from none to syntergistic with the applied ethylene. In subsequent tests seeds of purslane, redroot pigweed, and giant foxtail (Setaria faberi Herrm.), a species not responsive to ethylene, were examined for germination response to 14 low molecular weight hydrocarbon gases other than ethylene. Some stimulation by the olefins propylene and propadiene was found for purslane and pigweed. Propionaldehyde and butyraldehyde were slightly stimulatory to purslane only.

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.

The effect of droplet size on retention, absorption, and translocation of 14C-glyphosate was studied in glyphosate-resistant corn. Fine, medium, and coarse spray droplets were studied using a track-sprayer equipped with commercially available nozzles. Glyphosate-resistant corn was used to obtain measurements at field use rates in the absence of phytotoxicity. Spray retention on corn leaves was calculated based on recovered glyphosate per leaf area, and retention was higher with application of fine droplets (47%) than with application of coarse (38%) and medium (37%) droplets. Absorption in corn leaves was directly correlated with droplet size and reached a plateau 1 d after treatment (DAT) for all droplet sizes. Based on glyphosate recovered 3 DAT, coarse droplets showed the highest absorption (49%), followed by medium (35%) and fine (30%) droplets. Percentage of translocation also increased with droplet size, and translocation was primarily toward strong sink tissues such as roots and young leaves. Our results show that large droplets have slightly reduced retention in corn but have increased absorption resulting in increased translocation of glyphosate to growing sink tissues.

Field experiments were conducted on eight weed species to determine if chlorimuron, fomesafen, imazethapyr, or sulfentrazone at two rates (labeled and one-half the labeled rate) were complementary tank mixtures with glyphosate at 210 and 420 g ai ha−1. Laboratory experiments were conducted on barnyardgrass, pitted morningglory, Palmer amaranth, and velvetleaf using radiolabeled glyphosate, chlorimuron, and imazethapyr to determine the absorption and translocation pattern of these herbicides applied alone and in combination. In the field, glyphosate plus chlorimuron tank mixtures were generally additive. Adding chlorimuron did not decrease absorption or translocation of 14C-glyphosate by barnyardgrass, pitted morningglory, or velvetleaf. Adding glyphosate increased absorption of 14C-chlorimuron by Palmer amaranth and velvetleaf. All four fomesafen plus glyphosate rate combinations were antagonistic to goosegrass, sicklepod, Palmer amaranth, and velvetleaf, and three of the four were antagonistic to barnyardgrass and entireleaf morningglory. Fomesafen decreased absorption and translocation of 14C-glyphosate in barnyardgrass, pitted morningglory, and velvetleaf. Ninety percent of glyphosate plus imazethapyr combinations were additive or synergistic, with all rate combinations synergistic for pitted morningglory. Adding glyphosate to imazethapyr increased absorption of 14C-imazethapyr by Palmer amaranth and velvetleaf. Glyphosate plus sulfentrazone tank mixtures were antagonistic at all rate combinations for barnyardgrass and Palmer amaranth and at three of the four combinations for goosegrass and entireleaf morningglory, indicating that these herbicides are not complementary in tank mixtures.

Greenhouse and field experiments were conducted to evaluate the potential for antagonistic or synergistic interactions from tank mixtures of glyphosate plus a selective herbicide applied postemergence. In the greenhouse, glyphosate at 420 g ae ha−1 plus 28% liquid urea-ammonium nitrate (28% UAN) provided at least 89% control of common lambsquarters and common ragweed. Glyphosate at 1,680 g ha−1 plus 28% UAN provided less than 81% control of velvetleaf and less than 75% control of ivyleaf morningglory. Tank mixing bentazon at 1,120 g ai ha−1 with glyphosate at 420 g ha−1 synergistically increased control of velvetleaf. Tank mixtures of glyphosate plus a selective herbicide were predominately additive in control of common lambsquarters, common ragweed, and velvetleaf. Several tank mix combinations of chlorimuron or imazethapyr plus glyphosate plus 28% UAN were antagonistic in control of ivyleaf morningglory. In the field, glyphosate at 840 g ha−1 plus 28% UAN provided at least 88% control of common lambsquarters and velvetleaf in 1994. However, glyphosate at 840 g ha−1 plus 28% UAN provided only 60% control of velvetleaf in 1995. Tank-mixing bentazon or CGA-248757 with glyphosate at 420 g ha−1 increased velvetleaf and common lambsquarters control in 1995. In general, adding chlorimuron, imazethapyr, or thifensulfuron to glyphosate plus 28% UAN did not increase control of common lambsquarters or velvetleaf. Tank mixing imazethapyr with glyphosate plus 28% UAN antagonized velvetleaf control in 1994 and in 1995. The tank mixture of thifensulfuron at 2 g ha−1 plus glyphosate at 420 g ha−1 plus 28% UAN increased soybean injury in the field in 1994. However, tank mixing chlorimuron, imazethapyr, or thifensulfuron with glyphosate plus 28% UAN did not increase soybean injury in the greenhouse or in the field in 1995.

Greenhouse studies indicated that 3,6-dichloro-o-anisic acid (dicamba) or its metabolic derivative was strongly accumulated in meristematic tissues of Tartary buckwheat (Fagopyrum tataricum (L.) Gaertn.) and wild mustard (Sinapis arvensis L.) following both foliar and root uptake. In barley (Hordeum vulgare L.) and wheat (Triticum vulgare L.), it was distributed throughout the plants. Detoxification of dicamba occurred in all four species though not at equal rates, and a common major metabolite was identified chromatographically as 5-hydroxy-3,6-dichloro-o-anisic acid. A minor metabolite, 3,6-dichlorosalicylic acid, was found in barley and wheat but not in Tartary buckwheat or wild mustard. The four species tolerated dicamba treatment in the order of wheat, barley, wild mustard, and Tartary buckwheat. This ranking corresponds with the ability of the plants to detoxify dicamba and is inversely related to the extent of dicamba absorption and translocation in them.