Sustainable rangeland management will require successional strategies to deal with the expanding weed problem. These strategies must be consistent with the view that plant communities are dynamic and technology is used to enhance the natural processes and mechanisms that direct vegetation change. The goal is to shift the dynamics toward a desired plant community. A unified conceptual model is necessary to direct the development and application of successional weed management systems. We propose using a resource management model as a conceptual basis for successional weed management. This model is based on the primary causes of succession: site availability, differential species availability, and differential species performance. This model provides the mechanistic framework necessary for developing successional weed management systems and it is meant to enhance communication among rangeland weed managers and scientists.

Dithiopyr and prodiamine are new preemergence herbicides for crabgrass control in tall fescue and bermudagrass turf. A field experiment was conducted over a 2-yr period to determine the lowest rates of dithiopyr and prodiamine needed to control large crabgrass effectively in tall fescue and common bermudagrass. Prodiamine applied at 0.17 kg ai/ha in each of two applications controlled 92 to 93% of large crabgrass by late August in common bermudagrass and 78 to 85% in tall fescue. Dithiopyr applied at 0.11 kg ai/ha in each of two applications controlled 85 to 89% of large crabgrass in common bermudagrass, but the control was not consistent across years at any rate in tall fescue. Dithiopyr applied at 0.56 kg/ha on tall fescue controlled 91% of large crabgrass in 1993, but only 38% in 1994. The higher control with dithiopyr in common bermudagrass was due to greater competition with germinating large crabgrass seeds and emerging seedlings during the summer, compared with little or no competition from tall fescue. The quality of tall fescue and common bermudagrass treated with dithiopyr and prodiamine was generally higher in plots where large crabgrass was effectively controlled than in plots with poor control. In general, dithiopyr did not control large crabgrass as effectively in tall fescue as in common bermudagrass, while the control with prodiamine was similar for the two turfgrass species.

Field studies to determine the effect of flame cultivation on cotton pests and beneficial insects were conducted in 1994. Mortality of caged tarnished plant bugs and convergent lady beetles located at the soil surface and canopy heights of 10 and 20 cm above the ground in 25- to 30-cm cotton was recorded after flaming at two liquid propane-gas (LP-gas) pressure settings with or without water-shield protection. Air temperatures, measured proximal to insects during flame treatments, were lower at higher levels in the cotton canopy regardless of LP-gas pressure and the presence or absence of water-shield protection. A mortality of 100% was observed at the soil surface for both insect species in all treatments. Plant bug and lady beetle mortality was lower at the 10- and 20-cm heights relative to soil surface values after flaming at 100 kPa LP-gas pressure. After treatments at 175 kPa, only the mortality of lady beetles at 20 cm above the ground was lower than mortality obtained at the soil surface and 10-cm height. Although flaming induced lady beetle mortality, these beneficial insects were not affected to the same extent as plant bugs at higher canopy levels.

Field studies were conducted over four years to determine sugarcane response to clomazone applied POST at 1.1 and 2.2 kg ai/ha. Sugarcane leaves present when clomazone was applied were chlorotic 2 WAT. Injury 4 WAT ranged from 0 to 38% following single applications in March and April. Chlorosis 4 wk after the May application of a March plus May sequential treatment was only greater than for the single March application in 1992. The number of harvestable sugarcane stalks following treatments with clomazone was intermediate between those produced with standard applications of either metribuzin at 2.6 kg ai/ha (least injurious) or terbacil as a single treatment at 1.8 kg ai/ha in March and April or as a 1.8 plus 0.9 kg/ha March plus May sequential treatment. Sugar recovery as an average of all herbicide treatments was lower for the sequential applications than for the single applications. Averaged over single and sequential treatments, theoretically recoverable sugar levels with clomazone at both rates was intermediate between the recoveries obtained with standard applications of metribuzin (highest) and terbacil. Gross cane and sugar yields following all applications of clomazone at 1.1 kg/ha and following single applications of clomazone at 2.2 kg/ha in March and April were similar to the metribuzin standard. Sugar yields following the sequential treatment of clomazone at 2.2 kg/ha, although lower than the metribuzin standard, were similar to the standard terbacil sequential treatment.

Yellow nutsedge is a common and troublesome weed in the nursery industry. A selective postemergence herbicide is not available for yellow nutsedge control in most nursery crops. The effectiveness of MON 12000 for PRE and POST control of yellow nutsedge was evaluated in selected field-grown nursery crops. Preemergence control of yellow nutsedge 4 weeks after treatment (WAT) increased from 68% to 95% as MON 12000 rate increased from 0.03 to 0.28 kg ai/ha. At 9 WAT, control ranged from 16 to 73%. MON 12000 at 0.14 kg/ha provided similar PRE control of yellow nutsedge as metolachlor at 2.2 kg/ha. Four WAT, MON 12000 applied POST at 0.03 kg/ha controlled 73% and controlled 86% with the 0.28 kg/ha rate. MON 12000 at 0.14 and 0.28 kg/ha applied POST controlled yellow nutsedge better than bentazon at 1.12, chlorimuron at 0.01, imazaquin at 0.14, or glyphosate at 3.33 kg/ha. MON 12000 injured the foliage of azalea, crape myrtle, cotoneaster, and Japanese holly. Injury was most severe to cotoneaster. MON 12000 reduced azalea, cotoneaster, and crape myrtle shoot fresh weight compared to hand-weeded plots in at least one study. Metolachlor at 2.2 and 4.5 kg/ha caused little injury to the nursery species tested.

Studies were conducted from 1993 through 1995 to determine broomrape control from imazethapyr applied to pea seeds. Soaking seeds for 1 to 10 min in 0.0001% to 1% imazethapyr (Pursuit-10) solutions or coating seeds with imazethapyr at 0.025 to 0.30 mg/g (herbicide/seed, equivalent to 5 to 60 g/ha for a seeding rate of 200 kg/ha) did not affect seed germination or crop emergence and growth. Broomrape control with imazethapyr applied by coating seed at 0.1 to 0.2 mg/g (herbicide/seed, 20 to 40 g/ha) or by soaking seeds for 5 min in 0.1% to 1% imazethapyr solution was from 75 to 80% and 50 to 70%, respectively. Pea biomass and seed yield increased, compared to broomrape-infested nontreated control, when pea seeds were treated with imazethapyr and imazapyr at 5 g/ha was applied late postemergence, as a result of the excellent broomrape control.

Field experiments were established to evaluate coffee senna control in cotton with registered PRE, early POST-directed (EPD), and late POST-directed (LPD) herbicides and with pyrithiobac applied PRE and early POST. Fluometuron and norflurazon applied PRE controlled coffee senna 87 and 65%, respectively, 6 wk after planting (WAP). Methazole applied EPD controlled coffee senna 96% 2 wk after application, whereas fluometuron and MSMA applied EPD controlled 70 and 16%, respectively. LPD applications of cyanazine, diuron, and prometryn averaged only 45% control 8 WAP. MSMA applied LPD controlled coffee senna only 12%. Fluometuron and norflurazon applied PRE followed by methazole applied EPD and cyanazine applied LPD controlled coffee senna 98 and 99% 6 WAP, respectively. PRE, early POST, and PRE followed by early POST applications of pyrithiobac controlled coffee senna 88, 89, and 95% 6 WAP, respectively. Seed cotton yields were similar to weed-free cotton with all treatments except MSMA applied EPD and the LPD treatments, all of which provided unacceptable (12 to 49%) control 8 WAP.

A computer model which selects least cost herbicide programs given a minimum desired level of weed control could provide growers with economical weed management options. Using an integer programming approach, a herbicide selection model was developed for corn production under Pennsylvania conditions. Models for three rotations (corn-soybean, corn-corn, and corn-alfalfa) under three tillage systems (conventional tillage, reduced tillage, and no-till) that evaluated 21 soil-applied and 13 postemergence herbicide options for 24 weeds were developed. Each model minimizes the cost of a herbicide program subject to a desired level of weed control. By selecting the weed species to be controlled and the level of control desired, customized herbicide programs can be generated. The models can also be used to evaluate the cost of changing the level of control desired for an individual weed species or set of weeds.

Three soybean production systems coupled with conventional tillage (CT) or no-tillage (NT) were evaluated for metolachlor and metribuzin loss in water from runoff. Metolachlor and metribuzin concentrations were similar in runoff from CT or NT soybean following winter wheat, with stubble burned, and CT or NT monocrop soybean. However, metolachlor and metribuzin concentrations were higher in runoff water from the CT doublecrop soybean planted into winter wheat stubble than NT. In all 3 yr of the study, the majority of total losses occurred in the first three runoff events. Total loss of metolachlor in runoff from the doublecrop system with winter wheat stubble present was 0.6% in 1991 to 1.1% in 1990 from CT, and 0.2% in 1990 to 0.6% in 1991 from the NT. Similar results were observed from the CT and NT doublecrop with burned winter wheat stubble and the monocrop systems. Metribuzin loss patterns were similar to metolachlor, but the fraction lost was slightly higher. Multi-variate regression was used to describe and predict herbicide losses, and R2values ranged from 0.66 to 0.98 for both metolachlor and metribuzin.

The effect of imazethapyr and imazamethabenz on following crops was tested in southern Alberta, on Dark Brown and Brown Chernozemic soils, to assess the potential restrictions placed on cropping sequences by the use of these herbicides. Imazamethabenz reduced the yield of sugarbeet seeded one year after application. After imazethapyr application there is risk of yield loss with flax, corn, meadow bromegrass, mustard, sunflower, timothy, and wheat seeded one year later; canola seeded up to two years later; and sugarbeet and potato seeded up to three years later. Legume crops and intermediate wheatgrass may be seeded the year after application with little risk of yield loss. The required recropping intervals limit the use of imazethapyr for weed control in pea, alfalfa, or dry bean in cropping sequences that include sugarbeet, canola, or potato.

Decision making in weed control is complex and time-consuming. Moreover, the structure of the available information does not facilitate the comparison of different herbicides. Indeed, information format can be the limiting factor in the performance of sophisticated computer programs intended to supply appropriate advice on weed control treatments. A relational database for decision support on chemical weed control has been developed. It uses a detailed structure by subdividing the information where possible. The database includes programs for entering, updating, and printing data, as well as programs for retrieving information and giving treatment advice. The information access on herbicides is organized around searches based on a specific crop and multiple weed species at their respective growth stages. Optimization of the selected herbicides is carried out and supplies the lowest number of herbicides controlling all the chosen weeds. Information on critical parameters for herbicide application such as varietal restrictions, rotational crops, and compatibility with other products is also interactively available.

Field studies were conducted to determine the effect of soil pH ranging from 5.1 to 7.1 on imazaquin persistence on a Grundy silty clay loam (2.8% OM). Imazaquin residues were equivalent at pH 5.5 or higher but persisted longer at pH 5.1. Corn shoot growth was not different at any soil pH. Corn grain yields in 1993 were lower in imazaquin-treated plots than in the check plots at the lowest pH (5.1), where initial imazaquin soil concentrations were between 10 and 13 μg/kg across all pH levels. Corn grain yields were not affected by herbicide carryover in 1994 where soil imazaquin residues were well below critical levels at planting. Yields were lower where soil pH was less than 5.5.

Sweet corn and pumpkin were planted no-tillage (NT) into cover crop residue treatments of vetch, rye, crimson clover, and ladino clover controlled with glyphosate, and a bare ground conventional tillage (CT) control. Objectives included evaluation of crop growth, yield, and weed suppression in NT versus CT treatments. Herbicide application was also investigated, with a plus and minus herbicide treatment (alachlor plus cyanazine for sweet corn, or ethalfluralin for pumpkin) as the main factor in the factorial experiment, and cover crops the subfactors. Weed control 4 wk after planting was dependent upon cover crop. The fewest weed numbers and least biomass were found in the ladino clover plots, but clover regrowth and subsequent competition with the cash crop were severe. Herbicides also affected weed biomass at 4 wk after vegetable planting, with least biomass in herbicide-treated plots. Neither cover crop nor herbicide treatment significantly affected weed weight by 8 wk after planting or pumpkin fruit weight at harvest. Pumpkin yield was not influenced by herbicide application. The vetch cover, although harboring greatest weed biomass, produced the greatest total yield (ear weight) in sweet corn. When averaged over cover crop, sweet corn yields were higher in herbicide-treated plots than in untreated ones. Both sweet corn and pumpkin maturity were generally delayed in the absence of herbicide treatments or in the presence of cover crop residues, especially clover and rye residues.

Full-season interference and critical duration of early-season ivyleaf morningglory interference with cotton were measured in experiments at two Oklahoma locations. Densities ranging from 0 to 64 weeds/10 m of row were evaluated for their effects on cotton lint yield, fiber quality, and harvest efficiency. Analyses using linear-linear regression proved superior to linear, curvilinear, and linear-plateau models. Lint yield reductions of 36.9 kg/ha (or 5.9%) were recorded at Perkins for each weed/10 m up to 8.7. At densities greater than 8.7/10 m, yield was reduced an additional 3.0 kg/ha (0.5%) for each additional weed. At Chickasha, lint yield was reduced 29.7 kg/ha (or 3.9%) for each weed/10 m up to 9.0 with an additional lint loss of 3.6 kg/ha (0.7%) for each weed above the density of 9.0/10 m. Yield losses were over 80% before fiber length was also reduced. Mechanical harvest was not possible at densities above 8 weeds/10 m at Chickasha and above 16 weeds/10 m at Perkins. In critical duration experiments conducted at Perkins, lint yield was reduced 52.9 kg/ha (or 11.2%) for each week weed removal was delayed up to 9.5 wk. An additional 1.0 kg/ha (0.2%) was lost for each week of interference after 9.5 wk. At Chickasha, lint yield was reduced 49.0 kg/ha (or 7.8%) for each week of interference up to 11.0 wk with an additional loss of 1.2 kg/ha (0.2%) for each week thereafter.

Two biotypes of field bindweed differing in their susceptibility to glyphosate were used to determine if diluent or carrier volume and additional surfactant could overcome differences in intraspecific response to glyphosate. In greenhouse studies, glyphosate (formulated product) was applied at 1.68 kg/ha in three diluent volumes (142, 189, and 237 L/ha), with and without 1 % (v/v) additional amphoteric surfactant. Nonparametric and ordinal categorical analyses indicated that field bindweed biotype, diluent volume, and surfactant significantly increased glyphosate phytotoxicity 7 DAT. Only biotype and volume were significant 21 DAT. The tolerant biotype was less injured at the 189 and 237 L/ha volumes than the susceptible biotype. Field bindweed injury was similar at a diluent volume of 142 L/ha for both biotypes. These greenhouse studies suggest that control of field bindweed may be improved with glyphosate by using low spray volume in concert with additional surfactant.

Recently, roundleaved mallow, redroot pigweed, and annual smartweed species have become problems for flax producers in the Canadian prairies. The use of thifensulfuron to enhance weed control with commonly used herbicide mixtures was tested at Indian Head, SK and Morden, MB. Visible crop injury and reduced height, yield, and quality occurred at rates as low as 0.5–1.0 g/ha. Improved control of broadleaf weeds was not consistent and antagonism of grassy weed control with sethoxydim occurred.

Field and greenhouse experiments were conducted to investigate the influence of two cationic alkylamine surfactants, three nonionic surfactants, and ammonium sulfate on the performance of glyphosate (isopropylamine salt) against quackgrass. In the field experiments, the reinfestation of quackgrass in the year after treatment was zero or negligible in all plots that received the recommended rate of glyphosate (1.44 kg ae/ha). At one-fourth of the recommended rate (0.36 kg ae/ha) the glyphosate-adjuvant combinations also severely inhibited the reinfestation of quackgrass. At the low herbicide rate and without ammonium sulfate, lipophilic surfactants were less effective than more hydrophilic surfactants. In the greenhouse experiments, the glyphosate doses giving 50% response (ED50) were determined. Ranking of the surfactants according to the ED50 for glyphosate demonstrated a similar result to that observed in the field experiment. Ammonium sulfate added to the glyphosate-surfactant combinations reduced the ED50 for glyphosate 5-fold. The field and greenhouse experiments demonstrated that two hydrophilic nonionic surfactants enhance glyphosate efficacy against quackgrass to the same extent as the hydrophilic cationic tallowamine surfactant. These two nonionic surfactants have a relatively low toxicity to non-target organisms and may substitute the more toxic alkylamine surfactants.

The results of two experiments involving the effects of safeners on chloroacetanilide and dicamba injury in corn are reported here. The first experiment was conducted to determine whether MON-4660, CGA-154281, dichlormid, and MON-13900 would increase corn tolerance to alachlor, metolachlor, and acetochlor, respectively, over a range of soil temperatures (12 to 37 C). All three herbicides, particularly acetochlor, reduced corn shoot lengths at temperatures less than 32 C. The safeners effectively increased corn tolerance to their respective chloroacetanilide herbicide at all soil temperatures. However, acetochlor injury was not completely overcome by dichlormid or MON-13900 when temperatures were less than 27 C. The second experiment was conducted to determine whether these safeners would alleviate injury caused by chloroacetanilides applied with dicamba in sensitive (Pioneer 3320) and tolerant (Cargill 7567) corn hybrids at soil temperatures ranging from 17 to 27 C. Chloroacetanilides (4.5 kg ai/ha) plus dicamba (0.6 kg ae/ha) reduced shoot lengths in both hybrids, with acetochlor/dicamba combinations causing the greatest reductions. All four safeners increased corn tolerance to chloroacetanilide/dicamba tank-mixtures equally.

The effects of rye planting time, rye seeding rate, and rye/weed management systems on weed control and soybean yield were determined in field experiments near Arlington, WI from 1992 to 1994. Insufficient precipitation in 1992 resulted in limited soil moisture, less ground cover, less weed control, and lower soybean yields than in 1993 and 1994. The higher rye seeding rate provided more ground cover and better weed control than the lower rate in all years; however, it reduced soybean vigor. The optimum rye seeding rate was 112 kg/ha. The rye-only system reduced weed shoot biomass by 90, 82, and 60%, in 1992, 1993, and 1994, respectively, relative to the no-rye weedy check treatment. Killing rye 45 d after planting soybean gave optimum weed control. In 1993, rye alone suppressed the weeds without decreasing crop yield, but in 1994 crop yield was decreased due to inadequate weed control by rye. The results indicate that the rye living mulch technique can adequately control weeds without causing soybean yield reduction if weed pressure is low, ground cover and soil moisture are adequate and rye interference is minimal.

Triazine-resistant (TR) common waterhemp was reported and confirmed in Fillmore County, NE in 1990. A survey of 81 fields was conducted to characterize the occurrence of common waterhemp and grower practices. Sampled fields fell into three categories: suspected by growers to contain resistant plants (26 fields), randomly selected (28 fields), and adjacent to fields containing TR common waterhemp (27 fields). Resistant plants were found in 64% of all fields. Resistance was confirmed in 92% of the fields suspected by growers to contain resistant plants. Adjacent fields were no more likely to contain resistant plants than randomly chosen fields. Crop rotation did not significantly affect occurrence of resistance. Resistance was associated with the grower, indicating movement of resistant seed between fields via equipment. Atrazine, bromoxynil plus atrazine, and bentazon plus atrazine provided less than 75% postemergence control of TR common waterhemp, while primisulfuron, dicamba plus atrazine, primisulfuron plus dicamba, dicamba, 2,4-D ester, and metribuzin plus bentazon gave over 85% control.