Location: Crop Protection and Management Research
2018 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. Develop management tactics that effectively disrupt the relationship between bacterial diseases and weed hosts, such as occurs in organic onion production.
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. Improve cover crop mulching systems in cotton that are used to control glyphosate-resistant Palmer amaranth, such that they also better control other common, problematic weeds.
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
Uniform peanut stands are essential for optimum, cost-effective weed control. However, skippy, non-uniform peanut stands are common in organic peanut production due to no seed protectants available that are suitable for certified organic production systems. The uniformity of peanut stand affects weed emergence, with weeds commonly emerging in the voids. A previously reported experimental technique was used to create a skippy peanut stand at recurring intervals. In these organic trials, successful weed control using cultivation required a uniform peanut stand. When peanut stands were skippy, weeds were not adequately controlled by cultivation and supplemental planting offset from the original row did not adequately improve peanut yields compared to the same systems with a uniform peanut stand. These results differ from earlier research using the same experimental technique in conventional production systems which used herbicides to control weeds in the voids. These preliminary results indicate a small margin for error when growers plant organic peanut and they must seed the crop when conditions are conducive for establishing a uniform stand.
Accomplishments
1. Mechanical weed control significantly reduces handweeding cost in organic Vidalia® sweet onion. Organic Vidalia® sweet onion is a highly profitable production system, but weed control is difficult. Research has consistently shown that repeated cultivation with a tine weeder provides significant weed control. Despite the proven benefits of cultivation with a tine weeder, weed control is inconsistent, particularly when persistent rainfall during the winter months affects both the scheduling and performance of cultivation. Inconsistency is shown by densities of cutleaf evening primrose (the most common and troublesome weed of organic onion production) reduced by 59 to 92% by cultivation compared to the non-cultivated control in numerous trials from 2007 to 2015. Regardless of weed control performance, weeds escaping control by cultivation are very large at the time of harvest and undoubtedly cause losses. Given the substantial profit margin of organic Vidalia® sweet onion, it was hypothesized that a single (i.e. once-over) handweeding to control weeds escaping control by cultivation would be beneficial and cost-effective. Cultivation four times at biweekly intervals reduced densities of cutleaf eveningprimrose and onion yields were similarly improved by cultivation. A single handweeding after cultivation further reduced final weed densities and increased onion yields. Cultivation followed by handweeding increased onion yields enough to justify the cost of handweeding, and returns from handweeding investment averaged $3,040-$9,380/A when integrated with cultivation. ARS researchers at Tifton, Georgia, showed that handweeding used to supplement cultivation in organic onion production is an economically viable option due to the measured yield response from improved weed control and the high profit margin of the crop. In locations with history of intense weed pressure, the need for supplemental handweeding should be anticipated due to likelihood of numerous weed escapes.
2. Improved weed control consistency in sugarbeet grown for biofuel. Sugarbeet grown for biofuel is being investigated as an alternate cool-season crop in the southeastern U.S. In traditional sugarbeet production regions of the U.S., the crop is grown as a warm-season crop and the weed species diversity differs from weeds in the southeastern U.S. Previous research identified phenmedipham + desmedipham as the most effective option to control cutleaf evening primrose in sugarbeet grown for biofuel. However, when phenmedipham + desmedipham rates were lowered enough to be cost-effective, weed control was inconsistent. Sweep cultivation improved weed control consistency, but not enough to protect sugarbeet yields. Recent trials by ARS researchers at Tifton, Georgia, evaluated phenmedipham + desmedipham combined with residual herbicides. Both S-metolachlor and dimethenamid combined with phenmedipham + desmedipham improved cutleaf eveningprimrose control consistency in sugarbeet. Since dimethenamid and S-metolachlor are both registered for use on large-acreage crops nationwide, their cost is low compared other specialized sugarbeet herbicides. Cost-effective cool-season weed control in sugarbeet grown for biofuel has now been achieved.
3. Specialized implements improve in-row weed control in organic peanut. In-row weed control is difficult in organic peanut production. Since peanut is very tolerant of aggressive early-season cultivation, implements that bury peanut seedlings were evaluated to see if these implements improve the overall weed control. Two implements offer potential; rolling cultivator and finger-weeder. The blades of the rolling cultivator are metal, while the finger-weeder teeth are a semi-rigid plastic. Both operate using the same principal; peanut are tilled in-row to control weeds that escape conventional cultivation. When used in conjunction with a tine weeder, ARS researchers at Tifton, Georgia, found that both the rolling cultivator and finger-weeder improved in-row control of smallflower morningglory and annual grasses compared to the tine weeder alone. Despite being mechanically aggressive, both the rolling cultivator and finger weeder controlled weeds in organic peanut better during short-term drought compared to periods of monsoonal rainfall, both of which were encountered in 2018. This observation highlights the risks of weed control in organic crop production.
Review Publications
Johnson, W.C., Dutta, B., Sanders, F., Luo, X. 2017. Interactions among cultivation, weeds, and a bio-fungicide in organic Vidalia sweet onion. Weed Technology. 31:890-896.
Johnson, W.C., Luo, X. 2018. Weed control using ammonium nonanoate and cultivation in organic Vidalia sweet onion production. Weed Technology. 32:90-94.
