Understanding the environmental variables influencing the phenological development of weeds is essential for simulation model development. Temperature and photoperiod are important variables governing the phenological development of weeds. Growth cabinet studies were conducted to characterize the phenological development of Setaria viridis in response to variations in temperature and photoperiod and to determine the duration of the juvenile phase and the effect of temperature and photoperiod on reproductive development. Setaria viridis was adapted to a temperature range from 6.5 to 47 C. Phenological development of S. viridis was described accurately in terms of thermal days (cumulative day degrees above a base temperature) and biological days (Bd: chronological days at the optimum temperature and photoperiod). Four developmental phases of S. viridis were described: (1) a juvenile (photoperiod insensitive) phase of 2.6 Bd; (2) a photoperiod-sensitive inductive phase of 2.2 Bd; (3) a photoperiod-sensitive postinductive phase of 6.0 Bd; and (4) a photoperiod-insensitive inductive phase of 10.9 Bd. Photoperiod sensitivity of S. viridis did not differ with stage of development when expressed as a rate. Interpretation of constant sensitivity to photoperiod will simplify simulation of weed phenology in mechanistic models.

The current emphasis on reducing pesticide use has led to increased interest in alternative weed control methods. The purpose of this study was to evaluate the potential of hilling, in combination with subsequent interseeding of cover crops, to control weeds in potato and to determine the impact of these actions on potato yield and insects. Each cover crop, either hairy or lana vetch, oats, barley, red clover, or a combination of oats and hairy vetch, was interseeded, following hilling, 3, 4, or 5 wk after planting (WAP). Hilling and interseeding treatments were compared to a no-cover treatment and a chemical standard, metolachlor (1.7 kg ai ha−-1) plus linuron (1.7 kg ai ha−-1). Cultivation associated with the interseeding operation and cover crops reduced weed density 20 to 27% by 3 wk after interseeding. To prevent excessive competition, cereals interseeded 3 or 4 WAP and vetches interseeded 3 WAP were controlled at a height of 25 to 35 cm using fluazifop (0.22 kg ai ha−-1) plus metribuzin (0.28 kg ai ha−-1). Because of slow growth, it was necessary to control red clover (interseeded 3 WAP), only in 1996, at a height of 15 cm. Control of cereals resulted in a dead mulch that provided 0 to 95% weed control, whereas legumes regrew after herbicide application and provided 45 to 70% weed control. However, an adequate minimum of 70% weed control was achieved only with the chemical standard, cereals (1995 and 1996), and legumes (1996) interseeded 3 WAP when controlled with herbicides. In a dry season (1995), potato yields were highest with the chemical standard, whereas in a season with adequate precipitation (1996), potato yields equivalent to those of the chemical standard were obtained with early interseeding (3 WAP) of cereals. The observed changes in the densities of potato leafhopper and of its natural enemies were unlikely to have any effect on potato yield because of low overall insect populations. Thus, the results suggest that in good growing conditions, early (3 WAP) interseeding of cover crops supplemented with postemergence herbicides may both suppress weeds with 70% less herbicide (ai ha−-1) and provide yields comparable to those associated with conventional potato production.

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.

Interactions between initial weed seedling density and postemergence herbicide and mechanical weed control were studied in two field experiments conducted between 1994 and 1996. Increasing seedbank densities of velvetleaf (0 to 500 seed m–2) in soybean or common sunflower (250 to 2,500 seed 1.3 m–2) in corn or soybean were established at Lincoln and Mead, NE, respectively. Emerged seedlings were treated with increasing intensities of weed control from none to bentazon alone or with interrow cultivation. A positive linear relationship between initial seedling density and density of surviving seedlings was consistently observed. As initial seedling density increased, more survivors were present after treatment. As intensity of weed control increased, the number of seedling survivors decreased. Resulting reproductive fitness decreased with increasing management intensity but remained positive when regressed against surviving seedling densities. Weed management outcomes were dependent on initial seedling density, such that the absolute number of survivors increased, while proportion of survivors appeared constant within the density ranges studied. These research findings emphasize the need to account for weed infestation level when assessing efficacy of weed management systems and provide evidence that patchy weed distributions may persist in part because of the need for considerably higher management intensities in high density patch centers.

Yellow toadflax is an introduced ornamental perennial that has become a serious weed in Wisconsin peppermint production. Glyphosate is the only labeled herbicide that controls yellow toadflax in peppermint. Other control strategies, either mechanical or cultural, have little effect on the weed in solid stands of peppermint. Previous research has suggested that the root mining moth Eteobalea serratella Tr. (Lepidoptera: Cosmopterigidae) may provide effective biological control of yellow toadflax. Therefore, experiments were conducted to determine the host specificity of E. serratella and to quantify E. serratella root mining of yellow toadflax. In larval and adult no-choice tests, E. serratella attacked only yellow toadflax. Other plant species collected from E. serratella's natural range were not attacked, neither those closely related to yellow toadflax in the family Scrophulariaceae, nor those closely related to peppermint in the family Labiatae. Root mining by E. serratella reduced root biomass an average of 20% compared to nontreated plants. Peppermint competition alone or in conjunction with root mining by E. serratella reduced root biomass 52 and 65%, respectively, compared to nontreated plants under noncompetitive conditions. E. serratella is host specific for yellow toadflax and demonstrates potential as a biological control agent by contributing to the reduced competitive ability of yellow toadflax in peppermint.

Field experiments were conducted in Louisiana over 2 yr to evaluate the influence of full-season interference from johnsongrass, common cocklebur, or hemp sesbania at densities of 2.5, 0.5, and 2.0 plants m–1 of row, respectively, and simulated insect defoliation of soybean on weed and soybean growth. Defoliation at R2 (full bloom) and R5 (beginning seed development) soybean growth stages was accomplished by removal of zero, one, or two leaflets per soybean trifoliate, which approximated 0, 33, and 66% defoliation, respectively. Height and dry weight of all weeds were not affected by soybean defoliation level or defoliation stage. Soybean height 3 wk after defoliation at R5 was not influenced by weed interference, soybean defoliation level, or defoliation stage in either year. Averaged across soybean defoliation levels and stages in 1994, johnsongrass, common cocklebur, and hemp sesbania reduced soybean yields 30, 15, and 14%, respectively. In 1995, johnsongrass reduced soybean yield 35%. As soybean defoliation level increased, a linear decrease in soybean yield was observed. Averaged across weeds and soybean defoliation stages, 33 and 66% defoliation reduced soybean yield 6 and 20% in 1994 and 12 and 33% in 1995, respectively. Defoliation at R5 resulted in 10% lower yield than defoliation at R2 in one of two years. Yield reduction due to combinations of weeds and soybean defoliation was additive.

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.

The adoption of conservation tillage systems has been challenged by concerns potential weed species shifts. A 9-yr study from 1988 to 1996 was conducted Delhi, Ontario, on a loamy sand soil to evaluate the effect of tillage systems (conventional [CT] and no-till [NT]), cover crop Secale cereale, and nitrogen (N) rate (0, 50, 100, 125, 150, and 200 kg N ha−1) on monocrop Zea mays L. (corn) yield and changes in the composition of the weed flora. CT consisted of spring moldboard plowing followed by cultivation with a tooth cultivator. Weed counts were taken in the last 3 yr of the study (1994, 1995, and 1996) prior to postemergence herbicide application and then again 2 to 3 wk after herbicide treatment. Composition or the weed flora was analyzed by canonical discriminant analysis (CDA). The relationship between weed density and tillage system was not consistent. Weed species composition differed between CT and NT systems. Chenopodium album and Amaranthus retroflexus were associated with CT and Digitaria sanguinalis with NT. N rate and cover crop did not affect weed density or species composition. Proper management of weeds with herbicides appeared to minimize any long-term effect on the weed flora resulting from varying N rates. Zea mays yields did not differ between CT and NT systems but were greater in both systems with a cover crop at the higher N rates. Disturbance caused by tillage was more important than N rate and cover crop as a mechanism influencing composition of the weed flora.

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.

Weed management can be a significant challenge in cropping systems, partly because the effects of tillage systems on weed seedbank and seedling population dynamics are not well understood. Field research was conducted from 1994 to 1996 in established tillage plots consisting of moldboard plow (MP), chisel plow (CP), and no-tillage (NT). The objectives were to determine the effects of long-term tillage systems on the timing and duration of Setaria spp. emergence and percentage cumulative emergence from the soil seedbank and to investigate the effect of tillage on Setaria spp. density and seed production following glyphosate application at Setaria spp. heights of 5, 10, and 15 cm. NT contained a greater number of Setaria spp. seed in the 0- to 1-, 1- to 3-, and 3- to 6-cm depths than MP or CP systems. There was little difference between the three tillage systems at depths greater than 6 cm. Setaria spp. emergence was greater in NT than in MP or CP in 1994 and 1996 and greater than in MP in 1995. There was a substantial increase in Setaria spp. emergence in NT between 3 and 4 weeks after planting (WAP) in 1994 and between 5 and 6 WAP in 1995 and 1996. Significant emergence did not occur past 5 to 6 WAP in 1994 and 1995 but continued over a longer period of time in 1996. Setaria spp. plants consistently reached targeted herbicide application heights 4 to 9 d earlier in NT than in CP and MP. In 1994, final Setaria spp. density was greater in NT compared to CP and MP at the 5- and 10-cm herbicide application timings. When glyphosate was applied to 15-cm-tall Setaria, very few weeds were present following application across all tillage systems. In 1995, NT resulted in greater Setaria spp. density than MP or CP across all application timings. There was no difference in final Setaria spp. density between MP and CP across all glyphosate timings in 1994 and 1995. Seed production was negligible in MP and CP, regardless of glyphosate timing. In NT, however, significant seed production occurred, especially with early application. Results indicate that the effectiveness of nonresidual herbicides for Setaria faberi Herrm. control is influenced by tillage system and the timing of application.

Critical period experiments were conducted in 1997 near Milton, WI, and in 1998 near Edgerton, WI, in which Eriochloa villosa (Thunb.) Kunth emergence was characterized and related to Zea mays L. yield loss and weed seed production. At Milton, 2.7% of the spring seedbank emerged, and at Edgerton, 14.5% of the spring seedbank emerged. Peak time of E. villosa emergence occurred early in the season at both locations, accounting for 84% of the total season emergence at Milton by 37 d after planting and 90% of the total season emergence at Edgerton by 27 d after planting. A secondary peak in emergence occurred at each site, accounting for 14% of the total season emergence at Milton between 38 and 52 d after planting and 8% of the total season emergence at Edgerton between 28 and 43 d after planting. Following the secondary peak in emergence at each site, approximately 2% of the total season emergence occurred. Zea mays grain yield was reduced when E. villosa interfered with Z. mays past the V11 stage at Milton and the V3 stage at Edgerton. Zea mays yield reductions at Edgerton were greater, occurred under shorter periods of weed interference, and were associated with greater E. villosa biomass than yield reductions at Milton. Eriochloa villosa that emerged after the V2 Z. mays stage at Milton and the V3 stage at Edgerton did not cause crop yield loss; however, these plants produced seed at 143 and 63% of the initial spring seedbanks, respectively.

A better understanding of the influence of various crop and weed management practices on spatiotemporal dynamics of weeds could improve the design of integrated weed management systems. We examined the influence of 18- and 76-cm soybean row spacings on emergence pattern and spatial aggregation of giant foxtail, common lambsquarters, and velvetleaf seedling cohorts. In addition, we characterized the soil seedbank and determined the quantitative and spatial relationship between the seedbank and seedling populations. Viable seeds of about 10 weed species and twice as many species of seedlings were identified in the weed community. Giant foxtail and common lambsquarters were the predominant species in the seedling and seedbank population, respectively, each accounting for 60 to 70% of the total weed species density. Emergence of giant foxtail, common lambsquarters, and velvetleaf depleted 12 to 33%, < 2% and 12 to 49% of the seedbank in the upper 10 cm of the soil profile. Peak time and periodicity of weed emergence was not influenced by soybean row spacing, and peak time of emergence of giant foxtail, common lambsquarters, and velvetleaf occurred 3 to 4, 3 to 6, and 3 to 9 weeks after soybean planting (WAP), respectively. Magnitude of giant foxtail emergence 5, 6, and 9 WAP was 98, 96, and 76% greater in 76- than in 18-cm row soybeans only when the population of 76-cm row soybeans was 57% lower than the 18-cm soybeans in 1997. Giant foxtail and common lambsquarters seeds in the seedbank were aggregated in 1996 and 1997 according to the Taylor power law (TPL) and the negative binomial distribution (NBD). The TPL and the NBD were similar in describing the spatial aggregation of giant foxtail and common lambsquarters but not some velvetleaf seedling cohorts. The spatial aggregation of seedlings varied among cohorts for different weed species and was likely due to species-specific biological characteristics that influence seed dispersal, germination, and seedling emergence. Within a 1.5-ha area, aggregation declined with decreasing density. Within a 24-m2 area, the level of aggregation of all weed species decreased as seedling densities increased. These results indicated that soybean row spacing influenced neither weed emergence pattern nor weed spatial aggregation; thus, several management decisions can be similar in 18- and 76-cm row soybeans.

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.

Two separate open grassland areas within Sugarloaf Ridge State Park, Sonoma County, CA, were burned for three consecutive years (1993–1995 [Site A] and 1995–1997 [Site B]) for control of yellow starthistle. Burns were conducted in late June to early July following seed dispersal and senescence of desirable grasses and forbs but prior to viable seed production in yellow starthistle. After the first year burn, there was no significant reduction in yellow starthistle cover the following spring and summer. Despite the lack of control, the first year burn reduced the yellow starthistle soil seedbank by 74% and the number of seedlings the following spring by 83%. However, total plant diversity and species richness increased dramatically in the burned areas. This was due primarily to an increase in the number of native broadleaf species. A second burn the next summer (1995–1997 site) reduced seedbank, seedling density, and summer vegetative cover the following year by 94, 92, and 85%, respectively, while maintaining significantly higher native plant cover and richness. A third consecutive summer burn decreased yellow starthistle seedbank and seedling density by 96, 98, and 85%, respectively, in the 1995–1997 burn site. Three consecutive years of burning in the 1993–1995 site reduced yellow starthistle seedbank and seedling density by over 99% and summer vegetative cover by 91%. These results indicate that prescribed burning can be an effective tool for the management of yellow starthistle and can have a long-term benefit on native broadleaf diversity and richness.

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.

A plant response study was conducted in the greenhouse to evaluate interactions between imazamox and acifluorfen, fomesafen, or lactofen. Antagonistic interactions between imazamox and acifluorfen or fomesafen affected control of I. hederacea and A. theophrasti 21 d after treatment. Combinations of imazamox plus lactofen also antagonized control of I. hederacea. I. hederacea absorbed less 14C-imazamox when applied with nonlabeled acifluorfen, but absorption by A. theophrasti was not affected by the combination. Addition of nonlabeled lactofen did not affect absorption of imazamox in A. theophrasti or I. hederacea. Addition of acifluorfen or lactofen reduced translocation of 14C-imazamox to foliage above and below the treated leaf and the root in both A. theophrasti and I. hederacea. Antagonism of A. theophrasti control was due to reduced translocation of imazamox. Antagonism of I. hederacea control was due to reduced translocation of imazamox when applied with lactofen or reduced absorption and translocation when applied with acifluorfen.

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.

Emergence characteristics, including initial time of emergence, magnitude of emergence, and mean time of emergence, of Amaranthus rudis Sauer, Setaria faberi Herrm., Eriochloa villosa (Thunb.) Kunth, and Abutilon theophrasti Medik. were studied in central Iowa during the first 3 yr following burial of seed collected and buried in the fall of 1994 or 1995. Although the initial emergence date varied among years, the emergence sequence among species was consistent. Eriochloa villosa and A. theophrasti were the first species to emerge, with initial emergence occurring between April 28 and May 10. Initial emergence dates for these species were the same, except for 1995 when A. theophrasti emerged 4 d prior to E. villosa. Amaranthus rudis was the last species to emerge, with initial emergence ranging from 5 to 25 d after A. theophrasti. First-year emergence in 1995 was 8% for A. theophrasti, 7% for A. rudis, 41% for E. villosa, and 33% for S. faberi, based on the number of buried seed. Proportional emergence during the first year following burial in 1996 was similar to 1995 for A. theophrasti and S. faberi, but greater emergence was observed in 1996 for E. villosa and A. rudis. During the 3 yr of both studies, cumulative emergence of the two grass species (43 to 71%) was higher than for the broadleaf species (13 to 35%). A high percentage of the total annual emergence of E. villosa occurred within the first 2 wk of initial emergence, whereas a high percentage of A. rudis emergence occurred late in its emergence period. Emergence characteristics of the four species were consistent among years and seed sources.

Two field experiments were carried out from 1993 to 1995 to evaluate the critical period of weed control and to develop suitable weed management practices for jungle rice, horse purslane, and cockscomb in soybean. Horse purslane was more competitive during early growth stages (up to 45 days after sowing [DAS]) and cockscomb was more competitive during later growth stages, whereas jungle rice was competitive throughout the growing season. The critical period of weed control was found to be 30 to 45 DAS. Weed-free maintenance up to 45 DAS resulted in a 74% increase in grain yield of soybean over the unweeded control. Keeping soybean weed free for 45 d or allowing weeds to remain in the crop for less than 30 d resulted in no significant yield loss. Sequential application of a reduced rate of soil-applied trifluralin 1.0 kg ha–1 (0.67 ×) with postemergence fluazifop 0.75 kg ha–1 (0.75 ×) or a reduced rate of soil-applied trifluralin or pendimethalin at 1.0 kg ha–1 (0.67 ×) followed by hand hoeing 35 DAS provided better control of a broad spectrum of weeds than a single application of a postemergence herbicide applied at reduced or recommended rates. Integration of reduced rates of soil-applied herbicides with post-emergence herbicides or hand hoeing 35 DAS produced soybean yields similar to the hand-weeded treatment. Compared to the weed-free or integrated weed control, a single application of soil-applied or postemergence herbicide did not control a broad spectrum of weeds and reduced soybean yield.

A significant limitation in using sorption coefficients (Kd) to predict solute transport through natural soils is the spatial variability of soil properties over large field areas. Spatial variability in Kd for imazethapyr was determined on representative samples from a 31.4-ha field, covering a pH range from 4.9 to 7.6 and an organic carbon (OC) range from 1.45 to 5.80 g kg−-1. Kd varied from 0.18 to 3.78 across the field, with an average value of 1.56. The analysis of Kd variability showed two distinct patterns in spatial distribution: areas in which pH > 6.25 and Kd < 1.5, where Kd variation is based primarily on pH, and areas in which pH < 6.25 and Kd > 1.5, where other soil properties, i.e., OC content, have a significant influence on Kd variation. Based on soil pH distribution, an easily measured property, the field was divided into two potential management areas. This separation allowed identification of portions of the field where herbicide sorption would be minimal, with a relatively higher potential for leaching (i.e., areas with Kd < 1.5), and provided a rationale for site-specific imazethapyr application.