The critical period for weed control (CPWC) identifies the phase of the crop growth cycle when weed interference results in unacceptable yield losses; however, the effect of planting date on CPWC is not well understood. Field studies were conducted in 2004 and 2005 at Urbana, IL, to determine CPWC in sweet corn for early May (EARLY) and late-June (LATE) planting dates. A quantitative series of treatments of both increasing duration of interference and length of weed-free period were imposed within each planting-date main plot. The beginning and end of the CPWC, based on 5% loss of marketable ear mass, was determined by fitting logistic and Gompertz equations to the relative yield data representing increasing duration of weed interference and weed-free periods, respectively. Weed interference stressed the crop more quickly and to a greater extent in EARLY, relative to LATE. At a 5% yield-loss level, duration of weed interference for 160 and 662 growing-degree days (GDD) from crop emergence marked the beginning of the CPWC for EARLY and LATE, respectively. When maintained weed-free for 320 and 134 GDD, weeds emerging later caused yield losses of less than 5% for EARLY and LATE, respectively. Weed densities exceeded 85 plants m−2 for the duration of the experiments and predominant species included barnyardgrass, common lambsquarters, common purslane, redroot pigweed, and velvetleaf. Weed canopy height and total aboveground weed biomass were 300% and 500% higher, respectively, for EARLY compared with LATE. Interactions between planting date and CPWC indicate the need to consider planting date in the optimization of integrated weed management systems for sweet corn. In this study, weed management in mid-June–planted sweet corn could have been less intensive than early May–planted corn, reducing herbicide use and risk of herbicide carryover to sensitive rotation crops.

Giant foxtail density in corn was greater under no-till and chisel plow tillage systems than conventional or till plant. Giant foxtail density in no-till was 1400 shoots/m2 56 days after corn planting compared to 170 under conventional tillage. Velvedeaf density was greater under conventional tillage than all other tillage systems. Velvetleaf density was 120 plants/m2 56 days after corn planting under conventional tillage compared to 20 in no-till. Control of giant foxtail was often less under no-till or chisel plow conditions than conventional or till plant with the same herbicide treatment. Giant foxtail control with metolachlor treatments was affected less by tillage than similar treatments containing alachlor. Velvedeaf control was less with conventional tillage than other tillage systems when less than 1.7 kg/ha of atrazine was applied. Corn injury was not influenced by tillage systems. Corn yield was not affected by tillage systems under weed-free conditions. Several herbicide treatments resulted in corn yield similar to the weed-free under conventional tillage, but no herbicide treatment produced corn yield similar to the weed-free control under no-till conditions.

Purple nutsedge (Cyperus rotundus L.) develops as a series of shoots connected by bulbs, rhizomes, and tubers. The leaves contain parallel, collateral vascular bundles with the majority of the photosynthetic cells concentrated in the sheathing girdle around each bundle. The upper leaf surface consists of a single layer of large epidermal cells covered by a thick cuticle. Stomates occur only in the lower surface. The vascular bundles vary from collateral to amphivasal as they pass from the leaves through the bulb into the rhizomes and tubers. Newly developing rhizomes and tubers appear white and fleshy with a parenchymatous epidermis and cortex. Mature rhizomes appear brown and wiry with a deteriorated outer cortex and a lignified inner cortext and endodermis. Tubers and bulbs form similarly at the rhizome apices with each accumulating starch. The interconnecting vascular system appears to remain intact throughout the growing season.

Managing fertilizer in cropping systems may be an important component of integrated weed management programs. A field study was conducted to determine the effect of various application methods of the 15N-enriched nitrogen (N) fertilizer on N uptake in green foxtail, wild mustard, and spring wheat and on crop–weed competition. N application methods consisted of ammonium nitrate in solution applied broadcast on the soil surface, applied in pools on the soil surface at 20-cm intervals between every second wheat row, and point injected 10 cm deep at intervals similar to those of the surface pools. An unfertilized control treatment was also included. N uptake by green foxtail throughout the growing season was often greater from surface broadcast than from surface pools or point-injected N and was sometimes greater from surface pools than from point-injected N. In contrast, N uptake by wild mustard was rarely affected by the fertilizer placement method. In the presence of weeds, the ranking of N uptake by wheat usually was point injected > surface pools > surface broadcast. Weed biomass was often greater with surface broadcast than with either surface pools or point-injected N. In the absence of weeds, wheat yields were similar across the three N application methods. However, in the presence of green foxtail, wheat yields were greater with point-injected N than with surface broadcast N in two of the 3 yr and with surface pools of N in one of the 3 yr. In the presence of wild mustard, wheat yields were greater with surface pools and point-injected N compared with the unfertilized control in two of the 3 yr, whereas yields with broadcast N were never greater than the unfertilized control. Study findings suggest that point-injected N results in suppressed weed growth, not by reduced N uptake by weeds but instead by greater N uptake by wheat that increases its competitiveness with weeds. Information gained in this study will be utilized to develop a more integrated program for weed management in spring wheat.

An experiment was conducted at five locations in Nebraska to determine the extent of demise of weed seed in soil when seed production was eliminated from 1975 through 1979 in corn (Zea mays L.). Weed yields, weed seed production, and corn yields were determined under four weed management levels in 1980. Annual broadleaf weed seed were more prevalent than grass seed in cultivated soil throughout the study. The population of viable weed seed in soil declined 95% during the 5-yr period that weed seed production was eliminated. Weed seed buildup recovered to within 90% of the 1975 level during 1980 at Concord and Clay Center but remained low at Lincoln, North Platte, and Scottsbluff. Thus, seed longevity in soil was sometimes sufficient to withstand modern weed control methods and still reinfest a field after 5 yr of eliminating weed seed production. Corn yields were maintained 1 yr with minimum weed management effort following 5 yr of no weed seed production.

Growth, competitiveness, and fecundity differences between resistant (R) and susceptible (S) biotypes affect management strategies. This greenhouse research examines growth, competitiveness, and fecundity of chlorsulfuron-resistant (R) and -susceptible (S) kochia collected from North Dakota and Kansas. In noncompetitive studies, sulfonylurea R and S kochia biotypes from Kansas and North Dakota had similar growth rates and seed production. All biotypes produced about 12000 seeds per plant. Competitiveness of R and S biotypes from Kansas was similar. Interbiotype competition reduced R and S plant biomass more than intrabiotype competition. The relative competitiveness of the R and S biotypes was 0.75 and 0.85, respectively. The resistance trait did not reduce growth, seed production, and competitiveness of R compared to S kochia in greenhouse studies.

Upon germination, one or more rhizomes grew from the apical end of each yellow nutsedge (Cyperus esculentus L.) tuber. Each rhizome developed a basal bulb upon exposure to light. No significant differences in germination percentages existed between four lots of tubers which differed about fivefold in weight. The weight of plant material produced correlated significantly with the fresh weight of the tuber from which it emanated. When tubers germinated three successive times, over 60% of the tuber dry weight, carbohydrate, oil, starch, and protein were consumed during the first germination; but less than 10% of these constituents were uitlized during each of the next two germinations. Plants weighed significantly more after 43 and 91 days of growth with tubers attached throughout the period than when tubers were detached after emergence.

Weed seedbanks have been studied intensively at local scales, but to date, there have been no regional-scale studies of weed seedbank persistence. Empirical and modeling studies indicate that reducing weed seedbank persistence can play an important role in integrated weed management. Annual seedbank persistence of 13 summer annual weed species was studied from 2001 through 2003 at eight locations in the north central United States and one location in the northwestern United States. Effects of seed depth placement, tillage, and abiotic environmental factors on seedbank persistence were examined through regression and multivariate ordinations. All species examined showed a negative relationship between hydrothermal time and seedbank persistence. Seedbank persistence was very similar between the two years of the study for common lambsquarters, giant foxtail, and velvetleaf when data were pooled over location, depth, and tillage. Seedbank persistence of common lambsquarters, giant foxtail, and velvetleaf from October 2001 through 2002 and October 2002 through 2003 was, respectively, 52.3% and 60.0%, 21.3% and 21.8%, and 57.5% and 57.2%. These results demonstrate that robust estimates of seedbank persistence are possible when many observations are averaged over numerous locations. Future studies are needed to develop methods of reducing seedbank persistence, especially for weed species with particularly long-lived seeds.

Information on weed responses to soil fertility levels is needed to aid development of fertilizer management strategies as components of integrated weed management programs. A controlled environment study was conducted to determine shoot and root growth response of 22 agricultural weeds to fertilizer phosphorus (P) applied at 5, 10, 20, 40, or 60 mg kg−1 soil. An unfertilized control was included. Wheat and canola were included as control species. Shoot and root growth of all weeds increased with added P, but the magnitude of the response varied greatly among species. Many weeds exhibited similar or greater responses in shoot and root biomass to increasing amounts of soil P compared with wheat or canola. With increasing amounts of P, 17 weed species increased shoot biomass more than wheat, and 19 weed species increased shoot biomass more than canola. However, only 10 weed species exhibited greater increases in root biomass than canola, and no weed species increased root biomass more than wheat with added P. Canola was among species taking up the greatest percentage of available P at all P doses. However, percentage P uptake by wheat relative to other species varied with P dose. Only four weed species extracted more P than wheat at low P levels, but 17 weed species extracted more P at high soil P levels. These findings have significant implications as to how soil fertility may influence crop–weed competition.

Seed dormancy does not play a role in the germination ecology of curly dock (Rumex crispus L. ♯ RUMCR). This study confirms reports that freshly matured seeds are nondormant, and it shows that buried seeds exposed to natural seasonal temperature changes remain nondormant. From October 1981 through June 1983, seeds exhumed at monthly intervals germinated 80 to 100% at all thermo-periods. These results do not support suggestions that seeds of curly dock buried in soil enter dormancy. However, the results do explain why seeds of this species in the Beal and Duvel buried-seed experiments germinated when exhumed at various times during the growing season.

Seed of 41 accessions of Crotalaria representing 35 species were analyzed for the concentration of total pyrrolizidine alkaloids (PAs) and for oral toxicity to 1-week-old chicks. All accessions were assayed for the presence of monocrotaline and spectabiline by thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC). High concentrations of PA were found in the seed of Crotalaria spectabilis Roth # CVTSP (3.85%), C. retusa L. (2.69%), C. alata Leveille (1.60%), C. quinquefolia L. (1.19%), and C. argyrolobioides Bak. (1.01%). The seed of Crotalaria argyrolobioides produced toxic signs but no deaths when fed at 10 mg/g of body weight, whereas the seed of the other four species were 100% lethal when fed to chicks in one dose at 2 to 6 mg/g of body weight. Seed of other species contained less than 0.6% PA and were nontoxic to chicks fed one dose at 10 mg/g of body weight. Monocrotaline was identified in 17 accessions and spectabiline in 7. No species contained both monocrotaline and spectabiline.

Mobility and sorption of radiolabeled sulfometuron {2-[[[[(4,6-dimethyl-2-pyrimidinyl)amino] carbonyl] amino] sulfonyl] benzoic acid} and imazapyr {(±)-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-pyridinecarboxylic acid} were evaluated in five soils representative of the major soil types in Alabama. Thin-layer soil chromatography indicated that sulfometuron was slightly more mobile (average Rf = 0.86) than imazapyr (average Rf = 0.79). Sulfometuron was more subject to sorption (average kd = 0.29) than imazapyr (average kd = 0.09) in batch equilibrium experiments. Least sorption of both herbicides occurred on clay soils, and maximum sorption occurred on a sandy clay. Similar results were obtained with a soil solution recovery technique. Sorption of both herbicides was exhanced by temporary reduction in soil water content and by lowering soil pH. The pH response of imazapyr was greater than that of sulfometuron.

Resistance to acetolactate synthase (ALS)-inhibitor herbicides due to continuous and repeated selection is widespread in many troublesome weed species, including Palmer amaranth, throughout the United States. The objective of this research was to investigate the physiological and molecular basis of resistance to ALS inhibitors in a chlorsulfuron-resistant Palmer amaranth population (KSR). Our results indicate that the KSR population exhibits a high level of resistance to chlorsulfuron compared with two known susceptible populations, MSS and KSS from Mississippi and Kansas, respectively. MSS is highly susceptible to chlorsulfuron, whereas KSS is moderately sensitive. Dose–response analysis revealed that KSR was more than 275-fold more resistant compared with KSS. Nucleotide sequence analysis of the ALS gene from the plants that survived chlorsulfuron treatment revealed the possibility of evolution of both target site–based and non–target site based resistance to ALS inhibitors in the KSR population. The most common mutation (Pro-197-Ser) in the ALS gene associated with resistance to the sulfonylureas in many weed species was found only in 30% of the KSR population. A preliminary malathion study showed that the remaining 70% of resistant plants might have cytochrome P450–mediated non–target site resistance. This is the first report elucidating the mechanism of resistance to ALS inhibitors in Palmer amaranth from Kansas. Presence of both target site– and non–target site based mechanisms of resistance limits the herbicide options to manage Palmer amaranth in cropping systems.

The depth distribution of weed seed, their location in relation to soil aggregates, and the size distribution of soil aggregates were determined in fields that had been under continual conventional or reduced tillage for more than 12 years. Soil samples were taken at three depths in the spring and fall of 1982, and soil aggregates were classified into seven size classes by dry, rotary sieving. Subsamples were deflocculated and washed through sieves to recover weed seed. In the spring, 85% of all seed in the reduced-tillage and 28% of those in the conventional-tillage soil were in the 0- to 5-cm-depth layer. Conventional tillage incorporated weed seed uniformly into various soil aggregate classes, whereas with reduced tillage more seed accumulated in the unaggregated fraction of the soil. In the fall, weed seed distribution in relation to soil depth and among soil aggregate classes was similar for both tillage regimes.

A fundamental component of modeling crop interference is the effect of understory photosynthetic photon flux density (PPFD) on weed architecture and growth. The effect of decreased PPFD on spaced redroot pigweed dry matter accumulation, distribution, and plant architecture was quantified by increasing corn density and delaying weed planting date. As canopy-transmitted PPFD declined, total dry matter accumulation decreased and relative dry matter distribution was greater to main-stem components than to branch components. Increased rectangularity in understory weed architecture was associated with a concomitant decrease in branch number. The proportion of leaf area and dry matter in the upper segment of the redroot pigweed increased as PPFD declined with increased corn density. Results suggest that changes in plant architecture, as influenced by canopy-transmitted PPFD, may be as important as those of total dry matter and leaf area when describing and predicting the effects of crop-weed interference.

A reduced-till (RT) experiment determined the effect of rye seeding density and method of kill on rye biomass persistence, weed suppression, and tomato yield. ‘Wheeler’ rye was seeded at 56, 110, and 170 kg ha−1 in the fall of 1988 and 1989 in Champaign, IL; Lexington, KY; and Lafayette, IN. One wk before transplanting tomatoes, rye was desiccated either by applying glyphosate at 1.1 kg ha−1 and mowing (RT-glyphosate) or by mowing alone (RT-mowed). Both methods left rye residues on the soil surface as a mulch. Seeding density did not affect rye biomass, weed control, or tomato yield. Rye biomass differed depending on location and year, ranging from 320 to 1150 gm−2. Rye was completely killed by glyphosate, but regrew following mowing. RT-glyphosate suppressed weeds for 4 to 8 wk, which was similar to the suppression from conventional tillage (fall plowing with spring disking and harrowing) with trifluralin and metribuzin (CT-herbicide). Weed communities varied, but redroot pigweed and giant foxtail were present at all three locations. All treatments required supplemental weed management for commercially acceptable control. Except for IL in 1990, processing tomato yields (kg ha−1) in RT-glyphosate with hand-weeding were equal or better than yields in the CT treatments.

The management of glyphosate-resistant Palmer amaranth has been a challenge in southern United States cropping systems. Registration of dicamba-resistant crops will provide an alternative management option to control herbicide-resistant Palmer amaranth populations, particularly those having resistance to herbicide Groups 2, 3, 5, 9, 14, and 27. However, repeated use of sublethal doses of dicamba may lead to rapid evolution of herbicide resistance, especially in Palmer amaranth—a species with a strong tendency to evolve resistance. Therefore, selection experiments with dicamba were conducted on Palmer amaranth using sublethal doses. In the greenhouse, a known susceptible Palmer amaranth population was subjected to sublethal dicamba doses for three generations (P1–P3). Susceptibility of the individuals to dicamba was evaluated, and its susceptibility to 2,4-D was characterized. Based on the greenhouse study, following three generations of dicamba selection, the dose required to cause 50% mortality increased from 111 g ae ha−1 for parental individuals (P0) to 309 g ae ha−1 for the P3. Furthermore, reduced susceptibility of the P3 to 2,4-D was also evident. This research presents the first evidence that recurrent use of sublethal dicamba doses can lead to reduced susceptibility of Palmer amaranth to dicamba as well as 2,4-D. Here, we show that selection from sublethal dicamba doses has an important role in rapid evolution of Palmer amaranth with reduced susceptibility to auxin-type herbicides.

Vineyard weed communities were examined under four dormant-season cover-crop systems representative of those used in the north-coastal grape-growing region of California: no-till annuals (ANoT) (rose clover, soft brome, zorro fescue), no-till perennials (PNoT) (blue wildrye, California brome, meadow barley, red fescue, yarrow), tilled annual (AT) (triticale), and a no-cover-crop tilled control (NoCT). Treatments were carried out for 3 yr in the interrows of a wine grape vineyard. Glyphosate was used to control weeds directly beneath the vines, in the intrarows. Treatments significantly impacted weed biomass, community structure, and species diversity in the interrows. Orthogonal contrasts showed that tillage, and not the presence of a cover crop, impacted interrow weed biomass. Distance-based redundancy analyses (db-RDA) revealed significant effects of the cover-crop systems and of tillage on weed community structure in the interrows. For scarlet pimpernel and spiny sowthistle, the combination of ANOVA and orthogonal contrasts confirmed their association with the tilled treatments, as revealed by db-RDA. This same approach identified the association between California burclover and the no-till treatments. Our findings of no significant effects of the cover-crop systems on weed biomass, community structure, or diversity in the intrarows demonstrate that the impacts the cover-crop management systems had on the interrows did not carry over to adjacent intrarows. In addition, the fact that the cover crops did not affect vine yield, growth, or nutrition relative to the no-cover-crop control suggests that cover crops are likely to minimize soil erosion from winter rains, which is the primary purpose of vineyard cover cropping in northern California, without adversely affecting vine health or weed control.

Growth chamber and laboratory experiments were conducted to evaluate foliar absorption and translocation of the isopropylamine (Ipa) or trimethylsulfonium (Tms) salts of glyphosate in Abutilon theophrasti and Setaria faberi. Glyphosate was applied at 250 g ai ha−1. Absorption of 14C-glyphosate in both formulations increased similarly in both species over 72 h. Seventy-two hours after treatment (HAT), A. theophrasti absorbed 26% of applied 14C of both formulations, whereas S. faberi absorbed 43% of both formulations. The addition of 1% ammonium sulfate (AMS) to both formulations increased the amount of herbicide absorbed to 35% in A. theophrasti and 60% in S. faberi 72 HAT. Translocation of absorbed 14C-glyphosate in both formulations (with or without AMS) was greater for S. faberi than A. theophrasti 72 HAT. For A. theophrasti, 17% of 14C absorbed in Ipa and Tms formulations were translocated out of the treated leaf, whereas additions of AMS increased the amount translocated to 25 and 23% for Ipa and Tms, respectively. The largest portion of translocated 14C moved to tissues above the treated leaf. For S. faberi, 37% of absorbed 14C in both Ipa and Tms formulations were translocated out of the treated leaf 72 HAT, whereas 42% translocated when 1% AMS was added to both formulations. In S. faberi, the largest portion of 14C-glyphosate translocated to tissues below the treated leaf. In summary, no differences were found between Ipa and Tms formulations in both weed species.

Absorption and translocation of glyphosate [N-(phosphonomethyl)glycine] with and without adjuvants were examined in field bindweed (Convolvulus arvensis L. # CONAR) to develop an understanding of the influence of selected adjuvants and environment before application on glyphosate activity. Light intensity and humidity during plant development resulted in differences in 14C-glyphosate absorption. When applied in water or with an oxysorbic (20 POE) (polyoxyethylene sorbitan monolaurate) adjuvant, an average of 9% of the glyphosate was absorbed in plants grown in high light intensity, low humidity (HLLH) before treatment, compared to an average of 21% in plants grown in low light, high humidity (LLHH) before treatment, respectively. Amounts of epicuticular wax on HLLH field bindweed were almost three times as great as on LLHH leaves and may explain absorption differences. No differences in glyphosate absorption were observed between glyphosate applied with oxysorbic or no adjuvant even though the oxysorbic adjuvant effectively reduces surface tension. Absorption was increased two- to threefold with a polyethoxylated tallow amine adjuvant (MON 0818) compared to no adjuvant. Unlike absorption without adjuvant or with oxysorbic adjuvant, there were few absorption differences in plants grown in different environments before application. Absorption continued for 24 to 36 h after application regardless of adjuvant. Reductions in MON 0818 concentration and subsequent necrosis resulted in increased movement of radioactivity away from the site of application.