Sulfentrazone is a herbicide that has been observed to injure crops in an unpredictable manner. Therefore, experiments were conducted to determine whether root absorption of sulfentrazone was dependent on the pH of the rooting medium. Studies were initiated to examine sulfentrazone uptake of whole plants from soil and hydroponic solution, as well of excised roots in solution. These experiments demonstrated that transpiration decreased as soil pH decreased and herbicide rate increased; it was our intention to use this measure as a description of herbicide injury. Likewise, plants grown for 24 h in 14C-sulfentrazone hydroponic solution accumulated a greater herbicide concentration in roots as solution pH decreased below 6.5. This trend of increased absorption with reduced solution pH was again demonstrated when excised cotton roots were placed for durations of 10 to 120 min in hydroponic solution containing 14C-sulfentrazone. However, when excised roots were placed in solution containing the weak acid herbicide glyphosate, no trend of increased absorption was observed with changes in solution pH. Therefore, it was concluded that the accompanying change in solubility, as sulfentrazone was converted from the ionic to the neutral form, was responsible for the increased absorption by plant roots. Localized differences in soil pH could be responsible for greater sulfentrazone uptake and explain the unpredictable patterns of injury that have been observed.

There is great value in quantifying and reporting weed seed production as a component of herbicide efficacy evaluations for two reasons. First, visual weed control ratings and associated measurements such as weed density and biomass are not sufficient indicators of fecundity. Second, knowledge of fecundity associated with herbicide treatments can guide the development of effective management programs that impact long-term weed population dynamics and reduce the risk of herbicide resistance.

Field studies have shown that weed density and biomass were lower in crops following incorporation of brassica cover crops compared with fallow but have not determined whether weed-suppressive effects are solely a consequence of reduced establishment, as evidenced in our companion paper, reduced growth of established plants, or both. In 2002 and 2003, canola and yellow mustard were seeded in early May, mowed in early July, and the residues incorporated. Green bean and redroot pigweed were then planted at fixed densities. Plant height and biomass were measured weekly; leaf area and biomass of component plant parts were measured at three harvests. Based on analysis of variance (ANOVA) at discreet sampling points, growth of redroot pigweed and green bean in monoculture or mixture were similar following fallow and incorporated brassica cover crops. However, based on aboveground biomass fitted to a Richards function, redroot pigweed growth in monoculture was reduced by the yellow mustard cover crop compared with fallow in both years (P = 0.007), but the magnitude of this effect was small; the canola cover crop did not affect growth (P = 0.179). Brassica cover crops did not reduce redroot pigweed growth when it was grown in mixture with green bean (P ≥ 0.382). Redroot pigweed competition reduced green bean yield, but incorporated brassica cover crops did not affect green bean growth and yield, nor did they confer a competitive advantage to the crop. Thus, brassica cover crops may suppress the growth of established weed and crop plants, but the magnitude of suppression was less than previously documented for effects on weed establishment.

Although the problem of herbicide resistance is not new, the widespread evolution of glyphosate resistance in weed species such as Palmer amaranth (Amaranthus palmeri S. Wats.), common waterhemp (Amaranthus rudis Sauer), and kochia [Kochia scoparia (L.) Schrad.] raised awareness throughout the agricultural community of herbicide resistance as a problem. Glyphosate-resistant weeds resulted in the loss of a simple, single herbicide option to control a wide spectrum of weeds that gave efficacious and economical weed management in corn (Zea mays L.), soybean [Glycine max (L.) Merr.], and cotton (Gossypium hirsutum L.) crops engineered for tolerance to this herbicide and planted over widespread areas of the South and Midwest of the United States. Beyond these crops, glyphosate is used for vegetation management in other cropping systems and in noncrop areas across the United States, and resistance to this herbicide threatens its continued utility in all of these situations. This, combined with the development of multiple herbicide-resistant weeds and the lack of commercialization of herbicides with new mechanisms of action over the past years (Duke 2012), caused the weed science community to realize that stewardship of existing herbicide resources, extending their useful life as long as possible, is imperative. Further, while additional herbicide tolerance traits are being incorporated into crops, weed management in these crops will still be based upon using existing, old, herbicide chemistries.

Soil surface applications of crabgrass (Digitaria spp.) herbicides prevented rooting of Bermudagrass (Cynodon dactylon L., var. 328) from the stolon nodes. Inhibition of root development was confined to the areas of the stolon which were in direct contact with dimethyl 2,3,5,6-tetrachloroterephthalate (DCPA). Placement of the herbicides at various soil depths reduced the number of roots developing below the treatment; however, normal root growth occurred above the treated layer. Under field conditions, certain crabgrass herbicides delayed normal rooting of Bermudagrass from nodes for 12 weeks. Established root systems were not appreciably altered by the herbicidal treatments.

Greenhouse and laboratory studies were conducted to determine the degree of dominance of the monogenic sulfonylurea herbicide resistance trait in diploid sugarbeet by comparing the response of homozygous and heterozygous resistant sugarbeet to primisulfuron, thifensulfuron, and chlorimuron on the whole plant and acetolactate synthase (ALS) enzyme level. Progeny tests suggested that the monogenic sulfonylurea herbicide resistance was semidominant. Subsequently, heterozygous resistant (R-1) and homozygous resistant (R-2) sugarbeet lines were sprayed with increasing rates of primisulfuron, thifensulfuron, and chlorimuron, and herbicide rates required for 50% growth reduction (GR50) were determined. GR50 values were also determined for homozygous susceptible sugarbeet lines (S-1 and S-2). The GR50 values indicated that the R-2 sugarbeet was 377, 269, and 144 times more resistant to primisulfuron, thifensulfuron, and chlorimuron, respectively, than susceptible S-2 sugarbeet. In contrast, R-1 sugarbeet was only 107, 76, and 57 times more resistant to primisulfuron, thifensulfuron, and chlorimuron, respectively, than S-1 sugarbeet, indicating at least a twofold difference in the magnitude of resistance between homozygous resistant and heterozygous resistant sugarbeet lines. ALS enzyme activity analysis were consistent with whole plant results. Thus, based on these two, maximum crop resistance can be obtained by developing homozygous resistant cultivars.

Added surfactant and high relative humidity enhanced penetration of 4-amino-3,5,6-trichloropicolinic acid (picloram) into leaves of Canada thistle (Cirsium arvense (L.) Scop.). Leaves and roots of Canada thistle readily absorbed picloram after which it was translocated in both phloem and xylem. Picloram tended to accumulate in young, growing leaves following both foliar and root uptake. In field-grown plants, injury symptoms appeared in young shoots as far as 1 m from treated shoots. The results are in harmony with a “source-to-sink” pattern of picloram translocation in the phloem. Leaves, but not roots, retained a substantial portion of the picloram absorbed. Following foliar application, small amounts of picloram were exuded by the roots into surrounding soil.

Field studies were conducted to evaluate the influence of herbicide application timing on weed control in no-till soybean production. Row spacing generally had no effect on weed control. Herbicide treatments containing chlorimuron plus metribuzin applied as many as 45 days prior to planting in 1988 and 1989 controlled broadleaf weeds throughout the growing season. Imazaquin applied 45 and 30 days prior to planting provided poor control of common cocklebur in 1989. Giant foxtail control was inconsistent with all herbicide treatments. Soybean yields subsequent to early preplant herbicide applications were greater than or equal to those in which applications were made at planting when late-season weed control was adequate. Herbicides applied preemergence did not control high densities of common lambsquarters in 1989.

Rhizome johnsongrass grown in the greenhouse and treated with glyphosate at 1680 g ai ha−1 at an early (3- to 4-leaf) or late (6- to 8-leaf) growth stage displayed injury within a week. Plants treated with CGA-136872 or DPX-V9360 at 40 g ai ha−1 at both growth stages displayed injury 1 to 2 weeks later. CGA-136872 did not prevent regrowth at either growth stage. No regrowth occurred from DPX-V9360 or glyphosate-treated plants. Foliar absorption by greenhouse-grown plants within 24 h of application was greater with 14C-glyphosate than with 14C-DPX-V9360 or 14C-CGA-136872. More 14C-DPX-V9360 was absorbed than 14C-CGA-136872. Growth stage influenced glyphosate absorption (more by younger plants) but not CGA-136872 or DPX-V9360 absorption. Translocation of the 14C-CGA-136872 and 14C-DPX-V9360 out of the treated leaf was less than 20% of the absorbed label and was less than glyphosate translocation. Growth stage of rhizome johnsongrass at the time of treatment had no effect on the distribution of radiolabeled herbicides within 24 h.

Greenhouse and growth chamber studies were conducted to determine the cause of previously observed differences in the effectiveness of bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] when applied at various times of the day to velvetleaf (Abutilon theophrasti Medic). Leaves of velvetleaf moved rhythmically from a near horizontal day position to a near vertical night position with the tip of the blade pointed downward. Plants were sprayed with bentazon at various times of day. The amount of spray retained and the degree of control decreased as the leaves moved toward the vertical downward night position. This suggested that a major cause of the time-of-day effect with bentazon treatments on velvetleaf is variation in the amount of spray intercepted and retained as a function of varying leaf angles.

Laboratory studies were conducted to determine the effect of various factors on germination of tropical soda apple. Tropical soda apple seed were found to be moderately photoblastic with 30% germination occurring in the dark. Germination increased in response to green (545 nm) and red light (650 nm) to 75 and 66%, respectively, indicating phytochrome regulation. No germination occurred in response to blue (450 nm) and far-red light (750 nm). Germination increased from 4 to 64% between 10 to 30 C, but no germination was found at 5 and 40 C. Maximum germination occurred at 30 C. A second degree polynomial (Y= −48.7 + 7.9X – 0.2X2, R2 = 0.76) best fit the data. Seed germinated over a broad range of pH (2 to 14) with the highest germination occurring at pH 8 and no germination occurring at pH 2 and 14 (Y = −63.3 + 30.5X – 1.9X2, R2 = 0.95). Seed germination declined as osmotic potential increased from 0 to −1.0 MPa (Y = 76.7 – 12.9X + 5.2X2, R2 = 0.87). Seedling emergence was maximum from depth of 3 to 6 cm, but no seedling emergence occurred when seed were planted below 12 cm (Y = 10.2 + 17.8X – 1.6X2, R2 = 0.93). Optimum germination occurred at a depth of 5.6 cm. Mechanical and sulfuric acid scarification increased the rate of germination but not the overall percentage. Tap water or hot water increased the rate of germination 26% and KNO3, GA3, or ethephon increased it 53%. Tropical soda apple seed germinated in response to variable environmental and edaphic conditions which could allow establishment in diverse ecosystems.

In greenhouse studies, Fusarium lateritium Nees ex Fr. was effective in suppressing the growth of velvetleaf (Abutilon theophrasti Medic. # ABUTH) and prickly sida (Sida spinosa L. # SIDSP). Sprays containing conidia of the fungus and fungus-infested sodium alginate-kaolin clay granules controlled both weeds in corn (Zea mays L.), cotton (Gossypium hirsutum L.), and soybeans [Glycine max (L.) Merr.] without causing a reduction in dry weight of the crops. In field tests, postemergence foliar applications of the fungus controlled velvetleaf and prickly sida 40 and 80%, respectively, in 1982, but controlled only 27 and 34% of velvetleaf and prickly sida, respectively, in 1983. Preemergence applications of the fungus in the granule formulation controlled velvetleaf and prickly sida 46 and 59%, respectively, in 1982. In 1983, this treatment controlled the weeds 35 and 38%, respectively.

Five presumed biotypes were identified among field bindweed (Convolvulus arvensis L. ♯3 CONAR) clones collected from a field population near Lafayette, IN. Consistent variations in leaf morphology, floral characteristics, and accumulation of shoot and root biomass were found between biotypes when grown in a controlled environment. The biotypes also differed in their flowering capacity. The earliest flowering biotype formed flowers 23 days before the latest and produced 19 times more flowers per plant, which indicated further differences in seed production potential between biotypes. Pollination studies helped to differentiate biotypes within the population and showed that the presumed biotypes were self-incompatible. Vegetative reproduction potential of the biotypes varied from 1.8 to 74.5% in the number of root buds that developed into shoots. The variability in growth and reproduction observed between these field bindweed biotypes may explain the survival and adaptability of a population of this weed as environmental conditions and control practices change.

Competitive interactions of wheat and wild oats in relation to variations in plant density of both species were investigated in field studies under irrigation and added fertilizer. Competition studies included comparisons by both additive and replacement series. The additive series was used to study plant responses to competition under fixed density of wheat and increasing density of wild oats. The replacement series was used to study plant responses to competition under constant total plant density with differing proportions of wheat and wild oats. On a per plant basis, shoot dry weight and leaf area index of wild oats were less than those of wheat at anthesis. However, the replacement series experiment indicated that wheat and wild oats were equivalent in competitiveness. Relative density of wild oats gave a better regression fit for wheat yield than did absolute wild oats density (r2 values of 0.83 and 0.61, respectively). Yield of wheat grain was linearly proportional to relative density of wild oats.

Two rapid, nondestructive assays were developed and tested for their potential in differentiating glyphosate-resistant from glyphosate-susceptible biotypes of horseweed. In one assay, leaves of glyphosate-resistant and -susceptible corn, cotton, and soybean plants, as well as glyphosate-resistant and -susceptible horseweed plants, were dipped in solutions of 0, 300, 600, and 1,200 mg ae L−1 glyphosate for 3 d, and subsequent injury was evaluated. In the second assay, plant sensitivity to glyphosate was evaluated in vivo by incubating excised leaf disc tissue from the same plants used in the first assay in 0.7, 1.3, 2.6, 5.3, 10.6, 21.1, 42.3, and 84.5 mg ae L−1 glyphosate solutions for 16 h and measuring shikimate levels with a spectrophotometer. The leaf dip assay differentiated between glyphosate-resistant and -susceptible crops and horseweed biotypes. The 600 mg L−1 rate of glyphosate was more consistent in differentiating resistant and susceptible plants compared with the 300 and 1,200 mg L−1 rates. The in vivo assay detected significant differences between susceptible and glyphosate-resistant plants of all species. Shikimate accumulated in a glyphosate dose–dependent manner in leaf discs from susceptible crops, but shikimate did not accumulate in leaf discs from resistant crops, and levels were similar to nontreated leaf discs. Shikimate accumulated at high (≥ 21.1 mg ae L−1) concentrations of glyphosate in leaf discs from all horseweed biotypes. Shikimate accumulated at low glyphosate concentrations (≤ 10.6 mg L−1) in leaf discs from susceptible horseweed biotypes but not in resistant biotypes. Both assays were able to differentiate resistant from susceptible biotypes of horseweed and could have utility for screening other weed populations for resistance to glyphosate.

Wild oat continues to reduce spring wheat yields and profits despite the wide spread use of herbicides. Further reductions in the occurrence of wild oat could be achieved with the development of competitive cropping systems. Field studies were conducted to investigate the effects of wheat seed size and seeding rate on wild oat demographic processes under a range of wild oat densities. Spring wheat competitiveness increased as seed size and seeding rate increased, significantly reducing wild oat biomass and seed production. Averaged across all other factors, spring wheat plants derived from large seed reduced wild oat panicle numbers 15% and biomass and seed production 25% compared with small seed. Increasing spring wheat seeding rate from 175 to 280 plants m−2 reduced the number of panicles 10% and wild oat biomass and seed production 20%. The combined effect of large seed plus increased seeding rate reduced wild oat biomass and seed production 45%. Results demonstrate that the use of large seed size and increased seeding rates can improve wheat competitiveness and provide an effective means to reduce wild oat biomass and seed production.

The efficacies of bentazon and fomesafen in controlling annual weeds in dry and edible pod beans in New York State were investigated in greenhouse and field experiments. Dose responses to bentazon and fomesafen were studied for four weed species (ragweed, velvetleaf, eastern black nightshade, and hairy nightshade) under greenhouse conditions. Herbicides were applied at cotyledon to two-, two- to four-, and four- to six–true leaf stages, both with and without a crop oil concentrate (bentazon) or a nonionic surfactant (fomesafen). Field studies were conducted for 2 yr for all weed species except eastern black nightshade, for which no adequate field populations were found. Field studies confirmed greenhouse results, indicating that weed control could be improved by the use of an adjuvant, but there were exceptions. In general, adjuvant usage improved the efficacy of fomesafen more than it did with bentazon. The minimum rates of herbicide required for effective and consistent control was dependent on the particular combination of weed species, herbicide and its rate of application, growth stage at which the application was made, and adjuvant usage.

In greenhouse and controlled-environment tests, velvetleaf (Abutilon theophrasti Medic. ♯ ABUTH) and prickly sida (Sida spinosa L. ♯ SIDSP) control with Fusarium lateritium Nees ex Fr. was affected by inoculum concentration and plant growth stage at the time of inoculation, by postinoculative air temperature, and by dew period duration and temperature. A higher level of control of both weeds was obtained with inoculum concentrations of 7.5 × 105 and 1.5 × 106 macroconidia/ml when the weeds had fewer than seven true leaves. Postinoculative air temperatures above 25 C significantly reduced control of both weeds. Mortality of both species was higher at dew temperatures of 25 C than at temperatures of either 15 or 35 C. At least 12 h of dew at 25 C was required to achieve satisfactory control of prickly sida, while a dew period of 16 h was required to achieve a comparable level of control of velvetleaf.

The adsorption of 2-(ethylamino)-4-(isopropylamino)-6-(methylthio)-s-triazine (ametryne) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) varied greatly among 34 Puerto Rican soils was studied using the agitated slurry technique. Adsorption was expressed as a distribution coefficient (Kd), which is the ratio of the amount of herbicide adsorbed to the amount in the equilibrium solution. In general, diuron was adsorbed to a greater degree than ametryne. Adsorption of ametryne was positively correlated with organic matter and silt content but negatively correlated with pH of the soil. The inclusion of soil pH in a multiple regression analysis contributed the highest increase in explanation for adsorption of ametryne. Adsorption of diuron was highly correlated with organic matter content and cation exchange capacity. A correlation was found between adsorption of diuron and content of magnesium, and soil texture. Cation exchange capacity was the only inclusion soil property which, in addition to organic matter, significantly contributed to the adsorption of diuron. Temperature appeared to have a greater effect on the adsorption of diuron than on the adsorption of ametryne. Conversely, the effect of pH on the adsorption of ametryne was significantly greater than that of diuron.

Use of glyphosate in controlling Orobanche aegyptiaca (broomrape), a parasitic weed on dicotyledonous crops, was examined by determining glyphosate dose response and 14C-labeled glyphosate absorption, translocation, and metabolism patterns in Vicia sativa (common vetch) that is tolerant of low levels of glyphosate and Brassica napus (oilseed rape) that has been genetically engineered to be glyphosate resistant. Glyphosate provided excellent suppression of O. aegyptiaca growth in both V. sativa and B. napus. Absorption and translocation of 14C-glyphosate was similar between parasitized and nonparasitized V. sativa plants. 14C-Glyphosate was metabolized up to 32% in V. sativa, which could account for some of the tolerance of V. sativa to glyphosate. Approximately 27% of translocated 14C-glyphosate accumulated in O. aegyptiaca attachments on V. sativa. Absorption and translocation patterns of 14C-glyphosate were similar between parasitized and nonparasitized B. napus plants. Nearly one-third (31%) of the translocated radioactivity was found in O. aegyptiaca attachments on B. napus. No metabolism of 14C-glyphosate was detected in B. napus.