Palmer amaranth (Amaranthus palmeri S. Watson) is the most problematic weed of cotton (Gossypium hirsutum L.)-cropping systems in the U.S. Southeast. Heavy reliance on herbicides has selected for resistance to multiple herbicide mechanisms of action. Effective management of this weed may require the integration of cultural practices that limit germination, establishment, and growth. Cover crops have been promoted as a cultural practice that targets these processes. We conducted a 2-yr study in Georgia, USA, to measure the effects of two annual cover crops (cereal rye [Secale cereale L.] and crimson clover [Trifolium incarnatum L.]), a perennial living mulch (‘Durana®’ white clover [Trifolium repens L.]), and a bare ground control on A. palmeri population dynamics. The study was conducted in the absence of herbicides. Growth stages were integrated into a basic demographic model to evaluate differences in population trajectories. Cereal rye and living mulch treatments suppressed weed seedling recruitment (seedlings seed−1) 19.2 and 13 times and 12 and 25 times more than the bare ground control, respectively. Low recruitment was correlated positively with low light transmission (photosynthetic active radiation: above canopy photosynthetically active radiation [PAR]/below cover crop PAR) at the soil surface. Low recruitment rates were also negatively correlated with high survival rates. Greater survival rates and reduced adult plant densities resulted in greater biomass (g plant−1) and fecundity (seeds plant−1) in cereal rye and living mulch treatments in both years. The annual rate of population change (seeds seed−1) was equivalent across all treatments in the first year but was greater in the living mulch treatment in the second year. Our results highlight the potential of annual cover crops and living mulches for suppressing A. palmeri seedling recruitment and would be valuable tools as part of an integrated weed management strategy.

Kochia [Bassia scoparia (L.) A. J. Scott] is a problematic weed species across the Great Plains, as it is spreading fast and has developed herbicide-resistant biotypes. It is imperative to understand key life-history stages that promote population expansion of B. scoparia and control strategies that would provide effective control of these key stages, thereby reducing population growth. Diversifying weed control strategies has been widely recommended for the management of herbicide-resistant weeds. Therefore, the objectives of this study were to develop a simulation model to assess the population dynamics of B. scoparia and to evaluate the effectiveness of diverse weed control strategies on long-term growth rates of B. scoparia populations. The model assumed the existence of a glyphosate-resistant (GR) biotype in the B. scoparia population, but at a very low proportion in a crop rotation that included glyphosate-tolerant corn (Zea mays L.) and soybean [Glycine max (L.) Merr.]. The parameter estimates used in the model were obtained from various ecological and management studies on B. scoparia. Model simulations indicated that seedling recruitment and survival to seed production were more important than seedbank persistence for B. scoparia population growth rate. Results showed that a diversified management program, including glyphosate, could provide excellent control of B. scoparia populations and potentially eliminate already evolved GR B. scoparia biotypes within a given location. The most successful scenario was a diverse control strategy that included one or two preplant tillage operations followed by preplant or PRE application of herbicides with residual activities and POST application of glyphosate; this strategy reduced seedling recruitment, survival, and seed production during the growing season, with tremendous negative impacts on long-term population growth and resistance risk in B. scoparia.

Studies were conducted from 1987 to 1990 to measure the dynamics of sicklepod established at subeconomic threshold populations in a peanut-cotton-corn cropping system. The experimental site had no native populations of sicklepod prior to initiation of the study. Main plots were crops in the rotation sequence plus continuous summer fallow (no crop). Subplots were: sicklepod established in the initial year of the study, sicklepod established every year of the study, and no sicklepod. Sicklepod was established at subeconomic threshold densities to simulate weed survival and seed production in fields where economic thresholds were the basis for weed management decisions. Sicklepod growing alone in fallow plots produced more seed per plant, resulting in significantly more seedlings throughout the study than sicklepod growing with crops. Sicklepod growing in corn produced the fewest seed per plant. Seed produced from subeconomic threshold densities established only in the first year caused 7-, 21-, and 20-fold increases in sicklepod populations during the next three seasons compared to the nontreated control.

Some annual weeds, perennial herbs, and suffrutescent half shrubs are unique in that their populations are cyclic in nature and relatively short-lived. This variability presents a confounding factor that makes management decisions about control of these weeds difficult. This paper presents a procedure, using Markov processes, that gives additional information for managing cyclic weed populations. The Markov chain model is used to evaluate the interyear patterns of broom snakeweed survival when growing on New Mexico rangeland.

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.

Additional information on mechanical weed management systems is needed so producers can develop systems that meet their production and weed control goals without sacrificing profitability. Field research was conducted at Rosemount, MN, in 1989, 1990, and 1991 to determine the effect of preplant tillage and soybean planting date on weed populations and effectiveness of mechanical weed control operations. Delaying soybean planting from mid-May to early-June reduced weed densities and yield losses from weeds. Weed control with a herbicide treatment was not affected by planting date, but control with rotary hoeing and cultivation was often increased by delaying soybean planting. Early-planted soybean usually yielded better following herbicide treatment than mechanical weed control. However, when preplant tillage and planting were delayed, weed densities were reduced and mechanical weed control operations usually resulted in soybean yield similar to the herbicide treatment. While delaying soybean planting reduced weed densities and improved mechanical weed control, there is potential for reduced soybean yields with delayed planting.

Horseweed emergence and survival were evaluated in no-tillage soybean and corn at Rosemount, MN, and Ames, IA, from the fall of 1992 through the summer of 1995. Most of the horseweed at both locations emerged in the fall. Winter survival of fall-emerged seedlings ranged from 59 to 91%. Timing and extent of horseweed emergence in spring varied by year and location. Spring emergence ranged from 5 to 32% of total emergence, with greater spring emergence at Rosemount than at Ames. Emergence occurred as late as early June at Rosemount and late May at Ames. Results indicated that horseweed may emerge well into the growing season, and spring emergence should be considered in weed management systems for no-tillage crop production.

A bioeconomic weed management model was tested as a decision aid for weed control in corn at Rosemount, MN, from 1991 to 1994. The model makes recommendations for preemergence control tactics based on the weed seed content of the soil and postemergence decisions based on weed seedling densities. Weed control, corn yield, herbicide active ingredient applied, and economic return with model-generated treatments were compared to standard herbicide and mechanical control treatments. Effects of these treatments on weed populations and soybean yield the following year were also determined. In most cases, the model-generated treatments controlled weeds as well as the standard herbicide treatment. The quantity of herbicide active ingredient applied decreased 27% with the seed bank model and 68% with the seedling model relative to the standard herbicide treatment. However, the frequency of herbicide application was not reduced. In 1 yr, seed bank model treatments did not control weeds as well as the standard herbicide or seedling model treatments. Corn yields reflected differences in weed control. Net economic return to weed control was not increased by using model-generated control recommendations. Weed control treatments the previous year affected weed density in the following soybean crop. In 2 of 3 yr, these differences did not after weed control or soybean yield. Although tactics differed, the bioeconomic model generally resulted in weed control and corn yield similar to the standard herbicide. The model was responsive to differing weed populations, but did not greatly after economic returns under the weed species and densities in this research.

The story of agriculture is the story of weed interference. After millennia of weed control we still have weeds. This situation has led many growers to observe that “the weeds always win.” One of the most important reasons weeds are so successful is their biodiversity. Biodiversity is an inevitable consequence of the struggle an individual weed species undergoes in the presence of neighbors, and by occupying a physical space in an agroecosystem. Weeds have evolved in response to cropping system practices by adapting and occupying niches left available in agroecosystems. Forces created by our cropping practices over evolutionary time have led to the weed diversity we observe today. Diversity underlies weed management in several important ways. A plant experiences diversity among its neighbors in at least five different ways. Weeds have adapted to selection in agroecosystems in several ways: (1) genetic variants within a species; (2) somatic polymorphism of plant parts; (3) success in diverse habitat microsites; (4) temporal adaptations within the community; and (5) floristic diversity of a community at higher levels than the species. Herein, weed diversity is discussed in this broader context, in terms of population behaviors that emerge as a consequence of the activities of individual components at lower levels of organization. Diversity is also discussed in terms of its implications for weed management. The potential exists to develop management strategies based on differences in weed and crop diversity. These strategies might be developed by characterization of weedy genetic and phenotypic diversity; enhancement of crop, cropping system, and agroecosystem diversity; and characterization of the spatial distribution of weed populations.

A bioeconomic model was tested as a decision aid for weed control in soybean at Rosemount, MN, from 1991 to 1994. The model makes recommendations for preplant incorporated and preemergence control tactics based on the weed seed content of the soil and postemergence decisions based on weed seedling densities. Weed control, soybean yield, herbicide use, and economic return with model-generated treatments were compared to standard herbicide and mechanical control systems. Effects of these treatments on weed populations and corn yield the following year were also determined. In most cases, the model-generated treatments controlled weeds as well as a standard herbicide treatment. Averaged over the 3 yr, the quantity of herbicide active ingredient applied was decreased by 47% with the seedbank model and 93% with the seedling model compared with a standard soil-applied herbicide treatment. However, the frequency of herbicide application was not reduced. Soybean yields reflected differences in weed control and crop injury. Net economic return to weed control was increased 50% of the time using model-recommended treatments compared with a standard herbicide treatment. Weed control treatments the previous year affected weed density in the following corn crop but had little effect on weed control or corn yield. The bioeconomic model was responsive to differing weed populations, maintained weed control and soybean yield and often increased economic returns under the weed species and densities in this research.

Seeds of many plant species require a brief exposure to light to induce germination. Modifying the light environment during soil tillage may alter weed emergence in agricultural systems. The objective of this research was to evaluate the effect of secondary tillage conducted during daylight and at night on the emergence of 13 summer annual weed species under the same soil and environmental conditions. Annual grass and large-seeded broadleaf species showed little response to light environment during tillage. However, emergence was often less when no tillage was conducted than with any tillage by light environment combination. Emergence of small-seeded broadleaf species often was lower when tillage was conducted during darkness than with tillage during the light. Reductions varied by experiment and species and ranged from 70% for common lambsquarters to less than 30% for Pennsylvania smartweed. Large variations in response also occurred within species.

Broadleaf signalgrass [Brachiaria platyphylla (Griseb.) Nash # BRAPP has recently become the dominant annual grass in certain fields of the North Carolina Coastal Plains. Previously, fall panicum (Panicum dichotomiflorum Michx. # PANDI) and large crabgrass [Digitaria sanguinalis (L.) Scop. # DIGSA] were the dominant annual grasses in the region. One of the possible reasons for the observed population shift could be production of inhibitors or stimulators by one species that affects the population dynamics of the other species. Studies were initiated to evaluate the effects of broadleaf signalgrass, large crabgrass, and fall panicum residue, applied as a mulch or soil incorporated, on five indicator species: the three weeds themselves, corn (Zea mays L.), and soybean [Glycine max (L.) Merr.]. At expected residue levels, the degree of inhibition or stimulation from fall panicum and broadleaf signalgrass was determined to be significant for some indicator species. When such responses were seen, the amount of residue necessary to produce these results was usually within the concentrations normally observed in field situations. Based on these results, it appears that the observed population shift is partially mediated by the production of inhibitors or stimulators through plant residue. Other factors such as differential herbicide selectivity and crop rotation are being investigated.

Knowledge of Palmer amaranth demographics and biology is essential for the development and implementation of weed management strategies. A field experiment was conducted to investigate the effects of Palmer amaranth density on seedling mortality, flowering initiation, and flowering progress throughout the growing season and biomass production and fecundity in wide-row soybean. The experimental design was a randomized complete block design with three levels of Palmer amaranth density-clusters: high, medium, and low. Palmer amaranth mortality rate was greater at high Palmer amaranth population density-cluster, reaching a peak within 30 to 40 d after Palmer amaranth emergence (DAE) (0.55 and 0.80 for 2014 and 2015, respectively), in comparison with mortality rate at medium and lower density-clusters. Likewise, as Palmer amaranth density increased, biomass and seed production per unit area of the weed also increased. Biomass production at the high density-cluster in 2014 was 664.7 g m−2 compared with 542.9 and 422.1 g m−2 at medium and low density-clusters, respectively. Similarly, biomass production at high density-cluster in 2015 was 100.6 g m−2 compared with 37.3 and 34.2 at medium and low density-clusters, respectively. In addition, seeds produced at high density-cluster were 1.5 million and 245,400 seeds m−2 for 2014 and 2015, respectively. Seed production was reduced by 29% and 54% in 2014 and by 65% and 75% in 2015 at medium and low density-clusters, respectively. Earlier flowering initiation (i.e., between 30 to 40 DAE) occurred in higher Palmer amaranth density-clusters, indicating a trade-off between reproduction and survival at high densities and more stressed environments for species survival. Palmer amaranth male-to-female sex ratio was greater at high densities, 1.3 and 1.9, compared with lower densities of 0.6 to 0.7 and 0.7 to 0.8 in 2014 and 2015, respectively. The plasticity of Palmer amaranth population and population-structure regulation, vegetative growth, and flowering shifts at various levels of intraspecific competition (i.e., high vs. low population density-clusters) and the trade-off between these biological transitions merits further investigation.

Glyphosate-resistant (GR) weeds have been a prime challenge to the sustainability of GR cotton-based production systems of the midsouthern United States. Barnyardgrass is known to be a high-risk species for evolving herbicide resistance, and a simulation model was developed for understanding the likelihood of glyphosate resistance evolution in this species in cotton-based systems. Under a worst-case scenario of five glyphosate applications in monoculture GR cotton, the model predicts resistance evolution in about 9 yr of continuous glyphosate use, with about 47% risk by year 15. A unique insight from this model is that management in response to GR Palmer amaranth in this system (a reactive response) provided a proactive means to greatly reduce the risks of glyphosate resistance evolution in barnyardgrass. Subsequent model analysis revealed that the risk of resistance is high in fields characterized by high barnyardgrass seedbank levels, seedling emergence, and seed production per square meter, whereas the risk is low in fields with high levels of postdispersal seed loss and annual seedbank loss. The initial frequency of resistance alleles was a high determinant of resistance evolution (e.g., 47% risk at year 15 at an initial frequency of 5e−8 vs. 4% risk at 5e−10). Monte Carlo simulations were performed to understand the influence of various glyphosate use patterns and production practices in reducing the rate and risk of glyphosate resistance evolution in barnyardgrass. Early planting and interrow cultivation are useful tools. Crop rotation is effective, but the diversity of weed management options practiced in the rotational crop is more important. Diversifying weed management options is the key, yet application timing and the choice of management option is critical. Model analyses illustrate the relative effectiveness of a number of diversified glyphosate use strategies in preventing resistance evolution and preserving the long-term utility of glyphosate in midsouthern U.S. cotton-based production systems.

Postdispersal processes play an important role in the regulation of weed population dynamics. Experiments were conducted at two locations in Arkansas to understand postdispersal loss of five arable weed species important to this region—barnyardgrass, johnsongrass, pitted morningglory, Palmer amaranth, and red rice—between seed dispersal in autumn and the production of fresh seeds the subsequent autumn. Total seed loss through predation, decay, germination (fatal or successful), and loss in viability was estimated, and the influences of residue level and seed burial depth (near ground vs. 5 cm deep) were also examined. On average, the active (i.e., viable) seedbank proportion in spring (5 mo after dispersal) ranged from 8 to 11% (barnyardgrass), 10 to 11% (johnsongrass), 20 to 23% (pitted morningglory), 4 to 6% (Palmer amaranth), and 5 to 10% (red rice) across the two locations. At 1 yr after dispersal, 0.7 to 1.5% of barnyardgrass, 7 to 8% of johnsongrass, 5 to 9% of pitted morningglory, about 1.5% of Palmer amaranth, and 0.2 to 0.7% of red rice were part of the active seedbank for the two locations. There was no evidence to suggest that establishing a vegetation cover (such as a rye cover crop) after harvest of the main crop could accelerate seed predation. Burial depth did not influence seed decay, but most (45 [pitted morningglory] to 99% [Palmer amaranth]) of the seeds retrieved from the predator feeding stations were found buried in the soil substrate, and thus, not available for most predator species. This suggests that practices that allow weed seeds to lie on the soil surface (such as no-till planting in autumn) are highly valuable in encouraging seed predation. The high levels of seed loss observed in this study indicate that seedbank management should be a vital component of integrated weed management strategies.

Field experiments were conducted in 1999 and 2000 to determine if (1) seed predation of redroot pigweed plants occurred in agricultural fields, (2) corn-cropping patterns could be manipulated to influence the quantity of weed seed predation, and (3) alterations in corn canopies affected the microenvironment, possibly influencing predator populations. Corn was planted in standard (75 cm) or narrow (37.5 cm) rows with corn population densities ranging from low to very high (2.5 to 10 plants m−2). The extent of seed predation occurring on terminal weed inflorescences in the treatments was evaluated. Predation levels of redroot pigweed were highly variable spatially and temporally. Coleophora lineapulvella Chambers (Lepidoptera: Coleophoridae) was the dominant predator of redroot pigweed seed. Seed predation was higher in 2000 than in 1999 (P < 0.05). On average, C. lineapulvella larvae attacked 11% of the inflorescences in 2000 and 3% of inflorescences in 1999. The proportion of damaged seeds per attacked inflorescence was as high as 93% in 2000 but only 42% in 1999. Row spacing and corn density did not affect levels of weed seed predation (P > 0.05). But canopies of closely spaced corn increased shading to redroot pigweed plants growing below the canopy, consequently decreasing total weed biomass and seed production.

To improve understanding of relationships between crop diversity, weed management practices, and weed population dynamics, we used data from a field experiment and matrix models to examine how contrasting crop rotations affect velvetleaf. We compared a 2-yr rotation system (corn–soybean) managed with conventional rates of herbicides with a 4-yr rotation (corn–soybean–triticale + alfalfa–alfalfa) that received 82% less herbicide. In November 2002, a pulse of velvetleaf seeds (500 seeds m−2) was added to 7- by 7-m areas within replicate plots of each crop phase–rotation system combination. Velvetleaf seed, seedling, and reproductive adult population densities, seed production, and seed losses to predators were measured during the next year. Velvetleaf seed production was greater in the 4-yr rotation than in the 2-yr rotation (460 vs. 16 seeds m−2). Averaged over 12 sampling periods from late May to mid-November 2003, loss of velvetleaf seeds to predators also was greater in the 4-yr rotation than in the 2-yr rotation (32 vs. 17% per 2 d). Modeling analyses indicated that velvetleaf density in the 4-yr rotation should decline if cumulative losses of seeds produced in the soybean phase exceeded 40%. Achieving such a level of predation appears possible, given the observed rates of velvetleaf seed predation. In addition, no tillage occurs in the 4-yr rotation for 26 mo after soybean harvest, thus favoring seed exposure on the soil surface to predators. Models that included estimates of seed predation indicated that to prevent increases in velvetleaf density, weed control efficacy in soybean must be ≥ 93% in the 2-yr rotation, but could drop to 86% in the 4-yr rotation. These results support the hypothesis that diverse rotations that exploit multiple stress and mortality factors, including weed seed predation, can contribute to effective weed suppression with less reliance on herbicides.

The effects of microtopographic position on soil microenvironment and weed populations in ridge-tilled soybean were evaluated on three farms in Iowa in 1989 and 1990. In both years, over all weed species (primarily giant foxtail, green foxtail, yellow foxtail, redroot pigweed, and Pennsylvania smartweed), seedling emergence was highest in late May and early June, with few seedlings emerging after mid-June. Weed populations were highest in May and early June, after which rotary hoeing and cultivation reduced weed numbers in all plots. Microtopographic position (row, shoulder, and furrow) had a large effect on soil microenvironment and weed populations. Furrows were the wettest position through most of the growing season. Rows were the warmest position early in the season and the coolest position late in the season. Cumulative weed emergence early in the season was closely related to growing degree days, which accumulated faster in the row position than the furrow position. Following rotary hoeing and cultivation, the row position had significantly more total weeds than the shoulder and furrow positions on all farms in August of both years.

Seeds from natural populations of fierce thornapple and velvetleaf collected in a corn-growing area in central Spain were incubated at a range of constant temperatures and water potentials to model the progress of germination on the basis of the accumulation of hydrothermal time. Previous to the germination tests, the seeds were treated in two different ways: (1) dark storage under dry conditions (nonchilled seeds), and (2) burying in the original soil at 10-cm depth during 2 mo in winter (naturally chilled seeds). The results indicated different mechanisms inhibiting germination in both weed species. Whereas fierce thornapple displayed some type of embryo dormancy, the lack of germination in velvetleaf appeared to be entirely due to the seed coat. On the other hand, significant differences between nonchilled and naturally chilled seeds in fierce thornapple were observed, mainly due to the decrease in the mean base water potential of the 50th percentile in the latter, which indicated a loss of dormancy by exposure of the seeds to natural conditions. Hydrothermal time appears to be a good description of the germination patterns in both weed species, but in the case of fierce thornapple, only for naturally chilled seeds. Thus, the development of the hydrothermal model in fierce thornapple raises some questions for consideration concerning the influence of the type of seeds (conditions of storage, pretreatments of the seeds before germination tests) on its germination capacity.

Herbicide-resistant barnyardgrass has become widespread in the rice production systems of the midsouthern United States, leaving few effective herbicide options for controlling this weed. The acetolactate synthase (ALS)- and acetyl-CoA carboxylase (ACCase)-inhibiting herbicides remain largely effective in Clearfield® rice production, but strategies need to be developed to protect the long-term utility of these options. A two-trait model was developed to understand simultaneous evolution of resistance in barnyardgrass to the ALS- and ACCase-inhibiting herbicides in Clearfield rice. The model was used to predict resistance under a number of common weed management scenarios across 1,000 hypothetical rice fields in the Mississippi Delta region and answer some key management questions. Under an ALS inhibitor–only program consisting of three annual applications of imidazolinone herbicides (imazethapyr or imazamox) in continuous Clearfield rice, resistance was predicted within 4 yr with 80% risk by year 30. Weed management programs that consisted of ALS- and ACCase-inhibiting herbicides such as fenoxaprop and cyhalofop greatly reduced the risk of ALS-inhibiting herbicide resistance (12% risk by year 30), but there was a considerable risk for ACCase resistance (evolving by year 14 with 13% risk by year 30) and multiple resistance (evolving by year 16 with 11% risk by year 30) to both of these mechanisms of action. A unique insight was that failure to stop using a herbicide soon after resistance evolution can accelerate resistance to the subsequent herbicide option. Further, a strong emphasis on minimizing seedbank size is vital for any successful weed management strategy. Results also demonstrated that diversifying management options is not just adequate, but diversity combined with timely herbicide applications aimed at achieving high efficacy levels possible is imperative.