Location: Crop Protection and Management Research
2016 Annual Report
Objectives
Objective 1: Identify the ecological and edaphic factors affecting the dynamics of invasive weed pests, especially perennial weeds and weeds of cool season crops, in the southeastern United States.
Sub-objective 1.A. Characterize weed phenology as it occurs under various cultural practices in an effort to develop management tactics that effectively disrupt weed establishment.
Sub-objective 1.B. Improve perennial weed management by targeting production and persistence of vegetative propagules.
Sub-objective 1.C. Determine if improved in-row weed control in organic peanut production can be improved by planting in a depressed seedbed and aggressive cultivation, without compromising disease management.
Objective 2: Improve cultural weed control strategies, including conservation tillage methods that can be used as part of integrated weed management systems for invasive and herbicide resistant weeds (e.g. Palmer amaranth, Italian ryegrass, etc.) of agronomic crops.
Sub-objective 2.A. Evaluate the influence of various edaphic factors on weed seed persistence in the soil seedbank.
Sub-objective 2.B. Determine the effect of high-biomass cover crop mulches on glyphosate-resistant Palmer amaranth growth.
Subobjective 2.C. Determine when viable Palmer amaranth seed production first occurs and characterize the timing of seed dispersal from the female plant during the growing season.
Approach
Research will be conducted in conventional and organic summer and winter crops in the southeastern coastal plain to develop weed management practices that utilize diverse tactics to minimize herbicide reliance.
Conventional weed management systems rely heavily on herbicides to minimize crop yield losses associated with weeds. Organic cropping systems have few approved herbicide options, and must rely primarily on weed control from cultural and mechanical practices. The occurrence of herbicide-resistant weeds has limited the efficiency of many herbicides in conventional systems. Studies will be initiated to evaluate a multi-tactic approach to managing weeds with a reduced reliance on herbicide tools. In the first objective, the factors that affect the dynamics of problematic weeds in conventional and organic systems, especially weeds of cool-season crops (e.g. Vidalia onion and sugar beet) and perennial weeds, will be characterized. The impact of the timing of control practices on weed establishment will be evaluated. Management practices that reduce vegetative propagules of problematic weeds will also be studied. The second objective will evaluate integrated diverse weed control tactics of herbicide-resistant weeds. The influence of rolled high-biomass cover crops and nitrogen management on weed seed persistence in the field soil seedbank will be determined. Herbicide-resistant weeds thrive in the current herbicide-dominated weed control systems, with selection pressure for herbicide-resistant weeds. These studies will address if there are additional management practices that can be altered to improve weed control and alter the selection pressures on weeds. The ultimate goal of these objectives is to improve grower profitability and reduce reliance on a limited set of herbicide resources that are rapidly declining in efficiency.
Progress Report
Herbicide resistant Palmer amaranth (Amaranthus palmeri) has rapidly become a dominant weed management issue in agronomic crops of the southeast U.S. 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 by ARS weed scientists in Tifton, Georgia to characterize Palmer amaranth suppression and light permeability from a range of rye biomass levels. In the absence of rye, there was approximately 80% Palmer amaranth emergence, while the highest rate of rye biomass (14,000 kg ha-1) prevented Palmer amaranth emergence. The highest level of rolled rye biomass reduced the amount of light at the soil surface to 13% full sunlight. Rye biomass of 5,370 kg ha-1 reduced light at the soil surface by 50%, which 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 to 33% relative to non-fertilized controls, averaged over all planting dates. To capture the benefits of Palmer amaranth suppression, rye as a cover crop must be intensively managed for optimum growth, not grown as an after-thought.
Accomplishments
1. Improved weed management in sugar beet grown for biofuel. Sugar beet, grown for biofuel as energy beet, is being considered as an alternate cool-season crop in the southeastern U.S. coastal plain. Weed control must be cost-effective to make this alternative cropping system feasible. Herbicides registered for use on sugar beet are costly. ARS researchers in Tifton, Georgia reduced rates of phenmedipham plus desmedipham and effectively controlled the most troublesome weed of sugar beet, cutleaf eveningprimrose, when used in conjunction with sweep cultivation, reducing weed control cost by >30%. This integrated approach to managing weeds provides adequate control of weeds and protects sugar beet yields comparable to multiple applications of costly herbicides. This unique cropping system will give growers an option to efficiently utilize crop land during the winter growing season in the southeastern U.S.
2. Intensive cultivation and seedbed profiles do not increase disease incidence in organic peanut production. Weed management in organic peanut production is based on intensive cultivation. While effective, weed control improvements are needed for sustainable organic peanut production. Altering the seedbed profile and using different cultivation implements slightly improved weed control in the peanut row, but there are concerns that these changes would increase incidence of soil-borne diseases of peanut due to more soil being displaced onto the peanut plant. ARS and University of Georgia researchers found that disease incidence organic peanut was not increased by intensive cultivation. These results provide additional knowledge that will be used to formulate a comprehensive organic peanut production system for the southeastern U.S.
3. Weed seed dormancy and longevity in the soil. Weed seed dormancy buffers weed populations from control efforts. This aspect of weed science remains a persistent mystery and provides substantial challenges to long-term weed control, particularly for noxious or invasive species. Tropical soda apple is an example of a noxious weed that infests rangeland and is spread by livestock that eat the fruit. Seed burial studies conducted in Tifton, Georgia in conjunction with University of Florida weed scientists found that tropical soda apple seed retain 8% viability after burial for eight years. While that germination percentage may intuitively seem small, those surviving seed can quickly re-infest sites where the noxious weed was thought to be eradicated. These results indicate the need to monitor infested sites for much longer periods of time to prevent further spread of the weed.
4. Intensive cultivation improves weed control and reduces herbicide use in conventional peanut production. Herbicide use is intensive in conventional peanut production, increasing production costs and likelihood for additional weed resistance to commonly used herbicides. Intensive cultivation is a cornerstone of weed management in organic peanut production and this practice offers potential for use in conventional peanut production. ARS researchers in Tifton, Georgia found that intensive cultivation with a tine weeder following a single application of metolachlor provided equivalent weed control and peanut yields compared to a full herbicide regime commonly used by conventional peanut growers. Cost savings are substantial and will greatly improve net returns by conventional peanut growers.
5. High residue cover crops to suppress Palmer amaranth have additional weed problems. Palmer amaranth is the most troublesome weed of cotton production and is resistant to many diverse groups of herbicides. High residue cover crops as a form of cultural weed control provide valuable suppression of Palmer amaranth. Regional research among ARS scientists at Tifton, Georgia and Auburn, Alabama, University of Georgia, and Auburn University has shown that Palmer amaranth was suppressed by high residue cover crops, but other weeds proliferated, specifically Texas millet (Urochloa texana). Swapping one weed control challenge for another is not acceptable. These results indicate that the entire weed management system for cotton grown in high-residue cover crops need to be restructured to adequately manage all weeds, not just Palmer amaranth.
ARS scientists at Tifton, Georgia met with one of the largest certified organic farms in Georgia (Vidalia) to discuss on-going weed management research in certified organic Vidalia sweet onion production. These growers are presently using many of the weed management practices developed earlier by the ARS scientists. It was the consensus of the ARS scientist and growers that these weed control practices are effective, but improvements are needed. Subsequent discussions were mutually beneficial, leading to the growers to implement other weed control strategies that will substantially reduce their weed control costs and the ARS scientists initiating weed control experiments to study observations made by the growers.
Review Publications
Johnson, W.C., Davis, J.W. 2015. Perpendicular cultivation for improved weed control in organic peanut production. Weed Technology. 29:128-134.
Johnson, W.C., Way, T.R., Beale, D.G. 2015. An undergraduate student project to improve mechanical control of perenial nutsedges with a peanut digger in organic crop production. Weed Technology. 29:861-867.
Webster, T.M., Grey, T.L. 2014. Halosulfuron reduced purple nutsedge (Cyperus rotundus) tuber production and viability. Weed Science. 62:637-646.
Webster, T.M., Grey, T.L. 2015. Glysphosate-resistant Palmer amaranth (Amaranthus palmeri) morphology, growth, and seed production in Georgia. Weed Science. 63:264-272.
Webster, T.M., Simmons, D.B., Culpepper, A.S., Grey, T.L., Bridges, D.C., Scully, B.T. 2016. Factors affecting potential for Palmer amaranth (Amaranthus palmeri) suppression by winter rye in Georgia, USA. Field Crops Research. 192:103-109. doi: 10.1016/j.fcr.2016.04.020.
Li, X., Grey, T.L., Webster, T.M., Anderson, W.F., Cutt III, G.S. 2015. Evaluation of control of napiergrass (Pennisetum purpureum) with tillage and herbicides. Invasive Plant Science and Management. 8(4):393-400.
