Field studies in conventional and vertical-tilled soybeans were conducted in 1992/93 and 1993/94 at four locations in Argentina to compare the efficacy of imazethapyr and haloxyfop for johnsongrass control. Control was evaluated by measuring frequency of johnsongrass-infested areas at the beginning of the 1992/93 and 1993/94 growing seasons. Relative johnsongrass leaf area and number of rhizome phytomers within infested areas also was evaluated. Herbicide application reduced frequency of infested areas in the 1992/93 growing season. Frequency of infested areas increased in check plots. No differences were found among herbicide treatments, suggesting that the effect of imazethapyr was as high as that observed for specific graminicides. Reduction in frequency of infested areas caused by herbicide treatments was accompanied to some extent by a decrease in intensity of infestation within infested patches assessed by measuring johnsongrass relative leaf area. Phytomer population levels, on the other hand, were reduced only after two consecutive years of herbicide application. No differences were found among herbicide treatments in terms of impact on both measures of weed abundance. Johnsongrass responded similarly to all treatments regardless of tillage system.

Field studies were conducted in 1987 and 1988 at Fayetteville and Clarkedale, AR, to evaluate various herbicide treatments for johnsongrass control in soybeans. Herbicide treatments were postemergence applications of sethoxydim, fenoxaprop, fluazifop-P, haloxyfop, quizalofop, imazethapyr, and clethodim. No significant soybean injury was noted in 1987 or 1988 at either location in the johnsongrass experiments. All treatments except those with fenoxaprop and imazethapyr, provided 70 to 90% johnsongrass control. Generally, soybean yields were reduced from treatments that controlled less than 60% of the johnsongrass.

Research was conducted to formulate a temperature-dependent population level model for johnsongrass seed germination and rhizome bud break. A nonlinear poikilotherm rate equation was used to describe development rate as a function of temperature, and a temperature-independent Weibull function was used to distribute development times for the population. Seed germination and initiation of rhizome bud break of johnsongrass were collected under constant temperature conditions to parameterize the model. Seed germination rate increased with temperature up to 36 C, then declined at 40 C. Rate of rhizome bud break increased with temperature up to 32 C, then rapidly decreased with further temperature increases. Rate of rhizome bud break was higher than for seed germination at temperatures of 32 C or below, but lower at higher temperatures. Time to first germination or bud break event was longer for seed than for rhizomes, but subsequent progression of development was higher for seed. A population level temperature-dependent model was developed by coupling the poikilotherm equation with the Weibull function. The model was validated against two independent seed germination and three independent rhizome bud germination data sets.

In the greenhouse, soil applications of primisulfuron (40 g ai ha–1) reduced growth of emerged rhizome johnsongrass plants more than nicosulfuron (40 g ai ha–1). Both herbicides reduced growth more when applied to foliage only; a further decrease in growth did not occur for applications to both soil and foliage. Primisulfuron did not completely prevent regrowth of johnsongrass with any application method. Nicosulfuron prevented regrowth when applied to the foliage and to both soil and foliage. In single-year field studies in corn at four dryland sites and two irrigated sites, 50:50 split applications of primisulfuron (40 g ai ha–1) and nicosulfuron (35 g ai ha–1) approximately 2 wk apart provided the most consistent rhizome johnsongrass control compared with early or late single applications when visually rated 8 wk after the first application. Nicosulfuron treatments were more effective than primisulfuron treatments at both dryland sites the first year and at one of two dryland sites the second year. Primisulfuron and nicosulfuron at the irrigated site the first year were equivalent in efficacy. Nicosulfuron was more effective than primisulfuron at the irrigated site the second year. Primisulfuron or nicosulfuron treatments more than doubled corn yields at the dryland sites both years regardless of application timing. Split applications of primisulfuron or nicosulfuron provided the highest yields (approximately 80% increase over untreated plots) at the irrigated site the first year. All treatments provided equivalent yield increases (approximately 50%) the second year.

Experiments were conducted from 1990 to 1994 to investigate the density-dependent effects of rhizome and seedling johnsongrass on the magnitude of full-season interference in field corn. Corn grain yield was decreased as rhizome johnsongrass density increased. A maximum density of 3 rhizome johnsongrass plants 9.8 m−1 of row was considered critical to avoid yield losses above 5% of full-season weed-free corn. Interference of johnsongrass plants originating from seed and growing at densities up to 128 plants 9.8 m−1 of row did not affect grain yield. Seedling johnsongrass plants growing at a constant density of 9.8 plants m−1 of row did not affect yield from corn planted at five densities.

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.

Field studies were conducted in second-ratoon crops of sugarcane infested with rhizome johnsongrass to determine the effects of aqueous diluent volumes ranging from 47 to 561 L ha-1 on the performance of asulam. Trend analyses of johnsongrass recovery, based on panicle numbers late in the growing season, indicated a quadratic response with panicle numbers being lowest when asulam at 2.8 kg ai ha-1 was applied in a diluent volume approaching 187 L ha-1. The response was similar regardless of whether droplet size or number was altered to obtain these diluent volumes. Differences in late-season johnsongrass control were reflected in harvested sugarcane stalk populations and net cane yields. In greenhouse studies, johnsongrass plants were thoroughly washed with water 24 h after treatment with asulam at 2.8 kg ha-1 at water volumes ranging from 47 to 374 L ha-1. Asulam absorption, as evidenced by reduction in the growth of johnsongrass, was higher when surfactant was included, but was not affected by diluent volumes in the range evaluated.

Field experiments were conducted at two locations in Virginia to evaluate the following herbicides: alachlor, diphenamid, diuron, metolachlor, napropamide, norflurazon, oryzalin, oxyfluorfen, paraquat, pendimethalin, and simazine. One experiment involved newly-transplanted apple trees; the others, three in apple and one in peach trees, involved one-year-old trees. Treatments were applied in the spring (mid-April to early-May). Control of annual weed species was excellent with several treatments. A broader spectrum of weeds was controlled in several instances when the preemergence herbicides were used in combinations. Perennial species, particularly broadleaf species and johnsongrass, were released when annual species were suppressed by the herbicides. A rye cover crop in nontreated plots suppressed the growth of weeds. New shoot growth of newly-transplanted apple trees was increased with 3 of 20 herbicide treatments and scion circumference was increased with 11 of 20 herbicide treatments compared to the nontreated control. Growth of one-year-old apple trees was not affected. Scion circumference of one-year-old peach trees was increased with 25 of 33 herbicide treatments.

Fumonisin B1 (FB1) sprayed on intact jimsonweed plants at 10 to 200 μg ml–1 caused chlorosis and necrosis on three- to four-leaf plants and reduced their height and biomass. Excised leaves of jimsonweed were necrotic on both surfaces after treatment with FB1 at 10 to 200 μg ml–1. The degree of damage increased with the concentration of FB1. The minimum amount of FB1 that damaged excised leaves was 0.05 μg. The symptoms of toxicity on sensitive plants varied with concentration of FB1. Symptoms observed included: chlorosis (creeping cucumber), necrosis (jimsonweed and common sunflower), black leaf lesions (sicklepod), curl of tissues (soybean), stunting, defoliation (hemp sesbania and northern jointvetch), and death (prickly sida, spurred anoda, northern jointvetch, and jimsonweed). Monocots (barley, bermudagrass, corn, johnsongrass, rice, sorghum, and wheat) were not visibly affected by FB1. This is the first evidence of the phytotoxic properties of fumonisin B1 against a wide variety of weeds and crop cultivars.

Ammonium sulfate enhanced the initial control of johnsongrass by glyphosate and SC-0224 applied in the fall or summer. Glyphosate at 0.42 kg ha-1 in combination with ammonium sulfate at 3.33 kg ha-1 provided levels of fall johnsongrass control and spring regrowth control comparable to glyphosate at 0.84 kg ha-1. Spring applications to mixed populations of seedling and rhizome johnsongrass showed that ammonium sulfate added to glyphosate or SC-0224 afforded little enhancement of herbicide efficacy. Glyphosate or SC-0024 at 0.42 kg ha-1 applied in combination with logarithmically decreasing rates of ammonium sulfate demonstrated reduced weed control with ammonium sulfate rates above 9.7 kg ha-1.

Variables affecting ultra-low-volume (ULV) application of herbicides for postemergence grass control in soybeans were evaluated in field experiments from 1988 to 1990. Air-assist applications of clethodim at 2.3 and 4.7 L ha–1 were compared with 94 and 187 L ha–1 applications with a conventional hydraulic sprayer. Rate of herbicide, volume of diluent, type of diluent, air pressure, and four other herbicides were evaluated. Clethodim at 28, 56, and 112 g ha–1 applied in paraffinic oil at 2.3 L ha–1 controlled johnsongrass better 10 wk after treatment than equivalent rates applied at 2.3 or 187 L ha–1 in water. At 2.3 L ha–1, barnyardgrass control was improved by applying clethodim in paraffinic oil rather than water. Johnsongrass control 10 wk after treatment was better with clethodim at 28 g ha–1 applied in paraffinic oil than when applied in soybean oil, cottonseed oil, No. 2 diesel fuel, kerosene, or jet A fuel. Low air pressures of 14 or 28 kPa resulted in better control of johnsongrass and barnyardgrass than higher pressures of 56 and 112 kPa. Clethodim, fluazifop-P, haloxyfop, quizalofop, or sethoxydim were more effective on johnsongrass and barnyardgrass when applied in paraffinic oil than in water at 2.3 and 4.7 L ha–1 with an air-assist sprayer.

Competitive relationships between johnsongrass, smooth pigweed, and soybean were examined in 1986 and 1987, and the adequacy of an additive response model (ARM) and product response model (PRM) in predicting yield reductions due to multispecies weed populations was assessed. A severe drought reduced soybean yields statewide in 1986, whereas ideal conditions for soybean production were experienced in 1987. Averaged over monospecific weed densities of 1, 2, 4, and 8 plants 4.6 m−1 of row, smooth pigweed intercepted 2.5 and 1.8 times more light than johnsongrass in 1986 and 1987, respectively. In multispecies populations having either 4 or 8 smooth pigweed plants 4.6 m−1 of row, light interception by johnsongrass was negligible. Averaged over monospecific weed densities, smooth pigweed produced 5160 and 1760 kg ha−1, while johnsongrass produced 1530 and 450 kg ha−1 dry weight in 1986 and 1987, respectively. In multispecies populations, smooth pigweed contributed more than 80% of the total weed biomass. As monospecific johnsongrass density increased, reductions in soybean seed yield were linear, whereas exponential response patterns adequately characterized reductions in soybean seed yield due to smooth pigweed interference. Based on calibrated monospecific responses, the ARM and PRM generally projected higher soybean seed yield reductions for multispecies weed populations than were observed. When crop production conditions were favorable and competitive effects of weeds were low, both models adequately predicted soybean seed yield reductions. When dry conditions unfavorable for crop production existed, the PRM best accommodated the interactive effects between johnsongrass, smooth pigweed, and soybean.

Experiments were conducted to evaluate the occurrence and significance of release of herbicide through subterranean parts of nicosulfuron-treated johnsongrass. In a bioassay, the rooting medium of johnsongrass treated foliarly with 50 or 100 μg nicosulfuron plant−1 was inhibitory to the radicle elongation of sorghum and corn indicating the increased toxicity of the rooting medium of nicosulfuron-treated johnsongrass. The study with 14C-nicosulfuron indicated a basipetal translocation of foliarly applied nicosulfuron in johnsongrass to its roots/rhizomes and also into the rooting medium. By 30 DAT, around 23% of the 14C-label absorbed by johnsongrass was found exuded into the rooting medium. Radiochromatogram scans of thin layer chromatography plates of rooting medium indicated unmetabolized nicosulfuron as the major 14C-labeled compound (56%). The study also revealed a subsequent uptake of exuded 14C by corn roots sharing the medium. On the whole, the amount of 14C-label recovered from untreated corn amounted to 4.3% of the total applied to johnsongrass. In another experiment, the presence of nicosulfuron in the rooting medium was detrimental to corn growth. Reductions in corn growth occurred at concentrations of 10−8 M nicosulfuron or greater in the rooting medium. The sensitivity of corn to root uptake was attributed to greater accumulation of nicosulfuron at a faster rate in the growing parts.

The objective of this research was to modify the senior author's previously developed method for predicting yield losses in maize crops attributable to johnsongrass. The new method is based on the following assumptions: a) relative leaf frequency determines relative biomass; b) total biomass is constant despite the crop/weed ratio; and c) biomass, and therefore leaf frequency, are related to grain yield. Experiments in Argentina from 1986 to 1989 supported the above hypotheses, and the new method was more accurate than the old for predicting relative species biomass in johnsongrass/maize mixtures. Maize grain yield reductions associated with weed interference were also more accurately predicted.

The utility, advantages, and disadvantages of several spectrofluorometric and in vivo spectrophotometric methods in plant science are reviewed, with emphasis on their use in weed science. Examples of the use of in vivo, dual-wavelength spectrophotometry to probe the effects of herbicides on cytochrome f oxidation/reduction, the P515 chromatic shift, phytochrome synthesis, and other processes are discussed. Use of in vivo spectrophotometry to examine effects of herbicides on phytylation of chlorophyllide, protochlorophyllide photoconversion, porphyrin accumulation, and other processes that result in spectral changes is discussed and examples are provided. Spectrofluorometric methods for pigment identification and quantification and for enzyme assays are illustrated with examples. Spectrofluorometric methods used in conjunction with high-pressure liquid chromatography (HPLC) and thin-layer chromatography (TLC) are discussed.

There were no differences in johnsongrass rhizome winter survival at central and southeastern locations in Pennsylvania in 1988 or 1989. At a 6-cm depth of burial, survival rates of 5 and 25% were found for rhizome segments 7.5 and 15.0 cm in length, respectively. Survival rates for rhizomes buried 25 cm averaged 86%, with no difference in survival between rhizome lengths. The majority of rhizome biomass produced by the biotype used in these studies was found at soil depths allowing a high percentage of survival.

In a 6-yr field study, johnsongrass growing in ‘Forrest’ soybeans for the entire growing season reduced yields 45% in the year with most rainfall and 69% in the year with least rainfall. Yields of soybeans grown without johnsongrass competition were 56% higher in the year with the most rainfall than in the year with least rainfall. Yields of soybeans with full-season competition were reduced 75% from those in the year with most rainfall as compared with those in the year with least rainfall. The number of weeks of johnsongrass competition required to significantly reduce soybean yields varied from 3 wk, in the year with least rainfall, to 8 wk, in the year with most rainfall. Nine applications of sethoxydim at 0.84 kg ai ha-1 or haloxyfop at 0.55 kg ai ha-1 each year did not reduce soybean growth or yields.

Haloxyfop-methyl toxicity to johnsongrass was reduced when sprayed in combination with 2,4-D in greenhouse studies. The antagonism was not seen when 2,4-D application followed haloxyfop-methyl treatment by 72 h or preceded it by 24 h. The antagonism of haloxyfopmethyl activity was evident regardless of chemical form of 2,4-D used (DMA, BOE ester, IPA, or acid). Absorption of 14C-haloxyfop-methyl into johnsongrass leaves was slowed by the presence of 2,4-D in the treatment solution but total absorption after 72 h was not affected. Translocation of 14C was decreased when the 14C-haloxyfop-methyl was applied in combination with 2,4-D. More haloxyfop-methyl, less haloxyfop, and more ether-soluble haloxyfop-methyl metabolites were found in treated leaves when haloxyfop-methyl was applied with 2,4-D compared to haloxyfop-methyl applied alone. The changes in haloxyfop-methyl translocation and metabolism caused by 2,4-D can quantitatively account for the antagonism observed.

Field experiments were conducted to evaluate competitive effects of johnsongrass and sicklepod on growth and yield of irrigated and nonirrigated soybeans. Johnsongrass reduced soybean growth early in the growing season. Sicklepod was competitive all season, but competitiveness was greatest during the soybean reproductive stage. Soybean yields were reduced 31% by sicklepod, 14% by johnsongrass, and 36% by both weeds growing together. Nonirrigated soybeans yielded less than irrigated, but percent yield reductions were similar. Johnsongrass dry matter production and seed yield were reduced more than sicklepod by soybean interference. Soybean and sicklepod interference reduced johnsongrass seed production 73 to 95%. Johnsongrass produced 245 to 1238 seeds per plant when interfering with soybeans and sicklepod. Sicklepod seeds per plant were reduced 6 to 31% by johnsongrass interference and 47 to 75% by soybeans and soybeans plus johnsongrass.