Greenhouse and laboratory experiments were conducted to determine the physiological basis for differential tolerance of four Zea mays L. hybrids to RPA 201772. Differences in Zea mays tolerance were quantified by determining the herbicide rate required to injure and reduce Z. mays height 50% (GR50). GR50 values indicated that the Z. mays hybrids ‘Pioneer 3751’ and ‘Pioneer 3737’ were less tolerant to RPA 201772 than the hybrids ‘Pioneer 3394’ and ‘Pioneer 3963.’ Experiments using 14C-RPA 201772 were conducted to determine if hybrid sensitivity was due to differential uptake, translocation, or metabolism of the herbicide. Differences in hybrid tolerance were primarily due to differential herbicide metabolism rates. The time required for 50% inactivation (T½) of RPA 201772 was 42 and 52 h for the more tolerant hybrids and 66 and 93 h for the more sensitive hybrids. Increased uptake of RPA 201772 was also a contributing factor to the sensitivity of one of the hybrids.

Absorption, translocation, and metabolism of 14C-glufosinate were studied in three annual and two perennial weed species. Young seedlings of Setaria faberi, Chenopodium album, Cassia obtusifolia, Solanum carolinense, and Asclepias syriaca were treated with foliar-applied 14C-glufosinate, and plant tissues were harvested 12, 48, and 72 h after treatment (HAT). Absorption of 14C-glufosinate was initially rapid, but increased only slightly after 12 h in all species. Glufosinate absorption was highest in S. carolinense (73% of applied radioactivity), followed by S. faberi (54%), C. obtusifolia (44%), C. album (41%), and A. syriaca (37%) 72 HAT. Translocation of radioactivity out of the treated leaf was species dependent and did not increase much with time in all weed species. S. carolinense and S. faberi translocated the highest amounts of absorbed radioactivity out of the treated leaf with 49 to 59% moving to the upper foliage. S. faberi translocated the highest amount of absorbed radioactivity to the roots (12 to 14%), while C. album translocated the least (2 to 3%). TLC analysis of plant extracts showed that 14C-glufosinate was not metabolized in S. faberi, C. obtusifolia, S. carolinense, and A. syriaca. A glufosinate metabolite with an Rf value matching that of methyl-phosphinico propionate was detected in C. album. Treatment with ammonium sulfate (AMS) increased glufosinate absorption in S. faberi and C. obtusifolia 12 HAT, but decreased absorption in C. album. Treatment with pelargonic acid (PA) did not affect glufosinate absorption in any of the species tested. Treatment with AMS and PA did not affect glufosinate translocation in any of the five weed species. Treatment with AMS and PA did not influence the metabolism of glufosinate in any of the five weed species studied. These results show that differential absorption and translocation seem to explain the greater sensitivity of the annual and perennial weeds to glufosinate. Treatment with ammonium sulfate may increase the efficacy of glufosinate in perennial weeds.

Variability in weed control following pyrithiobac applications has been observed under field conditions. The influence of temperature on this variability was investigated. Results from field studies performed over two growing seasons identified plant and air temperatures at the time of herbicide treatment that correlated with whole-plant efficacy differences. Based on the field data, weed control with pyrithiobac was acceptable at application temperatures of 20 to 34 C. To investigate a potential source of thermal limitations on pyrithiobac efficacy, the thermal dependence of in vitro inhibition of acetolactate synthase (ALS), the site of action for pyrithiobac, was examined. A crude leaf extract of ALS was obtained from Amaranthus palmeri. Relative inhibitor potency (I50) values were obtained at saturating substrate conditions for temperatures from 10 to 50 C. Regression analysis of field activity against I50 values showed the two data sets to be highly correlated (R 2 = 0.88). The thermal dependence of enzyme/herbicide interactions may provide another means of understanding environmental factors limiting herbicidal efficacy and predicting herbicide inhibition at the whole-plant level.

In glasshouse experiments, Panicum repens rhizomes with more than two nodes emerged from soil depths of 1 to 20 cm. Emergence from one-node rhizomes was greatly reduced at depths more than 5 cm. In contrast, culm emergence from ginger like rhizomes decreased when burial depth (BD) was greater than 20 to 30 cm. P. repens emerged from deeper soil levels with an increasing number of nodes per cutting. Culms emerged from 36% of total underground regenerative organs. About 75 and 95% of the emerged culms were recorded 30 and 60 d after burial, respectively, and 91% emergence was recorded from 20-cm BD. In field studies, only 9% of nodes produced culms following cross-plowing. About 61 and 67% of the emerged culms were recorded 50 and 60 d after land preparation, respectively. Results indicate that mechanical and chemical control methods could be most effective for P. repens when applied approximately 50 d after land preparation. Burying deeper than 30 cm by deep plowing and reducing rhizome length by cross-plowing might effectively reduce emergence of P. repens.

Kyllinga species are becoming more prevalent in turfgrass sites throughout North America. The effects of nitrate (50, 200, and 400 mg L−1), temperature (33/24, 25/17, 19/11 C day/night, respectively), and light on seed germination of three Kyllinga species were investigated in growth chambers. Nitrate concentrations did not stimulate Kyllinga species seed germination compared with untreated seeds. All Kyllinga species seeds failed to germinate in darkness but resumed germination once they were placed in light. This is an important pest management strategy because a dense, uniform turfgrass stand with its minimum light penetration to the soil would minimize Kyllinga species seed germination. Higher temperatures increased seed germination rate and percentage of each species after 8 wk. Maximal (> 90%) K. brevifolia germination occurred 2 to 4 wk after initiation in every seed study, whereas K. squamulata seeds germinated continuously. Minimal (< 10%) K. pumila seeds germinated until alternating diurnal temperatures were imposed.

Kyllinga species are becoming more common throughout the southeastern United States. Two species, Kyllinga brevifolia and Kyllinga squamulata, in particular are prevalent weeds in turfgrass. To better understand these weeds, growth chamber studies determined the growth of K. brevifolia, K. squamulata, and Cynodon dactylon × Cynodon transvaalensis as influenced by three temperature regimes (33/24, 25/17, 19/11 C day/night, respectively). Temperature influenced almost all aspects of Kyllinga species growth. Plant height of both Kyllinga species increased nearly twofold after 8 wk at high temperatures. Plants were mowed each week to 2.5 cm; both species produced more than twice as many clippings by 8 wk at high (33/24 C) temperatures than at low (19/11 C) temperatures. Destructive analysis at 8 wk revealed that K. brevifolia shoot and root weight increased with decreasing temperature, whereas K. squamulata shoot and root weights were not affected by temperature. Shoot weight percentage for both Kyllinga species increased from 59% in medium temperatures to 69% in high temperatures. K. brevifolia shoot weight percentage decreased to 53% in low temperatures, whereas K. squamulata shoot weight percentage increased to 72%. K. brevifolia inflorescences formed at 2, 3, and 5 wk in high, medium, and low temperatures, respectively, whereas K. squamulata flowered immediately in all temperatures. C. dactylon × C. transvaalensis and Kyllinga species growth were similar within each temperature regime throughout the 8-wk study.

Bioeconomic weed management models help growers achieve appropriate weed management with less herbicide by matching management to the weed population in a field. Growers, however, will not use bioeconomic models unless cost-effective methods to sample their weed populations are identified. Counting and identifying seeds and seedlings is the most time-consuming and costly part of sampling weed populations. The time required for this process was investigated and modeled as a first step in developing sampling plans for growers using WEEDCAM, a bioeconomic model for weed management in Zea mays L. in Colorado. The time required to count and identify seeds or seedlings was recorded for 9,405 soil cores (5 cm in diameter and 10 cm deep) and 9,726 quadrats (18-cm band over 1.52 m of crop row) collected or examined in eight corn fields in eastern Colorado. The time required to count and identify seeds was best described using a log-linear regression with time increasing with the number of seeds and species and the amount of sand in the core. The average cost of determining there are no seeds in a core is $1.07 for a core from a field with 37% sand and $4.32 if the field has 88% sand. The average cost of counting and identifying 36 seeds of four species is $2.70 and $10.88 for cores with 37 and 88% sand, respectively. The time required to count and identify seedlings was best described using a log-linear regression with time increasing with the number of seedlings and species. Classifying seedlings as grass or broadleaf did not improve the model. The average cost of determining that a quadrat is weed-free is $0.02. The average cost of counting and identifying 37 seedlings is $0.05, $0.07, $0.19, and $0.26 per quadrat for 1, 2, 4, and 6 species, respectively. The cost of identifying seeds and seedlings in eastern Colorado Z. mays fields to use WEEDCAM is estimated as $2.71 per core for the seed bank and $0.08 per quadrat for seedlings.

Site-specific weed management recommendations require knowledge of weed species, density, and location in the field. This study compared several sampling techniques to estimate weed density and distribution in two 65-ha no-till Zea mays–Glycine max rotation fields in eastern South Dakota. The most common weeds (Setaria viridis, Setaria glauca, Cirsium arvense, Ambrosia artemisiifolia, and Polygonum pensylvanicum) were counted by species in 0.1-m2 areas on a 15- by 30-m (1,352 points in each field) or 30- by 30-m (676 points in each field) grid pattern, and points were georeferenced and data spatially analyzed. Using different sampling approaches, weed populations were estimated by resampling the original data set. The average density for each technique was calculated and compared with the average field density calculated from the all-point data. All weeds had skewed population distributions with more than 60% of sampling points lacking the specific weed, but very high densities (i.e., > 100 plants m−2) were also observed. More than 300 random samples were required to estimate densities within 20% of the all-point means about 60% of the time. Sampling requirement increased as average density decreased. The W pattern produced average species densities that often were similar to the field averages, but information on patch location was absent. Weed counts taken on the 15- by 30-m grid were dependent spatially and weed contour maps were developed. Kriged maps presented both density and location of weed patches and could be used to establish management zones. However, grid-sampling production fields on a small enough scale to obtain spatially dependent data may have limited usefulness because of time, cost, and labor constraints.

Greenhouse experiments were conducted to evaluate the effect of the fungus Dactylaria higginsii on the interference of Cyperus rotundus (purple nutsedge) with Lycopersicon esculentum (tomato ‘Agroset'). Cyperus rotundus plants established from tubers were planted at initial densities equivalent to 40, 80, 160, and 320 tubers m−2 with L. esculentum in 35-cm-diam pots. Cyperus rotundus plants were inoculated by spraying with a spore suspension of D. higginsii containing 0.5% Metamucil as a carrier. The treatments were Metamucil only, 104 conidia ml−1 + Metamucil, or 106 conidia ml−1 + Metamucil. In the absence of D. higginsii, C. rotundus at densities of 80, 160, and 320 tubers m−2 reduced L. esculentum yield by 14, 68, and 70%, respectively. In contrast, yield of L. esculentum treated with D. higginsii at 106 conidia ml−1 was equal to that in the weed-free control. The rate of disease increase (r G) was higher in treatments with 106 conidia ml−1 (r G = 0.126 to 0.136) compared to 104 conidia ml−1 (r G = 0.046 to 0.050). At 106 conidia ml−1D. higginsii reduced the interference from C. rotundus, controlled C. rotundus better, and increased L. esculentum yield compared to the weedy checks.

Field experiments were conducted in 1997 and in 1998 to determine the effects of density and time of emergence on Eriochloa villosa growth and seed production in Zea mays. E. villosa was transplanted at four densities (3, 9, 27, and 81 plants m−2) to simulate emergence at four Z. mays growth stages (VE, V2, V5, and V10). Compared to E. villosa plants that emerged with Z. mays plants, total above-ground E. villosa biomass at maturity of plants grown at 3 plants m−2 was reduced by 54, 97, and 99% when emergence was delayed until the V2, V5, and V10 stages of Z. mays, respectively, in 1997. In 1998, total aboveground E. villosa biomass at maturity was reduced by 70, 87, and 99% when emergence was delayed until the V2, V5, and V10 stages of Z. mays, respectively. E. villosa aboveground vegetative biomass per plant at maturity was linearly related to seed production per plant in each year. E. villosa seed production m−2 decreased nonlinearly as density decreased and time of emergence was delayed. Based on estimated model parameters, maximum seed production was 57,100 and 12,700 seeds m−2 in 1997 and 1998, respectively. Within time of emergence, E. villosa density did not affect seed mass per seed, however, seed mass of late-emerging cohorts was less than that of early-emerging cohorts. Time of weed emergence relative to the crop was a very important factor in determining biomass and seed production. Results suggest that late-emerging plants may not be very important to long-term management of E. villosa.

Four field experiments were conducted to measure the effects of seven Ipomoea hederacea (L.) Jacq. densities on Gossypium hirsutum L. lint yield, stripper-harvest efficiency, and fiber properties. The seven densities were 0, 2, 4, 6, 8, 10, and 12 weeds 10 m−1 of row. Data were used to develop prediction models to compare with those previously constructed that used fewer experiments and fewer weed densities in this range. Gossypium hirsutum lint yield in kilograms per hectare and as a percentage of the weed-free control best fit a linear regression model. Gossypium hirsutum lint yield reductions for each increase of one weed 10 m−1 of row ranged from 30.7 to 36.2 kg ha−1 at Chickasha and from 35.4 to 36.4 kg ha−1 at Perkins. Lint yield reductions for each weed 10 m−1 of row ranged from 3.8 to 6.9% at Chickasha and from 3.9 to 6.0% at Perkins. All plots could be mechanically stripper harvested, except for the 12-weed density at Chickasha in 1994 and the 10- and 12-weed densities at Perkins in 1996. Harvest efficiencies were not significantly different in any experiment. The only fiber properties to display significant differences were micronaire and strength at Chickasha in 1994. Prediction models calculated herein were highly similar to those previously constructed.

A spatially explicit metapopulation neighborhood model was extended to encompass the seed bank dynamics of the annual weed Alopecurus myosuroides Huds. (blackgrass), growing in U.K. winter wheat crops established after noninversion tillage. The effects of the initial spatial pattern of infestation, herbicide, and combine harvesting on seed bank densities and on expected yield losses of the infested crop were studied within a 634 m2 area. In the absence of herbicide, all seed bank populations were large and typical of values in the literature; those with patchy initial distributions spread quickly over the entire field. The effects of intraspecific competition ensured that even after 10 yr, the average seed bank density from three patterns with the same initial density consistently retained the same rank order: the initially uniform pattern consistently ranked largest; a patchily distributed moderate infestation ranked next; a more patchily distributed heavy infestation ranked least. Expected grain yield losses sometimes exceeded 40%. With the introduction of a herbicide, seed densities declined exponentially, but the rank order with regard to spatial pattern remained. Relatively dense patches occasionally persisted for longer than 10 yr. The economic threshold for treatment was achieved within 2 and 5 yr, depending on the infestation pattern, but the model predicted that it would take many more than 10 yr before infestations could be completely eliminated. The effects of harvesting by combine were modeled. In all cases, the speed of the spread of infestation along a row appeared to be largely due to the combine rather than to natural dispersal or other cultivation practices. The proportion of seeds removed by the combine was an important determinant of the number of years required to drive the population below the economic threshold and of the probability of long-term eradication of the weed metapopulation.

Root development and the associated acquisition of water and nutrients are an important part of weed competitiveness. Characterization of root morphological development, however, is inherently problematic because of the complexities of soil–plant interactions. In this study, we used hydroponically grown plants and digital imaging to examine root characteristics of Glycine max and the competing weeds Senna obtusifolia and Amaranthus palmeri. The purpose was to define inherent differences in root length and surface area that would contribute to growth responses during the establishment phase in the field. The methodology involved growing plants for 16 to 22 d, dissecting and staining root segments, mounting subsamples on slides, and imaging using a stereomicroscope and digital camera. Microscopy was required because of the small diameters of a significant proportion of the weed roots. With plants of similar root fresh weights (4.5 to 5.0 g), counting of individual roots revealed that 5. obtusifolia and A. palmeri had 2 and 3.7 times more roots than G. max (4,616 and 7,781 vs. 2,120, respectively). The imaging analyses indicated that roots of S. obtusifolia and A. palmeri had 2.9 and 5 times more length than G. max (10,042 and 17,192 cm vs. 3,418 cm, respectively). Furthermore, the analysis of length in different root diameter classes indicated that weed roots were noticeably finer then those of G. max. Approximately 84% of S. obtusifolia root length was contributed by roots in the 0.1- to 0.25-mm range, whereas 45% of the G. max roots were in the 0.1- to 0.25-mm range and 48% were in the 0.25- to 0.75-mm range. In contrast, 68% of A. palmeri length was contributed by roots smaller than 0.1 mm in diameter with 26% in the 0.1- to 0.25-mm range. Based on the expression of root characteristics observed here, root systems of these weed species would have finer roots with much greater length that would occupy a much larger volume of soil than those of G. max. Presumably, this would result in a competitive advantage in the acquisition of water and nutrients, especially when availability is limited.

The robustness of competitive attributes of cereals such as rapid and uniform seedling emergence, tillering, early biomass accumulation and canopy closure, and height advantage over weeds have not yet been tested under environmental conditions typical of no-till (NT) cropping systems. Our objective was to evaluate the effects or NT practices on growth and productivity of Avena sativa, Triticum aestivum, Hordeum vulgare, and associated weeds. The experiment was conducted on a Kamouraska clay at La Pocatière, QC, in 1994, 1995, and 1996. Avena sativa, T. aestivum, and H. vulgare were grown under tilled and NT practices. Cereal growth parameters were measured six (1994) or seven (1995) times between planting and the 11th week after planting but only once in 1996. Grain yields and yield components were determined at crop maturity. Avena sativa and H. vulgare populations were little affected by tillage, whereas T. aestivum populations were reduced by 16 to 20% in NT systems. Growth in height in NT systems was either similar or greater than in tilled systems in all three cereals. Cereal leaf area index (LAI) and biomass accumulation was also comparable between tillage systems, except for T. aestivum LAI in 1994, which was greater in tilled plots on two sampling dates. Response of annual dicots to tillage was inconsistent in all crops. Annual monocots dominated in some but not all NT systems. Perennial dicots dominated in NT systems, whereas perennial monocots were more abundant in tilled systems in all three cereals. Avena sativa and T. aestivum yields in NT plots were comparable or greater than in tilled plots, in spite of having either lower test weights (A. sativa) or lower 1,000-grain weights (T. aestivum). NT T. aestivum productivity was maintained in spite of reduced plant establishment. Hordeum vulgare yields were also similar across tillage systems, except in 1995, when yields in tilled plots were greater than in NT plots. The height advantage observed for NT H. vulgare did not result in improved yields. All three cereals, and particularly A. sativa, appeared well suited to NT systems, despite the pressure provided by different weed groups, compared to tilled systems. However, results suggest that NT production of cereals could benefit from improved attention to perennial dicot control and crop seedling establishment, particularly for T. aestivum.

Two field experiments were conducted from 1993–1994 through 1995–1996 growing seasons in Harney County, OR, to determine the relative competitive abilities of Elymus elymoides (squirreltail) a native perennial range grass, and Taeniatherum caput-medusae (medusahead), an exotic annual grass weed. The 1993–1994 growing season was very dry, 1994–1995 was dry, and 1995–1996 was wetter than average. One experiment tested seedlings vs. seedlings in each of three seasons. The second experiment tested seedlings plus second- and third-year established E. elymoides plants vs. 77 caput-medusae over 2 yr. Biomass, seed production, and soil moisture utilization 15, 30, 45, and 60 cm deep by the two species were measured. A randomized block design with factorial arrangement was used, with 25 2.25-m2 plots per block. Initial seeding densities of each species were 0, 10, 74, 550, and 4,074 seeds m−2 in all combinations of density. In the seedling vs. seedling experiment, intraspecific competition by 77 caput-medusae on itself was always significant (P ≤ 0.10) for both biomass and seed production. Interspecific competition by E. elymoides seedlings on T. caput-medusae biomass and seed production was not significant (P ≥ 0.10) in 2 of 3 yr and was always less than intraspecific competition by 77 caput-medusae. Only 0.4% of E. elymoides seed germinated, and no seed was produced in the very dry first year, but 84% of remaining seed was viable for the next year, which had better moisture conditions for germination and establishment. Interspecific competition affected (P ≤ 0.10) E. elymoides seedling biomass and seed production throughout the study. Intraspecific competition affected (P ≤ 0.10) seedling E. elymoides seed production in the dry year but not in the wetter than average year. In the mature E. elymoides experiment, intraspecific competition by T. caput-medusae on weight and seed production per plant was greater than interspecific competition from E. elymoides. Seedling/mature E. elymoides reduced T. caput-medusae weight per plant in the dry year but the effect was not biologically significant. Larger, mature Eelymoides plants produced 600 to 3,000 seeds per plant during the wet year; neither intra- nor interspecific competition was a factor. Taeniatherum caput-medusae was better able to access deeper soil moisture and was more aggressive at extracting soil moisture than were E. elymoides seedlings in the wet year. Cold soils and low oxygen due to wet soils may have restricted E. elymoides seedling root activity. Mature E. elymoides plants did not appear restricted by cold soils or low oxygen. Established second- and third-year E. elymoides plants were able to compete for soil moisture down to 45 cm. The generally greater interspecific competitive effects of T. caput-medusae on E. elymoides than vice versa suggested that it will be difficult to establish an E. elymoides stand in an existing T. caput-medusae community without first suppressing T. caput-medusae. Individual E. elymoides plants did establish and were productive with and without T. caput-medusae competition.

Crop rotation is thought to reduce weed density and maintain species diversity, thus preventing the domination of a few problem weeds. Because cropping sequence dictates other agricultural management practices, variations in weed populations between cropping systems may be the direct result of crop rotation, the result of different weed management practices associated with crop rotation, or both. Studies that fail to separate the effects of crop rotation from weed management may generate misleading results. A 10-yr crop rotation study was undertaken to study the dynamics of the standing weed vegetation in Zea mays L., Glycine max L., and Triticum aestivum L. The present paper compared total weed density and diversity between monocultures and rotations under three levels of weed management. Weed management accounted for 37.9% of the variation in total weed density, whereas crop rotation accounted for only 5.5%. Weed density varied between monocultures and rotations in plots where herbicides were applied. The effectiveness of rotations in reducing weed density was dependent upon the crop. Margalef's species richness index (DMG), a measure of diversity, varied among weed management strategies, with 38.4% of the variance attributed to this factor. In the 10th year, when all plots were sown with Z. mays, few cumulative effects of crop rotation were apparent, with two exceptions. In weedy and herbicide-treated plots, weed density was higher on plots cropped with Z. mays the previous year. Also, under these weed management treatments, including a cereal in the crop rotation reduced weed density. Crop rotation, when used in combination with herbicides, provides additional weed control and is therefore an effective tool in integrated weed management.

Experiments were conducted in an environmentally controlled growth chamber to determine the effects of temperature (10/5 or 21/7 C, day/night), soil moisture (7, 14, or 20%), timing (preemergence [PRE] or postemergence [POST]), and rate (9 or 18 g ai ha−1) of application on MON 37500 efficacy on Bromus secalinus L. and toxicity to Triticum aestivum L. MON 37500 reduced B. secalinus plant density an average of 40% but did not reduce T. aestivum density. PRE treatments reduced B. secalinus density 40% compared to 12% with POST applications when plants were grown at 10/5 C. Soil moisture level also influenced plant density, with 7.9 plants per pot when soil moisture was maintained at 7%, compared to 8.5 plants per pot with 14 or 20% soil moisture. MON 37500 reduced B. secalinus dry weight more at 18 g ha−1 than at the 9-g ha−1 rate when grown at 21/7 C, but no rate response occurred at 10/5 C. PRE applications of MON 37500 at 10/5 C decreased B. secalinus dry weight 22% more than PRE applications at 21/7 C or POST applications under either temperature regime. However, PRE applications of MON 37500 at 21/7 C decreased T. aestivum biomass an average of 16%, compared to 3% or less with other treatments. At 21/7 C, B. secalinus dry weight decreased 46% as soil moisture increased from 7 to 20%. Bromus secalinus was 10 to 12% less susceptible to MON 37500 when grown at 7% soil moisture at 10/5 C than when grown at the same temperature at 14 or 20% soil moisture. Triticum aestivum injury was greater at 20% soil moisture under each temperature regime.

Euphorbia esula is an exotic perennial weed that is estimated to infest 650,000 ha in North and South Dakota, Montana, and Wyoming. The estimated annual economic effect of E. esula infestations in the four-state area is about $130 million. We present the results of a survey of ranchers, local decision makers, and public land managers of grazing and nongrazing property from a five-county area in North and South Dakota, Montana, and Wyoming. A total of 565 ranchers, local decision makers, and public land managers were surveyed, which resulted in 267 completed questionnaires. The main objective of the survey was to evaluate respondent's perspectives of managerial, institutional, and social factors that may affect the rate and extent of implementation of various E. esula control strategies. The reasons for not using the various E. esula controls generally fell into environmental, financial, and educational categories. In many cases, little can be done to remove environmental constraints; however, financial constraints may be addressed through cost-share programs either offered locally or through state agencies. The other main reason for not using some controls was a lack of knowledge to work with various controls (e.g., grazing and biological controls). These obstacles could be addressed with workshops, demonstrations, or educational bulletins. These educational tools could be provided by universities or governmental agencies, as respondents indicated this was their choice for information dissemination. Disagreements among the survey groups were not substantial, and many share similar concerns in controlling the weed.

Field experiments were conducted from 1995 to 1997 to test approaches to managing atrazine-resistant Datura stramonium. Eight weed management programs in a Zea mays and Glycine max rotation were evaluated for their effects on the dynamics of atrazine-resistant and -susceptible D. stramonium populations. Overall D. stramonium density and relative abundance of resistant (R) biotypes were greatly reduced in no-till Secale cereale L. (rye) cover crop management programs without triazine compared to conventional-tillage systems with the application of triazine herbicides. The negative effects of no-till on D. stramonium were greater under a G. max–Z. mays—G. max (SCS) rotation than under a Z. mays–G. max—Z. mays (CSC) rotation. A cropping system involving more G. max phases under no-till reduced both the resistant and susceptible D. stramonium populations. Results from this study support the use of soil management, crop rotation, and negative cross-resistant herbicides to manage atrazine-resistant biotypes.