2012 Annual Report
1a.Objectives (from AD-416):
1. Develop integrated systems of weed management for organic agronomic and vegetable cropping systems, such as peanut, cotton, cucurbits and dry bulb onion respectively, in the southeastern coastal plain.
2. Identify the ecological and edaphic factors affecting the reproduction, spread, and survival of invasive, herbicide-reistant, and herbicide tolerant weed pests of agronomic and vegetable crops in the southeastern coastal plain, including, but not limited to pigweeds, common bermudagrass, and perennial nutsedges.
3. Combine effective chemical and cultural control measures into integrated systems for the management of key species of herbicide resistant and invasive weeds of agronomic and vegetable crops in the southeastern coastal plain, such as pigweeds and Benghal dayflower.
1b.Approach (from AD-416):
Research will be conducted to determine weed management practices that will: A) Manage weeds without conventional herbicides in organic systems and B) reduce reliance on a dwindling number of herbicide tools in conventional systems.
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, integrated weed management systems will be developed in organic agronomic and vegetable cropping systems. Cultural and mechanical weed management strategies will be employed to prevent seedling establishment and reduce propagule persistence in the soil. The second objective will determine the factors that affect the reproduction and persistence of herbicide-resistant and herbicide tolerant weed pests. Specific studies will include the effect of cover crops on weed establishment, growth, and fecundity, as well as determining the factors that affect persistence of the soil seedbank. The third objective is to combine effective chemical and cultural control measures into integrated systems for the management of key weed species. Weed growth and reproduction as affected by crop stand and presence of cover crops will be evaluated. Ultimately fulfillment of these objectives will improve grower profitability and reduce reliance on a limited set of herbicide resources that are rapidly declining in efficiency.
Cultivation is a proven component in the management of weeds in organic Vidalia sweet onion production. However, delays in the initial cultivation due to wet soils reduce the overall effectiveness of cultivation. In this case, improved performance using herbicides derived from natural sources would be useful. These herbicides were found to be much more efficacious when applied using a sprayer calibrated for a high output (>50 gal./A) compared to sprayers with a normal calibration (approximately 25 gal./A). When herbicides derived from natural products were applied in this manner, early season weed control was significantly improved and was synergistic with cultivation using a tine weeder. Research on multi-tactic approaches for effective glyphosate-resistant Palmer amaranth management continues. Winter cover crops rolled horizontal to form a thick mulch mat will help to hinder establishment of the small-seeded Palmer amaranth. Palmer amaranth populations in the southern United States have resistance to four different herbicide classes (mechanisms of action) and based on the genetic variability within the population, resistance to other herbicides is likely to already exist within a population. Repeated use of a particular herbicide class will select for this resistance, by allowing plants to establish and reproduce. Successful stewardship of the current herbicide technologies depends on minimizing the number of plants that are under herbicide selection pressure. The physical barrier of the rolled cover crop mulches reduces the established Palmer amaranth plant density. However, the mulch will bind some herbicides, keeping them from reaching the soil surface, reducing Palmer amaranth control. In addition, weeds that are able to emerge where the cover crop mulch was thin or moved during planting of the summer crop, may have greater growth than those growing without the cover crop. Research continues on ways to maximize consistent ground coverage of the mulch in order to minimize the safe sites for Palmer amaranth establishment.
Importance of uniform peanut stand for optimum, cost-effective weed control. Skippy, non-uniform peanut stands are common in conventional peanut production, despite seed treated with combinations of protective fungicides. An experimental technique was developed that created a skippy peanut stand at recurring intervals. ARS researchers in Tifton, Georgia determined weed control required additional herbicides and was more costly compared to peanut with a uniform peanut stand. In addition, residual herbicides effectively controlled weeds in the voids compared to ineffective weed control using postemergence herbicides. In all cases, it was more cost-effective to invest in additional herbicides to control escaped weeds than to destroy the skippy stand and replant.
Yield loss potential in cotton reduced with delayed establishment of Palmer amaranth. Effective control of glyphosate-resistant Palmer amaranth is predicated on the use of soil applied herbicides that prevent weed emergence. However, the duration of weed control from these herbicides is governed by various factors which can result in escaped weeds. ARS researchers in Tifton, Georgia determined that when Palmer amaranth was established at the 3- to 8-leaf stages of cotton, crop yield loss was approximately 6% for every Palmer amaranth in 20 feet of cotton row. This simulates the failure of preplant herbicides, which are the critical tools for managing this weed, as options for controlling glyphosate-resistant Palmer amaranth in glyphosate-resistant cotton systems are very limited once the weed has emerged. When Palmer amaranth was established during simulated herbicide failures from timings of late postemergence and layby applications (12- to 17-leaf stages of cotton), there was no effect on cotton yield. However, one of the tenets of managing this herbicide resistant species is reducing the population densities in the soil seedbank in an effort to reduce and improve future weed management; all Palmer amaranth establishment timings produced viable seed that returned to the soil seedbank. Growers must be diligent in managing escaped weeds, even the relatively low Palmer amaranth densities evaluated in this study.
Plant spacing and cultivation are critical for weed control in organic Vidalia onion. Weed control is a limiting factor for organic onion production. ARS researchers in Tifton, Georgia found techniques of weed management that offered promise in organic peanut production. Cultivation with a tine weeder effectively controlled weeds when onion were spaced at 3 plants/30 cm row compared to 2 plants/30 cm. While onion is not an effective competitor with cool-season weeds, weeds were more effectively controlled using cultivation when onion had a denser spacing of transplants compared to onion spaced at the conventional spacing of 2 plants/30 cm row.
Benghal dayflower seed viability less than 2% after 42 months. Benghal dayflower was the first weed species to become troublesome in glyphosate-resistant cotton fields in Georgia, due to its natural tolerance to glyphosate. Effective Benghal dayflower management strategies have been developed, but herbicide costs in Georgia alone exceeded $1 million annually to control this exotic, tropical invasive weed. The duration of intensive herbicide programs needed to prevent new seed return and delete the soil seedbank (i.e. the persistence of Benghal dayflower seeds) was not known. ARS researchers in Tifton, Georgia in collaboration with scientists from North Carolina State University and University of Florida, conducted seed burial studies in the three states over a 60-month period. In North Carolina, Benghal dayflower seed viability was less than 1% after 42 months; in Georgia seed viability was less than 2% after 36 months, and in Florida seed viability was less than 7% after 39 months. Intensive management of this tropical weed species using herbicides such as metolachlor, should continue at least 39 to 48 months to minimize the soil seedbank populations of Benghal dayflower.
Alternatives to conventional polyethylene mulches. Most commercial vegetable production in the southeastern uses transplants grown on seedbeds covered with thin-film mulch. It is costly to procure, remove from the field, and dispose thin-film mulches. Alternative biodegradable mulching materials will eliminate the cost associated with removal and disposal. In preliminary trials, ARS researchers in Tifton, Georgia evaluated an alternative mulching material made from processed cotton gin trash. This material was successfully applied using conventional mulch application equipment and adequately suppressed Palmer amaranth when treated post-application with linseed oil, black latex paint, or dark-colored paint derived from natural plant-based products. The rolled cotton fiber mat made from cotton gin trash offers great potential as an alternative mulching material since it is fully biodegradable, made from a common waste material, and serves as a value-added product to the cotton industry.
Crop row spacing and cultivation are critical for weed control in organic peanut. Weed control is a limiting factor for organic peanut production. ARS researchers in Tifton, Georgia found techniques of weed management that offered promise in organic peanut production. Cultivation with a tine weeder effectively controlled weeds when peanut were seeded at a uniform density of 20 seed/m. In contrast, if peanut were seeded in multiple rows at 10 seed/m or had skips in the peanut stand, then weeds were not controlled using any cultivation regime. The most effective cultivation regimes began just prior to peanut emergence.
Purple nutsedge sprouting increases with fluctuations in temperatures. Purple nutsedge is among the most troublesome weeds of vegetables in the southeastern United States and a substantial impediment in the search for methyl bromide alternatives. Greater understanding of the environmental cues that regulate tuber sprouting may assist in improving nutsedge management. ARS researchers in Tifton, Georgia evaluated the effect of daily temperature fluctuations on purple nutsedge tuber sprouting. Studies used early spring daily temperature fluctuations that corresponded to those conditions under black polyethylene mulch used in commercial vegetable production in the southeast United States. In the absence of daily temperature changes (i.e. constant temperature), purple nutsedge sprouting was 52%. When temperatures varied (a daily temperature fluctuation of 34 F degrees), purple nutsedge sprouting was 87%. The use of various types and colors of mulching material can affect daily temperature fluctuations, potentially shifting nutsedge emergence to a time when it can be effectively controlled, perhaps even stimulating emergence between vegetable crop seasons in order to increase the spectrum of herbicides available for control.
New peanut cultivars have less herbicide tolerance. Recent peanut cultivars have improved disease and nematode resistance and higher yield potential than the cultivars that they have replaced over the last 10 years. However, weed management is still a critical component that relies on herbicides, so herbicide tolerance of these new cultivars is significant. Chlorimuron is a herbicide applied relatively late in the season to control escaped weeds, especially Florida beggarweed. ARS researchers in Tifton, Georgia in collaboration with scientists from the University of Georgia, determined that two of three tested cultivars, including ‘Georgia-06G’ (the one most commonly planted in Georgia) and ‘TifGuard’, have increased susceptibility and suffer significant yield losses when sprayed with chlorimuron, compared to older cultivars. Based on these results, the University of Georgia Extension specialist is no longer recommending use of chlorimuron on Georgia-06G or TifGuard for control of Florida beggarweed. However, ‘Florida-07’ was sufficiently tolerant of chlorimuron to warrant continued use.
Palmer amaranth seed longevity less than 22% after three years. Palmer amaranth with resistance to glyphosate (trade name Roundup, among others) was first detected in Georgia, but has spread throughout the southern United States. The longevity of this small-seeded annual species in the soil seedbank will regulate the population dynamics in growers’ fields. ARS researchers in Tifton, Georgia assessed Palmer amaranth seed longevity in soil burial studies. Seed packets were buried at four depths, exhumed over a three-year period and tested for seed viability. Seed viability declined to 65 to 78% after six months of burial. As burial duration increased, so did the differences in seed viability among the burial depths. By 36 months, seed viability ranged from 9% (1/2 inch burial depths) to 22% (16 inch burial depth). The relatively rapid decline of seed viability at shallow soil depths may allow growers to alter their weed populations if weed seed production is prevented. One of the proposed methods of reducing potential seedling populations is to reposition weed seeds that are near the soil surface to depths below optimal emergence zones through plowing. Deeper buried seed will have greater longevity in the seedbank, so the practice should not be repeated annually. As herbicide resistance continues to develop in Palmer amaranth (currently resistant to four herbicide mechanisms of action), leveraging the inherent strengths of agroecosystems will garner increased significance in management of this species and others.
Grey, T.L., Webster, T.M. 2012. Transplant production. In: Russo, V.M., editor. Peppers: Botany, Production, and Uses. Oxfordshire, UK: CABI. p. 87-99.
Reberg-Horton, S.C., Grossman, J., Kornecki, T.S., Meijer, A.D., Price, A.J., Place, G.T., Webster, T.M. 2012. Utilizing cover crop mulches to reduce tillage in organic systems in the southeastern USA. Renewable Agriculture and Food System. 27(1):41-48.
Webster, T.M. 2010. Effect of autumn management on winter annual weeds prior to cotton planting. Journal of Cotton Science. 14:113-118.
Webster, T.M., Nichols, R. 2012. Changes in the weed species in the major agronomic crops of the United States: 1994/1995 to 2008/2009. Weed Science. 60:145-157.
Potter, T.L., Truman, C.C., Webster, T.M., Bosch, D.D., Strickland, T.C. 2011. Tillage, cover-crop residue management, and irrigation incorporation impact on fomesafen runoff. Journal of Agricultural and Food Chemistry. 59:7910-7915.
Sabila, M., Grey, T., Webster, T.M., Vencill, W., Shilling, D. 2012. Evaluation of factors that influence Benghal dayflower (Commelina benghalensis) seed germination and emergence. Weed Science. 60:75-80.
McCullough, P.E., Schwartz, B.M., Grey, T.L., Webster, T.M. 2012. Preemergence herbicides influence sprig establishment of 'TifEagle' bermudagrass. Weed Technology. 26(2):300-303.
Johnson, III, W.C., Davis, J.W. 2012. Techniques for Cynodon dactylon (L.) Pers. control suitable for use in fallow organic transition in the southeastern U.S. coastal plain. Crop Protection Journal. 39:63-65.
Johnson, W.C., Langston Jr, D.B., MacLean, D.D., Sanders, F.H., Torrance, R.L., Davis, J.W. 2012. Integrated systems of weed management in organic transplanted vidalia sweet onion production. HortTechnology. 22:64-69.
Wann, D.Q., Tubbs, R.S., Johnson, III, W.C., Smith, A.R., Smith, N.B., Culbreath, A.K., Davis, J.W. 2012. Tine cultivation effects on weed control, productivity, and economics of peanut under organic management. Peanut Science. 38:101-110.
Johnson, W.C., Boudreau, M.A., Davis, J.W. 2012. Implements and cultivation frequency to improve in-row weed control in organic peanut production. Weed Technology. 26:334-340.