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Optimal Conditions for Buckwheat [fagopyrum Esculentum Moench] Production as a Cover Crop for Weed Suppression in Florida

Permanent Link: http://ufdc.ufl.edu/UFE0025074/00001

Material Information

Title: Optimal Conditions for Buckwheat fagopyrum Esculentum Moench Production as a Cover Crop for Weed Suppression in Florida
Physical Description: 1 online resource (82 p.)
Language: english
Creator: Huang, Pei-Wen
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: buckwheat, cover, crop, suppression, weed
Horticultural Science -- Dissertations, Academic -- UF
Genre: Horticultural Science thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Buckwheat Fagopyrum esculentum Moench is an annual broadleaf grain crop, which also has utility as a cover crop. Cover crops are plants that are planted for purposes other than for food, feed, fuel, and fiber and provide ecosystem services such as erosion control and pest management. Buckwheat is considered to be a promising cover crop for weed management in sustainable and organic cropping systems since its rapid germination and growth permit competition for resources, and because of inhibition of weed seed germination and propagule sprouting and the potential for allelopathy. However, low tolerance of drought, frost and hot weather may limit the use of buckwheat in some locations. The goal of this study was to generate information on planting dates and termination practices for buckwheat to determine whether it can be used as a cover crop to suppress weeds effectively for sustainable and organic cropping systems in north central Florida. The objective of the first experiment was to determine the optimal planting period for buckwheat in spring and fall. Buckwheat growth and canopy closure in spring were better when planted between late April and early May than at earlier and later planting dates. During the spring cover cropping period, buckwheat suppressed weeds effectively and the persistence of suppression after buckwheat termination of total and monocot weed biomass was as effective as a harrowed control and better than a weedy fallow. Buckwheat also effectively suppressed weeds during the cover cropping period in fall with the optimal planting period in October to avoid hot temperatures in early fall and frost injury later in the season. Weed suppression after buckwheat termination in fall was not significantly different from that with the harrowed control and weedy fallow. In the second experiment, termination practices for buckwheat were compared. Buckwheat grown in spring and fall 2008 was terminated successfully with all of the practices used: rolling, flail mowing alone, light tillage alone, and the combination of flail mowing and light tillage. The loss of buckwheat residue biomass was greater with termination methods of light tillage alone and the combination of flail mowing and light tillage than with rolling or flail mowing alone. Buckwheat terminated with rolling generated the most long-lasting residue coverage in spring compared with the other termination practices. Although rolling and flail mowing were not effective for suppression of established weeds, there were fewer weed seedlings with buckwheat killed by rolling and flail mowing than killed by light tillage alone and the combination of flail mowing and light tillage. Light tillage alone may be a promising method to terminate buckwheat in fall to save the fuel cost of using the combination of flail mowing and light tillage. Rolling and flail mowing may be potential termination practices for spring and specifically for no-till cropping systems with similar effects on weed control, whereas the persistence of surface mulch generated by flail mowing was shorter than with rolling.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Pei-Wen Huang.
Thesis: Thesis (M.S.)--University of Florida, 2009.
Local: Adviser: Chase, Carlene A.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0025074:00001

Permanent Link: http://ufdc.ufl.edu/UFE0025074/00001

Material Information

Title: Optimal Conditions for Buckwheat fagopyrum Esculentum Moench Production as a Cover Crop for Weed Suppression in Florida
Physical Description: 1 online resource (82 p.)
Language: english
Creator: Huang, Pei-Wen
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: buckwheat, cover, crop, suppression, weed
Horticultural Science -- Dissertations, Academic -- UF
Genre: Horticultural Science thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Buckwheat Fagopyrum esculentum Moench is an annual broadleaf grain crop, which also has utility as a cover crop. Cover crops are plants that are planted for purposes other than for food, feed, fuel, and fiber and provide ecosystem services such as erosion control and pest management. Buckwheat is considered to be a promising cover crop for weed management in sustainable and organic cropping systems since its rapid germination and growth permit competition for resources, and because of inhibition of weed seed germination and propagule sprouting and the potential for allelopathy. However, low tolerance of drought, frost and hot weather may limit the use of buckwheat in some locations. The goal of this study was to generate information on planting dates and termination practices for buckwheat to determine whether it can be used as a cover crop to suppress weeds effectively for sustainable and organic cropping systems in north central Florida. The objective of the first experiment was to determine the optimal planting period for buckwheat in spring and fall. Buckwheat growth and canopy closure in spring were better when planted between late April and early May than at earlier and later planting dates. During the spring cover cropping period, buckwheat suppressed weeds effectively and the persistence of suppression after buckwheat termination of total and monocot weed biomass was as effective as a harrowed control and better than a weedy fallow. Buckwheat also effectively suppressed weeds during the cover cropping period in fall with the optimal planting period in October to avoid hot temperatures in early fall and frost injury later in the season. Weed suppression after buckwheat termination in fall was not significantly different from that with the harrowed control and weedy fallow. In the second experiment, termination practices for buckwheat were compared. Buckwheat grown in spring and fall 2008 was terminated successfully with all of the practices used: rolling, flail mowing alone, light tillage alone, and the combination of flail mowing and light tillage. The loss of buckwheat residue biomass was greater with termination methods of light tillage alone and the combination of flail mowing and light tillage than with rolling or flail mowing alone. Buckwheat terminated with rolling generated the most long-lasting residue coverage in spring compared with the other termination practices. Although rolling and flail mowing were not effective for suppression of established weeds, there were fewer weed seedlings with buckwheat killed by rolling and flail mowing than killed by light tillage alone and the combination of flail mowing and light tillage. Light tillage alone may be a promising method to terminate buckwheat in fall to save the fuel cost of using the combination of flail mowing and light tillage. Rolling and flail mowing may be potential termination practices for spring and specifically for no-till cropping systems with similar effects on weed control, whereas the persistence of surface mulch generated by flail mowing was shorter than with rolling.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Pei-Wen Huang.
Thesis: Thesis (M.S.)--University of Florida, 2009.
Local: Adviser: Chase, Carlene A.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0025074:00001


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OPTIMAL CONDITIONS FOR BUCKWHEAT [Fagopyrum esculentum Moench] PRODUCTION AS A COVER CROP FO R WEED SUPPRESSION IN FLORIDA By PEI-WEN HUANG A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2009 1

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2009 Pei-wen Huang 2

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To my lovely parents and all the members in my family who always support me with their love 3

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ACKNOWLEDGMENTS Financial support for my project was provided by a Sustainable Agriculture Research and Education Graduate Student Grant and a Sout hern Region Integrated Pest Management Enhancement Grant. I sincerely thank my advisor, Dr. Carlene Chase, for giving me this opportunity and financial support to pursue my degree. I am grateful for her help and patience to polish my language and scientific skills during these three years. I woul d like to thank my committee members, Dr. Bielinski Santos and Dr Xin Zhao, for their insightful thoughts and suggestions on my project and my research journey. I would like to show my appreciation to Khalid Omer for t echnical assistance. It was a nice experience to work with the cr ews in Citra. I have to th ank Buck Nelson, Tim Pedersen, Jeraldine Parker, and Leonard Novinger for th eir assistance during my field experiments. I would also like to thank all the faculty, sta ff and students of the Horticultural Sciences Department for their help and guidance throughout th e past three years. In addition, I am grateful to all of my friends in the US who made my life here so colorful, and esp ecially the people who helped me in the field during the holiday seasons. Finally, I would definite ly thank my parents, my relatives in Taiwan and the US, and all my friends in Taiwan for their endless love, encouragement, and support. 4

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TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ..........7 LIST OF FIGURES.........................................................................................................................9 ABSTRACT...................................................................................................................................10 CHAPTER 1 INTRODUCTION................................................................................................................. .12 Sustainable Cropping Systems...............................................................................................12 Integrated Weed Management................................................................................................13 Cover Crops............................................................................................................................15 Mechanisms of Weed Suppression..................................................................................15 Limitations.................................................................................................................... ...16 Termination.................................................................................................................... .16 Allelopathy.............................................................................................................................17 Buckwheat..............................................................................................................................18 Morphology and Biology................................................................................................18 Cover Crop Benefits and Limitations..............................................................................19 Mechanisms of Weed Suppression..................................................................................20 Objectives and Hypotheses.....................................................................................................21 2 OPTIMAL PLANTING PERIOD FOR BUCKWHEAT AS A COVER CROP FOR WEED SUPPRESSION IN FLORIDA..................................................................................22 Introduction................................................................................................................... ..........22 Materials and Methods...........................................................................................................24 Results and Discussion......................................................................................................... ..27 Buckwheat Growth..........................................................................................................27 Shoot biomass..........................................................................................................27 Plant height...............................................................................................................27 Buckwheat Canopy Closure............................................................................................28 Leaf area index (LAI)...............................................................................................28 Photosynthetically active radiation (PAR)...............................................................29 Ground cover............................................................................................................29 Weed Suppression...........................................................................................................30 Before termination....................................................................................................30 After termination......................................................................................................31 5

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3 MECHANICAL TERMINATION OF BUCKWHEAT USED FOR WEED SUPPRESSION IN FLORIDA...............................................................................................48 Introduction................................................................................................................... ..........48 Materials and Methods...........................................................................................................50 Results and Discussion......................................................................................................... ..52 Buckwheat Suppression...................................................................................................52 Residue Decomposition...................................................................................................53 Residue biomass.......................................................................................................53 Ground cover............................................................................................................53 Weed Suppression...........................................................................................................54 4 SUMMARY AND CONCLUSIONS.....................................................................................72 LIST OF REFERENCES...............................................................................................................75 BIOGRAPHICAL SKETCH.........................................................................................................82 6

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LIST OF TABLES Table page 2-1 Influence of planting date of buckwh eat and fallow treatment on weed biomass before buckwheat termination in spring 2007...................................................................40 2-2 Influence of planting date of buckwh eat and fallow treatment on weed biomass before buckwheat termination in spring 2008...................................................................41 2-3 Influence of planting date of buckwh eat and fallow treatment on weed biomass before buckwheat termination in fall 2007........................................................................42 2-4 Influence of planting date of buckwh eat and fallow treatment on weed biomass before buckwheat termination in fall 2008........................................................................43 2-5 Influence of planting date of buckwheat and fallow treatment on weed biomass after buckwheat termination in spring 2007...............................................................................44 2-6 Influence of planting date of buckwheat and fallow treatment on weed biomass after buckwheat termination in spring 2008...............................................................................45 2-7 Influence of planting date of buckwheat and fallow treatment on weed biomass after buckwheat termination in fall 2007...................................................................................46 2-8 Influence of planting date of buckwheat and fallow treatment on weed biomass after buckwheat termination in fall 2008...................................................................................47 3-1 Buckwheat density with different termination practices during residue decomposition after termination in fall 2008.............................................................................................64 3-2 Residue biomass by category with differe nt termination practices during residue decomposition after termination in spring 2008................................................................65 3-3 Residue ground cover by category with di fferent termination practices during residue decomposition after termination in spring 2008................................................................66 3-4 Residue ground cover by category with di fferent termination practices during residue decomposition after termination in fall 2008.....................................................................67 3-5 Weed biomass by category with different termination practices harvested 5 weeks after termination in spring 2008.........................................................................................68 3-6 Number of weeds by categor y averaged over termination practices at 3 and 5 weeks after termination in spring 2008.........................................................................................69 3-7 Weed biomass by category with different termination practices harvested 5 weeks after termination in fall 2008.............................................................................................70 7

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3-8 Number of weeds by cate gory averaged over termination practices at 0, 3, and 5 weeks after termination in fall 2008..................................................................................71 8

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LIST OF FIGURES Figure page 2-1 Air and soil temperatures during buckwheat cover cropping period in spring..................34 2-2 Air and soil temperatures during buckwheat cover cropping period in fall......................35 2-3 Buckwheat biomass in spring and fall in 2007 and 2008..................................................36 2-4 Buckwheat height in sp ring and fall in 2007 and 2008.....................................................37 2-5 Buckwheat canopy closure in sp ring averaged over years................................................38 2-6 Buckwheat canopy closure in fall averaged over years.....................................................39 3-1 Buckwheat density averaged over term ination practice during decomposition period in spring 2008....................................................................................................................57 3-2 Buckwheat density averaged over decomposition period with different termination methods in spring 2008......................................................................................................57 3-3 Buckwheat residue biomass remaining on the soil surface with different termination methods during the 5 weeks after termination in spring 2008...........................................58 3-4 Buckwheat residue biomass averaged ove r termination practices with decomposition period in fall 2008............................................................................................................ ..59 3-5 Buckwheat residue biomass averaged over decomposition period with different termination met hods in fall 2008.......................................................................................59 3-6 Buckwheat residue ground cover remaining on the soil surface with different termination methods during the 5 week s after termination in spring 2008.......................60 3-7 Buckwheat residue ground cover remain ing on the soil surface with termination methods during the 5 weeks after termination in fall 2008...............................................61 3-8 Number of weeds by category in res ponse to termination method averaged over decomposition period, spring 2008....................................................................................62 3-9 Number of weeds by category in res ponse to termination method averaged over decomposition period, fall 2008........................................................................................63 9

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Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science OPTIMAL CONDITIONS FOR BUCKWHEAT [Fagopyrum esculentum Moench] PRODUCTION AS A COVER CROP FO R WEED SUPPRESSION IN FLORIDA By Pei-wen Huang August 2009 Chair: Carlene A. Chase Major: Horticultural Science Buckwheat [ Fagopyrum esculentum Moench] is an annual broadleaf grain crop, which also has utility as a cover crop. Cover crops are plan ts that are planted for purposes other than for food, feed, fuel, and fiber and provide ecosystem services such as eros ion control and pest management. Buckwheat is considered to be a promising cover crop for weed management in sustainable and organic croppi ng systems since its rapid ge rmination and growth permit competition for resources, and because of inhibi tion of weed seed germination and propagule sprouting and the potential for allelopathy. However, low tolera nce of drought, frost and hot weather may limit the use of buckwheat in some lo cations. The goal of this study was to generate information on planting dates and termination pr actices for buckwheat to determine whether it can be used as a cover crop to suppress weeds effectively for sustaina ble and organic cropping systems in north central Florida. The objective of the first experiment was to determine the optimal planting period for buckwheat in spring and fall. Buckwheat growth and canopy closure in spring were better when planted between late April and early May than at earlier and later pl anting dates. During the spring cover cropping period, buckwheat suppressed weeds effectively and the persistence of suppression after buckwheat termination of total and monocot weed biomass was as effective as 10

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a harrowed control and better than a weedy fallow. Buckwheat also effectively suppressed weeds during the cover cropping period in fall with the optimal plan ting period in October to avoid hot temperatures in early fall and frost injury la ter in the season. Weed s uppression after buckwheat termination in fall was not significantly different from that with the ha rrowed control and weedy fallow. In the second experiment, termination practi ces for buckwheat were compared. Buckwheat grown in spring and fall 2008 was terminated successf ully with all of the practices used: rolling, flail mowing alone, light tillage alone, and the combination of flail mowing and light tillage. The loss of buckwheat residue biomass was greater wi th termination methods of light tillage alone and the combination of flail mowing and light ti llage than with rolling or flail mowing alone. Buckwheat terminated with rolling generated th e most long-lasting residue coverage in spring compared with the other term ination practices. Although rolli ng and flail mowing were not effective for suppression of established weeds, there were fewer weed seedlings with buckwheat killed by rolling and flail mowing than killed by light tillage alone and the combination of flail mowing and light tillage. Light tillage alone may be a promis ing method to terminate buckwheat in fall to save the fuel cost of using the comb ination of flail mowing a nd light tillage. Rolling and flail mowing may be potential termination prac tices for spring and specifically for no-till cropping systems with similar effects on weed control, whereas the persistence of surface mulch generated by flail mowing was shorter than with rolling. 11

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CHAPTER 1 INTRODUCTION Weed management in horticultural production can be a major challenge in conventional, as well as in transitional and organi c production systems. In conventional systems, fewer herbicides are available than for agronomic crops because of less acreage devoted to horticultural crops and the liability of herbicide damage to crops, which discourage research and development by agrochemical companies. Even when herbicides are available, they may not be used because of the possibility of injury to rota tional crops. In transitional and organic cropping systems, the use of synthetic herbicides is not permitted so alternative measures are needed. Effective nonchemical weed management strategies can assi st with weed management problems for which there are no registered herbic ides, reduce the use of herbic ides in conventional production systems, provide increased options for sustainable systems, and result in more profitable, more productive, and less labor-intensi ve organic production systems. The optimization of cultural weed management practices such as cover croppi ng can reduce the need for herbicides, limit the likelihood of herbicide-resistant weeds in c onventional systems, reduce the need for handweeding, and by decreasing the frequency of cultiva tions in organic production systems, limit the adverse effects of weed management on the environment. Sustainable Cropping Systems There is increased public concern about e nvironmental contamination, soil erosion and degradation, pesticide residues in foods, developm ent of herbicide-resistan t weeds, occurrence of new weed problems due to single control methods, and other social, economic and health-related impacts caused by conventional agriculture systems with high input of chemicals. As a result, there is increasing interest in the developmen t of agricultural ecosystems that depend on the 12

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management of ecological interact ions rather than synthetic chem icals to maintain economic and biological productivity (Liebman and Ohno, 1998). The definition of sustainable ag riculture is an integrated system of plant and animal production practices having a site-s pecific application that will ove r the long term satisfy human food and fiber needs, enhance environmental quality and the natu ral resource base upon which the agriculture economy depends, make the most e fficient use of nonrenewable resources and onfarm resources and integrate, wh ere appropriate, natura l biological cycles and controls, sustain the economic viability of farm operations, and enha nce the quality of life for farmers and society as a whole (USDA, 2008). In sustainable cropping systems, th e goal is to conserve, establish and maintain the soil at a high level of fertility with its sustainability involving social, economic, and ecological relationships at local, national, and global levels (Abdul-B aki and Teasdale, 1997). Some of the major practices incl ude crop rotation, reduced tillage use of animal manures, and cover crops (Lu et al., 2000). As for weed management in sustainable cropping systems, it is a challenge for farmers to control weeds effectively without herbicides, especially organic farmers who consider weeds to be their most serious problem (Baker and Smit h, 1987). Some approaches used to control weeds in sustainable agrosystems include cover crops, timely cultivation, rotation of crops, mulches, solarization, and tillage (Walz, 1997). Since weeds are a major f actor affecting agricultural production and employment, practi ces that render weed manageme nt more efficient, economic, and environmentally safe can contribute to maintaining a viable, sustainable agriculture system (Creamer et al., 1996). Integrated Weed Management In conventional vegetable cropping systems, a combination of tillage and herbicide is generally used to control weeds. However, ti llage may cause loss of soil structure and soil 13

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erosion (Melsted, 1954), and herbicides may cont aminate water resources (Thurman et al., 1991). For sustainable agroecosystems, the approach to controlling weeds is through the strategy of integrated weed management, which has b een defined as the application of multiple approaches, such as prevention, cu ltural, physical, biological, and ch emical strategies to enhance the competitiveness of crops over weeds (Aldrich and Kremer, 1997). Integrated weed management (IWM) not only focuses on killing or suppressing weeds, but also on the interaction with agroecosystems (Liebman et al., 2001). With IWM, multitactic and ecologically based strategies are used to reduce weed population unde r acceptable threshold levels and improve crop competitiveness (Mortens en et al., 2000). Management measures that provide greater weed species ric hness or more uniform distribu tion of species might result in more persistent weed control and may be more be neficial to the ecosystem than strategies in which only a few dominant weeds survive (Clements et al., 1994). IWM has th e ability to restrict weed population and regeneration with complement ary practices in a congregate management system (Hillger et al., 2006); and moreover, it al so helps growers to reduce herbicide costs, herbicide-resistant weeds and mode rate the negative social, healt h, and environmental impacts of agriculture (Swanton and Murphy, 1996). In organic cropping systems, synthetic chem icals are generally prohibited with the potential likelihood of environmental and economi c benefits. The National Organic Rule (USDA, 2002) recommends cultural practices as the first level of pest management and indicates that permitted weed management practices also include (1) mulching with fully biodegradable materials, (2) mowing, (3) lives tock grazing, (4) hand weeding a nd mechanical cultivation, (5) flame, heat, or electrical means, or (6) plas tic or other synthetic mulches which are removed from the field at the end of the growing or ha rvest season as the secondary level practices, and 14

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nonsynthetic herbicides as strategies in the th ird level. However, the only such herbicide registered for use in organic cropping systems is ammonium nonanolate, and The National List of Allowed and Prohibited Substances includes soap-based herbicides for use only for non-crop areas of organic farms and or namental crops (USDA, 2007). Cover Crops Cover crops are species that are planted sp ecifically to increase soil organic matter, maintain or increase nutrient availability, improve soil physical properties, prevent soil erosion, and in some cases, reduce soil-borne pathogens (Sarrantonio, 1994), but not planted for profit or economic value. Leguminous cover crops can also be used as green manures to provide nitrogen into the soil at the end of the gr owing season (Peoples et al., 1995). Mechanisms of Weed Suppression Using cover crops to suppress weeds became pop ular during the search for sustainable and environmentally-friendly farming methods (Shilling et al., 1985). The mechanisms of such an inhibition effect include crop and weed competition for resources of light, water and nutrients (Barnes and Putnam, 1983), the po tential to occupy the space before weeds (Barberi, 2002; Gallandt, 2004; Hatcher and Melander, 2003; Li ebman and Davis, 2000) during the growing period of cover crops, and alterati on of the soil physical environm ent, for instance, changes in light availability, soil temper ature, and soil moisture by liv ing cover crops (Teasdale and Daughtry, 1993) and cover crop residues (Teasdale and Mohler, 1993). Additionally, cover crops may be planted due to their a llelopathic ability to suppress weeds (Putnam and DeFrank, 1979). Weed seed mortality may also be greater in th e presence of cover crop residues because they may favor seed predators (Reader, 1991). Moreover when cover crops were used in combination with herbicides in various reduced tillage system s, better weed control and more consistent crop yield were obtained (Teasdale, 1996). Theref ore, weed populations and crop yield can be 15

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affected by using cover crops and management systems in both the long and short term (Ngouajio et al., 2003). Limitations However, there are also adverse effects of using cover crops. Th ey require additional management and the purchase of cover crop seed. Cover crops may cause soil moisture depletion and residues may interfer e with crop establishment, decrease soil temperature, and result in less predictable crop fertiliz er requirements, which may cause problems for growers (Teasdale, 1996). One of the concerns of utilizing cover crops in cooler climates is that soil will be cooled to temperatures that may delay crop production and becomes a disadvantage when the soil must be warmed quickly after a cold winter (Masiunas et al., 1995). Another nega tive effect caused by growing cover crops is that they may exploit soil moisture in areas without enough water recharge for the soil before subsequent crop planting and reduce crop yield; however, improvement of water conservation may also be accomplished by retaining crop residues on the soil surface (Unger and Vigil, 1998). Termination In the southeast, nonselective herbicides are commonly used for killing cover crops followed by a selective postemergence herbicide as needed, especially for grass cover crops (Worsham, 1991). However, with concerns abou t human and environment health, management practices with reduced chemical inputs are desirable especially for cropping systems in which limited herbicide products are permitted. Terminating cover crops mechanically is an alternative to the conventional strategies. Cover crop residues can be incorporated into the soil or terminated and retained on the surface of the soil as mulches to facilitate weed suppressi on in succeeding crops. It is recommended that cover crop termination should occur at least two weeks before cash crop 16

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planting for the timely recharge of soil mois ture (Hargrove and Frye, 1987). The success of termination strategies depend partially on the growth stage and species of the cover crops (Creamer and Dabney, 2002). Termination practices can also affect weed emergence and growth (Creamer et al., 1995). Allelopathy The definition of allelopathy is: a beneficial or detrimental effect from a donor plant to a receiving one via a chemical pathway (Rice, 1984 ). Putnam (1986) broadened this definition to include chemicals produced by actinomycetes, alg ae, fungi, or other microorganisms that may have association with the plants in the rhizopher e. Allelochemicals are usually produced in plants as secondary products and some primary metabo lites (Putnam, 1988) and major intermediates are also involved (Rice, 1984). Phytotoxic compounds occur in virtually all plants in organs such as leaves, stems, flowers, roots, seeds, and buds. Allel opathy is said to have occurred when these chemicals are exuded or releas ed into the environment at adequate rates and conducive conditions to interfere with the germination and growth of surrounding plants (Weston, 1996). Many cover crops release signi ficant levels of allelochemicals, which reduce weed emergence (Putnam et al., 1983). Such allelopathic plants can contribute to weed management when included as rotational crops and by using the crop residues (Weston, 1996). The allelopathic effect of cover cr ops may inhibit emergence or earl y growth of small-seeded crops such as lettuce (Lactuca sativa L.) or tomato ( Solanum lycopersicum L.) (Putnam et al., 1983). Caamal-Maldonado et al. (2001) planted legume cover crops with transplanted tomato and found that weed numbers, weed biomass, and tomato biomass were stimulated or suppressed depending on cover crop species. Therefore, it is needed to prevent negative effect such as interference caused by living cover crops through competitio n and allelopathy for optimizing use of allelopathic cover crops (Caamal-Maldonado et al., 2001). 17

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Using allelopathic plants to manage pests, such as weeds, insects, nematodes, and pathogens can have a positive effect on cr op yield while maintaining more pristine environmental conditions (Tsuzuki and Dong, 2003) Since there is a tr end towards increased herbicide resistance in weeds, th e use of allelopathic cover crops for weed management in crop production may also increase (Rizvi and Rizvi, 1992). Furthermore, growth inhibitors identified from allelopathic plants could also be a useful source for the future deve lopment of bioherbicides and pesticides (Putnam, 1988; Xuan et al., 2005). Buckwheat Buckwheat [ Fagopyrum esculentum Moench] is an annual broadleaf grain and belongs to the Polygonaceae family. Its origin is in temperate Ea st Asia and because of its wide adaptability to different environmental conditions, it has spread worldwide. Currently, it is grown in Australia, Argentina, North America, Europe, East As ia, and India as an important food and noodle ingredient and also as a summer cover crop to improve organic ma tter in the soil and reduce soil erosion (Iqbal et al., 2003). Morphology and Biology Buckwheat can grow to a height of about 100 cm and has a thick and succulent stem with some axillary branches. It has a single stout taproo t with several branched lateral roots. Most of the roots are in the upper 25 cm of soil (Valenzuela and Smith, 2002). Buckwheat prospers in a cool, moist environment but is not tolerant of fros t, and thus, it cannot be us ed as a winter annual crop. Buckwheat is not tolerant of drought either and wilts under a hot, dry environment, but will recover rapidly in the humid evening conditions (Sarrantonio, 1994). During the reproductive period, it produces clusters of small and whitish or pinkish flowers and small triangular fruits. Buckwheat can be planted in various soil types, even infertile, poorly tilled lands and soils with low pH. However, it does not tolerate hea vy, wet soils or soils with a large amount of 18

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limestone (Sarrantonio, 1994). Moreover, its grow th may be stunted with reduced grain yield when planted in light, well-drained soils w ith high nitrogen cont ent (Sarrantonio, 1994). Cover Crop Benefits and Limitations As a cover crop, buckwheat has a strong ability to suppress weeds due to its rapid growth, which results in rapid establishment of the canopy, competition for light, and interference with the growth of weeds (Oplinger et al., 1989). It can be used to suppress or eradicate Canada thistle ( Cirsium arvens e (L.) Scop.) (Eskelsen and Crabtree, 1995), quackgrass ( Agropyron repens (L.) Beauv.) (Cook, 1989), sowthistle ( Sonchus spp. L.), field bindweed ( Convolvulus arvensis L.), leafy spurge ( Euphorbia esula L.), Russian knapweed ( Centaurea repens L.), and perennial pepperweed ( Lepidium latifolium L.) (Oplinger et al., 1989). Buckwheat also possesses other desirable char acteristics such as: (1) attractiveness to beneficial insects like natural enemies that supp ress pests such as white flies and aphids (Frank and Liburd, 2005; Hooks et al., 1998) and pollinator s, (2) ease of kill with rapid decomposition, and (3) the potential to extr act soil phosphorus (Marks and Townsend, 1973). Because of its rapid growth and canopy closure, buckwheat coul d be used to provide soil cover for a short, warm period to protect and condition the soil pr ior to transplanting or sowing late-season crops (Sarrantonio, 1994). Its rapid decomposition rate a llows for seedbed preparation without residue interference and offers a shortterm improvement in soil tilt h, water-holding capacity, and nutrient availability for a subs equent crop (Sarrantonio, 1994). In addition to the benefits, there are also lim itations when using buckwheat as a cover crop or living mulch. Buckwheat living mulch may cause interspecies interference due to competition for light and nutrient resources and alle lepathy (Eskelsen and Crabtree, 1995). The allelochemicals in buckwheat may inhibit the germ ination and growth of small-seed crops such as lettuce, cress, carrot, and onion (Iqbal et al., 2002; Iqbal et al., 2003; Kato-Noguchi et al., 19

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2007). The timing for buckwheat termination is also important as the cover crop must be killed at 35 to 40 days after seeding to reduce the incidenc e of volunteer plants from seeds and buckwheat regrowth from cut stems (Bjrkman, 2006). Howeve r, even after buckwheat termination, it may has the potential to cause nitrogen deficiency in subsequent crops be cause of immobilization during residue decompositi on (Kumar et al., 2008). Mechanisms of Weed Suppression Because of its rapid germination and growth, buckwheat can be used as a weed suppressive cover crop to compete against weeds for resource s such as light, soil moisture, and nutrients. Buckwheat can also establish its canopy faster th an weeds to shade the soil surface and modify the soil environmental conditions to interfere wi th weed germination and growth (Eskelsen and Crabtree, 1995). Additionally, Kumar et al. (2008) suggested that nitrogen deficiency caused by uptake during buckwheat growt h, nitrogen immobilization duri ng residue decomposition, and fungal pathogens of weeds that grow on residue s are possible mechanisms of weed suppression, and the effectiveness will depend on species. Tang (1986) reported that buckwheat may ha ve the potential to release phytotoxic chemicals that inhibit the germination of wheat Tsuzuki and Yamamoto (1987) and Tsuzuki et al. (1987) found that there are th ree phenolic acids: ferulic aci d, caffeic acid, and chlorogenic acid and four fatty acids: palmitic acid, stearic acid, arachidic acid, and behenic acid in buckwheat. Iqbal et al. (2002) reported three other chemicals: fagomine, 4-piperidone, and 2piperidinemethanol, as allelochemicals in buckwh eat. Iqbal et al. (2003) isolated and identified other allelopathic constituents such as gallic ac id and (+)-catechin in th e ethyl acetate phase. In contrast, buckwheat shoots are repoted to contain the following allelochemicals: rutin, chlorogenic acid, and (-)-epicat echin (Golisz et al., 2007) and the weed-suppressive compounds in roots include: 4-hydroxyacetophenone, vanillic aci d, and gallic acid (Kalinova et al., 2007). 20

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Some flavonoids found in buckwheat such as cate chin, quercetin, and isoq uercitrin also have inhibitive effects on weeds (K alinova and Vrchotova, 2009). The isolated fagomine, 4-pipe ridone, and 2-piperidinemethanol reduced radicle elongation in lettuce seedlings by 50% (Iqbal et al. 2002) and gallic acid ha d stronger effect on inhibiting root and hypocotyl elongation th an (+)-catechin (Iqbal et al., 2003) The results of Tsuzuki and Dong (2003) indicated suppre ssion of barnyardgrass ( Echinochloa crusgalli L.) and monchoria ( Monochoria vaginallis P.) in rice fields of Japan. Powell amaranth ( Amaranthus powellii ) emergence and growth were reduced by buc kwheat residues (Kumar, 2009). Buckwheat seedlings were also suppressive to cress, lettu ce, timothy, and ryegrass seedlings due to release of allelochemicals that interfered with ne ighboring plants (KatoNoguchi et al., 2007). Hypotheses and Objectives Buckwheat is a promising weed-suppressive c over crop because of its rapid establishment and growth and the potential for allelopathy. Ho wever, its utility may vary with location, management practices, and environmental factors. Th erefore, it is hypothesized that there will be optimal planting periods in sp ring and fall during which buckwhe at would effectively suppress weeds in north central Florida and persistence of weed suppression with buckwheat would differ depending on the practices used to terminate the crop. The overall objective was to determine th e effectiveness of buckwheat for weed suppression when grown in north cen tral Florida. The specific objectiv es of this study were (1) to determine the optimal planting period for buckwh eat as a cover crop in Florida for effective weed suppression, and (2) to dete rmine the optimal termination practices for buckwheat when used as a cover crop to suppress weeds in Florida. 21

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CHAPTER 2 OPTIMAL PLANTING PERIOD FOR BUCKWH EAT AS A COVER CROP FOR WEED SUPPRESSION IN FLORIDA Introduction Weed management can be challenging for conve ntional growers of sp ecialty crops due to limited herbicide options. The conventional weed management approach, which is strongly dependent on the application of herbicides, was confronted with public concerns about environmental, economic, and health issues. Herb icide-resistant weeds ha ve been increasing in cropping systems in which herbicides are the pr imary means of weed management (Heap, 2007). In addition, a survey of organic growers indica tes that weeds are the major production problem in organic cropping systems (Walz, 1997). The use of cover crops is one way to maintain productivity and reduce the cost of and relian ce on synthetic chemicals in production systems where few products are re gistered or allowed. Cover crops could be used as an alternative to herbicides and as a part of an integrated weed management program to suppress weeds in conventional production systems and decrease the need for hand-pulling and cultivation in orga nic systems. Since tillage and herbicides are generally used in conventional cropping systems for weed management, it is possible that soil erosion and loss of soil physical properties can be caused by tillage, and that contamination of water bodies can result from inappr opriate herbicide use culmina ting in environmental problems (Melsted, 1954; Thurman, 1991). Cover crops can al so be an option to moderate these problems for more sustainable cropping systems (Sa rrantonio, 1994; Creamer and Baldwin, 2000). As a result, the development of agricultural ecosyst ems that depend on the management of ecological interactions rather than synthetic chemicals to maintain economic a nd biological productivity may lead to less adverse impacts on the environment (Liebman et al., 2001). 22

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Both living cover crops and cover crop residue s can effectively interf ere with weed seed germination and emergence (Teasdale, 1993). Th e mechanisms by which cover crops suppress weeds include resource competition during cover cr op growth, allelopathy, and modification of soil microclimate, such as light penetration, soil temperature, and soil moisture (Creamer et al., 1997). After termination, cover crop residue left on the soil surface can also be used as a physical barrier to weed emergence and to modify soil environmental conditions to eliminate light and temperature cues for weed seed germinati on (Teasdale, 1993; Hutchi nson and McGiffen, 2000; Ngouajio et al., 2003). Cover crop re sidues can also provide habitat for weed seed predators to lessen the weed seed bank (Read er, 1991; Pullaro et al., 2006). Buckwheat [ Fagopyrum esculentum Moench] is an annual broadleaf grain crop, which belongs to the Polygonaceae family and has been utilized as a summer cover crop for soil fertility improvement, soil erosion prevention, and weed suppression (S arrantonio, 1994). Its optimal growth may occur in cool and moist e nvironments, but it is in tolerant to frost and drought (Sarrantonio, 1994). It is generally used to provide ecosyst em services such as attracting beneficial insects (Frank a nd Liburd, 2005; Hooks et al., 1998 ) and pollinators, acquiring phosphorus (Marks and Townsend, 1973), prot ecting and conditioning the soil before transplanting or sowing late-season crops (Sa rrantonio, 1994), and suppr essing weeds (Oplinger et al., 1989). After termination, buckwheat re sidues can improve soil tilth, water-holding capacity, and nutrient availabil ity (Sarrantonio, 1994). Howeve r, Kumar et al. (2008; 2009) reported the contradictory result that buckwheat may reduce nitr ogen availability due to the interactions between buckwheat re sidue and soil during degradation. The mechanisms by which buckwheat inhibits weeds are thought to include its rapid germination and growth leading to competition with weeds for resources such as light, soil 23

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moisture, and nutrients, and alte ration of the soil physical envir onment to interfere with weed germination and growth by shading the soil surf ace (Eskelsen and Crabtree, 1995). It was also proposed that buckwheat can re lease phytotoxic chemicals that contribute to allelopathic suppression of weeds. The inhibition of wheat germination has been demonstrated (Tang, 1986). Additionally, several puta tive allelochemicals have been isol ated and identified from buckwheat: three phenolic acids, four fatty acids, and three other chemi cals (Tsuzuki and Yamamoto, 1987; Tsuzuki et al., 1987), fago mine, 4-piperidone, 2-piperidinemeth anol (Iqbal et al., 2002), gallic acid, (+)-catechin (Iqbal et al., 2003) rutin, chlorogenic acid, (-)-epicatechin (Golisz et al., 2007), 4-hydroxyacetophenone, vanillic acid, gallic acid (Kalinova et al ., 2007), catechin, quercetin, and isoqueicitrin (Kalinova and Vrchotova, 2009). Although it is known that buckwheat can be us ed as a cover crop for weed management, the utility of cover crops and thei r effectiveness in suppressing w eeds can vary greatly with local soil and climatic conditions. Since buckwheat is typically grown in summer in temperate areas, it was hypothesized that the optimal planting period for buckwheat w ould occur in spring or fall in Florida. Therefore, the objective of this study was to determine the optimal planting period for buckwheat as a cover crop in north central Flor ida for effective weed suppression in spring (from the end of February to late May) and fall (from late September to early December). Materials and Methods Experiments were conducted at the Plant Sc ience Research and E ducation Unit (PSREU) in Citra, Florida on a Hague series sand (loa my, siliceous, semiactive, hyperthermic Arenic Hapludalf) in spring and fall of 2007 and repeated in 2008. A buckw heat cover crop was compared with a weedy fallow and a harrowed cont rol at six planting dates. Plots were arranged in a split-plot design with main plot treatments arranged systema tic complete block design with three replications. Planting dates were assigned to the main plots (from Mar. 13 to May 22 in 24

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spring 2007, from Feb. 28 to May 8 in spring 2008, from Sep. 20 to Nov. 29 in fall 2007, and from Sep. 24 to Dec. 3 in fall 2008) and were separated by 2-week intervals. The subplot treatments were randomly allocated and includ ed buckwheat, the harrowed control, and the weedy fallow). Main plot size was 173.3 m (570 ft.) long and 27.4 m (90 ft.) wide, and each subplot size was 15.2 m (50 ft.) long and 1.8 m (6 ft.) wide with 3.0 m (10 ft.)-wide vertical alleys and 4.6 m (15 ft.)-wide horizontal alleys to separate each subplot. Soil temperatures were recorded with dataloggers1 at a depth of 10 cm (3.9 in.) in buckwheat plots during the cover cropping period, and air temperatures were obtained from the Florida Automated Weather Network website2 (Figures 2.1 and 2.2). At each planting date, fertilizer3 was applied to the whole strip before seeding the buckwheat. Buckwheat cv. Mancan (Seedland Inc., Wellborn, Florida) was drilled 2.54 cm (1 in.) deep in 17.78 cm (7 in.) inter-row spacing at a ra te of 56 kg/ha (50 lb/acre) with a John Deere 10-foot grain drill. The harrowe d control plots were harrowed with a KMC 10-foot spring tooth harrow at 2-week intervals to disturb the soil to control weeds. A linear overhead irrigation system was used only before seeding to ensure there was enough soil moisture for buckwheat seed germination and when adult buckwheat plants wilted by 2 pm. Buckwheat height, photosynthetically activ e radiation (PAR, with AccuPAR LP-80 Ceptometer4), and non-destructive leaf area index (LAI, with AccuPA R LP-80 Ceptometer) were determined weekly beginning at 2 weeks afte r planting (WAP) in sp ring 2007. However, this 1 WatchDog 100 Series Water Resistant Button Logger3619WD 2K (Spectrum Technologies, Inc., Plainfield, IL) 2 Florida Automated Weather Network website: http://fawn.ifas.ufl.edu/ 3 10-10-10 Granular Fertili zer, N-P-K 10-10-10, 6.8 kg (15 lb) N per acre (Southern States Cooperative Inc., Anthony, FL ) 4Decagon Devices, Inc., Pullman, WA 25

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was found to be too early for the measurement a nd it was changed to 3 WAP for the other three seasons. Both PAR above the canopy and under the canopy were measured, and the PAR percentage was calculated by dividing the under-canopy PAR by the above-canopy PAR for data analysis then multiplying by 100. The ground cover (GC) percentage of buckwheat was measured using a diagonal transect of 100 point s (Colbach et al., 2000) with the beaded-string method every week until 5 WAP. Moreover, weed densities were counted at the same time using a 0.25 m2 quadrat at four randomly-chosen sampli ng spots in each plot. In 2008, buckwheat emergence density was also evaluated in four randomly placed 0.25 m2 quadrats per plot. Above ground buckwheat biomass and weed biomass were harvested at 5 WAP from each plot within a 0.25 m2 quadrat at four randomly-chosen sampling spots, and then dried at 70 C for 7 days and weighed. Weed biomass samples were separated into monocot and dicot categories in spring 2007, and into sedge, grass, and broadleaf categor ies in the other three seasons. Buckwheat was terminated by mowing (John Deere 10-foot rotary mower) at 39 days after planting (DAP) in spring and fall 2007, 35 DAP in spring 2008, and 40 DAP in fall 2008. Non-buckwheat treatments were disked using an Athens 10-f oot disc harrow to si mulate standard field preparation for a subsequent transplanted ve getable crop. Weed emergence was monitored in four randomly placed 0.25 m2 quadrats per plot at 2, 4, and 6 weeks after buc kwheat termination. At the final emergence count, weeds from all quadr ats were cut at the soil surface, separated into monocot and dicot categories in spring 2007, and in to sedge, grass, and broadleaf categories in the later three seasons, and dried and weighed. Analysis of variance was perf ormed with SAS software, Version 9.0 (SAS, 2002) using the MIXED procedure and least square means were co mpared using the PDIFF option. If the effect 26

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of planting date was significant, PROC REG was used with the raw data to predict the optimal period for buckwheat planting. Results and Discussion Buckwheat Growth Shoot biomass Since there was an interaction between year a nd planting date for spring and fall trials, the results were reported by year. In spring 2007, bi omass was lower with March and early April planting dates, and higher when planted at da tes between late April and early May with the maximum (247.2 g/m2) occurring at the planting date of Ap r. 24 (Figure 2.3a). However, there was no significant effect of planting date on buc kwheat biomass in spring 2008 (Figure 2.3a). A late frost (Figure 2.1b) occurred during the growth of buckwheat planted on Mar. 27 and reduced buckwheat biomass severely. Buckwheat biomass in 2008 was lower because flooding in the lowest lying of the blocks resulted in seed rot. In fall 2007, biomass was lower when planted on Sep. 20 and Oct. 4 as a result of high temperatures and higher with planting dates be tween Oct. 18 and Nov. 15 with mild weather. Biomass then declined with the Nov. 29 planti ng date due to low temp eratures (Figure 2.2a, 2.3b). In 2008, maximum biomass (70.4 g/m2) occurred with the Sep. 24 planting date (Figure 2.3b). Biomass from the Oct. 8 planting date was lower because of low germination rate due to loss of seed viability. Although new seeds were used and germination rate with the planting date of Oct. 22 was better, buckwheat biomass was lo w because of low temperature and frost (Figure 2.2b). Plant height There was an interaction between year and planti ng date for spring and fall trials so that the results were reported by year. In spring 2007, buckwh eat plants were shortest when planted in 27

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March. Plant height increased with planting da tes between Apr. 10 and May 8 with maximum plant height occurring in early May and then decreased. In spring 2008, plant height increased with buckwheat planted at late r planting dates with the maximum (50.5 cm) occurring with the May 8 planting date (Figure 2.4a). Shorter buckwheat he ight with the Mar. 27 planting date was probably due to the late frost (Figure 2.1b) and the flooding of one of the blocks and may also account for the low R2 value of the 2008 regres sion model (Figure 2.4a). Although there was no difference among planting dates in the experiment of fall 2007, buckwheat height in fall 2008 ha d a significant response to plantin g date. It was highest (39.8 cm) at the planting date of Sep. 24 and then declin ed and increased along with temperature drop and rise (Figure 2.4b). Frost occurred between late October and mid November in 2008 and inhibited buckwheat growth severely. Buckwheat Canopy Closure The interaction between year and planting date for spring trials was not significant, so data were pooled. Although with the MIXED procedure th e effect of planting date on LAI, PAR, and GC in spring was significant, regression models were not significant. Therefore, differences between means were obtained by mean comparison (Figure 2.5). Leaf area index (LAI) In spring, averaged over years LAI was highest (1.78) with the planting date of Apr. 24 and lowest with planting dates of Mar. 27, Apr. 10, and May 8. LAI with the Mar. 13 planting date was intermediate (Figure 2.5a). In fall 2007, LAI was highest with plan ting dates between Oct. 18 and Nov. 15. Temperatures in late September and early Oc tober may have been too high, causing adverse effects on canopy establishment. Maximum LAI ( 0.71) occurred with buckwheat planted on Sep. 24 in 2008 (Figure 2.6a). Frost and low temperatures (Figure 2.2b) beginning in late October of 28

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2008 did not favor buckwheat canopy closure, and therefore, LAI was quite low when buckwheat was planted during this period (Figure 2.6a). Photosynthetically ac tive radiation (PAR) Averaged over years for spring experiments, PAR penetrating the canopy was lower when buckwheat was planted on Mar. 13 and late Apr. 10, yet it was highest with the Mar. 27 planting date and decreased with the Apr. 10 planting date and increased again with the planting date of May 8 (Figure 2.5b). With fall trials, PAR was higher with buckwh eat planted on Sep. 20 and Oct. 4, and it was lower when planted later than Oct. 4 in 2007. In 2008, although the effect of planting date was significant, there was no signifi cant regression model for PAR (F igure 2.6b). Poor buckwheat germination wit the Oct. 8 planting date due to loss of seed viability, and frost and low temperature in November gave unexpected results. Ground cover (GC) For spring, when averaged over years, GC was highest with planting dates of Mar. 13 and Apr. 24, lowest with buckwheat planted on Mar. 27, and intermediate at the planting dates of Apr. 10 and May 8 (Figure 2.5c). As for fall trials, GC was lower when buckwh eat planted prior to Oct. 18, and it was higher with planting dates between Oct. 18 and Nov. 15 in 2007. However, in 2008 there was no significant regression model for LAI, although the effect of planting date was significant with the analysis of variance (Fig ure 2.6c). Low buckwheat germination ra te with the Oct. 8 planting date due to loss of seed viability and frost and lo w temperature in November caused unexpected low GC values. 29

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Weed Suppression Before termination In spring 2007, monocot weeds were the pre dominant weed category. Weed biomass was lower with earlier planting dates than later pl anting dates, and monocots were the predominant weed species. Increases in total weed biomass and monocot weed biomass occurred with April and May planting dates in the weedy control wh ereas with buckwheat only the May 22 planting date resulted in increases. However, for th e May 22 planting date, to tal and monocot weed biomass in the weedy control were 3.7 and 3.3fold more, respectively, than with buckwheat. Dicot weed biomass was lowest with Mar. 13, Ma r. 27, and Apr. 24 planting dates. Dicot weed biomass averaged over planting date was 3.1-fold less with buckwheat than with the weedy control (Table 2.1). Total and monocot weed biom ass were lower with buckwheat than in the weedy fallow with April and May planting dates and averaged over al l dates for dicots. In spring 2008, sedge weeds were the predom inant weed category w ith earlier planting dates, and grass weeds were the predominant weed category at later planting dates. Total weed biomass was higher with planting dates in Apri l and May, and grass weed biomass was higher with the planting dates of Mar. 27 and later. Howe ver, there was no significa nt effect of planting date on sedge and broadleaf weed biomass. Averaged over planting date, total, sedge, and broadleaf weed biomass in buckwheat were 2.1 -fold, 1.9-fold, and 3.8-fold less than in the weedy control, respectively (Table 2.2). Weed biomass in fall 2007 was lo w with planting dates of Oct. 18 or later due to the cool weather. Total weed biomass was lowest with planting dates in October or later. Total weed biomass with buckwheat was lower than with the weedy fallow with Oct. and Nov. 15 planting dates. There was no effect of planting date on sedge weed biomass. Grass weed biomass was lower after the planting date of Oct. 18, but the effect of buckwheat did not differ from the 30

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weedy fallow. Broadleaf weed biomass did not di ffer from that of the weedy fallow except for being higher with the Oct. 4 planting date (Table 2.3). The predominant weed category in fall 2008 wa s broadleaf weeds. Averaged over fallow treatment, only for the broadleaf weed group was there a significant resp onse to planting date. Broadleaf weed biomass was highest with the Dec. 3 planting date. Biomass with all other planting dates were not significan tly different from each other. Averaged over planting dates, total, sedge, grass, and broadleaf weed bioma ss with buckwheat and the weedy fallow were not different (Table 2.4). After termination In spring 2007, total and monocot weed bioma ss were suppressed more effectively with buckwheat planted in March and April than in May (Table 2.5). In 2008, suppression of total and grass weed biomass persisted only with the Mar. 13 planting date. Se dge weed biomass was highest with the April 24 planting date, and broa dleaf weed biomass was highest with the Feb. 28 planting date (Table 2.6). Persistence of suppression of tota l and monocot weed biomass in 2007 by buckwheat was similar to the harrowed contro l but better than the we edy control (Table 2.5). In 2008, buckwheat weed suppression was intermediate between the harrowed and weedy control for total and grass weed biomass (Table 2.6). No persistence of dicot (broadleaf) weed suppression with buckwheat was apparent af ter cover crop termination in both years. In fall trials, there was no significant effect of fallow treatments during the six weeks following buckwheat termination in both years (Tables 2.7, 2.8). In 2007, total and grass weed biomass were higher when buckwheat was plante d in October, and sedge weed biomass was highest when buckwheat was planted on Oct. 4. Ho wever, the effect of planting date was not significant in all of the weed categories in 2008. Cold weathe r in winter suppressed weed germination and growth and resulted in low weed biomass regardless of species. 31

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Temperature is really critical to buckwheat growth and may cause adverse effects on biomass accumulation, plant height, and canopy clos ure. Teasdale et al. (2004) evaluated the effect of planting date with a hairy vetch c over crop and found that hairy vetch growth and development were influenced by temperature. Ground cover and biomass of hairy vetch were reduced due to insufficient growing degree days, which is consistent with our results that buckwheat growth and canopy closure were lower with buckwheat planted in early spring and late fall. In spite of the fact that buckwheat did not suppress weed biomass effectively during the cover cropping period in the fall trials, total, sedge, grass, and broadleaf weed biomass were lower with buckwheat than the weedy control in spring. The effectiveness of buckwheat at suppressing weeds during the cove r cropping period was also repor ted in other studies. In a greenhouse experiment of Xuan and Ts uzuki (2004), perennial buckwheat ( Fagopyrum cymosum (Trev.) Meisn.) suppresse d southern crabgrass ( Digitaria ciliaris (Retz.) Koel.), Digitaria adscendens Hem., and rape ( Brassica campestris L.). During buckwheat growth, weed biomass was lower by 75% (Iqbal et al., 2003) and 86% (Creamer and Baldwin, 2000) than the weedy controls; however, we did not obtain such effect ive suppression in our study. Kato-Noguchi et al. (2007) also found that buckwheat seedlings can inhibit cress ( Lepidium sativum L.), lettuce ( Lactuca sativa L.), timothy ( Phleum pratense L.), and ryegrass ( Lolium multiflorum Lam.) seedlings. After buckwheat incorporation, weed suppressi on can continue to occur during residue decomposition. Kumar et al. (2008) incorporated buckwheat residue s in soil to evaluate their effect on the suppression of Powell amaranth ( Amaranthus powellii S. Wats.), shepherds-purse ( Capsella bursa-pastoris (L.) Medicus), and corn chamomile ( Anthemis arvensis L.) and found 32

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that the biomass of all three weeds was reduced in buckwheat residue compared to bare soil. They indicated that the mechanisms by whic h buckwheat residue can suppress weeds may include not only allelopathy, but also biologi cal control effects of fungal pathogens and decreased nitrogen availability. In north-central Florida, air temperatures in la te February and March ar e still cool and frost is possible, which is harmful to buckwheat growth. The mild temperatures between late April and early May favored buckwheat growth and canopy closure. Buckwheat can suppress weeds effectively during the cover cropping period as well as continue to suppress total and monocot weed biomass after termination when planted at the beginning of May in spring. The potential optimal planting period for buckwheat in fall might be in October and prior to early November. The temperatures in late September might still be too hot for buckwheat growth, but first frost might occur in November and it would hinder buckwheat growth or even kill buckwheat. Although the persistence of weed suppression after buckwheat termin ation in fall is not apparent, buckwheat may still be useful for suppressing w eeds during the cover cropping period. Moreover, even though the effect of weed suppression w ith buckwheat may not be effective enough in a certain time of the year, it can still be used to provide other ecosyste m services. Additionally, it is reported that buckwheat is used in a biculture with legumes in summer in the Tallahassee area (Xin Zhao personal communication). Therefore, buckwheat can be utilized for attracting beneficial insects and pollinators, providing phosphorus and organic matter, and short-term coverage of the soil for neighboring or succeed ing crops; and other crops in the same cropping system can also provide resources for buckwheat to keep the balance of the agroecosystem. 33

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0 5 10 15 20 25 30 35 40 45 1-Mar15-Mar29-Mar12-Apr26-Apr10-May24-May7-Jun21-Jun5-Jul DateTemperature (C) Air T min Air T max Soil T min Soil T max !s 1st Growing Period 3rd Growing Period 4th Growing Period 6th Growing Period 2nd Growing Period 5th Growing Period (A) 0 5 10 15 20 25 30 35 40 45 21-Feb6-Mar20-Mar3-Apr17-Apr1-May15-May29-May12-Jun DateTemperature (C) Air T min Air T max Soil T min Soil T max 1st Growing Period 2nd Growing Period 3rd Growing Period 4th Growing Period 5th Growing Period 6th Growing Period (B) Figure 2-1. Air and soil temp eratures during buckwheat cove r cropping period in spring. A) 2007 and B) 2008. 34

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-10 -5 0 5 10 15 20 25 30 35 40 45 20-Sep4-Oct18-Oct1-Nov15-Nov29-Nov13-Dec27-Dec10-Jan DateTemperature (C) Air T min Air T max Soil T min Soil T max 1st Growing Period 2nd Growing Period 3rd Growing Period 4th Growing Period 5th Growing Period 6th Growing Period (A) -10 -5 0 5 10 15 20 25 30 35 40 45 20-Sep4-Oct18-Oct1-Nov15-Nov29-Nov13-Dec27-Dec10-Jan DateTemperature (C) Air T min Air T max Soil T min Soil T max 1st Growing Period 2nd Growing Period 3rd Growing Period 4th Growing Period 5th Growing Period 6th Growing Period (B) Figure 2-2. Air and soil temp eratures during buckwheat cove r cropping period in fall. A) 2007 and B) 2008. 35

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50 70 90 110 130 150Planting Date (day of the year) 0 100 200 300Weight (g/m2) 2007 2008 Y=2875-89.5X-0.9X2-0.003X3R2=0.66 (A) 250 270 290 310 330 350Planting Date (day of the year) 0 10 20 30 40 50 60 70 80Weight (g/m2) 2007 2008 Y=2875-89.5X-0.9X2-0.003X3R2=0.66Y=2819-18X+0.03X2R2=0.46 (B) Figure 2-3. Buckwheat biomass in spring and fa ll in 2007 and 2008. A) spring and B) fall. Shoot biomass was harvested at 39 DAP in fall and spring 2007 and 35 DAP in spring 2008. Data represent the means of three replica tions; however, the regression analysis was performed using the raw data. The planting date effect of spring 2008 was not significant so that no regression model is shown. Planting dates in spring 2007 were Mar. 13, Mar. 27, Apr. 10, Apr. 24, May 8, and May 22; in spring 2008 were Feb. 28, Mar. 13, Mar. 27, Apr. 10, Apr. 24, and May 8; in fall 2007 were Sep. 20, Oct. 4, Oct. 18, Nov. 1, Nov, 15, and Nov. 29; in fall 2008 were Sep. 24, Oct. 8, Oct. 22, Nov. 5, Nov, 19, and Dec. 3. 36

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50 70 90 110 130 150Planting Date (day of the year) 0 10 20 30 40 50 60 70 80Height (cm) 2007 2008 Y=269-7.77X+0.09X2-0.0003X3R2=0.85Y=29.67+0.14X R2=0.19 (A) 250 270 290 310 330 350Planting Date (day of the year) 0 10 20 30 40 50Height (cm) 2007 2008 Y=-10762+122X-0.4X2+0.0004X3R2=0.97 (B) Figure 2-4. Buckwheat height in spring and fall in 2007 and 2008. A) spring and B) fall. Height was measured at 35 DAP. Data points repr esent the means of three replications; however, the regression analys is was performed using the raw data. The planting date effect of fall 2007 was not significant, so that no regression equation is shown. Planting dates in spring 2007 were Mar. 13, Mar. 27, Apr. 10, Apr. 24, May 8, and May 22; in spring 2008 were Feb. 28, Mar. 13, Mar. 27, Apr. 10, Apr. 24, and May 8; in fall 2007 were Sep. 20, Oct. 4, Oct. 18, Nov. 1, Nov, 15, and Nov. 29; in fall 2008 were Sep. 24, Oct. 8, Oct. 22, Nov. 5, Nov, 19, and Dec. 3. 37

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507090110130150 0 1 2LAI 507090110130150 0 10 20 30 40 50 60 70 80PAR (%) 507090110130150Planting Date (day of the year) 0 10 20 30 40 50 60 70 80GC (%) aba aab a abab abb b aab(A) (B) ab(C) Figure 2-5. Buckwheat canopy closure in spring averaged over year s. A) leaf area index (LAI), B) photosynthetically active radiation (PAR) percentage and C) ground cover (GC) percentage. Measurements were taken at 35 DAP. Data represent the means of three replications and points with the same le tter means no significance according to the comparison by the PDIFF option at P 0.05. Regression models ar e not significant so that no equation is shown. Planting dates in spring 2007 were Mar. 13, Mar. 27, Apr. 10, Apr. 24, May 8, and May 22; in 2008 we re Feb. 28, Mar. 13, Mar. 27, Apr. 10, Apr. 24, and May 8. 38

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39 Figure 2-6. Buckwheat canopy closure in fall averaged over years. A) leaf area index (LAI), B) photosynthetically active radiation (PAR ) percentage, and C) ground cover (GC) percentage. Measurements were taken at 35 DAP. Data points represent the means of three replications; however, th e regression analysis was performed using the raw data. The planting date effects on PAR and GC in 2008 were not significant, so that no regression model is shown. Planting dates in fall 2007 were Sep. 20, Oct. 4, Oct. 18, Nov. 1, Nov, 15, and Nov. 29; in 2008 were Sep. 24, Oct. 8, Oct. 22, Nov. 5, Nov, 19, and Dec. 3. at canopy closure in fall averaged over years. A) leaf area index (LAI), B) photosynthetically active radiation (PAR ) percentage, and C) ground cover (GC) percentage. Measurements were taken at 35 DAP. Data points represent the means of three replications; however, th e regression analysis was performed using the raw data. The planting date effects on PAR and GC in 2008 were not significant, so that no regression model is shown. Planting dates in fall 2007 were Sep. 20, Oct. 4, Oct. 18, Nov. 1, Nov, 15, and Nov. 29; in 2008 were Sep. 24, Oct. 8, Oct. 22, Nov. 5, Nov, 19, and Dec. 3. 250270290310330350 0.00 0.20 0.40 0.60 0.80 1.00LAI0 20 40 60PAR (%)80 100 250270290310330350 2007 2008 2007 2008 250270290310330350Planting Date (day of the year) 0 10 20 30 40 50 60 70GC (%) 2007 2008 Y=401-4.2X+0.01X2-0.00002X3R2=0.53Y=21-0.1X+0.03X2R2=0.46Y=-20052+209X-0.7X2-0.0008X3R2=0.62Y=-24128-254X+0.9X2-0.001X3R2=0.92(C) (B) (A)

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Table 2-1. Influence of planting date of buckwheat and fallow treatment on weed biom ass before buckwheat termination in spring 2007. Weed biomass (g/m2) Total Monocot Treatment Buckwheat Weedy Buckwheat Weedy Dicota Planting date Mar. 13 2.8 aAb 8.9 aA 1.6 aA 6.8 aA 1.6 a Mar. 27 14.8 aA 14.1 aA 11.6 abA 12.5 aA 3.8 a Apr. 10 16.3 aA 78.9 bB 7.4 aA 56.5 bB 15.8 b Apr. 24 8.8 aA 91.2 bcB 7.3 aA 82.5 bcB 5.6 a May 8 22.9 abA 120.5 cB 18.4 abA 94.6 cB 15.2 b May 22 38.7 bA 143.8 cB 38.8 bA 127.4 dB 11.5 b Fallow treatment Buckwheat ----4.3 a Weedy ----13.5 b a The interaction between planting date and fallow treatment was not signif icant and main effects are reported. 40b Data are the least square means of three replications. LS mean s within a column followed by th e same lowercase letter are not significantly different when compar ed with the PDIFF option at P 0.05. Pairs of buckwheat and weedy fallow means followed by the same uppercase letter are also not significantly different at P 0.05.

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Table 2-2. Influence of planting date of buckwheat and fallow treatment on weed biom ass before buckwheat termination in spring 2008. Weed biomass (g/m2) Treatment Total Sedge Grass Broadleaf Planting date Feb. 28 27.08 aba 20.37 a 1.23 a 5.45 a Mar. 13 8.02 a 5.85 a 0.97 a 1.20 a Mar. 27 17.68 a 6.87 a 6.42 ab 4.40 a Apr. 10 52.00 b 15.53 a 30.90 b 5.57 a Apr. 24 37.60 ab 22.85 a 12.32 ab 2.43 a May 8 47.37 ab 16.57 a 27.20 ab 3.60 a Fallow treatment Buckwheat 20.45 a 10.19 a 8.65 a 1.59 a Weedy 42.81 b 19.15 b 17.69 a 5.96 b a Data are the least square means of three replications. LS means within a column followed by the same lowercase letter are not significantly different when compar ed with the PDIFF option at P 0.05. 41

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Table 2-3. Influence of planting date of buckwheat and fallow treatment on weed biom ass before buckwheat termination in fall 2 007. Weed biomass (g/m2) Total Sedge Broadleaf Treatment Buckwheat Weedy Buckwheat Weedy Grassa Buckwheat Weedy Planting date Sep. 20 8.54 cAb 9.57 bA 2.44 a 4.86 a 4.66 c 1.04 aA 0.46 aA Oct. 4 5.47 bA 10.41 bB 3.22 a 1.42 a 2.17 b 1.14 aA 5.42 bB Oct. 18 0.87 aA 3.61 aB 0.45 a 0.59 a 0.88 ab 0.69 aA 1.76 abA Nov. 1 0.97 aA 1.14 aA 0.45 a 0.47 a 0.04 a 0.49 aA 0.62 aA Nov. 15 1.17 abA 2.01 aB 0.50 a 0.41 a 0.39 a 0.53 aA 0.97 aA Nov. 29 0.98 aA 1.19 aA 0.55 a 0.31 a 0.10 a 0.39 aA 0.72 aA Fallow treatment Buckwheat ----1.15 a --Weedy ----1.60 a --a The interaction between planting date and fallow treatment was not signif icant and main effects are reported. b Data are the least square means of three replications. LS mean s within a column followed by th e same lowercase letter are not significantly different when compar ed with the PDIFF option at P 0.05. Pairs of buckwheat and weedy fallow means followed by the same uppercase letter are also not significantly different at P 0.05. 42

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Table 2-4. Influence of planting date of buckwheat and fallow treatment on weed biom ass before buckwheat termination in fall 2 008. Weed biomass (g/m2)a Treatment Total Sedge Grass Broadleaf Planting date Sep. 24 9.81 a 0.18 a 3.67 a 5.97 ab Oct. 8 16.02 a 0.30 a 5.42 a 10.30 a Oct. 22 6.50 a 1.08 a 0.55 a 4.88 a Nov. 5 7.67 a 0.32 a 0.02 a 7.28 a Nov. 19 14.25 a 0.52 a 0.08 a 13.65 a Dec. 3 39.66 a 0.43 a 0.40 a 38.82 b Fallow treatment Buckwheat 11.86 a 0.46 a 0.66 a 10.73 a Weedy 19.44 a 0.48 a 2.72 a 16.24 a a The interaction between planting date and fallow treatment was not signif icant and main effects are reported. b Data are the least square means of three replications. LS mean s within a column followed by th e same lowercase letter are not significantly different when compar ed with the PDIFF option at P 0.05. 43

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Table 2-5. Influence of planting date of buckwheat and fallow treatment on weed biom ass after buckwheat termination in spring 2007. Weed biomass (g/m2) Dicota Treatment Total Monocot Buckwheat Harrowed Weedy Planting date Mar. 13 50 a 38 a 8.7 aAb 7.1 aA 19.3 aA Mar. 27 90 ab 73 ab 23.2 aB 7.2 aA 23.6 aB Apr. 10 117 b 96 b 34.3 aB 21.6 aAB 14.0 aA Apr. 24 92 ab 68 ab 54.7 aB 6.8 aA 6.1 aA May 8 168 bc 155 c 16.6 aA 5.3 aA 5.9 aA May 22 202 c 192 c 6.8 aA 6.6 aA 5.1 aA Fallow treatment Buckwheat 98 a 75 a ---Harrowed 119 ab 110 ab ---Weedy 142 b 126 b ---a The interaction between planting date and fallow treatment was significant and simple effects are reported. 44b Data are the least square means of three replications. LS mean s within a column followed by th e same lowercase letter are not significantly different when compar ed with the PDIFF option at P 0.05. Pairs of fallow treatment means followed by the same uppercase letter are also not si gnificantly different at P 0.05.

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Table 2-6. Influence of planting date of buckwheat and fallow treatment on weed biom ass after buckwheat termination in spring 2008. Weed biomass (g/m2)a Treatment Total Sedge Grass Broadleaf Planting date Feb. 28 63.6 bb 16.0 ab 34.3 ab 13.4 b Mar. 13 5.3 a 2.1 a 0.3 a 2.9 a Mar. 27 59.7 b 11.6 a 42.6 b 5.5 a Apr. 10 95.3 b 15.9 ab 77.5 b 2.0 a Apr. 24 68.1 b 22.5 b 42.6 b 3.0 a May 8 91.2 b 14.6 ab 72.6 b 4.0 a Fallow treatment Buckwheat 66.5 ab 12.1 a 49.0 ab 5.5 a Harrowed 44.6 a 12.5 a 27.1 a 5.1 a Weedy 80.5 b 16.8 b 58.9 b 4.8 a a The interaction between planting date and fallow treatment was not signif icant and main effects are reported. b Data are the least square means of three replications. LS mean s within a column followed by th e same lowercase letter are not significantly different when compar ed with the PDIFF option at P 0.05. 45

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Table 2-7. Influence of planting date of buckwheat and fallow treatment on weed biom ass after buckwheat termination in fall 20 07. Weed biomass (g/m2) Broadleafa Treatment Total Sedge Grass Buckwheat Harrowed Weedy Planting date Sep. 20 1.80 ab 0.49 a 0.11 a 1.96 b 0.95 b 0.68 ab Oct. 4 5.10 b 3.95 b 0.70 b 0.20 a 0.87 ab 0.27 a Oct. 18 2.66 ab 1.08 a 0.36 ab 0.90 a 1.26 b 1.55 b Nov. 1 1.42 a 0.30 a 0.10 a 0.74 a 1.09 b 1.23 b Nov. 15 0.52 a 0.24 a 0.09 a 0.25 a 0.16 a 0.17 a Nov. 29 1.86 a 0.29 a 0.09 a 1.49 ab 1.39 b 1.54 b Fallow treatment Buckwheat 2.05 a 0.88 a 0.25 a ---Harrowed 2.09 a 0.95 a 0.19 a ---Weedy 2.53 a 1.35 a 0.29 a ---a The interaction between planting date and fallow treatment was significant and simple effects are reported. 46b Data are the least square means of three replications. LS mean s within a column followed by th e same lowercase letter are not significantly different when compar ed with the PDIFF option at P 0.05.

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47Table 2-8. Influence of planting date of buckwheat and fallow treatment on weed biom ass after buckwheat termination in fall 20 08. Weed biomass (g/m2) Sedgea Treatment Total Buckwheat Harrowed Weedy Grass Broadleaf Planting date Sep. 24 0.56 ab 0 a 0 a 0.10 a 0.06 a 0.46 a Oct. 8 0.42 a 0.13 a 0.07 a 0.07 a 0.02 a 0.31 a Oct. 22 8.11 a 1.10 a 0.80 a 0.37 a 0.01 a 7.44 a Nov. 5 4.06 a 0.20 a 0.03 a 0.17 a 0 a 3.88 a Nov. 19 1.76 a 0 a 0 a 0 a 0 a 1.84 a Dec. 3 6.31 a 0.37 a 0.33 a 0.37 a 0.04 a 5.92 a Fallow treatment Buckwheat 2.12 a ---0.01 a 1.79 a Harrowed 4.92 a ---0.01 a 4.79 a Weedy 3.57 a ---0.04 a 3.34 a a The interaction between planting date and fallow treatment was significant and simple effects are reported. b Data are the least square means of three replications. LS mean s within a column followed by th e same lowercase letter are not significantly different when compar ed with the PDIFF option at P 0.05.

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CHAPTER 3 MECHANICAL TERMINATION OF BUCKWHEA T USED FOR WEED SUPPRESSION IN FLORIDA Introduction Buckwheat [ Fagopyrum esculentum Moench] is an agronomic species of the Polygonaceae family that originated in temperate East Asia and spread worldwide because of its wide adaptability to various envir onmental conditions (Edwardson, 1996) Buckwheat can be used as a cover crop for short-term soil coverage, improve ment of phosphorus availability, and weed control (Sarrantonio, 1994). Buckwheat can be uti lized as a weed suppressive cover crop because of its rapid establishment and growth (Stone, 1906). It is also considered to have allelopathic potential (Kumar, 2009; Tang, 1986). Rapid decom position of buckwheat residue minimizes the interference with seedbed preparation that is common with more recalcitrant residues (Kumar et al., 2009). Cover crops can be used to pr ovide a number of ecosystem services in conventional and organic cropping systems as well as in conservati on tillage and no-till syst ems such as preventing soil erosion (Flach, 1990) and runoff, modifyi ng soil physical properties (Beale et al., 1955), improving soil fertility (Smith et al., 1987), reduc ing nitrate leaching (Dinnes et al., 2002), and suppressing weeds (Teasdale, 1993). To focus on w eeds, both living cover crops and cover crop residues can be used for weed suppression. Additionally, cover crop residues can inhibit weeds through mechanical impedance, changes in soil environmental conditions, and by release of allelochemicals (Creamer et al., 1996). Cover crops are usually managed with herbic ides because of their effectiveness and efficiency, whereas alternative means are require d to terminate cover cr ops in systems with limitations on chemical application. To termin ate cover crops mechanically, options include 48

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incorporation into the soil by disking or plow ing and undercutting, rolling or mowing the cover crop to retain residues on the soil su rface as mulches (Creamer et al., 1995). Incorporation of cover crop resi dues is generally chosen for species used as green manures using various forms of tillage (Pieters, 1927) or for biofumigants for plasticulture production systems (Gamliel and Stapleton, 1997). Timing of in corporation is important to have cover crops killed early in the season w ith high biomass and enough time for degradation and nutrient release (Wagger, 1987). Mowing is a commonly used method to terminate cover crops and produce surface mulches (Creamer et al., 1995). A flail mower can kill cover crops successfully and even enhance weed suppression by cutting cover crops into small pieces. Although finely cut residues may shorten the weed suppression period due to a rapid decomposition rate of the mulch (Creamer et al., 1995), rapid degr adation may facilitate seedbed preparation for the subsequent crop without residues becoming enta ngled in tillage equipment. Rolling and roll-chopping kill cover crops by breaking, cutting, crushing, or crimping stems (Dabney et al., 1991). They can produce surface mulches with intact plant residues and result in longer persistence and weed suppr ession duration (Creamer and Dabney, 2002). Rolling is regarded as a better option than mowing due to its efficiency of operation at higher speed, lower cost for equipment maintenance, and lower consumption of fuel, though the equipment may generate vibrations harmful to human hea lth (Raper et al., 2004). Optimal practices for cover crop termination may vary with cover crop species, subsequent crop species, and cropping systems; and their success depends upon the growth stages of cover crops. Because buckwheat itself can be weedy, to a void volunteer plants causing a weed problem in subsequent cash crops, Sarrantonio (1994) sugg ested that the optimal incorporation time for 49

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buckwheat cover crops is 7 to 10 days after flowering and before seed set, and Bjrkman (2006) indicated that 35 to 40 days after seeding is id eal for buckwheat mowing or incorporation. While tillage and mowing are commonly used to kill buckwheat, rolling can also kill it effectively (Morse, 1995). Termination practices may influence allelochemi cal release by resulting in different sizes of residue fragments and decomposition rates, and they may also affect cover crop regrowth. Practices that maximize the amount of residue that is retained on the soil surface may be more effective at suppressing weeds by mechanical impedance. Ther efore, we hypothesized that persistence of weed suppression with buckwheat would differ depending on the practices used to terminate the cover crop. The objective of this study was to compare mechanical practices to determine the best for termination of the buckw heat cover crop and fo r persistence of weed suppression in Florida. Materials and Methods The experiments were conducted at the Plan t Science Research and Education Unit (PSREU) in Citra, Florida on a Hague series sa nd (loamy, siliceous, semiactive, hyperthermic Arenic Hapludalf) in spring and fall in 2008. Four buckwheat cover crop termination treatments were used: rolling, flail mowing alone, light till age alone, and the combination of flail mowing followed by light tillage. Rolling was accomplished w ith a 4 ft steel roller, flail mowing with a New Holland 918H flail mower, and light tillage with a roto-tiller. The experimental design was a randomized complete block with four repl ications. The field was 79.0 m (260 ft) by 32.8 m (108 ft) and was divided into pl ots 15.2 m (50 ft) long and 3.7 m ( 12 ft) wide with 6.1 m (20 ft)wide vertical and horizontal alleys to separate plots. Buckwheat cv. Mancan (Seedland Inc., Wellborn, Florida) was drilled on Apr. 15 and Nov. 6 at a depth of 2.5 cm (1 in) w ith a John Deere 10-foot grain dr ill. A 17.8 cm (7 in) inter-row 50

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spacing was used and the seeding rate was 56 kg/ ha (50 lb/acre). Prior to buckwheat planting, fertilizer1 was applied to the field at a rate of 6.8 kg (15 lb) N per acre. At 35 DAP in spring and 40 DAP in fall (because of differing rates of development), the buckwheat cover crop was terminated with the assigned practices. Before termination, buckwheat height was measured and shoot biomass samples were harvested using a 0.25 m2 quadrat from four randomly sele cted spots in each plot. After termination, each plot was separated into two parts: one for ground cover measurement and the other for grab samples. Ground cover (GC) perc entage of buckwheat residues was determined weekly using the beaded-stri ng method with 100 points for a di agonal transect (Sloneker and Moldenhauer, 1977). Grab samples (Ruffo and Bo llero, 2003) were also collected weekly from four randomly selected spots in each plot using the 0.25 m2 quadrat to estimate residue degradation on the soil surface weekly for 5 weeks. Grab samples were dried at 70 C for 7 days and weighed to determine residue biomass. At 3 and 5 weeks after termination (WAT), we ed counts were taken by species using four 0.25 m2 quadrats per plot. Weed counts were also ta ken at 0 WAT right after termination in fall because it was found that the established weeds ha d important influences on the results. Weed biomass samples were harveste d at 5 WAT from four 0.25 m2 quadrats per plot. After samples were dried at 70 C for 7 days, weeds were separated into sedge, grass, and broadleaf categories and weighed. Analysis of variance was perf ormed with SAS software, Version 9.0 (SAS, 2002) using the MIXED procedure and least square means were compared using the PDIFF option. Regression models were fitted using the REG procedure of SAS and SigmaPlot (Version 11.0) with the 1 10-10-10 Granular Fertili zer, N-P-K 10-10-10, 6.8 kg (15 lb) N per acre (Southern States Cooperative Inc., Anthony, FL ) 51

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means to determine the nature of a significant re sponse with each termin ation practice during the post-termination residue decomposition period. Results and Discussion Buckwheat Suppression In spring, averaged over termin ation practices, buckwheat co ntrol was 93% (Figure 3.1). Buckwheat population averaged over decompositi on periods of 3 WAT a nd 5 WAT was greatest when killed with rolling (14.6 plants/m2) and lowest when killed with the combination of flail mowing and light tillage (2.0 plants/m2) (Figure 3.2). The numbers of buckwheat plants that survived rolling and flail mowing we re higher than with light tillage and the combination of flail mowing and light tillage. This is because ev en though these practices can kill buckwheat successfully and reduce buckwheat density, buckwheat may regrow from the old plant since rolling and flail mowing cannot destroy the root system. In the fall trial, after buckwh eat termination, termination practices of lig ht tillage alone and the combination of flail mowing and light tillage did suppress buckwheat dens ity, whereas rolled and flail mowed buckwheat densities were still high (Table 3.1). Rolling and flail mowing did not effectively kill buckwheat because buckwh eat had been stunted by frost. Since buckwheat was still short and succulent at termination, most of the plants survived the roller and the flail mower. The density of buckwheat remaining afte r rolling and flail mowing at 5 WAT was lower than at 0,1,2,3, and 4 WAT due to fr eezing injury and plant senescence. Because of an early frost in late October in 2008, buckwheat growth was not as good as expected. Mean buckwheat height was only 12.5 cm and stems were still succulent at termination (data not shown). This resulted in a low killing ra te when rolling and flail mowing were used for termination due to problems of regrowth and volunteer plants. 52

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Residue Decomposition Residue biomass Buckwheat residue decomposition rate was fast est with the combination of flail mowing and light tillage, and slowest with rolling in spring (Figure 3.3; Table 3.2). The remaining residue reached 50% at approxima tely 30 days after terminati on (DAT) for rolling, 8 DAT for flail mowing, 10 DAT for light tillage, and 5 DAT for the combination of flail mowing and light tillage. At 5 WAT, the level of decomposition was greatest for the combination of flail mowing and light tillage (85.1 % weight loss), and lowest for rolling (48.7 % weight loss). In fall, averaged over termination practices, no residue decomposition occurred dur ing the 5 weeks after termination (Figure 3.4). However, residue biom ass was lowest with termination methods of light tillage alone and the combination of flail mowing and light tillage, highest with rolling, and intermediate with flail mowing averag ed over decomposition period (Figure 3.5). Ground cover Analysis of variance indicated significant differences in GC among termination practices. For the experiment in spring, there was 23% to 30% loss of GC by 1 week after termination (Table 3.3). Buckwheat terminated by flail mowi ng, light tillage, and the combination of flail mowing and light tillage result ed in a 50% reduction in GC by approximately at 19 DAP, 22 DAP, and 21 DAP, respectively; however, by 35 DAT GC with rolling wa s still greater than 50% (Figure 3.6). In fall, light tillage and the combination of flail mowing and light tillage reduced buckwheat GC by 18% and 25%, respectively, by 1 WAT (Figure 3.7). Because of the failure to effectively terminate buckwheat with rolling and flail mowing, there were still respectively 88 % and 79 % GC remaining on th e plots due to live buc kwheat (Table 3.4). 53

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Weed Suppression The effect of termination practices on total, sedge, grass, and broadleaf weed biomass during the spring trial was not si gnificant (Table 3.5). However, total and grass weed population densities were suppressed by rolling and flail mowing, br oadleaf weed population was suppressed by rolling, flail mowing, and light till age (Figure 3.8). Sedge weed population did not differ with termination Although the differences in weed densities at 3 WAT and 5 WAT were not significant (Table 3.6), we found that roll ing and flail mowing had limited impact on weeds present at buckwheat termination, yet they coul d terminate buckwheat e ffectively. Therefore, even though these practices can generate surf ace mulches and suppress weed seed germination, weeds that were already establis hed recovered easily. Light tillage and the combination of flail mowing and light tillage can di srupt established weeds, yet they disturb the soil and may stimulate germination of weed seed s or sprouting of vegetative propa gules in this disturbed layer. Moreover, the high temperature in summer favored weed growth and caused high weed biomass. In fall, there was less total, sedge, and broadl eaf weed biomass with buckwheat terminated by light tillage and the combinati on of flail mowing and light tillage than with rolling and flail mowing alone, but there was no significant effect of termination method on grass weed biomass (Table 3.7). Rolling and flail mowing did not suppress total, grass, and broadleaf weed population compared with incorpor ation practices of li ght tillage alone and the combination of flail mowing and light tillage; and there was no significant difference in sedge weed population densities in response to termination practices (Fig ure 3.9). This was also because rolling and flail mowing failed to kill established weeds, and weeds can still regrow well in cold weather. Light tillage and the combination of flail mowing and light tillage terminated buckwheat and weeds, and at the same time unlike in spring, the cold weather suppressed weed germination and emergence. Averaged over termination practices total and broadleaf weed population densities 54

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at 3 and 5 WAT were not signifi cantly different from their dens ities at termination, but sedge weed population density was higher by 3 and 5 WAT and grass weed population was lower at 5 WAT than at termination and at 3 WAT (Table 3.8). These responses may have been due to temperature decline in late fall and early wint er. Low temperature inhi bited grass weed seed germination severely, whereas sedge weed emerge nce was not affected by the low temperature. It was reported that buckwheat can be kill ed completely using mowing, undercutting, and rolling (Creamer and Dabney, 2002), while the le vel of buckwheat control in our study was lower at 93%. Our finding that the decomposition rate of buckwheat killed by flail mowing was faster than buckwheat killed by rolling was sim ilar to the results of Creamer and Dabney (2002). Practices providing surface mulches reduced tota l, grass, and broadleaf weed population, but incorporation practices increased total, grass, and broadl eaf weed population densities in our spring trial. Teasdale and Rosecr ance (2003) used a flail mower, a corn stalk chopper, and light and heavy disks to kill a hairy vetch cover crop. They found that the stalk chopper and the light disk generated surface mulches with the cover crop residue and suppressed initial broadleaf weed emergence better than the hea vy disk. However, in a comparison of flail mowing, undercutting, and sicklebar mowing, flail mowing did not lowe r broadleaf weed population and biomass, and the intact cover crop residues produced by undercutting and sicklebar mowing suppressed weeds more effectively than chopped residues pr oduced by flail mowing (Creamer et al., 1995). Buckwheat termination is more than 90% effective with rolling, flail mowing, light tillage, and the combination of flail mowing and light tillage when it grows in an appropriate environment. Buckwheat residue de gradation was faster when term inated by light tillage and the combination of flail mowing and light tillage than by rolling and flail mowing alone. Although there was no difference in the effectivene ss of termination practices on weed biomass 55

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suppression, this may have been because rolli ng and flail mowing effectively suppressed weed population densities in spring while incorporatio n practices may have stimulated germination and emergence of weeds. Since the results of lig ht tillage alone and the combination of flail mowing and light tillage were si milar, light tillage alone may be a potential practice to reduce cost and consumption of fuel rather than using the combination of flail mowing and light tillage. Fall would be the best season to utilize light tillage for buckwheat termination because high temperature germination cues that result from tillage are not as important for winter annuals as for summer annuals. Rolling may also be a promising practice for notill cropping systems for reducing new weeds with surface mulch. Flail mowing alone may be another promising practice for no-till cropping systems with similar effect on reducing new weeds by generating mulches; however, the residue may decompose rapidly and the persistence of control may be short. In addition, rolling and flail mowing work better on suppressing weed population in spring than in fall. 56

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071421283542Days after termination 0 50 100 150Buckwheat density (plants/m2) R2=0.999 Y= 134.5 18.1 X X= 7 Y= 1133.2 9*10-21X Figure 3-1. Buckwheat density averaged over termination practice dur ing decomposition period in spring 2008. Data are the means of four replications. Regression analysis was performed on the means. 0 2 4 6 8 10 12 14 16 R M LT M+LTTermination practicesBuckwheat density (plants/m2)a b c d Figure 3-2. Buckwheat density averaged over decomposition period with different termination methods in spring 2008. Termination methods included rolling (R), flail mowing (M), light tillage (LT), and the combination of flail mowing and light tillage (M+LT). Bar with the same letter means no significance between treatments according to the comparison by the PDIFF option at P 0.05. 57

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Y=34.5+68.9*exp(-0.05*X) R 2 =0.90Days after termination 01020304 0 % of biomass remaining 0 20 40 60 80 100 0 20 40 60 80 100 R 2 =0.998Days after termination 010203040% of biomass remaining X = 9.4 Y = 40.4 0.03X Y = 100 6.37X (A) (B) R 2 =0.95Days after termination 0102030 40 0 20 40 60 80 100 % of biomass remaining 0 20 40 60 80 100 Y = 42.5 0.27X Y = 97.7 4.15X X = 14.2Y = 100 10.8XDays after termination 010203040% of biomass remaining R 2 =0.99 Y = 25.1 0.25X X = 7.1 (C) (D) Figure 3-3. Buckwheat residue biomass remainin g on the soil surface with different termination methods during the 5 weeks after termina tion in spring 2008. (A ) rolling, (B) flail mowing, (C) light tillage, and (D) the combin ation of flail mowing and light tillage. Data are the means of four replications. Regression an alysis was performed on the means. 58

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0 1 2 3 4 5 6 7 8 9 10 0714212835Days after terminationResidue biomass (g/m2) Figure 3-4. Buckwheat residue biomass averaged over termination practic es with decomposition period in fall 2008. Analysis with PROC MIXED indicated no significant effect of days after termination (P=0.98). 0 2 4 6 8 10 12 14 16 18 R M LT M+LTTermination practiceResidue biomass (g/m2) c b a a Figure 3-5. Buckwheat residue biomass averag ed over decomposition period with different termination methods in fall 2008. Terminati on methods included rolling (R), flail mowing (M), light tillage (LT), and the comb ination of flail mowing and light tillage (M+LT). Bar with the same letter means no significance between treatments according to the comparison by the PDIFF option at P 0.05. 59

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Y=-106.3+202.3*exp(-0.007*X) R 2 =0.82Days after termination 01020304 0 Ground cover (%) 0 20 40 60 80 100 0 20 40 60 80 100 Y=-19.8+117.1*exp(-0.03*X) R 2 =0.94Days after termination Ground cover (%)(A) (B) 010203040 Y=34.5+64.4*exp(-0.07*X) R 2 =0.96Days after termination 01020304 0 Ground cover (%) 0 20 40 60 80 100 0 20 40 60 80 100 Y=-14.7+113.3*exp(-0.03*X) R 2 =0.99Days after termination Ground cover (%)(C) (D) 010203040 Figure 3-6. Buckwheat residue ground cover remaining on the soil surface with different termination methods during the 5 weeks afte r termination in spring 2008. (A) rolling, (B) flail mowing, (C) light tillage, and (D) the combination of flail mowing and light tillage. Data are the means of four replica tions. Regression analysis was performed on the means. 60

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Days after termination 01020304 0 Ground cover (%) 0 20 40 60 80 100 Y = 95.67 0.74XR 2 =0.64Days after termination 0 20 40 60 80 100Ground cover (%)(A) (B) 010203040 Y = 100 11.96X Days after termination 01020304 0 Ground cover (%) 0 20 40 60 80 100 R 2 =0.996Y = 22.4 0.37X X = 6.7 R 2 =0.99Days after termination 0 20 40 60 80 100 010203040Ground cover (%) Y = 31.8 0.61X Y = 100 14.53X X = 4.9 (C) (D) Figure 3-7. Buckwheat residue ground cover re maining on the soil surface with termination methods during the 5 weeks after termin ation in fall 2008. (A) rolling, (B) flail mowing, (C) light tillage, and (D) the combin ation of flail mowing and light tillage. Data are the means of four replications. Regression an alysis was performed on the means. The effect of days after termin ation for rolling is not significant and no regression curve is shown. 61

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0 50 100 150 200 250 TotalSedgeGrassBroadleafWeed typesWeed density (plants/m2) R M LT M+LT c b a a b a b a b a a a a a a a Figure 3-8. Number of weeds by category in response to term ination method averaged over decomposition period, spring 2008. Termination methods included rolling (R), flail mowing (M), light tillage (LT), and the comb ination of flail mowing and light tillage (M+LT). Bar with the same letter means no significant difference between treatments according to the comparison by the PDIFF option at P 0.05. 62

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63 Figure 3-9. Number of weeds by category in response to term ination method averaged over decomposition period, fall 2008. Termination methods included rolling (R), flail mowing (M), light tillage (LT), and the comb ination of flail mowing and light tillage (M+LT). Bar with the same letter means no significant difference between treatments according to the comparison by the PDIFF option at P 0.05. 0 20 40 TotalSedgeGrWeed typesWeed60 80 100 120 140 160 180 200 assBroadleaf density (plants/m2)b c a a a a a a a b b a b c a M+LT LT M Ra

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Table 3-1. Buckwheat density with different termination practices during residue decomposition after termination in fall 2008. Buckwheat density (plants/m2) Termination practice Week 0 Week1 Week 2 Week 3 Week 4 Week 5 Rolling 64.0 aDa 52.0 bB 56.8 cBC 61.0 cC 56.8 bBC 13.3 bA Flail mowing 67.0 aD 60.0 cC 49.8 bB 46.3 bB 56.8 bC 13.5 bA Light tillage 65.0 aB 0.5 aA 0.3 aA 0.8 aA 1.8 aA 1.3 aA Flail mowing + light tillage 66.0 aB 0 aA 0.3 aA 0.5 aA 0.5 aA 1.5 aA a Data are the least square means of four replications. LS means within a column followed by the same lowercase letter are not significantly different when compar ed with the PDIFF option at P 0.05. LS means within rows followed by the same uppercase letter are also not significan tly different at P 0.05. 64

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Table 3-2. Residue biomass by category with different termination practices during residue decomposition af ter termination in spring 2008. Residue biomass (g/m2) Termination practice Week 0 Week1 Week 2 Week 3 Week 4 Week 5 Rolling 201.8 cAa 186.0 cC 132.4 bC 110.1 bC 97.7 aB 103.5 aC Flail mowing 198.6 cA 109.6 bB 80.6 aB 77.5 aB 76.6 aB 79.6 aBC Light tillage 203.7 cAB 130.4 bB 85.4 aB 69.6 aB 82.3 aB 61.9 aB Flail mowing + light tillage 226.4 bB 56.1 aA 49.5 aA 40.3 aA 48.8 aA 33.6 aA a Data are the least square means of four replications. LS means within a column followed by the same lowercase letter are not significantly different when compar ed with the PDIFF option at P 0.05. LS means within rows followed by the same uppercase letter are also not significan tly different at P 0.05. 65

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Table 3-3. Residue ground cover by categor y with different terminati on practices during residue d ecomposition after terminatio n in spring 2008. Ground cover (%) Termination practice Week 0 Week1 Week 2 Week 3 Week 4 Week 5 Rolling 77.4 cDa 59.4 cBC 64.3 cC 55.3 cB 46.0 cA 40.8 cA Flail mowing 71.0 bcD 50.1 bC 47.1 bC 38.1 bB 23.1 aA 20.8 aA Light tillage 68.0 bD 49.4 bC 38.9 bB 37.0 bB 32.8 bAB 24.5 bA Flail mowing + light tillage 43.9 aD 33.6 aC 27.9 aBC 23.7 aB 17.0 aAB 13.1 aA a Data are the least square means of four replications. LS means within a column followed by the same lowercase letter are not significantly different when compar ed with the PDIFF option at P 0.05. LS means within rows followed by the same uppercase letter are also not significan tly different at P 0.05. 66

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Table 3-4. Residue ground cover by categor y with different terminati on practices during residue d ecomposition after terminatio n in fall 2008. Ground cover (%) Termination practice Week 0 Week1 Week 2 Week 3 Week 4 Week 5 Rolling 15.5 bBa 16.2 cB 15.2 cAB 12.3 bA 14.3 cAB 13.7 bAB Flail mowing 14.2 abB 12.9 bB 11.9 bAB 10.7 bAB 9.7 bA 11.3 bAB Light tillage 12.6 abB 2.3 aA 2.4 aA 1.9 aA 1.6 aA 1.0 aA Flail mowing + light tillage 12.3 aB 3.1 aA 3.4 aA 2.0 aA 1.9 aA 1.3 aA a Data are the least square means of four replications. LS means within a column followed by the same lowercase letter are not significantly different when compar ed with the PDIFF option at P 0.05. LS means within rows followed by the same uppercase letter are also not significan tly different at P 0.05. 67

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Table 3-5. Weed biomass by category with different termination practices harvested 5 weeks after termin ation in spring 2008. Weed biomass (g/m2) a Termination practice Total Sedge Grass Broadleaf Rolling 119.2 30.1 42.7 50.9 Flail mowing 111.2 21.7 42.9 46.7 Light tillage 91.8 26.0 43.0 22.8 Flail mowing + light tillage 71.0 23.4 34.8 12.8 P value 0.18 0.88 0.92 0.07 a Data are the least square mean s of four replications. The eff ect of termination practice on s uppression weed biomass in all ca tegories was not significant when compared at P 0.05. 68

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Table 3-6. Number of weeds by category av eraged over termination practices at 3 a nd 5 weeks after termination in spring 2008. Weed count (plants/m2) a Time after termination Total Sedge Grass Broadleaf Week 3 140.56 56.19 34.69 49.69 Week 5 124.63 46.06 37.56 41.00 P value 0.27 0.68 0.27 0.14 a Data are the least square means of four replications. The effect of time after termination on weed biomass was not significant at P 0.05.. 69

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Table 3-7. Weed biomass by category with different termination practices harveste d 5 weeks after termination in fall 2008. Weed biomass (g/m2) Termination practice Total Sedge Grass Broadleaf Rolling 111.25 ba 0.51 ab 0.50 a 110.30 b Flail mowing 101.60 b 0.70 b 2.33 a 98.60 b Light tillage 2.45 a 0.48 a 0.05 a 1.95 a Flail mowing + light tillage 4.20 a 0.20 a 0.05 a 3.98 a a Data are the least square means of four replications. LS means within the same column followed by the same letter are not significantly different when compared by the PDIFF option at P 0.05. 70

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71Table 3-8. Number of weeds by category averaged over termination practices at 0, 3, and 5 weeks after termination in fall 2008 Weed count (plants/m2) Time after termination Total Sedge Grass Broadleaf Week 0 82.38 4.06 aa 1.81 b 76.50 Week 3 94.38 11.00 b 2.50 b 80.88 Week 5 102.19 14.38 b 0.06 a 87.75 P value 0.38 <0.001 0.004 0.73 a Data are the least square means of four replications. LS means within the same column followed by the same letter are not significantly different when compared by the PDIFF option at P 0.05.

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CHAPTER 4 SUMMARY AND CONCLUSIONS Buckwheat [ Fagopyrum esculentum Moench] is a promising cover crop for weed management in sustainable and organic croppi ng systems and is generally used as a summer cover crop in temperate areas. Since the humid s ubtropical climate of Florida results in local conditions that are considerably different from the rest of the country, recommendations for cover crop species and the time of planting deve loped in other parts of the US may not be applicable to Florida. Termination practices may influence weed emergence and growth by providing or eliminating germination cues throug h soil disturbance or mu lches or by allelopathy, whereas they may also affect residue decompositi on rate and the persistence of the surface mulch or allelopathic effect. Therefore, it was hypothesized that the optimal planting period for buckwheat would occur with mild weather in spring or fall in north central Florida and persistence of weed suppression after a buckwheat cover crop would differ depending on the practices used to terminate the crop. The objec tives were to determine the optimal planting period for buckwheat used as a cover crop in Fl orida for effective weed suppression in spring and fall and to compare mechanical termination practices to determine the best practice for terminating a buckwheat cover crop that allows for persistence of weed suppression after termination. In the study to determine the optimal planti ng period, we chose planting dates from late February to late May in spring and late Septembe r to early December in fall. In early spring, cool temperatures suppressed weed emergence and grow th and late frost during February and March may cause adverse effects on buckwheat growth However, warmer te mperatures in May speeded up weed growth and did not favor buckwheat growth. As for fall, temperatures in early fall might be too hot for buckwheat, whereas early frost in late fall may even kill buckwheat. The 72

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effect of buckwheat on weed suppr ession was not significant in late fall because low temperature not only suppressed weed growth but also sl owed down buckwheat growth. Therefore, our results suggest that optimal planting period in spring may be on or around May 1, and in fall it may be on or around Oct. 16. In the second study, termination practices of rolling, flail mowing, light tillage, and the combination of flail mowing and light tillage were compared to assess their effects on buckwheat termination, residue decomposition, and persisten ce of weed suppression. All of the practices resulted in the death of more than 90% of the buckwheat. Rolling allows for retention of cover crop residues as an organic mulch, however, ro lled buckwheat residue was not sufficient to suppress established weeds. Flail mowing choppe d and shredded buckwheat residue into small pieces, which accelerated the decomposition rate of the residue. Light tillage and the combination of flail mowing and light tillage could disturb the top layer of soil and cause disruption of established weeds; however, they ma y also stimulate weed seed germination within this layer of soil. Incorporati on reduces the amount of residue on the soil surface by distributing it within the upper layer of the soil. Although the remaining surface residues may be insufficient for a mulch, incorporation may improve allelopathic suppressi on of weed growth. Depending on the cropping system, flail mowing and rolling ma y have utility for no-till and reduced tillage production systems. Although there was no differen ce in weed suppression with the two, the persistence of surface coverage with flail mowing may not be as good as with rolling. Light tillage alone may be more appl icable in fall when cooler te mperatures can inhibit weed germination and emergence and reduce the adverse e ffect caused by disturbing the soil. It is also a promising method for saving energy and limiting fuel cost since light tillage alone was as effective as a combination of flail mowing and light tillage. 73

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As a result, it is recommended that buckwheat can be a useful short-term cover crop for weed suppression in Florida with the optimal planting period at or around May 1 for subsequent summer crops, and around Oct. 16 for subsequent winter crops. Even though hot weather in Florida is not appropriate for buckwheat growth and reduces the effectiveness of buckwheat for weed suppression, buckwheat may still play a ro le in sustainable cropping systems by providing ecosystem services such as nectar sources fo r beneficial insects, phosphorus source, organic matter, and short-term soil cover for neighboring or subsequent crops. Termination practices can be chosen depending on the production systems or buckwheat planting seasons. Rolling and flail mowing can be favorable practices in spring or for no-till cropping systems to provide organic mulch, although the mulch generated by rolling pe rsists longer than that produced by flail mowing. It may be better to utilize light tillage for buckwheat termination in fall. Light tillage may also facilitate allelochemi cal release for weed suppression. 74

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LIST OF REFERENCES Abdul-Baki, A.A. and J.R. Teasdale. 1997. Sustaina ble production of fresh-market tomatoes and other summer vegetables with organic mulc hes. U.S. Department of Agriculture, Agriculture Research Service, Farm ers Bulletin No. 2279, Revised. pp.1. Aldrich, R.J. and R.J. Kremer. 1997. Principles in Weed Management. Iowa State University Press, Ames, IA. Baker, B.P. and D.B. Smith. 1987. Self identified research needs of New York organic farmers. Amer. J. Alternative Agric. 2:107. Barberi, P. 2002. Weed management in organic agriculture: are we addr essing the right issues? Weed Res. 42:177. Barnes, J.P. and A.R. Putnam. 1983. Rye residues contribute weed suppr ession in no-tillage cropping systems. J. Chem. Ecol. 9:1045. Beale, O.W., G.B. Nutt, and T.C. Peele. 1955. Th e effects of mulch tillage on runoff, erosion, soil properties, and crop yields. Proc. Soil Sci. Soc. Am. 19:244. Bjrkman, T. 2006. Encouraging the use of buckwh eat cover crops for weed control by reducing the risk of volunteer seedlings. NYS IPM Program Report. Online at: http://www.nysipm.cornell.edu/grantspgm/projects/proj06/veg/bjorkman.asp Caamal-Maldonado, J.A, J.J. Jimnez-Osornio, A. Torres-Barragn, and A.L. Anaya. 2001. The use of allelopathic le gume cover and mulch species for weed control in cropping systems. Agron. J. 93:27. Clements, D.R., S.F. Weise, and C.J. Swant on. 1994. Integrated weed management and weed species diversity. Phytoprotection 75:1. Colbach, N., F. Dessaint, and F. Forcella. 2000. Evaluating field-scale sampling methods for the estimation of mean plant densit ies of weeds. Weed Res. 40:411. Cook, J. 1989. Wiping out witchg rass. Horticulture 67:34. Creamer, N.G. and K.R. Baldwin. 2000. An eval uation of summer cover crops for use in vegetable production systems in No rth Carolina. HortScience 35:600. Creamer, N.G. and S.M. Dabney. 2002. Killing cove r crops mechanically : review of recent literature and assessment of new research resu lts. Amer. J. Alternative Agric. 17:32. Creamer, N.G., M.A. Bennett, and B.R. Stinner. 1997. Evaluation of cover crop mixtures for use in vegetable production systems. HortScience 32:866. Creamer, N.G., M.A. Bennett, B.R. Stinner, J. Cardina, and E.E. Regnier. 1996. Mechanisms of weed suppression in cover crop-based production systems. HortScience 31:410. 75

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Creamer, N.G., B. Plassman, M. A. Bennett, R.K. Wood, B.R. S tinner, and J. Cardina. 1995. A method for mechanically killing cover crops to optimize weed suppression. Amer. J. Alternative Agric. 10:157. Dabney, S.M., M.W. Buehring, and D.B. Reginell i. 1991. Mechanical cont rol of legume cover crops. Pages 146 in W.L. Hargrove, ed. Cover Crops for Clean Water. Soil Water Conservation Society, Ankeny, IA. Dinnes, D.L., D.L. Karlen, D.B. Jaynes, T.C. Kaspar, J.L. Hatfield, T.S. Colvin, and C.A. Cambardella. 2002. Nitrogen management strate gies to reduce nitrat e leaching in tiledrained midwestern soils. Agron. J. 94:153. Edwardson, S. 1996. Buckwheat: Pseudocer eal and nutraceutical. Pages 195 in J. Janick, ed. Progress in New Crops. ASHS Press, Alexandria, VA. Eskelsen, S.R. and G.D. Crabtree. 1995. Th e role of allelopathy in buckwheat ( Fagopyrum sagittatum ) inhibition of Canada thistle ( Cirsium arvense ). Weed Sci. 43:70. Flach, K.W. 1990. Low-input agricu lture and soil conservation. J. Soil Water Conserv. 45:42. Frank, D.L. and O.E. Liburd. 2005. Effects of living and synthetic mulch on the population dynamics of whiteflies and aphids, their associ ated natural enemies, and insect-transmitted plant diseases in zucchini Environ. Entomol. 34:857. Gallandt, E.R. 2004. Soil improving practices fo r ecological weed management. Pages 267 in Inderjit, ed. Principles and Practices in Weed Management: Weed Biology & Weed Management. Kluwer Academic, Dordrecht, The Netherlands. Gamliel, A. and J.J. Stapleton. 1997. Improvement of soil solarization with volatile compounds generated from organic Amendm ents. Phytoparasitica 25:S31S38. Golisz, A., B. Lata, S.W. Gawronski, and Y. Fujii. 2007. Specific and total activities of the allelochemicals identified in buc kwheat. Weed Biol. Manag. 7:164. Hargrove, W.L. and W.W. Frye. 1987. The need for legume cover crops in conservation tillage production. Pages 1 in J.F. Power, ed. The Role of Legumes in Conservation Tillage Systems. Soil Conservation Soci ety of America, Ankeny, IA. Hatcher, P.E. and B. Melander. 2003. Combin ing physical, cultural and biological methods: prospects for integrated non-chemical weed management strategies. Weed Res. 43:303 322. Heap, I. 2007. International survey of herbicide resistant weeds. Online at: http://www.weedscience.org/ Hillger, D.E., S.C. Weller, and K.D. Gibson. 2006. Emergent weed communities associated with tomato production systems in Indiana. Weed Sci. 54:1106. 76

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Hooks, C.R.R., H.R. Valenzuela, and J. Defrank. 1998. Incidence of pest and arthropod natural enemies in zucchini grown in living mu lches. Agri. Ecosys. Environ. 69:217. Hutchinson, C.M. and M.E. McGiffen, Jr. 2000. Co wpea cover crop mulch for weed control in desert pepper production. HortScience 35:196. Iqbal, Z., S. Hiradate, A. Noda, and Y. Fujii. 2003. Allelopathic activity of buckwheat: isolation and characterization of ph enolics. Weed Sci. 51:657. Iqbal, Z., S. Hiradate, A. Noda, S. Isojima, and Y. Fujii. 2002. Allelopathy of buckwheat: assessment of allelopathic potential of ex tract of aerial parts of buckwheat and identification of fagomine and other related alkaloids as allelochemicals. Weed Biol. Manag. 2:110. Kalinova, J. and N. Vrchotova. 2009. Level of catech in, myricetin, quercetin and isoquercitrin in buckwheat ( Fagopyrum esculentum Moench), changes of thei r levels during vegetation and their effect on the grow th of selected weeds. J. Agric. Food Chem. 57:2719. Kalinova, J., N. Vrchotova, and J. Triska. 2007. Exudation of allelopathic substances in buckwheat ( Fagopyrum esculentum Moench). J. Agric. Food Chem. 55:6453. Kato-Noguchi, H., H. Sugimoto, and M. Yamada 2007. Buckwheat seedlings may inhibit other plant growth by allelopathic subs tances. Environ. Control Biol. 45:27. Kumar, V., D.C. Brainard, and R.R. Bellinder. 2008. Suppression of Powell amaranth ( Amaranthus powellii ), shepherds-purse ( Capsella bursa-pastoris ), and corn chamomile ( Anthemis arvensis ) by buckwheat residues: role of nitrogen and fungal pathogens. Weed Sci. 56:271. Liebman, M. and A.S. Davis. 2000. Integration of soil, crop, and weed management in lowexternal-input farming systems. Weed Res. 40:27. Liebman, M. and T. Ohno. 1998. Crop rotatin and legume residue effects on weed emergence and growth: applications for weed management. Pages 181 in J.L. Hatfield, D.D. Buhler, and B.A. Stewart, eds. Integrated Soil and Weed Management, Ann Arbor Press, Chelsea, MI. Liebman, M., C.L. Mohler, and C.P. Staver. 2001. Ecological Manageme nt of Agricultural Weeds. Cambridge University Pr ess, Cambridge, United Kingdom. Lu, Y., K.B. Watkins, J.R. Teasdale, and A.A. Abdul-Baki. 2000. Cover crops in sustainable agriculture. Food Rev. Int. 16:121. Marks, C.F. and J.L. Townsend. 1973. Buckwheat. University of California. Cover crops resource page. Online at: http://www.sarep.ucdavis.edu/ccrop/ Masiunas, J.B., L.A. Weston, and S.C. Weller. 1995. The impact of rye cover crops on weed populations in a tomato croppi ng system. Weed Sci. 43:318. 77

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BIOGRAPHICAL SKETCH Pei-wen was born in 1984 in Taipei, Taiwan. She grew up and lived there for twenty-two years until shortly after she rece ived her Bachelor of Science in horticulture from National Taiwan University in Taipei, Taiwan in June 2006. Upon graduation, Pei-wen decided to travel to the United States to pursue further study in horticulture. She was accepted into the Master of Science program in the Horticultural Sciences Depa rtment of the University of Florida in August 2006. The topic of her masters research focuse d on optimizing buckwheat cover crops for weed suppression in Florida. Pei-wen has presented the results of her research at the Southern Weed Science Society meeting in 2008, the American Society of Horticultural Science meeting in 2008, the joint meeting of the Southern Weed Scie nce Society and the Weed Science Society of America in 2009, and the Florida W eed Science Society meeting in 2009. She is also a member of the Honor Society of Agri culture: Gamma Sigma Delta. 82