Weed Management Systems Research Articles

Cover Crop and Postemergence Herbicide Integration for Palmer amaranth Control in Cotton

Field experiments were conducted to evaluate the integration of cover crops and POST herbicides to control glyphosate-resistant Palmer amaranth in cotton. The winter-annual grasses accumulated the greatest amount of biomass and provided the most Palmer amaranth control. The estimates for the logistic regression would indicate that 1540 kg ha−1would delay Palmer amaranth emerging and growing to 10 cm by an estimated 16.5 days. The Palmer amaranth that emerged in the cereal rye and wheat cover crop treatments took a longer time to reach 10 cm compared to the hairy vetch and crimson clover treatments. POST herbicides were needed for adequate control of Palmer amaranth. The glufosinate-based weed control system provided greater control (75% vs 31%) of Palmer amaranth than did the glyphosate system. These results indicate that a POST only herbicide weed management system did not provide sufficient control of Palmer amaranth, even when used in conjunction with cover crops that produced a moderate level of biomass. Therefore, future recommendations for GR Palmer amaranth control will include integrating cover crops with PRE herbicides, overlaying residual herbicides in-season, timely POST herbicide applications, and hand weeding in order to achieve season-long control of this pest.

Biological control of broad-leaved dock infestation in wheat using plant antagonistic bacteria under field conditions.

Conventional weed management systems have produced many harmful effects on weed ecology, human health and environment. Biological control of invasive weeds may be helpful to minimize these harmful effects and economic losses incurred to crops by weeds. In our earlier studies, plant antagonistic bacteria were obtained after screening a large number of rhizobacteria for production of phytotoxic substances and effects on wheat and its associated weeds under laboratory conditions. In this study, five efficient strains inhibitory to broad-leaved dock and non-inhibitory to wheat were selected and applied to broad-leaved dock co-seeded with wheat both in pot trial and chronically infested field trial. Effects of plant antagonistic bacteria on the weed and infested wheat were studied at tillering, booting and harvesting stage of wheat. The applied strains significantly inhibited the germination and growth of the weed to variable extent. Similarly, variable recovery in losses of grain and straw yield of infested wheat from 11.6 to 68 and 13 to 72.6% was obtained in pot trial while from 17.3 to 62.9 and 22.4 to 71.3% was obtained in field trial, respectively. Effects of plant antagonistic bacteria were also evident from the improvement in physiology and nutrient contents of infested wheat. This study suggests the use of these plant antagonistic bacteria to biologically control infestation of broad-leaved dock in wheat under field conditions.

The Ideology of U.S. Agriculture: How Are Integrated Management Approaches Envisioned?

ABSTRACTThe ease and affordability of the herbicide glyphosate and the development of glyphosate tolerant crops has encouraged a simplified weed management system in the U.S. However, the overuse of this technological package in recent years has led to an explosion in the number of herbicide resistant weeds and a decrease in the effectiveness of glyphosate. In addition to this bio-chemical problem, many growers appear to be stuck within a farming ideology that focuses on a reduction in time/labor, an increase in profitability and a vision of ever-increasing efficiencies of scale. This may prevent them from conceptualizing and implementing solutions that fall outside of a simple herbicide application plan. The objective of this paper is to begin describing the current ideology of industrial corn and soybean farmers in the United States, as well as how this ideology contributes to farmers’ resistance to complex weed management strategies.

Differential Response of Palmer Amaranth (Amaranthus palmeri) Gender to Abiotic Stress

Knowledge of Palmer amaranth biology and physiology is essential for the development of effective weed management systems. The aim of this study was to investigate the response of Palmer amaranth gender to nutrient deficiency and light stress. Differential gender responses were observed for all the growth, phenology, and photochemistry parameters measured. Female plants, for example, invested more in height, stem, and total dry weight, whereas male plants invested more in leaf area and leaf dry weight. The growth rate of females was higher than that of male Palmer amaranth plants, although both followed similar declining trends as the experimental period progressed. Initiation of flowering of female plants occurred 6 to 8 d earlier compared with male plants. Nitrogen and to a certain extent phosphorous were the most influential nutrients that affected measured parameters in both Palmer amaranth genders, particularly under high light intensity. Electron transport rate and chlorophyll content of female Palmer amaranth plants compared with male plants was lower at high light intensity in combination with nitrogen and phosphorous deficiencies. There is a potential to manipulate Palmer amaranth population structure by altering microenvironments at the field level.

Moving integrated weed management from low level to a truly integrated and highly specific weed management system using advanced technologies

SummaryIntegrated weed management (IWM) is one of the most commonly referred to approaches for sustainable and effective weed control in agriculture, yet it is not widely practiced, likely because current IWM systems fail to meet performance expectations of growers. The effectiveness and value of IWM systems should increase with increasing application specificity and true integration made possible with contemporary advances in technology, information systems and decision support. IWM systems can be classified based on their degree of application specificity and level of integration of tactics. In the application specificity pathway, a tactic is applied at a range of scales, from subfield to plant specific. In the integration pathway, multiple weed control tactics are combined in a synergistic manner. We hypothesise that the full value of IWM can and will be realised only when current and emerging technological innovations, information systems and decision tools are synergistically combined for use in real time. The True IWM system we envision requires automation and robotic technologies, coupled with information and decision support systems that are available or emerging but not yet enabled, in a proven integrated platform. Examples of low‐level, traditional and precision IWM systems are discussed, and research needs for a True IWM system are presented. We conclude that the immediate call should be for a long‐term investment in R&D and education (both theoretical and empirical) to develop and implement True IWM systems, an effort best accomplished in a public–private partnership where all essential entities are fully engaged and adequately resourced, including growers from all countries who will utilise IWM.

Herbicide resistance in weeds and its management

Repeated use of herbicides with similar modes of action for weed control in wheat has resulted in evolution of multiple herbicide resistance in Phalaris minor, which could threaten the sustainability of the rice-wheat cropping system in north-western India. Herbicide resistance could also become a problem in direct seeded rice and soybean crops, which rely heavily on acetolactate synthase (ALS) and acetyl coenzyme A carboxylase (ACCase) inhibiting herbicides. As discovery of new herbicide modes of action has slowed dramatically, all effort should be made to increase the effective life of existing herbicides and make weed management cost-effective and efficient. Early detection of resistance can facilitate timely adoption of alternative tactics and minimise the financial impact on farmers. Structured surveys of resistance affected areas can provide required information for site-specific recommendations as expression of herbicide resistance tends to be highly site-specific. Greater emphasis is needed on weed management systems based on sound knowledge of weed ecology. Refinement is needed in seeding machinery for high residues systems, improvements in equipment for inter-row cultivation and application technology of herbicides. Farmer-participatory research and adoption of improved agronomy could delay evolution of resistance in weeds and enhance the sustainability of Indian cropping systems. This paper has reviewed and discussed herbicide resistance in weeds in Indian context.

Measuring Interference from Midseason Tall Morningglory (Ipomoea purpurea) to Develop a Model for Teaching Weed Seedbank Effects on Chile Pepper

Tactics that target seedbanks are important components of weed management systems; however, such tactics can be difficult to adopt because consequences of seedbank reduction are often unclear. This study developed model-based software to provide insights on the economic outcomes, in the context of chile pepper production, of additions to tall morningglory seedbanks. Data for the model were derived from this and previous studies. In this study, field experiments were conducted to determine chile pepper yield and harvest efficiency responses to mid-season tall morningglory infestations. The experimental treatments were factorial combinations of herbicide (pendimethalin-treated, nontreated) and tall morningglory density (0, 4, 8, 12, 16, 20 plants 10-m row–1). Treatments were installed 9.5 weeks after crop seeding. Data collected included fresh weight of marketable chile peppers and time required for one individual to harvest 10-m of crop row, which was used to calculate the amount of chile pepper harvested in 1 min (harvest efficiency). Results indicated that crop yield was not influenced by tall morningglory density, pendimethalin treatment and interactions between tall morningglory density and pendimethalin. Harvest efficiency was influenced by tall morningglory density but was not influenced by herbicide treatment or interactions between herbicide treatment and tall morningglory density. Each additional tall morningglory plant decreased the amount of chile pepper harvested in 1 min by 9.7 g. The results of this and previous studies were used to develop model-based software that presents tall morningglory seedbank density effects on: (1) tall morningglory seedling densities after pendimethalin, (2) time requirements for hand-hoeing after pendimethalin, and (3) time requirements for hand-harvesting to acquire yield goals. The model-based software is intended to be used in the instruction of weed seedbank management strategies. By presenting seedbank density effects on weed control outcomes and crop production expenses, the model-based software might promote adoption of seedbank reduction strategies.

Competitiveness of Winter Rice Varieties against Weed under Dry Direct Seeded Conditions

Aerobic rice or dry direct seeded rice is highly vulnerable to weeds because of lack of “head start” over weeds and standing water layer to suppress weeds. The risks of chemical control and the huge cost involvement in mechanical control demand an eco-friendly and cost-effective integrated weed management. Weed competitive rice cultivar may be considered as a viable tool for integrated weed management. In these circumstances, an experiment was designed to evaluate weed competitiveness of some selected winter rice varieties under aerobic soil conditions. The study was conducted during dry season (February to June) 2016 at the Agronomy Field Laboratory and Weed Management Laboratory, Bangladesh Agricultural University, Mymensingh. Fourteen rice varieties namely, BRRI dhan28, BRRI dhan29, BRRI dhan47, BRRI dhan50, BRRI dhan55, BRRI dhan58, BRRI dhan59, BRRI dhan67, Binadhan-5, Binadhan-6, Binadhan-8, Binadhan-10, BRRI hybrid dhan3 and Agrodhan-14 were grown under weedy and weed-free conditions. Plots with no rice were also maintained to observe the natural growth of weed in absence of rice. The experiment was conducted with split-plot design with three replications. Weeding regime was allocated in main plot and rice variety was allocated in sub-plot. Results showed that rice varieties varied widely in their yield performances and weed competitiveness. Among rice varieties, BRRI dhan59 allowed the minimum weed growth (19.2 g m-2) while Binadhan-5 allowed the maximum weed growth (62.8 g m-2). Grain yield ranged from 1.85 t ha-1 (BRRI dhan55) to 3.92 t ha-1 (Binadhan-5) under weed-free condition and between 0.41 t ha-1 (BRRI dhan55) and 2.06 t ha-1 (BRRI dhan59) under weedy condition. Weed inflicted relative yield loss ranged from 43.4% to 82.1% among the varieties. BRRI dhan59 allowed the least yield penalty (43.4%) while Binadhan-5 had the maximum yield penalty (82.1%) due to weed competition. Although Binadhan-5 is the most productive variety (3.92 t ha-1) for aerobic culture but its weed inflicted relative yield loss is higher (82.1%) than many other varieties with low yield potential. On the other hand, BRRI dhan59 appeared as the most weed competitive variety (only 43.4% relative yield loss) with productivity of 3.84 t ha-1. Therefore, weed competitive variety should be considered as a vital tool while designing integrated weed management system for aerobic rice.

Assessment of an Integrated Weed Management System in No-Till Soybean and Corn

The objective of this study was to evaluate weed control, crop yields, potential soil loss, and net returns to management of an integrated weed management system in no-till corn and soybean compared to an herbicide-based strategy. The integrated weed management system reduced herbicide inputs by delayed cover crop termination, herbicide banding, and high-residue cultivation (reduced herbicide [RH]), while the other system used continuous no-tillage and herbicides to control weeds (standard herbicide [SH]). Research was conducted within the Penn State Sustainable Dairy Cropping Systems Experiment, where corn and soybean are each planted once in a 6-yr crop rotation. In this 3-yr study, weed density and biomass were often greater under RH management, but weed biomass never exceeded 19 g m–2in corn and 21 g m–2in soybean. Corn yield and population did not differ in any year, and net returns to management were $33.65 ha–1higher in RH corn due to lower herbicide costs and slightly, though not significantly, higher yields. Soybean yield was lower in RH compared to SH in 2 of 3 yr, and was correlated with soybean population and cover crop residue. Net financial returns were $43.69 ha–1higher in SH soybean compared to RH. Predicted soil loss never exceeded T (maximum allowable soil loss) for any treatment and slope combination, though soil loss was 100% greater on a 10% slope under RH management (vs. SH) due to cultivation.

Does intercropping play a role in alleviating weeds in cassava as a non-chemical tool of weed management? – A review

Cassava is an important root and tuber crop in the tropics which requires intensive weed management at its early growth stages. This review emphasizes the potential of exploiting the option of intercropping as a non-chemical tool of weed management in cassava. The appropriateness and the significance of including intercropping solely and as a component of an integrated weed management system in cassava are further discussed. Literature suggests that intercropping is a successful option in managing weeds when the spatial and temporal compatibility of intercrop combinations is being achieved. In widespread cassava-based intercropping systems, intercropping itself has proven its ability to alleviate weeds up to 30–60%, or even up to 100% with the selection of a better compatible crop mixture such as the cassava-pumpkin intercrop combination. A number of studies conducted to ascertain the appropriate spatial and temporal compatibility levels of many intercrop combinations have provided evidence of their weed suppressive ability in cassava-based intercropping systems. The focus of such studies towards the basic agronomic, physiological and biochemical determinants of crop-weed interactions seems rather constricted. In conclusion, intercropping is suggested to be exploited as an effective weed management tool in cassava preferably through further research, prior to endorsing it as a proper alternative to chemical weed control measures, especially for the resource–poor farmers who probably can neither afford herbicides nor labour-intensive cultural methods for weed management.

Alternatives to Atrazine for Weed Management in Processing Sweet Corn

Atrazine has been the most widely used herbicide in North American processing sweet corn for decades; however, increased restrictions in recent years have reduced or eliminated atrazine use in certain production areas. The objective of this study was to identify the best stakeholder-derived weed management alternatives to atrazine in processing sweet corn. In field trials throughout the major production areas of processing sweet corn, including three states over 4 yr, 12 atrazine-free weed management treatments were compared to three standard atrazine-containing treatments and a weed-free check. Treatments varied with respect to herbicide mode of action, herbicide application timing, and interrow cultivation. All treatments included a PRE application of dimethenamid. No single weed species occurred across all sites; however, weeds observed in two or more sites included common lambsquarters, giant ragweed, morningglory species, velvetleaf, and wild-proso millet. Standard treatments containing both atrazine and mesotrione POST provided the most efficacious weed control among treatments and resulted in crop yields comparable to the weed-free check, thus demonstrating the value of atrazine in sweet corn production systems. Timely interrow cultivation in atrazine-free treatments did not consistently improve weed control. Only two atrazine-free treatments consistently resulted in weed control and crop yield comparable to standard treatments with atrazine POST: treatments with tembotrione POST either with or without interrow cultivation. Additional atrazine-free treatments with topramezone applied POST worked well in Oregon where small-seeded weed species were prevalent. This work demonstrates that certain atrazine-free weed management systems, based on input from the sweet corn growers and processors who would adopt this technology, are comparable in performance to standard atrazine-containing weed management systems.

Intrarow Weed Removal in Broccoli and Transplanted Lettuce with an Intelligent Cultivator

The performance of the Robovator (F. Poulsen Engineering ApS, Hvals⊘, Denmark), a commercial robotic intrarow cultivator, was evaluated in direct-seeded broccoli and transplanted lettuce during 2014 and 2015 in Salinas, CA, and Yuma, AZ. The main objective was to evaluate the crop stand after cultivation, crop yield, and weed control efficacy of the Robovator compared with a standard cultivator. A second objective was to compare hand weeding time after cultivation within a complete integrated weed management (IWM) system. Herbicides were included as a component of the IWM system. The Robovator did not reduce crop stand or marketable yield compared with the standard cultivator. The Robovator removed 18 to 41% more weeds at moderate to high weed densities and reduced hand-weeding times by 20 to 45% compared with the standard cultivator. At low weed densities there was little difference between the cultivators in terms of weed control and hand-weeding times. The lower-hand weeding time with the Robovator treatments suggest that robotic intrarow cultivators can reduce dependency on hand weeding compared with standard cultivators. Technological advancements and price reductions of these types of machines will likely improve their weed removal efficacy and the long-term viability of IWM programs that will use them.

A Survey of Weed Management in Organic Small Grains and Forage Systems in the Northwest United States

A lack of information regarding weed control, relative to conventional systems, has left organic growers largely on their own when devising weed management systems for organic crops. As interest in organic weed management increases, researchers need more information regarding the type and number of weed control practices undertaken on organic farms. A survey of certified organic growers was conducted in five states in the northwest United States to identify organic weed management programs and what grower and farm-operation characteristics were factors in weed management program design. Three types of weed management programs, with varying diversity in weed control practices, were identified. Stepwise binary logistic regression indicated that the likelihood of an organic grower using a more-diverse weed management program increased if the grower engaged in grain production and as the number of crops produced on an organic farm operation in 1 yr increased. The probability of operating a more-diverse weed management program also increased as a grower's education level increased. Organic hectarage operated was positively correlated with weed management program diversity, and with the adoption of cultural controls. Additionally, awareness of weeds as a factor causing yield loss was correlated with increased weed management program diversity. An increased awareness among researchers of the differing needs and abilities of organic growers in managing weeds on their farms will improve communication and outreach efforts when assisting growers with designing organic weed management programs.

Weed management in cotton (Gossypium hirsutum L.) through weed-crop competition: A review

The agriculture sector is embracing energy efficient conservation systems and technological innovations to meet the ever increasing demand for food, fibre, and fuel in tune with the rapidly increasing human population. The genetic modification of plants is one of the technological innovations that is adopted rapidly across the world. In cotton, many major producing countries have adopted herbicide-tolerant genetically modified crops. Over-reliance on herbicides for weed management in both genetically modified and conventional systems has led to the rapid evolution of herbicide-resistant weeds. Poor weed management can cause up to 90% yield loss in cotton. Undoubtedly, integration of non-chemical methods and diversifying weed control options would ensure the sustainability of available weed management options, including herbicides. Increasing crop competitiveness is one of the approaches that could be integrated with the current weed management systems. Choosing cultivars with early vigour, use of narrow row planting, orienting crop rows with regard to sunlight, and adjusting planting density are some of the approaches that could enhance the competitiveness of crops over weeds. Review of the available literature on cotton indicates weed suppressive benefits by enhancing crop competitiveness through increasing planting density and narrow row spacing. Early canopy closure in narrow row spaced systems would suppress many problem weeds. In addition, herbicide efficacy may be increased due to competition offered by a dense crop stand, which may reduce herbicide selection pressure on weeds. However, the use of narrow row spacing is still in an infant stage in many cotton-growing countries and the success may depend on the environment, soil type, and resource availability. This review analyses and reports the potential benefits of increasing crop competition as a weed management option and also highlights research to be undertaken to ensure the adoption of different strategies on a much wider scale.

High N2O variations induced by agricultural practices in integrated weed management systems

Integrated weed management aims to decrease the dependence of cropping systems on herbicides by using a combination of several agricultural practices. Environmental impacts of individual practices under various conditions are already known. However, there is scarce knowledge on the impact of combining several practices. Therefore, we studied N2O emissions of weed management cropping systems that use differing practices such as crop diversity, tillage, and herbicide pressure, during about 1 year. Data were compared with a conventionally managed cropping system. Results were also simulated using the NOE model. Results show a large variation of N2O emissions, ranging from 177 g N2O-N/ha for intensive tillage integrated weed management to 362 g N2O-N/ha for the conventional cropping system, 777 g N2O-N/ha for the no herbicide cropping system and 5226 g N2O-N/ha for no-till integrated weed management. Most N2O emissions occurred in spring, despite the absence of fertilizing related N2O peaks. The lowest emissions occurred in autumn and winter. Emissions are explained by interactions between specific soil potential denitrification rate, soil bulk density, temperature, soil water, and inorganic N contents. N2O emissions are accurately predicted by the model NOE, with a global Nash-Sutcliffe coefficient of efficiency equal to 0.80.

Efecto del manejo de las malezas en macadamia sobre la población del barrenador de la nuez Ecdytolopha torticornis Meyrick (Lep: Tortricidae).

The effect of three weed management systems -macadamia husk as a mulch, bare ground and weeds growing freely - on the population ofthe nutborer, Ecdytolopha torticornis, were studied from June to September 1992 at Oriente Farm, Turrialba, Costa Rica. The smallest number of the nutborer's eggs was recorded on the bare ground treatment, followed by the unweeded treatment and the husk treatment. Although the macadamia husk as a mulch has been used to sol ve the problem of residues from nut processing, the results suggest that the use of this practice might increase the nutborer population by concentrating odours which could attract the adults, or by disrupting the parasitoid or predator population. There is a need for testing other mulches such as plastics, other plant residues, sawdust and others, and to study alternative weed management systems, including the use of cover crops in the mid-rows.

Effects of Water on Recovery of Weed Seedlings Following Burial

Recovery of common agricultural weeds after burial by soil was studied in four greenhouse and three field experiments. Species studied included velvetleaf, Powell amaranth, common lambsquarters, barnyardgrass, and giant foxtail. Seedlings were bent over before burial to simulate the effect of the impact of soil thrown by a cultivator. Altogether, more than 35,000 seedlings were marked and observed for recovery. No seedlings recovered from 4 cm of burial. Recovery from complete burial under 2 cm of soil ranged from 0 to 24% depending on the experiment, species, and watering treatment, but recovery greater than 5% was rare. Large-seeded species tended to recover from complete burial under 2 cm of soil better than small-seeded species. The study did not reveal a difference in recovery of grasses relative to broadleaf weeds. Overall, seedlings tended to recover best when water was applied daily after burial, worst when water was applied once on the day of burial, and to an intermediate extent when no water was applied. However, difference in recovery between the no-water and watering-once treatments were usually small. Also, many experiment by species combinations showed no significant differences among watering treatments. When even a small portion of the seedling was left exposed, recovery generally exceeded 50%. Organic weed management systems commonly use burial of weed seedlings with tine weeders and soil thrown by sweeps and hilling disks to control weeds in crop rows. Recovery from burial could pose a substantial weed management problem in some circumstances, particularly for large-seeded weed species. Maximizing burial depth is important for limiting recovery. Recovery from burial can be minimized by withholding irrigation for several days after hilling-up operations.

Weed Control in Clean Agriculture: A Review1

ABSTRACT Weed control is consider the major obstacle for the growers in the organic farming. Lower plant productivity in organic farming mainly related to the poor weed control. It is widely known, in most cases, that losses caused by weeds exceeded the losses from any category of agricultural pests. Under water-stress condition, weeds can reduce crop yields more than 50% through moisture competition alone. In the light of the environmental and toxicological problems created by herbicides, it has become necessary to develop the safety methods for controlling weeds. Soil Solarization, Mulching, Biodegradable Mulch, Natural Herbicides, Hot Water, and Agronomic Practices have been successfully adopted in many countries as safe methods for controlling weeds in the organic farming. In addition, there are some promising new and non-traditional measures such as Fresnel Lens, Electrical Weed Control, Lasers, etc which could be employed for controlling the weeds in organic farming. Also the agronomic practices such as choice of competitive varieties, stale seedbeds had a significant impact on weeds. The growers in organic farming should keep these three points in mind: 1) start clean stay clean successful, 2) Prevention is always better than treatment and, 3) One year's seeds will lead to seven year's weed infestation. Successful and sustainable weed management systems are those that employ combinations of techniques rather than relying on one method. The objectives of this paper are to review some safe weed control methods in the clean agricultural.

Evaluation of Herbicide Programs for Use in a 2,4-D–Resistant Soybean Technology for Control of Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri)

Two separate field experiments were conducted over a 2-yr period in Fayetteville, AR, during 2012 and 2013 to (1) evaluate POST herbicide programs utilizing a premixture of dimethylamine (DMA) salt of glyphosate + choline salt of 2,4-D in a soybean line resistant to 2,4-D, glyphosate, and glufosinate and (2) determine efficacy of herbicide programs that begin with PRE residual herbicides followed by POST applications of 2,4-D choline + glyphosate DMA on glyphosate-resistant Palmer amaranth. In the first experiment, POST applications alone that incorporated the use of residual herbicides with the glyphosate + 2,4-D premixture provided 93 to 99% control of Palmer amaranth at the end of the season. In the second experiment, the use of flumioxazin, flumioxazin + chlorimuron methyl, S-metolachlor + fomesafen, or sulfentrazone + chloransulam applied PRE provided 94 to 98% early-season Palmer amaranth control. Early-season control helped maintain a high level of Palmer amaranth control throughout the growing season, in turn resulting in fewer reproductive Palmer amaranth plants present at soybean harvest compared to most other treatments. Although no differences in soybean yield were observed among treated plots, it was evident that herbicide programs should begin with PRE residual herbicides followed by POST applications of glyphosate + 2,4-D mixed with residual herbicides to minimize late-season escapes and reduce the likelihood of contributions to the soil seedbank. Dependent upon management decisions, the best stewardship of this technology will likely rely on the use multiple effective mechanisms of action incorporated into a fully integrated weed management system.

Factors affecting potential for Palmer amaranth (Amaranthus palmeri) suppression by winter rye in Georgia, USA

Herbicide resistant Palmer amaranth (Amaranthus palmeri) has rapidly become a dominant weed management issue in agronomic crops of the Southeast US. The small size of Palmer amaranth seeds, relative to other common weeds, provides an opportunity to use physical weed control through high-biomass, rolled cover crop mulches, in conjunction with herbicide tools. Experiments were conducted to characterize Palmer amaranth suppression and light permeability from a range of rye biomass levels. There was an inverse relationship between Palmer amaranth emergence and rye biomass that was described by a log-logistic regression model. In the absence of rye, there was approximately 80% Palmer amaranth emergence, while the highest rate of rye biomass prevented Palmer amaranth emergence. A log-logistic regression model also described the amount of photosynthetic active radiation transmitted through rye mulch in a green house experiment. The highest level of rye biomass reduced the amount of light to 13% of full sunlight, while 5370kgha−1 of rye caused a 50% reduction of light transmission; a similar level of rye biomass (P=0.93) reduced Palmer amaranth emergence by 50%. Effective suppression of Palmer amaranth will depend upon the ability to produce high-biomass rye. Field experiments evaluated changes in planting date, seeding rate, and nitrogen application on rye biomass production. Maximum rye biomass in April occurred when rye was planted prior to middle-November. However, a 50% reduction in rye biomass resulted from middle-December planting of rye, providing growers with a short planting interval for high-biomass rye production. Additionally, rye seeding rate did not increase rye biomass accumulation indicating that delays in autumn sowing cannot be overcome with plant density. Finally, nitrogen fertilizer applied at planting consistently increased rye biomass production 23–33% relative to non-fertilized controls averaged over all planting dates. Additional research is needed to evaluate how repeated high-biomass cover crop systems affect weed management systems, other pest complexes, and soil moisture status in the sandy soils of the southeast Coastal Plain.