A Bromus tectorum L. (downy brome) biotype with cross-resistance to sulfosulfuron has been identified. The resistant biotype was selected with primisulfuron in Poa pratensis L. (Kentucky bluegrass) plots near Madras, OR. The plots received two treatments of 20 g ai ha−1 primisulfuron in the fall of 1993, 1994, and 1995. In 1995, control of B. tectorum decreased and greenhouse studies confirmed that the biotype was resistant to primisulfuron. Cross-resistance to sulfosulfuron also was confirmed in greenhouse studies. Metabolism of sulfosulfuron rather than an insensitive site of action is the likely cause of the cross-resistance.

The effect of the organophosphate insecticide malathion on the response of resistant rigid ryegrass population SLR 31 to the herbicides trifluralin, pendimethalin, clomazone, and triallate was investigated. The insecticide and herbicides were soil-applied prior to emergence of plants grown under controlled conditions. In the absence of malathion, the resistant population exhibited significant resistance to the four herbicides compared with a susceptible population. Levels of resistance, as determined by comparison of herbicide rates required to inhibit growth by 50%, were 35, 11, 2.4, and 2.4 for pendimethalin, trifluralin, triallate, and clomazone, respectively. Malathion had a synergistic effect on pendimethalin in the resistant population but not in the susceptible population. Malathion had no synergistic effect on trifluralin, triallate and clomazone. Resistance to triallate and clomazone was found despite the fact that the resistant population had never before been selected with these herbicides. This resistance, selected by other herbicides, further indicates that the use of alternative herbicides to control multiple resistant weeds is unlikely to be a successful resistance management approach.

Growth chamber experiments were conducted to determine the effects of UV light and riboflavin on photolysis of 2,4-D applied to Zea mays leaves. Droplets of 100 mg L−114C-2,4-D were applied to Z. mays leaves with and without 10 mg L−13H-riboflavin and exposed to either UV-enhanced or UV-attenuated polychromatic light in a time-course assay. Photolysis of nonabsorbed 14C-2,4-D residues on Z. mays leaves was sensitized by riboflavin regardless of UV light regime, but a larger percentage of nonabsorbed herbicide was degraded under UV-enhanced light compared to UV-attenuated light. Riboflavin was almost completely photolyzed during the first 10 h of exposure; yet, photolysis of 14C-2,4-D surface residues in treatments containing riboflavin increased from 59% at 10 h of exposure to 87% at 42 h of exposure. In corresponding treatments without riboflavin, photolysis of 14C-2,4-D surface residues was 37% at 10 h of exposure and 84% at 42 h of exposure. In contrast, only 7% of the 14C-2,4-D deposited on glass microscope slides was degraded after 42 h of exposure in the absence of riboflavin, whereas 59% was degraded in the presence of riboflavin. Photolysis of 2,4-D on Z. mays leaves in treatments without riboflavin suggests that certain epicuticular component(s) of Z. mays acted as photosensitizers or catalytic agents that promoted photolysis of nonabsorbed 2,4-D residues.

Laboratory experiments were conducted to assess the influence of surfactants applied with or without nitrogen on MON 37500 foliar absorption by Bromus tectorum, Bromus japonicus, Aegilops cylindrica, Triticum aestivum, Chorispora tenella, and Lactuca serriola. MON 37500 absorption in B. tectorum and B. japonicus increased from 40% 24 h after treatment (HAT) to 48% 48 HAT, averaged across surfactants with no added nitrogen. Averaged across nitrogen source and species, nonionic surfactant, ethylated seed oil, and organosilicate provided comparable enhancement of MON 37500 absorption (56 to 68%), whereas crop oil concentrate provided only 27 to 29% absorption under the same conditions. Averaged across species and surfactant class, urea ammonium nitrate had the greatest effect on MON 37500 absorption (68%), compared to ammonium sulfate (59%) or no nitrogen (40%). Nitrogen, regardless of the type, significantly improved foliar absorption of MON 37500. MON 37500 absorption by species was 71, 63, 57, 57, 49, and 38% in C. tenella, B. japonicus, T. aestivum, A. cylindrica, B. tectorum, and L. serriola, respectively, when averaged across surfactants and nitrogen. Densely pubescent B. japonicus leaves did not retain significant amounts of MON 37500 following a primary leaf wash.

Reports of unacceptable shattercane (Sorghum bicolor) control with acetolactate synthase (ALS)-inhibiting herbicides prompted the investigation of 29 fields in central and south-central Nebraska for ALS-resistant (ALSr) shattercane. These fields were located in three distinct geographical areas designated C, G, and P. Shattercane from 13 fields spanning all three areas was resistant to 80 g ai ha−1 (2 X field rate) primisulfuron. Accessions C and G were more resistant than accession P to primisulfuron and nicosulfuron. Accessions C and G were susceptible to imazethapyr, whereas accession P was resistant. The ALS resistance was associated with alterations in the ALS enzyme. Primisulfuron I50 values for ALS from ROX (forage sorghum), C, G, and P were 7, 8,510, 8,870, and 714 nM, respectively; nicosulfuron I50 values were 647, 4,110, 4,070, and 1,460 nM, respectively; and imazethapyr I50 values were 5,440, 13,100, 11,800, and 51,700 nM, respectively. Based on cross-resistance and enzyme sensitivities, at least two biotypes are represented in the three accessions of ALSr shattercane. Shattercane individuals from accessions C, G, and P were intercrossed to determine if the ALSr genes in each of the accessions were at independent loci. All the F2 populations were resistant to 80 g ai ha−1 primisulfuron, suggesting that the ALSr alleles in the three accessions are at the same locus or possibly linked loci. When the C, G, and P accessions were crossed with the wild type (WT), comparisons between the F1, susceptible, and resistant populations showed that primisulfuron resistance was expressed as a dominant, partially dominant, and additive trait for the C, G, and P accessions, respectively. The differences in ALSr allelic interactions indicate that primisulfuron resistance developed independently in each of the three accessions.

The ability of Medicago sativa to suppress weed growth may provide a viable alternative to chemical weed control and allow crop producers to reduce herbicide inputs. Quantitative information regarding the suppressive effect of M. sativa on weed populations in current cropping systems is lacking. A survey was conducted in Manitoba, Canada, in 1993 and 1994 to investigate weed populations in commercial cereal fields that had been preceded by either M. sativa hay or cereal grain crops. A total of 117 fields were surveyed; approximately half from each field type. Principle component analysis indicated that the inclusion of M. sativa in crop rotations resulted in weed communities different from those of continuous cereal fields. Naturally occurring populations of Avena fatua, Cirsium arvense, Brassica kaber, and Galium aparine were lower in cereal fields that had previously contained M. sativa than in cereal fields that had been preceded by a cereal crop. Lower field uniformity values for C. arvense and Avena fatua indicated that these weeds were also more patchy in the M. sativa rotations. Population differences between field types were nonsignificant for Amaranthus retroflexus, Chenopodium album, and Polygonum convolvulus, and although populations of Taraxacum officinale and Thlaspi arvense were greater in M. sativa/cereal fields than in continuous cereal crops. No consistent effect of field type on Setaria viridis populations was observed. These results show that M. sativa effectively suppressed some, but not all, of the weeds found in the study area. Including M. sativa hay crops in crop rotations can be part of an integrated weed management strategy for weeds such as A. fatua, B. kaber, and C. arvense.

Redroot pigweed is a troublesome weed in the sorghum-growing regions of North America. In 1994 and 1995, field studies were conducted at two locations near Manhattan, KS, to determine the influence of redroot pigweed density and environmental conditions on physiological determinants of redroot pigweed growth: duration of plant growth, light interception, radiation-use efficiency, and dry matter partitioning. In addition, specific leaf area was determined. Redroot pigweed was seeded at monoculture densities of 2, 4, and 12 plants m−1 of row each year at each location. Duration of redroot pigweed growth was not influenced by plant density. Light interception was defined as a simple exponential function of leaf area index. Specific leaf area did not change over the season and averaged 135 cm2 g−1. Partitioning of redroot pigweed dry matter was not influenced by plant density or environmental conditions but did not change within vegetative and reproductive stages. Radiation-use efficiency was not influenced by redroot pigweed density; the most reliable estimate was 1.74 g dry matter MJ−1 of intercepted photosynthetically active radiation. Physiological determinants described were not affected by redroot pigweed density or environmental conditions and therefore provide a starting point for the development of a redroot pigweed growth module. The module could be coupled with available crop growth models (e.g., the sorghum growth model SORKAM) to simulate redroot pigweed–sorghum competition.

The relative leaf area of weeds is a good predictor of the outcome of weed-crop competition. However, this variable has not been used in decision-making tools for integrated weed management because leaf area cannot be measured quickly. A powerful image analysis system for measuring leaf cover (the vertical projection of plant canopy on the ground) has been developed and validated. This research was conducted to compare the efficiency of weed relative leaf area and relative leaf cover in predicting corn yield loss. Field studies were conducted in 1996 and 1997 using varying densities of common lambsquarters, barnyardgrass, common lambsquarters plus barnyardgrass, and a natural weed community. Corn grain yield and biomass loss varied with weed infestation type and year. Values of the relative damage coefficient of weeds (q) were smaller in 1997 compared with 1996. For both years, the relative leaf area of weeds was an adequate predictor of corn yield loss (r2 varied from 0.61 to 0.92). The precision of the predictions was not influenced by the leaf area sampling period (four- or eight-leaf stage of corn). In general, smaller values of q and m (predicted maximum yield loss) were obtained as a consequence of using the relative leaf cover of weeds in model fitting. However, percentages of variation explained by the model (from 0.67 to 0.90) were similar to values obtained with the relative leaf area. On the basis of the residual mean squares, neither of the variables could be declared superior to the other in yield loss prediction. The development of weed control decision-making tools using the relative leaf cover of weeds may require improvements prior to being used in weed management systems. Such improvements would include use of appropriate sampling and image-processing techniques, development and validation of empirical models specific to individual situations, and proper identification of the crop growth stage at which leaf cover must be assessed.

Four field experiments were conducted in Oklahoma to measure full-season Palmer amaranth interference on cotton lint yield and fiber properties. Density of the weed ranged from 0 to 12 plants 10 m−1 of row. Cotton lint yield vs. weed density fit a linear model for densities ⩽ 8 weeds row−1 at Perkins and Chickasha in 1996 and at Alms in 1997. At Perkins in 1997, all densities fit a linear model. For each increase of 1 weed row−1, lint yield reductions were 62 kg ha−1 (or 10.7%) and 58 kg ha−1 (or 11.5%) at Perkins and at Chickasha in 1996, respectively. At Perkins and Alms in 1997, for each 1 weed row−1, lint yield was reduced 71 kg ha−1 (or 5.9%) and 112 kg ha−1 (or 8.7%), respectively. Lint yield vs. end-of-season weed volume fit a linear model except at Alms in 1997. For each increase of 1 m3 of weed plot−1, cotton lint yield in 1996 was reduced by 1.6 and 1.5% at Perkins and Chickasha, respectively. In 1997 at Perkins and Altus (⩽ 6 weeds), each increase of 1 m3 of weed plot−1 reduced lint yield 1.6 and 2.3%, respectively. Lint yield vs. end-of-season weed biomass fit a linear model in all four experiments. Lint yield was reduced 5.2 to 9.3% for each increase of 1 kg of weed biomass plot−1. Fiber analyses revealed significant differences for micronaire (fiber fineness) among weed densities in two experiments, marginal significance in a third, and none in a fourth. An intermediate number of weeds often resulted in improved fiber micronaires in these environments. No other fiber properties were influenced by weed density.

Some species of the genus Echinochloa are troublesome weeds in rice fields. The taxonomy of this genus leads to confusion in many cases due to its great morphological diversity. Because of the differential sensitivity to the herbicide quinclorac shown by Echinochloa spp., it was necessary to assess the botanical and molecular characterization of this weed. Echinochloa colonum, E. oryzoides, and E. oryzicola were very susceptible to quinclorac treatment; by contrast, E. crus-galli and E. hispidula showed some degree of natural tolerance. Physiological and molecular results agreed with the botanical classification of the genus Echinochloa in Flora Europea. The importance of these results is due to yield losses produced by the infestation of Echinochloa and the need for a strategy for Echinochloa management depending on the distribution of the Echinochloa species.

Near isolines of ‘Nugaines’ winter Triticum aestivum that differed in height were planted with and without Aegilops cylindrica to determine the effect of plant height on competition against A. cylindrica. The isolines had either reduced height gene Rht1, Rht2, Rht1 plus Rht2, or neither Rht genes and averaged 79, 77, 51, and 101 cm tall, respectively, when grown with or without competition from A. cylindrica. Plants with fewer reduced height genes had the faster rates of height and weight gain, which are important traits for enhanced competitiveness. When growing in competition with A. cylindrica, the shortest isoline allowed the greatest amount of A. cylindrica seed production but did not have the lowest T. aestivum yield. However, when compared to the A. cylindrica-free control, the shortest isoline had the greatest percent yield loss. The tallest isoline reduced A. cylindrica seed production the most, and T. aestivum yield reduction due to A. cylindrica on a percent basis was the least when averaged over 2 yr. When competing against A. cylindrica, the tallest isoline did not always have the largest yield and yield parameters, and the shortest isoline did not always have the smallest yield and yield parameters. There is a cost to the T. aestivum plant to produce extra stem biomass that may reduce yield potential of taller plants and reduce the advantage gained by being taller than the surrounding weeds.

Glycine max fields with Sesbania exaltata and Senna obtusifolia were selected to evaluate weed effects on insecticide spray droplet deposition within the crop canopy and soybean looper (Pseudoplusia includens) mortality. Weedy and weed-free treatments with and without thiodicarb insecticide application were included. Soybean looper populations in plots with S. exaltata and S. obtusiolia were approximately half those found in weed-free plots. Thiodicarb reduced soybean looper larval survival in the absence of weeds or when weeds were present in three of four experiments. Dye-sensitive cards placed in the top, middle, and bottom thirds of the G. max canopy showed droplet coverage to be highest on cards placed in the top of the canopy. Weeds reduced insecticide coverage in the top of the G. max canopy 26 to 43%. Thiodicarb deposition within the middle and bottom levels of the canopy was not reduced by weeds. Leaflets from the top, middle, and bottom thirds of the crop canopy were collected immediately after insecticide application to bioassay for insecticide deposition. For both years, mortality of soybean looper larvae on leaflets collected from the middle or top of weedy and weed-free G. max treated with thiodicarb was equivalent. Assuming that an economic return can be expected, soybean looper management may not need to be altered when moderate infestations of S. exaltata and S. obtusifolia are present in G. max in late season.

Field studies were conducted near Tifton, GA, from 1995 to 1997 to measure Cyperus esculentus (yellow nutsedge) interference with ‘Fancipack’ Cucumis sativus (cucumber) using a response prediction experiment with a natural infestation of C. esculentus. Cucumis sativus was direct-seeded each year. Plots (1.8 by 1.8 m) were established immediately after crop emergence. Cyperus esculentus plants were counted 2 wk after crop emergence in each plot, at which time four weed-free plots were randomly established. C. esculentus densities ranged from 0 to 955 plants m−2. Total yield, plant biomass, and C. sativus stand were regressed against C. esculentus density and biomass. Regression analysis showed a 5% reduction in C. sativus yield with a C esculentus infestation of approximately 15 plants m−2. A uniform C. sativus stand maximized its competitive ability and minimized interference from C. esculentus. Cyperus esculentus was more competitive and reduced C. sativus yields when C. sativus stands were low and nonuniform.

Field experiments were conducted from 1994 through 1996 in dry-seeded and water-seeded Oryza sativa to compare weed control and O. sativa yield in conventional tillage and reduced tillage (stale seedbed and Triticum aestivum cover crop) systems with no in-season herbicide, propanil applied early postemergence (EPOST), and propanil EPOST followed by propanil plus molinate (dry-seeded production) or granular molinate (water-seeded production). The T. aestivum cover crop reduced Echinochloa crus-galli, Heteranthera limosa, Ammania coccinea, and Cyperus esculentus infestation and reduced the need for in-season herbicides in some but not all experiments. Oryza sativa grain yield was affected by both tillage systems and herbicide programs; however, these treatment factors influenced yield independently. Increasing the number of in-season herbicide applications increased weed control and O. sativa yield in some but not all experiments.

The WEEDSIM bioeconomic model was developed in Minnesota and was designed to support weed management decisions for both soil-applied and postemergence weed control programs in Zea mays and Glycine max. In this research, we adapted the WEEDSIM weed management model to Michigan by modifying the crop yield loss functions and herbicide efficacy ratings. We then validated the components of the model and determined whether WEEDSIM led to more profitable weed management than recommendations from SOYHERB or CORNHERB, computer decision aids based solely on herbicide efficacy and cost. The crop year significantly influenced the weed-free yield in Z. mays and G. max, but the weed—crop interference function did not change each year. Total weed seed increased in the untreated compared with the weed-free control over the 3-yr period. Weed seed did not increase significantly in WEEDSIM preemergence/postemergence (PRE/POST), WEEDSIM postemergence, or CORNHERB or SOYHERB treatments compared with the weed-free control, although annual grass seedling density at the time of postemergence herbicide application had increased by 1995 in the WEEDSIM postemergence treatment in G. max because of a 2,4-D amine application only in Z. mays in 1994. WEEDSIM PRE/POST and CORNHERB provided excellent weed control in all three years, and WEEDSIM PRE/POST resulted in gross margins over weed control costs equal to or greater than CORNHERB recommendations. In G. max, Chenopodium album and annual grass control was excellent in all three years for WEEDSIM PRE/POST, WEEDSIM postemergence, and SOYHERB treatments. The highest average gross margin for the 3-yr study was from mechanical weed control in 76-cm-wide rows of G. max ($806 ha−1) and from SOYHERB in 38- and 19-cm-wide rows of G. max ($776 and $808 ha−1, respectively). WEEDSIM recommendations controlled weeds and maintained crop yield in both Z. mays and G. max.

Laboratory studies were conducted to evaluate 14C-flumetsulam mobility in two Mississippi soils of varied texture and organic matter content following delays in irrigation. Mobility was evaluated using packed soil columns, 25 cm deep, under unsaturated–saturated flow conditions. Irrigation timings included 0, 3, and 5 d after flumetsulam application. Flumetsulam mobility (defined as the amount collected in leachate) decreased from 45% to no more than 20% of the applied in the Prentiss sandy loam soil when irrigation was delayed 3 or 5 d. With the Okolona soil, flumetsulam recovery in the leachate was 21, 14, and 6%, respectively when irrigation occurred 0, 3, and 5 d after application. Flumetsulam proved to be mobile when irrigation immediately followed application, with 6 to 45% recovered in the leachate from all soils evaluated. The Prentiss soil retained 6% of the applied flumetsulam in the upper 5 cm and the Okolona soil retained 22% when irrigation immediately followed flumetsulam application. When the irrigation interval was delayed at least 3 d, the Okolona soil retained 40% in the upper 5 cm, whereas the Prentiss soil retained 10%. Flumetsulam mobility was dependent on irrigation timing and soil type.

Metsulfuron sorption, dissipation, and leaching were studied in six Colorado soils. Sorption was studied in the laboratory by batch equilibration of soil horizons from the surface to 1 m deep found in each study area. Kd was correlated to several soil parameters with pH (-0.773) and percent organic matter (OM) (0.666) the strongest, although low. Kd generally decreased with depth at each site and ranged from 0.10 to 0.83 among surface soils. The lowest Kd was in the soil with the highest pH. Leaching and dissipation were studied by high-performance liquid chromatography/ultraviolet analysis of field samples collected 1 m deep. Bromide ion as conservative tracer was applied concurrently with metsulfuron at the four dryland and two irrigated sites. In most cases, metsulfuron was found only in upper horizons, and the half-life ranged from 11.8 to 27.7 d, the shortest being in the soil with highest percent OM and lowest pH. Very little leaching below the surface horizon was detected in any soil.

Starch levels, used as a measure of plant stress, were not consistently reduced in root or crown tissue of purple loosestrife plants after 2 yr of severe Galerucella calmariensis or Galerucella pusilla (Coleoptera: Chrysomelidae) defoliation. Early in the season, defoliation from Galerucella spp. approached 100%, but the majority of Lythrum salicaria plants regrew by the end of August, resulting in an average reduction of 81% of the aboveground biomass compared to the control. The stress imposed by Galerucella spp. defoliation was less than that achieved from more severe stress imposed by mechanical shoot clipping at 2- or 4-wk intervals from June to October. Both shoot-clipping treatments killed the majority of plants after one growing season. Galerucella spp. feeding reduced plant stature, which may reduce competitiveness. However, considering the extensive carbohydrate reserves present in the large woody crowns of Lythrum salicaria, it will require in excess of 2 yr of consistent, severe leaf defoliation to cause plant mortality. A combination of stresses, such as winter crown injury, or other biological control agents in addition to Galerucella leaf defoliation may be required for plant mortality.