Field studies were conducted from 1997 to 1999 to evaluate the influence of weed competition and herbicides on glyphosate-resistant soybean at the Belleville Research Center at Belleville, IL. Season-long weed competition did not delay maturity or reduce soybean population. However, season-long weed competition reduced soybean height by 10% and grain yield by 68%. Competition from weeds 30 cm in height or less did not increase days to maturity or reduce plant population, height, or grain yield. Chlorimuron plus thifensulfuron caused 6% height reduction at 21 d after treatment. Despite the injury from herbicides, none increased days to maturity or reduced plant height, population, or grain yield of soybean. The soil-applied herbicides alachlor, imazaquin, and pendimethalin and the postemergence herbicides acifluorfen, bentazon, chlorimuron, imazethapyr, and thifensulfuron did not reduce yield of glyphosate-resistant soybean.

A mesorhizotron and scanning system was modified to study the development of Russian thistle root systems during the 1996 and 1997 growing seasons at Lind, WA. Our imaging equipment combined the full profile images afforded by conventional rhizotrons with the portability of cylinder-based minirhizotron systems at a fraction of the cost of either system. Root development of Russian thistle in early spring was rapid and extensive compared with shoot growth. In 1996, 30 d after planting (DAP) Russian thistle roots were at least five times as long as the corresponding plant's shoots. During the next 20 d, shoots grew a maximum of 20 cm, whereas roots grew a maximum of 120-cm deep. Maximum root elongation rate reached 2 to 3 mm/cm2/d at the 70- to 120-cm depths 30 to 50 DAP in 1996 and 55 to 70 DAP in 1997. More than one (multiaxial grouping) Russian thistle root was often observed growing through the same soil channels. After the rapid early season growth, roots began to shrink or die back until shoots were clipped to simulate wheat harvest. Within 7 d after harvest, roots regenerated in old root channels. Our mesorhizotron system is a promising inexpensive tool for monitoring root morphological development of Russian thistle under field conditions.

Dissipation of imidazolinone herbicides imazaquin and imazethapyr under no-till field conditions was not affected by straw mulch level (1,300, 3,600, or 5,900 kg/ha) in either of 2 yr of field data. Half-lives averaged 7 d for imazaquin and 8 d for imazethapyr. Chlorimuron half-lives averaged 5 d. Imazaquin and imazethapyr residues did not injure corn (Zea mays) the growing season following application. Injury from chlorimuron to corn planted the next year was greatest where applications were made to the highest straw level, reducing corn yield 6%. In laboratory experiments, uniformly cut wheat (Triticum aestivum) straw placed over sand intercepted all three herbicides equally. Herbicide interception was 16, 43, 69, and 89% for straw levels of 840, 1,680, 3,360, and 6,720 kg/ha, respectively. In the laboratory, simulated rainfall removed more imazaquin and imazethapyr from straw than chlorimuron, and approximately 10% of the applied imazaquin or imazethapyr and 20% of the applied chlorimuron remained on the straw after application of 1.0 cm of water. Imazaquin and imazethapyr dissipated 59 and 20%, respectively, during a 7-d incubation period in the greenhouse. Imazaquin dissipation was equal between sand and straw, whereas imazethapyr dissipation was primarily from sand. Chlorimuron did not dissipate from either straw or sand. This indicated that chlorimuron, a sulfonylurea, was more stable than either imidazolinone under these conditions.

We examined the effect of a postemergence application of fluazifop-P on root uptake, translocation, and metabolism of 14C-terbacil in ‘Kent’ strawberry. Fluazifop-P had no effect on uptake of 14C-terbacil from a nutrient solution over 48 h, nor did it affect the proportional distribution of 14C-label in the plants. The 14C-label was readily translocated to the foliage where it tended to accumulate in vascular tissue. Fluazifop-P reduced the overall conversion of 14C-terbacil to metabolites by about 50%. Thin layer chromatography of methanol extracts of leaflets, petioles, crowns, and roots separated the 6-hydroxymethyl derivative, and several unidentified metabolites, including a major water-soluble metabolite at the origin. At least 50% of the latter could be converted to the 6-hydroxymethyl metabolite by β-glucosidase, but other metabolites could be separated in other solvent systems. The levels of metabolites were generally two- to threefold higher in plants treated with terbacil alone than in those pretreated with fluazifop-P. Therefore, we conclude that the interaction observed between these herbicides in the field results from fluazifop-P inhibiting detoxification of terbacil by strawberry.

Small-scale African farmers typically have few resources for the management of weeds and soil fertility. Weed composition varies with soil properties and crops compete with weeds for available nutrients. We conducted field surveys in four districts to study the effects of cropping systems, fallow, landscape position, and soil characteristics on weed composition and evaluated the value of weed species density in predicting soil fertility conditions. Weed density was higher with annual than for banana-based cropping systems with higher relative densities of hairy beggarticks and garden spurge and lower relative densities for Cyperus spp. and johnsongrass in the banana-based systems. The relative densities of weed species varied with the length of the postfallow period, landscape position, drainage, and soil properties. Long-term rotation of annual and perennial cropping systems may be more effective than fallow in reducing total weed numbers, but fallow may be effectively used to reduce the relative densities of tropical spiderwort, Cyperus spp., and goosegrass. Relative densities of blue couch, goosegrass, garden spurge, Cyperus spp., oxalis, and johnsongrass varied more with nutrient supply than for other species. Soil properties had less effect on the distributions of tropic ageratum, hairy beggarticks, tropical spiderwort, and smallflower galinsoga. High relative densities of blue couch and garden spurge were generally associated with low soil nutrient levels. Goosegrass, johnsongrass, and oxalis were associated with higher nutrient levels. Information on the relationship of soil nutrient levels with weed flora was used to develop a decision guide to aid farmers in the identification of areas in their fields with severe nutrient deficiencies.

Field studies in 1998 and 1999 determined the response of grain sorghum and sunflower to soil residues of MKH 6561 and MON 37500 in a failed winter wheat recropping situation. Averaged across years, MON 37500 at 30 g ai/ha reduced sunflower density by 39%. MKH 6561 at 30 or 45 g/ha did not reduce sunflower density, growth, or late-season biomass, whereas MON 37500 reduced late-season biomass in both years. Neither herbicide affected grain sorghum plant density in 1998, but MON 37500 decreased sorghum density 80% in 1999. Sorghum growth, biomass, and yield were limited more severely by MON 37500 in 1999 than in 1998. MKH 6561 had little or no effect on grain sorghum during either year. Differences in crop response to MON 37500 between years was likely because of wheat plant size at the time of application and greater interception and metabolism of the herbicide in 1998 crop year.

Greenhouse studies were conducted to determine yellow nutsedge response to selective placement of a 5-cm layer of norflurazon-treated soil above, below, or above plus below pregerminated nutsedge tubers. Norflurazon at 1.68 kg ai/ha applied postemergence (POST) as a foliar, soil, or foliar plus soil treatment was also evaluated. Soil concentrations of norflurazon at 1.5 mg/kg (w/w) applied below or above plus below nutsedge tubers reduced yellow nutsedge shoot numbers at least 69%, shoot height at least 71%, shoot dry weights at least 77%, and root tuber dry weights at least 89%. Yellow nutsedge growth reduction with norflurazon was greatest when norflurazon was placed in the soil profile below or above plus below the yellow nutsedge tubers. Placement of norflurazon above yellow nutsedge tubers did not reduce shoot number, height, or dry weight until the foliar portion of plants were removed 39 d after treatment. POST application of norflurazon did not reduce initial yellow nutsedge shoot number regardless of application method. Foliar-only POST application of norflurazon was less effective for reducing numbers of emerged yellow nutsedge than application to soil only or soil plus foliage.

Field studies were conducted in 1996 and 1997 to evaluate response of eight peanut cultivars to flumioxazin applied preemergence (PRE) at 71 g ai/ha. Peanut cultivars evaluated include ‘NC 12C’, ‘NC 7’, ‘VAC 92R’, ‘NC-V 11’, ‘NC 10C’, ‘AT VC 1’, ‘NC 9’, and the experimental breeding line ‘N9001OE’. Visible injury 3 wk after planting in 1996 was 3% or less regardless of cultivar. In 1997, all cultivars were injured 15 to 28% with flumioxazin PRE, except VC 1, which was injured 45%. No visible injury was observed at 5 and 9 wk after planting. Flumioxazin did not influence the incidence of early leaf spot, late leaf spot, southern stem rot, cylindrocladium black rot, or tomato spotted wilt virus. Flumioxazin did not affect percentage of extra-large kernels, sound mature kernels, other kernels, and total yield.

A survey of county extension agents was conducted in 1998 to determine the most troublesome weeds in corn, cotton, forages and pastures, peanut, small grains, soybean, tobacco, and vegetables in Georgia. The most troublesome weed statewide averaged over all crops was sicklepod. It was the most troublesome weed in cotton and soybean and among the four most troublesome weeds in corn, peanut, tobacco, and vegetables. Sicklepod was found in each of the nine climatological districts and in all the crops surveyed. Perennial nutsedge species were the second most troublesome weeds in Georgia. They ranked as the most troublesome weeds in tobacco and vegetables and were among the top five most troublesome weeds in corn, cotton, peanut, and soybean. Pigweed species were ranked third averaged over all the crops surveyed and were the second most troublesome weeds in cotton and vegetables and among the top five most troublesome species in corn, soybean, and tobacco. Morningglory species were listed as troublesome in six of the eight crops surveyed and ranked fourth overall. Similarly, Texas panicum was found in all districts and was the fifth most troublesome weed species. Texas panicum was the most troublesome weed in corn and among the top five most troublesome weeds in peanut, soybean, and tobacco. Florida beggarweed was the most troublesome weed in peanut, the second most troublesome weed in tobacco, and a top-10 weed species in corn, cotton, soybean, and vegetables, resulting in a ranking of sixth overall. Wild radish, large crabgrass, and tropic croton were the seventh through the ninth most troublesome weeds. Wild radish was the most troublesome weed of small grains and the sixth most troublesome weed of vegetables. Large crabgrass was the second most troublesome weed of forages and pastures and was reported in six other crops. Tropic croton was a troublesome weed in seven of the eight crops surveyed and was among the top five most troublesome weeds of cotton and peanut. The 10th most troublesome weed overall was bahiagrass, the most troublesome weed of forages and pastures.

Several herbicide-based weed management programs for glyphosate-tolerant cotton were compared in eight field studies across Alabama during 1996 and 1997. Weed management programs ranged from traditional, soil-applied residual herbicide programs to more recently developed total postemergence (POST) herbicide programs. Pitted morningglory and sicklepod control was best achieved with fluometuron applied preemergence (PRE) followed by (fb) a single POST over-the-top (POT) application of glyphosate fb a POST-directed application of glyphosate. Annual grass control was better with the preplant incorporated (PPI) programs at two of three locations in both years. Treatments that included at least one glyphosate POT application gave increased grass control over no glyphosate or pyrithiobac POT. Velvetleaf control was improved with the addition of glyphosate POT. A herbicide program using no POST herbicides yielded significantly less seed cotton than any program using POST herbicides at one location. PRE- and POST-only weed management programs at another location produced more seed cotton and gave greater net returns than PPI programs. Similarly, net returns at that same location were equivalent for both PRE- and POST-only programs, and less for PPI programs. POST-only programs yielded highest amounts of seed cotton and netted greater returns.

Field studies were conducted in 1998 and 1999 at Simcoe and Exeter, ON, to evaluate the tolerance of nine sweet corn cultivars, representing a range of genotypes, to RPA 201772 applied preemergence at 105 and 210 g ai/ha. The location × year interaction was significant for visible plant injury, plant height, and yield. All cultivars showed significant injury at Simcoe in 1999, especially at 210 g/ha. Calico Belle was the most sensitive cultivar, with RPA 201772 resulting in severe injury at Simcoe in 1999. Increased injury in 1999 was due to significant rainfall (48.5 mm) within 10 d of RPA 201772 application at Simcoe. Calico Belle also showed injury at Exeter in 1999 and at Simcoe in 1998. Plant height of all cultivars was reduced at the Simcoe location in 1999. Yield of five cultivars was also reduced at Simcoe in 1999. Sensitivity to RPA 201772 increased as application rate increased. Plant height and yield was not reduced at Exeter.

Research was conducted in 1997 and 1998 to evaluate narrow row spacing and glufosinate in glufosinate-resistant corn. Glufosinate-resistant corn was planted in 51- and 102-cm row spacings at the same plant populations. Herbicide treatments included glufosinate alone and in different herbicide combinations. Atrazine plus glufosinate enhanced Palmer amaranth control compared to glufosinate alone. Control of johnsongrass, ivyleaf morningglory, entireleaf morningglory, Texas panicum, smellmellon, browntop panicum, and toothed spurge with glufosinate was greater than 82%. Common sunflower control with glufosinate was greater than 95%. Atrazine followed by glufosinate applications provided at least 94% control of all species and was the most consistent herbicide system used. Row spacing had little effect on weed control. Crop injury to glufosinate-resistant corn was minimal with glufosinate and atrazine plus glufosinate combinations.

Greenhouse studies were conducted to evaluate weed control from various formulations of glyphosate alone and in combination with postemergence herbicides. Tank mixtures did not increase barnyardgrass control 2 wk after treatment (WAT) when compared with glyphosate alone; however, tank mixtures did reduce barnyardgrass fresh weight 4 WAT when compared with glyphosate alone in several instances. Antagonism was observed when chlorimuron was combined with all formulations of glyphosate 4 WAT, but control was not reduced when compared with glyphosate alone. Selective herbicides added to glyphosate had an additive or antagonistic effect on prickly sida fresh-weight reductions. Antagonism of pitted morningglory fresh-weight reductions occurred when glyphosate was combined with all herbicides except acifluorfen, which had an additive effect. Fomesafen or lactofen effectively controlled hemp sesbania 2 WAT without the addition of glyphosate. Acifluorfen and chlorimuron combined with glyphosate Cheminova, Monsanto, or Zeneca reduced hemp sesbania fresh weight nearly twofold more than glyphosate alone.

Purple loosestrife is an invasive, exotic perennial weed of wetlands throughout much of the United States and Canada. Once established, it forms dense colonies to the near exclusion of more desirable vegetation. Established plants are prolific seed producers. Our purpose was to investigate the germination of purple loosestrife seeds at a wide range of constant or alternating temperatures from 0 through 40 C. Purple loosestrife seeds germinate over a wide range of temperatures. We define optimum germination as not less than the maximum observed minus its confidence interval at the 0.01 level of probability. Optimum germination occurred over a considerable range of temperatures: Only 10, 15, or 20 C for 16 h alternating in each 24 h with 35 C for 8 h always supported optimum germination. Wide ranges in diurnal temperature fluctuations were conducive to maximum germination. These ranged from a maximum of 35 degrees with 0/35 C to 5 degrees with 30/35 C. No one constant temperature supported optimum germination.

Field trials evaluated weed control in corn and soybean with CGA-248757 and flumiclorac. CGA-248757 and flumiclorac were applied to 5-, 45-, and 60-cm-tall velvetleaf. Visual control ratings 7 d after treatment (DAT) indicated the greatest control of velvetleaf in corn with CGA-248757 applied to velvetleaf when 5 or 45 cm tall, whereas the greatest control with flumiclorac occurred when applied to 5-cm-tall velvetleaf. However, ratings 21 DAT to 60-cm-tall velvetleaf indicated velvetleaf control with both herbicides was greatest when applied to plants 45 or 60 cm tall. CGA-248757 or flumiclorac applied postemergence (POST) following metolachlor or metolachlor plus atrazine preemergence (PRE) improved velvetleaf control compared with metolachlor plus atrazine alone. Similarly, tank mixtures of CGA-248757 or flumiclorac with 2,4-D following metolachlor PRE provided greater velvetleaf control compared with metolachlor PRE followed by 2,4-D POST. Velvetleaf control in corn was greater when CGA-248757 or flumiclorac were tank mixed with nicosulfuron plus atrazine or nicosulfuron plus dicamba. In soybean, metolachlor plus metribuzin plus clomazone PRE, metolachlor PRE followed by flumiclorac POST, and metolachlor plus metribuzin PRE followed by CGA-248757 or flumiclorac POST provided greater than 85% control of giant foxtail, common lambsquarters, redroot pigweed, and velvetleaf 56 d after POST treatments (DAPO) in 1997. Additionally, glyphosate applied late postemergence (LPOST) and tank mixtures of glyphosate with CGA-248757 or flumiclorac LPOST provided greater than 90% control of these same weed species in 1997. However, only metolachlor plus metribuzin PRE followed by CGA-248757 or flumiclorac POST and imazethapyr plus CGA-248757 or flumiclorac POST provided greater than 80% giant foxtail, common lambsquarters, redroot pigweed, and velvetleaf control 56 DAPO in 1998.

The number of papers published on allelopathy was low until the 1970s but has accelerated rapidly in recent years. Alleged allelopathic activity has been suggested for numerous weed and crop species; however, many studies employed preliminary bioassays (leachates or extracts in the absence of soil). The importance of substratum and of abiotic and biotic stresses (nutrients, shade, herbides, plant diseases, herbivores), plant density, habitat, and climate has been realized in recent years. Laboratory bioassay, the first step to investigate probable involvement of allelopathy, should be designed to correspond to field conditions. Studies need to focus on the effects of (1) climatic, environmental, and habitat factors on allelopathic potential; (2) abiotic and biotic soil components on fate and biological activity of allelochemicals; and (3) allelochemicals on soil nutrient dynamics, microbial ecology, and other abiotic and biotic components. Recently, attempts to find crop cultivars with a competitive allelopathic edge have been made. Interaction of professionals from different disciplines is needed to understand the complexity of the ecosystem. The following are important topics for discussion: allelopathy and the transition to sustainable agriculture; physiological and biochemical studies; using transgenes to produce allelopathic crops; microbial allelochemicals and pathogens as weed biological agents; bioassays for allelopathy (problems and solutions); pollen allelopathy; and an ecological perspective of allelopathy.

BAY MKH 6562 [flucarbazone-sodium (proposed)], an acetolactate synthase (ALS)-inhibiting herbicide of the sulfonylaminocarbonyltriazolinone family, provides postemergence wild oat control in wheat. Whole-plant dose responses and in vitro ALS sensitivity assays were used to evaluate the magnitude and nature of cross-resistance to BAY MKH 6562 in a wild oat accession (AR1) with metabolism-based resistance to imazamethabenz, an ALS inhibitor of the imidazolinone family. An imazamethabenz-susceptible wild oat accession (AHS2), five BAY MKH 6562-resistant wild oat accessions, AN104, AN205, AN307, AN406, and ASB11, and wheat were also evaluated. AHS2 and AR1 dose responses to BAY MKH 6562 indicated a resistant/susceptible (R/S) herbicide dose required to cause 50% growth reduction (GR50) ratio of 200. Inhibition of ALS from the AHS2 and AR1 wild oat by BAY MKH 6562 was similar, with a concentration of herbicide required to cause 50% inhibition of enzymatic activity (I50) of 0.007 µmoles, indicating that cross-resistance was not due to an altered ALS enzyme. The GR50 for BAY MKH 6562 for the AN104, AN205, AN307, AN406, and ASB11 wild oat accessions was 0.23, 0.07, 0.23, 0.22, and 0.12 kg ai/ha, respectively, and the R/S ratio to the GR50 value for the AHS2 accession was 230, 70, 230, 220, and 120, respectively. Studies on ALS sensitivity to BAY MKH 6562 indicated that the I50 for the AN104, AN205, AN307, AN406, and ASB11 wild oat accessions was 5.2, 0.003, 0.008, 9.8, and 0.007 µmoles, respectively, and the R/S ratio to the I50 value for the AHS2 accession was 759, 0.5, 1, 1,444, and 1, respectively. Of the five wild oat accessions resistant to BAY MKH 6562, accessions AN104 and AN406 had high R/S I50 ratios indicative of an altered target site and accessions AN205, AN307, and AR1 had low R/S I50 ratios indicative of resistance based on metabolic degradation. Hard red spring wheat (2371) was 800-fold tolerant to BAY MKH 6562 and inhibition of ALS from wheat by BAY MKH 6562 was similar to that of ALS from the susceptible accession AHS2.

Spot treatments with hexazinone (0.03, 0.046, and 0.06% ai [v/v]) were evaluated for their effects on thistle density, seed production, and crown vetch vigor from 1991 to 1994 to determine if long-term usage of hexazinone caused damage to the vetch and exacerbated the thistle problem. Thistle densities were reduced within a year at any concentration of hexazinone and seed production was reduced or eliminated. Environment had a large impact because initial thistle densities varied widely among years. The amount of precipitation during July of the previous year accounted for 89% of the variation in initial thistle densities in the subsequent year. Vetch injury from hexazinone was also best explained by the amount of precipitation during July. Vigorously growing crown vetch in wet years may have suppressed the thistles enough to reduce density the following year while drought years reduced the competitiveness of the crown vetch, resulting in higher initial densities of thistles in the following year.

Allelopathy has been suggested as a mechanism of interference in several weed species. Allelochemicals released from certain weed species influence the growth and yield of crop species. Several laboratory studies present circumstantial evidence of the occurrence of allelopathy as a causative agent in weed–crop agroecosystems. Field evidence, however, is still lacking. In this paper, the significance of field studies is argued in terms of a multifaceted approach to allelopathy, and mugwort is used as an example. Previous research demonstrated the allelopathic potential of mugwort; however, experiments were not carried out in a natural environment. Inderjit and Foy (1999) have demonstrated that chemical characteristics (pH, inorganic ions, and phenolics) of soil amended with mugwort leaf leachate were altered when compared to unamended soil. We have analyzed the mugwort-infested field soil and compared its chemical characteristics with those of amended soils. No definite trend, in terms of influence of mugwort on soil chemistry, was observed. Results indicate the importance of field studies in order to obtain ecologically relevant data from laboratory studies. Field situations are often complex in terms of the presence of interfering flora. Cyanobacteria, for example, play an important role in weed–crop interactions in rice paddy soils. Allelochemicals released from weed species present in the paddy field may influence nitrogen-fixing cyanobacteria in addition to their phytotoxic effects to the paddy crop. Significance of phytotoxic effects of weed species on crop growth, and N2-fixing potential of cyanobacteria in paddy soils is discussed.

Studies were conducted in 1997 and 1998 at the Northeast Mississippi Research and Extension Center to investigate the effects of row spacing (76, 38, and 19 cm), soybean population, and three weed management systems on sicklepod growth and seed production. The cultivars ‘Hartz 5088RR’ (glyphosate-tolerant) and ‘Hutcheson’ (a conventional cultivar) were used in two separate studies. The average soybean populations over cultivars and year were 245,000 (low), 481,000 (medium), and 676,000 (high) plants/ha. The three weed management systems were: no (untreated), one, and two herbicide applications. In the glyphosate-tolerant system, one or two postemergence (POST) applications of glyphosate were used, whereas in the conventional system, flumetsulam plus metolachlor preemergence was used alone (single) or followed by chlorimuron POST (sequential). Reducing soybean row spacing from 76 cm, coupled with increased soybean population, reduced sicklepod population up to 80%. Except for Hutcheson in 1998, reducing row spacing and increasing soybean population also reduced sicklepod seed production in both the untreated and the single applications. A single herbicide application reduced sicklepod population up to 68% from untreated plots. However, except for Hartz 5088RR in 1998, the sequential application did not further reduce sicklepod population. In a shading study, partial shading increased sicklepod height but reduced dry weight. However, as shading level increased from 65 to 80 and 95%, height was also reduced.