The phenology and biomass dynamics of Typha subulata were studied for 2 yr under two natural conditions, a marsh and an irrigation channel. Populations demonstrated a spring to summer growth pattern, with peak aboveground biomass in spring. The phenological phases identified for both populations were identical, but plants growing in the marsh averaged 38 and 28% less aboveground biomass and height, respectively, than plants growing in the channel. Noticeable differences were found in the timing of both phenological phases and biomass cycle between growth seasons, suggesting that the susceptibility of cattail to weed control during the seasons could vary between years.

Yield loss of hard red spring wheat due to competition from common milkweed was measured in grower fields in Minnesota. Wheat yield loss was measured using the area of influence and additive competitive methods. Simple linear regression of wheat yield and percentage wheat yield loss against distance from a common milkweed shoot gave r2 values of 0.013 and 0.015, respectively, indicating limited value of the area of influence model for common milkweed in spring wheat. In an additive competition model, wheat yield was reduced 47% at the highest density of 12 common milkweed shoots m−2. Coefficients of determination for percentage yield loss regressed against common milkweed shoot density were 0.548, 0.547, and 0.529 for simple linear, nonlinear rectangular hyperbolic, and linear square root function models, respectively. Regression of percentage yield loss against common milkweed biomass resulted in r2 values of 0.566, 0.645, and 0.658 for simple linear, nonlinear rectangular hyperbolic, and linear square root function models, respectively. Restrictions of common milkweed density due to factors other than competition limited yield loss response to the simple linear phase of both the nonlinear rectangular hyperbolic and the linear square root function models previously described.

Controlled environment experiments were completed to determine the effect of temperature on giant foxtail and fall panicum germination, emergence, and growth. Giant foxtail seed germination decreased when exposed to a constant 30 C compared to 20 C. Germination also decreased in the alternating 20/30 C temperature regime when the hours of exposure to 30 C as compared to 20 C increased. Fall panicum required alternating temperatures of 14 C (9 h)/28 C (15 h) to germinate. Giant foxtail seed germination exceeded 60% 4 d after exposure to an alternating temperature of 7 C (9.4 h)/20 C (14.6 h). Conversely, fall panicum seed did not germinate at the 7 C (9.4 h)/20 C (14.6 h) temperature regime and required a minimum of 7 d exposure to alternating temperatures of 13 C (8.7 h)/26 C (15.3 h) for 88% of the seed to germinate. The greatest emergence of giant foxtail and fall panicum was from 1 cm and 1 to 2.5 cm, respectively. Less than 5% of the giant foxtail and fall panicum seed emerged from 7.5 cm. The growth of giant foxtail seedlings was five times greater than that of fall panicum at each temperature regime tested. Incorporation of this information into bioeconomic models could result in accurate predictions of weed germination for effective weed management strategies.

Laboratory experiments were conducted to determine the effect of light, oxygen, and submergence on bur beggarticks seed germination. Fresh and dry stored seeds exhibited dormancy, requiring thiourea and light for germination. Red light was more effective than far-red or blue light in breaking dormancy. Both an oxygen-saturated and anoxic atmosphere completely inhibited germination. Hypoxic conditions (5 and 10% oxygen) led to an increase in germination compared to normoxia (21% oxygen). Primary dormancy was eliminated by 1 yr of soil burial. Seeds exhumed after soil burial had greater germination percentage than fresh or lab-stored seeds after a brief far-red irradiation, but did not germinate in the dark. The induction of secondary dormancy increased with increasing durations of submergence. Finally, sowing at different soil depths (0 to 8 cm) showed that germination and emergence occurred mainly with shallow burial (0.5 to 1.0 cm).

Experiments were conducted at two sites for 2 yr in the Pacific Northwest dryland cropping region to determine if seeding rate of small-red lentil could enhance weed control with herbicides and increase lentil seed yield. At Pendleton, OR, and LaCrosse, WA, lentil was planted at 22 or 44 kg ha−1 in one direction in all plots. In one-half of the plots, lentil was cross-seeded at right angles with an additional 22 kg ha−1 to provide seeding rates of 22, 44, 22 + 22, and 44 + 22 kg ha−1. Seeding rate main plots were split into three herbicide treatments and an untreated control. Total weed density was reduced by increasing seeding rate at Pendleton both years when averaged over all herbicide treatments. Seeding rate reduced total weed density to a greater extent when herbicides did not adequately control weeds or when herbicides were not applied at Pendleton in 1992. Increased seeding rate also reduced total weed dry weight at Pendleton in 1992 and 1993 and at LaCrosse in 1993. The suppressive effect of increased seeding rate on weed dry weight was more evident when herbicides were not used or when herbicides gave only partial control. Herbicides generally reduced weed density, but the effectiveness of individual treatments was related to the weed species present and environmental conditions present in each experiment. Lentil aboveground dry weight production increased with seeding rate at both locations; however, only in 1 yr did lentil seed yield increase with seeding rate. The primary benefit from increased seeding rate in this study was to reduce weed density and dry weight.

Field research was conducted in 1994 and 1995 to monitor the dissipation of flumetsulam in five Mississippi soils for a period of 16 wk. These soils allowed evaluation of flumetsulam persistence across three soil series with a relatively constant soil pH of 6.0, as well as across three soils of varied pH from 6.0 to 7.6 within a constant soil series. Bioavailability was determined with a rice root length bioassay. Predicted half-lives ranged from 20 to 26 d in 1994 and from 23 to 46 d in 1995. Persistence decreased with increasing cumulative rainfall and decreasing organic matter content (within a range of 1.2 to 3.5%). Soil pH had no measurable effect on flumetsulam persistence.

Norflurazon adsorption and dissipation under field and laboratory conditions, and distribution within the soil profile were determined in three soils representative of cotton-growing regions of the southeastern U.S. Norflurazon adsorption was greater in soil from 0 to 8 cm in a Lexington silt loam (Tennessee) and a Beulah silt loam (Mississippi) than in a Dothan loamy sand (Georgia). Adsorption was directly related to organic matter. Norflurazon degradation under controlled conditions in soil from 0 to 8 cm from each state was not different among locations, with half-lives ranging from 63 to 167 d. Degradation at 30 C in soil from the 30- to 45- and 60- to 90-cm depths was not different among locations, and was slower at the 60- to 90-cm depth than in surface soil. Norflurazon dissipation was more rapid under field conditions than under laboratory conditions, with half-lives ranging from 7 to 79 d in the 0- to 8-cm soil horizon. Dry field conditions slowed norflurazon dissipation. Norflurazon was not detected below 15 cm in the profile in any soil, and concentrations in the 8- to 15-cm soil zone were < 36 ppbw 112 d after treatment.

Resistance to fluazifop-P and quizalofop-P, (aryloxyphenoxypropionates) and sethoxydim (cyclohexanedione) was identified in 2 populations of johnsongrass in both field and greenhouse studies. The cropping history (1983–1991) of the sites indicated 1 or more annual applications of a graminicide (primarily fluazifop-P) since the early 1980s. Under field conditions, control of resistant seedling and rhizome johnsongrass (R91F) with fluazifop-P, quizalofop-P, fenoxaprop-ethyl, and sethoxydim was less than 35%. Clethodim provided up to 80% control of R91F. Under greenhouse conditions, ratios (R/S) of the I50 values (amount of herbicide required to inhibit plant growth by 50%) of resistant (2 sites: R91F and R91S) to susceptible (S91H) seedling (20–30-cm height) plants were > 388 (fluazifop-P), > 15 (quizalofop-P), and from 2.3 (R91S) to 3.4 (R91F) (both sethoxydim). For rhizome (30–45 cm height) plants, the R/S ratios were > 388 (fluazifop-P), > 16 (quizalofop-P), and 2.8 (R91S) to 8.5 (R91F) (both sethoxydim). Labeled rates (in kg ai ha−1) of fluazifop-P (0.10 and 0.21), quizalofop-P (0.039 and 0.08), and sethoxydim (0.21 and 0.21) were applied on seedling and rhizome plants, respectively, and resulted in little or no control of resistant johnsongrass. Greenhouse studies indicated registered rates of clethodim (0.10 and 0.14 kg ai ha−1 for seedling and rhizome plants, respectively) effectively controlled the resistant populations, but tolerance was measured for both seedling and rhizome plants at sublethal doses (down to 0.007 and 0.009 kg ai ha−1, respectively), with I50 ratios ranging from 1.5 (R91S) to 2.1 (R91F) for seedling plants and 4.5 (R91S) to 4.8 (R91F) for rhizome plants. Control of resistant seedling and rhizome johnsongrass under field conditions was adequate with glyphosate at 0.84, glufosinate at 0.84, and sulfosate at 0.84 kg ai ha−1, indicating no cross-resistance.

Laboratory studies were conducted to evaluate flumetsulam adsorption and mobility in seven Mississippi soils of different organic matter content, pH, and texture. Adsorption isotherms were determined for all soils using a 1:1 (soil: water) technique. In six of seven soils, Freundlich n constants were close to unity, suggesting a partitioning-like adsorption mechanism for flumetsulam. Mobility was examined using packed soil columns. 14C-flumetsulam recoveries in leachate ranged from 1 to 70% and were influenced by both organic matter content and soil pH. However, the effects of organic matter content and soil pH were not independent. Consequently, clear relationships between flumetsulam mobility and either organic matter content or soil pH were not established across all soils. However, among soils of similar pH (7.5 ± 0.3), mobility decreased linearly (R2 = 0.75) as organic matter content increased from 0.7 to 3.6%. Across soils with similar organic matter content (3.9 ± 0.3%), mobility increased linearly (R2 = 0.98) as soil pH increased from 5.3 to 7.2. Net adsorption constants (Kd) provided a more accurate assessment of flumetsulam mobility across all soils than Koc.

Field experiments were conducted to determine the growth, development, and seed yield response of scentless chamomile when planted at various dates throughout the growing season. Scentless chamomile emerged within 6 to 15 d of planting and required 75 to 131 growing degree days (GDD). Flowering occurred within 56 to 82 d of planting and required 584 to 845 GDD. Plants that emerged in mid-July and onward did not flower in that season but survived as winter annuals. Spring-planted scentless chamomile matured within 97 to 127 d and required 906 to 1,176 GDD. Plants that emerged in May and June attained more leaf area and biomass than plants that emerged in mid-summer. Plants that emerged early in the growing season produced three-to fourfold more seed than plants that emerged at later dates. Scentless chamomile seed production ranged from 71,000 to 256,000 plants−1 depending on planting date and existing environmental conditions. Results are discussed in terms of its potential to become an increasingly troublesome weed in crops of the Canadian prairies and possible management strategies for its control.

This research examined the effect of duration of the warm cycle and number of fluctuating temperature/light cycles on goosegrass seed germination. Less than 10% goosegrass seed germination occurred at constant temperatures of 20, 25, and 35 C, while 99% germinated at daily fluctuating temperature regimes of 20 C for 16 h and 35 C for 8 h with light. Increasing the number of fluctuating temperature cycles increased goosegrass seed germination. Fluctuating temperature cycles with 22 h at 20 C and 2 h at 35 C with light strongly stimulated germination, but not as much as the cycle with 8 h at 35 C. Goosegrass seed germination ceased once the fluctuating temperature regime was discontinued, and constant 20 C with 8 h light daily was reimposed. Germination occurred only on resumption of fluctuating temperature cycles. Seed incubated for several weeks in a moist environment at constant 20 C had 81 to 91% germination with two cycles of 20 C for 16 h and 35 C for 8 h, compared to 14% for fresh seeds.

Many growers perceive that reduced herbicide inputs and greater reliance on mechanical methods will result in increased weed management problems over time. Previous research has shown short-term benefits of integrated weed management strategies, but information concerning the long-term implications of these strategies is lacking. Therefore, research was conducted from 1992 through 1995 to determine the influence of full-rate preemergence broadcast, half-rate preemergence broadcast plus cultivation, and full-rate preemergence band plus cultivation treatments on weed population and seedbank dynamics in no-tillage (NT), chisel plow (CP), and moldboard plow (MP) systems in both continuous corn (CC) and corn—soybean (CS) rotation. Seventeen weed species were identified in the plant population and seedbank across all treatments, but common lambsquarters, giant foxtail, and redroot pigweed dominanted. Vertical distribution of weed seeds in soil was influenced by tillage, with 74, 59, and 43% of the total viable seed of these species less than 10 cm deep in NT, CP, and MP, respectively. In contrast, seed dormancy in the spring was not influenced greatly by tillage and averaged 79, 10, and 42% for common lambsquarters, giant foxtail, and redroot pigweed, respectively, across tillage methods. Crop rotation was not a major factor influencing weed population and seedbank dynamics. From 1992 to 1995, reduction of shoot biomass and seed production of the predominant weed species were similar among weed management treatments in most tillage and crop rotation systems. The influence of weed management treatments on the seedbank was also similar, with up to 50, 95, and 92% less seed of common lambsquarters, giant foxtail, and redroot pigweed, respectively, in the seedbank in 1995 than in 1992. Corn and soybean grain yields were similar among weed management treatments and were greater than the nontreated check in each tillage and crop rotation treatment. These results indicate that reduced herbicide inputs plus interrow cultivation were as effective as full-rate herbicide inputs for the management of several annual weed species in both conventional- and conservation-tillage systems over 4 yr.

The response of peanut to low rates of MSMA under weed-free conditions was studied in 1992 and 1994 near Tifton, GA. MSMA was applied 40, 70, 100, or 130 days after emergence (DAE). At each application timing, MSMA was applied at 0, 90, 168, and 336 g ai ha−1. MSMA application timing did not affect peanut yield. Increasing rates of MSMA caused minor reduction in peanut yield. Analysis using atomic absorption spectroscopy showed elemental arsenic accumulation of ≤ 0.7 ppm in peanut kernels from MSMA applied 70 and 100 DAE. This approximates the time interval of peanut pod and kernel formation. MSMA applied 40 and 130 DAE resulted in little detectable arsenic in kernels. Low rates of MSMA, originating as drift from aerial applications to cotton or illegal and unsound applications to peanut for Florida beggarweed control, are not likely to reduce peanut yields. The greatest hazard from peanut exposure to MSMA is accumulation of arsenic in kernels, which would adversely affect peanut marketability and consumer demand.

Giant foxtail, woolly cupgrass, and wild-proso millet infest millions of hectares of land devoted to corn production in the midwestern U.S. Control of these species and effects on corn grain yield were evaluated at various timings using POST applications of nicosulfuron vs. applications of various PRE herbicides at 17 locations across the midwestern U.S. in 1992 and 1993. Nicosulfuron applied to 5 to 10 cm giant foxtail and woolly cupgrass provided greater control than that observed with selected PRE herbicides. Giant foxtail control with nicosulfuron averaged 88%, and control of woolly cupgrass averaged 77% across all sites. Nicosulfuron, applied to 5 to 10 cm wild-proso millet, provided a level of control similar to that of selected PRE herbicides. Corn grain yield was greater when giant foxtail was controlled POST with nicosulfuron vs. PRE control with selected soil-applied herbicides. Corn grain yields were similar when nicosulfuron was applied POST to 5 to 10 cm woolly cupgrass or wild-proso millet vs. PRE control of these grass weeds. Across a broad range of geographical locations, nicosulfuron, applied POST to 5 to 10 cm tall grass, provided greater or similar levels of weed control vs. the selected PRE herbicides, with no deleterious effect on grain yield.

Recent changes in technology, governmental regulation and scrutiny, and public opinion have motivated the agricultural community to examine current management practices from the perspective of how they fit into a sustainable agricultural framework. One aspect which can be incorporated into many existing farming systems is plant residue management (e.g., reduced tillage, cover crops). Many residue management systems are designed to enhance accumulation of plant residue at the soil surface. The plant residue covering the soil surface provides many benefits, including protection from soil erosion, soil moisture conservation by acting as a barrier against evaporation, improved soil tilth, and inhibition of weed emergence. This review summarizes recent literature (ca. last 25 yr) concerning the effects of plant residue management on the soil environment and how those changes impact herbicide interactions.

Activity, adsorption, mobility, and field persistence of one emulsifiable concentrate (EC) and three microencapsulated (ME) formulations of alachlor were studied with petri dish bioassay, based on root response of oats grown in sand or soil. Both bioassays indicated that activity of all formulations was increased with increasing herbicide concentration. EC-alachlor in sand showed the highest activity, while ME-alachlorL and ME-alachlorA, the lowest; the activity of ME-alachlorC was intermediate. In silty clay loam soil, EC-alachlor had the highest activity, while ME-alachlorL had the lowest; and ME-alachlorA showed intermediate activity and was similar to that of ME-alachlorC More alachlor was adsorbed on the soil or remained encapsulated (not biologically available) after ME-alachlorL and ME-alachorA application compared to EC-alachlor. An intermediate amount of alachlor was adsorbed or remained inside the capsules when the ME-alachlorC formulation was applied. Greater amounts of alachlor were leached through a silty clay loam after EC-alachlor application compared to the three ME-alachlor formulations. Biologically available alachlor was not detected below 15 or 10 cm after application of EC- and ME-alachlor formulations, respectively. All alachlor formulations applied alone or in mixture with atrazine showed similar field persistence. Biologically available alachlor was not detected in the 0- to 10-cm soil depth 30 d after their application. All alachlor formulations applied alone gave excellent control of redroot pigweed and black nightshade, but only partial control of jimsonweed. None of the herbicide treatments showed any detrimental effect on corn, and all of them increased corn yield to the level of weed-free control.