Rhizomes develop from the crowns of yellow nutsedge (Cyperus esculentus L.) shoots and the tips transform into shoots or tubers. Transformation of primary rhizomes developed by the original shoot of a plant is a valid index of plant response to treatment when compared with the transformation of all rhizomes (primary, secondary, tertiary, and others), developed by a plant under treatment.

The relationship between trifluralin dosages detected in the soil at seeding, initial crop injury, and subsequent recovery of spring wheat during the growing season was investigated in field experiments in 1986 and 1987. As the amount of trifluralin in the soil increased, both crop density and dry matter production decreased such that at 1 kg/ha the two were reduced by 37 and 50%, respectively, early in the season. As the season progressed, crop growth rates (CGRs) of wheat in trifluralin-treated plots exceeded those of wheat in the untreated plots. Maximum CGRs occurred between Zadok's growth stages 30 and 45 where trifluralin levels in the soil were 0.3 to 0.5 kg ai/ha at seeding. Recovery from trifluralin injury was characterized by enhanced net assimilation rates of surviving plants, increased tillering and greater dry matter production per plant. Wheat seed yield was only weakly correlated with trifluralin levels in the soil at seeding. From a linear regression model it was determined that a 35% reduction in plant dry weight from the trifluralin injury at the beginning of tillering would result in no more than a 10% reduction in seed yield at final harvest.

We applied 1,1′-dimethyl-4,4′-bipyridinium ion (paraquat) to quackgrass (Agropyron repens L. Beauv.) at different stages of growth and subjected these treated plants to different light regimes. After paraquat and light treatment, foliage was removed and the reduction in regrowth was measured as evidence of paraquat movement and its effect on the regenerative potential of the rhizomes. Paraquat or a toxic metabolite moved in quantity from point of leaf application. Sufficient basipetal movement occurred in both the dark and light to significantly affect regrowth. As quackgrass plants matured and developed more foliage, the effect of paraquat treatment increased (i.e., the regrowth capacity of the plants and the regenerative ability of their rhizome buds decreased). A period of darkness (24 hr) after paraquat application and before exposure of treated plants to light did not affect numbers of plants killed but did decrease rate of regrowth. Treated quackgrass foliage showed extensive paraquat injury when placed in light for 48 to 72 hr after treatment; but only parts of treated leaves were killed. Leaving treated foliage intact greatly increased the number of plants killed.

Percentage canopy reduction of threadleaf snakeweed [Gutierrezia microcephala (D.C.) Gray] and common goldenweed [Isocoma coronopifolia (Gray) Greene] was substantially greater when sprays were applied during the spring of a year characterized by above-average rainfall than in a drought year. Applied at 2 kg/ha or less, 2,4-D [(2,4-dichlorophenoxy)acetic acid] only partially controlled threadleaf snakeweed in either year. Picloram (4-amino-3,5,6-trichloropicolinic acid) or triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid} gave satisfactory control of threadleaf snakeweed at 0.5 or 1 kg/ha, depending upon year of application, whereas these treatments were effective against common goldenweed only in the “wet” year. Herbicide 3,6-dichloropicolinic acid tended to be less effective against either species than picloram or triclopyr. Approximately 1 and 2 yr after herbicides were applied, standing forage crop increased 37 and 25 kg/ha, respectively, with each percentage point reduction of threadleaf snakeweed canopy cover.

Herbicide absorption and translocation patterns were determined following a wet blade method of application to dogfennel stems. Treatment consisted of severing stems with a pruning shear blade containing 5 μl of [14C]-triclopyr or [14C]-clopyralid solution. More than 73 or 82% of applied [14C]-triclopyr or [14C]-clopyralid was absorbed immediately, and uptake increased to more than 90%, 60 min after treatment. Both radiolabeled compounds appeared in the roots as soon as 12 h after treatment. By 48 h, 5 to 7% of the [14C] label was present in the roots, and 7 to 10% was exuded from the roots.

Zoysia germplasm exhibit different levels of sensitivity to fluazifop-P-butyl, but the genetic factors responsible for such differences are unknown. Segregation patterns of the fluazifop-P-butyl tolerance trait were studied under greenhouse conditions. In total, 244 F1 lines were generated from multiple crosses between the tolerant line 5337-2 (non–target site tolerance) and three more-sensitive lines (123, 252, and 5330-23). Progeny segregation showed that fluazifop-P-butyl tolerance within zoysiagrass (Zoysia spp.) is expressed as a quantitative trait with a wide range of intermediate phenotypes between parental phenotypes. Transgressive segregation was extensive and largely favored susceptibility in most families, but was especially evident for 5337-2 × 123 and 5337-2 × 5330-23. The segregation patterns for biomass reduction and percent injury were different within reciprocal crosses and among three different family crosses. Reciprocal effects were observed in growth reduction for 5337-2 × 5330-23, in percent injury at 3 wk after the treatment (WAT), and for 5337-2 × 252 at 6 WAT. This indicated that fluazifop-P-butyl tolerance was not completely controlled by nuclear genetic factors in 5337-2 and maternal/cytoplasmic inheritance was also partially responsible. These results suggested that fluazifop-P-butyl tolerance may be attributed to multiple genetic mechanisms, which could present a challenge for future breeding efforts because of the difficulty of fixing multiple traits within a breeding population.

Edamame [Glycine max (L.) Merr.] differs from grain-type soybean in several aspects, one being that edamame seeds are 65% to 100% larger than grain-type soybean seed. Crop seed size has implications for weed management in grain-type soybean; however, the extent to which this observation holds true for edamame is unknown. Because weed interference continues to be a barrier to domestic edamame production, the objective was to quantify the effect of edamame seed size on the crop’s ability to tolerate weed interference (CT) and the crop’s ability to suppress weeds (WSA). Five edamame cultivars plus one grain-type cultivar were each sorted to create “small” and “large” seed size classes. Seed lots were included in a split–split plot design, whereby an additional experimental factor was presence or absence of velvetleaf (Abutilon theophrasti Medik.). Crop and weed emergence and growth were monitored through 8 wk after emergence (WAE). Crop plants from large seed had higher tolerance to A. theophrasti than plants from small seed, as evidenced by crop height, area, and biomass. Edamame seed size had little effect on WSA; however, crop cultivars differentially reduced A. theophrasti leaf area and biomass at 4 and 8 WAE. While both seed size and edamame cultivar influence early-season crop competitive ability, the magnitude of these factors on CT and WSA underscores the importance of considering them not as stand-alone tactics but rather as useful additions to a more comprehensive integrated weed management system.

Soybean [Glycine max (L.) Merr.] has recently become a popular rotational crop in the Canadian Northern Great Plains where herbicide-resistant (HR) soybean cultivars have been widely adopted. Intense reliance on herbicides has contributed to the development of HR weeds in soybean and other crops. Cultural weed management practices reduce the need for herbicides and lower the selection pressure for HR weed biotypes by improving the competitiveness of the crop. The effects of two row spacings, three target densities, and three cultivars on the critical weed-free period (CWFP) in soybean were evaluated as three separate experiments in southern Manitoba. In the row-spacing experiment, soybean grown in narrow rows shortened the CWFP by up to three soybean developmental stages at site-years with increased weed pressure. In the target density experiment, low-density soybean stands lengthened the CWFP by one soybean developmental stage compared with higher-density soybean stands. The effect of soybean cultivar varied among locations, yet tended to be consistent within location over the 2-yr study, suggesting that competitive ability in these soybean cultivars was linked to edaphic and/or environmental factors. Generally, the cultivar with the shortest days to maturity, which also had the shortest stature, consistently had a longer CWFP. Each of these cultural practices were effective at reducing the need for in-crop herbicide applications.

The use of herbicides for controlling weeds in the grape row is becoming resolved in mature vineyards. Oil and DNBP mixtures can be used because of the protective covering of old bark on the trunk. Herbicides such as CIPC, monuron and diuron can be used as a band treatment in the row because of the resistance to these chemicals where only a portion of the root zone is treated. Most of the above references caution about the use of these materials on young vineyards or transplants.

Effect of 3,5-dimethyltetrahydro-1,3,5,2H-thiadiazine-2-thione (DMTT) on the growth of Monterey pine (Pinus radiata) seedlings was studied at constant levels of soil moisture maintained by capillary fringes. A high content of water depressed the growth of plants but preserved their normal morphological balance. A low content of water produced succulent seedlings with inferior root systems. The results suggest that DMTT is better adapted to fine-textured soils with restricted aeration. Maintenance of water supply at field capacity of the soil also may improve the balance of nursery stock and its survival potential.

Band applications of the best residual preemergence herbicides were equal to timely shallow cultivations for weed control in soybeans, and were superior when wet weather delayed the early shallow cultivations. The ten-year average soybean yield reduction of 3.8 bushels per acre from weed competition despite normal control methods suggests that, under normal conditions, the net cost of herbicide treatment would not exceed the value of the soybeans lost through failure to control weeds. Coarse broad-leaved annuals were a minor part of the weed populations and there was no giant foxtail, so the benefits of herbicides would probably be greater with these weeds present.

Yellow rocket (Barbarea vulgaris R. Br.) was studied for 5 years under field conditions. Yellow rocket seeds germinated throughout the growing season, but vernalization of the rosettes formed was necessary for flowering. There appeared to be a minimum size plant that could be vernalized. In the spring vernalized rosettes formed flower stalks. As the flower stalk elongated, basal rosette leaves died, giving the impression of an annual. When the flower stalk was cut or seed matured, new rosette leaves developed from buds on the crown. This new growth was capable of going through the subsequent winter. Marked plants survived for at least 3 years. The ability to flower again and set viable seed in the same year after cutting appeared to be associated with the stage of the original flower stalk when cut. Yellow rocket is perennial, but it may not survive after its second year under conditions of disease or strong competition.

Carbon14 carboxy labeled 2,4-D with varied amounts of Tween 20 (polyoxyethylene sorbitan monolaurate) was applied to excised Kalmia angustifolia L. (Ericaceae) leaves and, after an absorption period, the radioactivity of the dried ground leaf tissue was measured by counting. Entry rate of 2,4-D into recently formed leaves decreased with increase in Tween 20, within the range of 2,4-D/Tween 20 ratios used (roughly 5/1 to 0.5/1). Entry rate of 2,4-D into older leaves increased with increase in Tween 20.

In field practice much of the application of herbicides to local infestations of bermudagrass (Cynodon dactylon L.) is done by hand on a spray-to-wet basis. With a fixed concentration of active ingredient in the spray mixture the dosage is determined by the volume used in wetting a particular stand of grass. The amount of herbicide deposited on the weed foliage may be affected by the addition of surfactants to the spray mixture. In orchards and vineyards where the tolerance of the crop to the herbicide is limited, variations in the stand may lead to local overdosage and injury to the trees and vines.

Sorghum is second only to wheat in importance in dryland crop production in the Southern Great Plains. Sorghum is planted during the season when several common annual weeds normally emerge. Proper pre-planting cultivation will often reduce the seriousness of weed infestations, but many times the crop is invaded by weeds. Under such conditions chemical weed control is of value. These studies were initiated to determine the effect of 2,4–dichlorophenoxyacetic acid (2,4–D) on the yield of weed-free sorghum.

The influence of buthidazole {3-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-4-hydroxy-1-methyl-2-imidazolidinone}, diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea], and atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] on fluorescence emission and post-luminescence decay in chlorella (Chlorella vulgaris Beyer) was studied. All three herbicides blocked photosynthetic electron transport and thus enhanced fluorescence. Buthidazole was the most active, a 79% increase in fluorescence being observed when algal cultures were exposed to 1 × 10-6 M herbicide for 5 min, and a 206% increase with an exposure time of 60 min. Postluminescence in chlorella exposed to 1 × 10-6 M buthidazole for 5, 10, 20, and 60 min was inhibited 15, 34, 58, and 69%, respectively. Diuron and atrazine inhibition of postluminescence was much less than that of buthidazole. The ratio of relative fluorescence to relative postluminescence can be sued to give a numerical value (C) for the phytotoxicity of this type of herbicide to a given plant species.

Eight substituted dinitroaniline herbicides were studied for relative effectiveness in controlling weeds and for tolerance of peas (Pisum sativum L.). Greenhouse trials comparing 10-5 M herbicide solutions added to washed silica sand indicated that trifluralin (α,α,α-trifluroro-2,6-dinitro-N,N,-dipropyl-p-toluidine) and dinitramine (N4,N4-diethyl-α,α,α-trifluoro-3,5-dinitrotoluene-2,4-diamine) were the most phytotoxic to peas. Field trials conducted over a 3-yr period using 0.84 and 1.68 kg/ha application rates demonstrated that all eight of the herbicides successfully controlled the weeds present. Use of trifluralin resulted in the lowest shelled pea yields at both rates of application. Significant reductions in yield occurred from the 1.68 kg/ha rate of trifluralin in 1973 and 1975 when compared with the untreated checks. While major differences did not occur between the other chemicals, the greatest average pea yields resulted from the use of oryzalin (3,5-dinitro-N4,N4-dipropylsulfanilamide), dinitramine, and fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)analine].

Dimethyl tetrachloroterephthalate (DCPA) caused swelling of a localized region of the hypocotyls of cucumber (Cucumis sativus L., var. Wis. SMR-15), tomato (Lycopersicon esculentum Mill., var. Heinz 1370), and jimsonweed (Datura stramonium L.). DCPA caused stem swelling in jimsonweed by affecting dividing and/or differentiating cells in the vascular cambium. As swelling proceeded, xylem tissue was increased over the controls and xylem cells were disorganized and no longer continuous. Swelling was induced at the site of DCPA application, and was reduced when DCPA concentration per unit of stem was lowered, even though the same amount per plant was applied. Uptake of 14C-DCPA from the treated stem region was slow, but the treated area had the highest specific activity.

Rapid strides are being made in understanding the fundamental regulation of plant growth, development, and responses to the environment due to recent advances in molecular biology. Current questions in weed science such as herbicide mechanisms of action, biodegradation, and mechanisms of weed resistance are equally approachable using such methodology. Efforts to introduce herbicide resistance into agronomically important crops are possible because of successful isolation and transfer of genes. Investigations of weed survival and competitive strategies based on developmental processes, such as seed dormancy, are currently underway using techniques designed to monitor and characterize differential gene expression. Molecular methodology also plays a key role in taxonomic studies of weed populations using restriction fragment length polymorphism (RFLP) mapping. The future potential for these and other techniques such as nucleic acid hybridization, polymerase chain reaction (PCR), gene transfer, and the use of transgenic plants is described.

A yellow nutsedge biotype (Res) from an Arkansas rice field has evolved resistance to acetolactate synthase (ALS)-inhibiting herbicides. The Res biotype previously exhibited cross-resistance to ALS inhibitors from four chemical families (imidazolinone, pyrimidinyl benzoate, sulfonylurea, and triazolopyrimidine). Experiments were conducted to evaluate alternative herbicides (i.e., glyphosate, bentazon, propanil, quinclorac, and 2,4-D) currently labeled in Arkansas rice–soybean production systems. Based on the percentage of aboveground dry weight reduction, control of the yellow nutsedge biotypes with the labeled rate of bentazon, propanil, quinclorac, and 2,4-D was < 44%. Glyphosate (867 g ae ha−1) resulted in 68 and > 94% control of the Res and susceptible yellow nutsedge biotypes, respectively, at 28 d after treatment. Dose-response studies were conducted to estimate the efficacy of glyphosate on the Res biotype, three susceptible yellow nutsedge biotypes, and purple nutsedge. Based on the dry weights, the Res biotype was ≥ 5- and ≥ 1.3-fold less responsive to glyphosate compared to the susceptible biotypes and purple nutsedge, respectively. Differences in absorption and translocation of radiolabeled glyphosate were observed among the yellow nutsedge biotypes and purple nutsedge. The susceptible biotype had less 14C-glyphosate radioactivity in the tissues above the treated leaf and greater radioactivity in tissues below the treated leaf compared to the Res biotype and purple nutsedge. Reduced translocation of glyphosate in tissues below the treated leaf of the Res biotype could be a reason for the lower glyphosate efficacy in the Res biotype. No amino acid substitution that would correspond to glyphosate resistance was found in the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene of the Res biotype. However, an amino acid (serine) addition was detected in the EPSPS gene of the Res biotype; albeit, it is not believed that this addition contributes to lower efficacy of glyphosate in this biotype.