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HS-795 Weed Control in Florida Citrus1 S.H. Futch2 1. This document is HS-795, one of a series of the Horticultural Sciences Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Date printed: February 2001. Please visit the EDIS Web site at http://edis.ifas.ufl.edu. 2. S.H. Futch, Extension Agent IV, Citrus REC, Lake Alfred, Florida, Horticultural Sciences Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611. The use of trade names in this publication is solely for the purpose of providing specific information. It is not a guarantee or warranty of the products named, and does not signify that they are approved to the exclusion of others of suitable composition. The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension Service/Institute of Food and Agricultural Sciences/University of Florida/Christine Taylor Waddill, Dean. Introduction Citrus is one of the most important agricultural crops in Florida, comprising 850,000 acres with an on-tree value of $1.15 billion for the 1998-99 crop year. Florida's citrus industry consists of oranges (78%), grapefruit (16%), and specialty varieties (6%). Control of weeds is a major economic cost, estimated at $172.8 million or 25% of annual production budget. Total production costs per acre is estimated at $850 which includes weed control cost of $205 per acre. An estimated 90% of the acreage receives some form of chemical weed control as well as mechanical control. Florida's favorable climate allows for weed germination and year-round growth. Weeds compete with citrus trees for nutrients, light, water, space and harbor insects and rodents that attack citrus trees. Additionally, weeds increase cold damage during radiation freezes, increase the incidence of Phytophthora foot rot, impede harvesting of citrus crops, interfere with low volume irrigation systems, and intercept soil-applied chemicals. Thus, the objective of today's citrus weed management program is the suppression of the undesirable effects of weed populations to an acceptable level. Weed Control Chemical weed control minimizes hand labor, increases tree growth, reduces damage to tree trunks, and improves movement within the grove. When properly selected and applied for specific tree age, scion, and soil type, herbicides will not injure healthy citrus trees. Positive responses in tree trunks and canopy volume have been noted when weeds are controlled which promotes greater production. Herbicides used in groves are divided into two groups: soil-applied preemergence (Table 1), and foliar-applied postemergence (Table 2). The postemergence herbicides can be further divided into systemic or contact. Commonly used preemergence herbicides for the control of grasses include: bromacil (Hyvar), norflurazon (Solicam), oryzalin (Surflan), and thiazopyr (Mandate). Preemergence herbicides for the control of broadleaf weeds include: diuron (Diuron, Karmex, Direx), oxyfluorfen (Goal), and simazine (Princep, Simazine). Soon to be released, azafenidin (Milestone) controls both broadleaf and grass weeds. Postemergence herbicides include non-selective paraquat (Gramoxone), glyphosate (Roundup), sulfosate (Touchdown), and selective grass control herbicides are fluazifop-P-butyl
Weed Control in Florida Citrus 2 (Fusilade) and sethoxydim (Poast Plus). Control of specific weeds is dependent on herbicide selection (Table 3), application rate, and weed species. For the effective control of all weed species (broadleaf and grasses) combinations of products may be necessary to achieve successful control. Weed control methods include preventative, chemical, chemical mowing, mechanical, biological, and hand labor. Hand labor is minimized due to its high cost and uncertain availability. Materials The average of three 120 days after treatment (DAT) visual ratings for twelve months were averaged to determine the annual weed control rating for selected herbicide combinations (Table 4). Rating percentages are for combinations of broadleaf, grasses, and sedges. The rating scale is 0 to 100, with 0% being complete ground cover and 100% completely weed free. Herbicide application rates varied due to weed intensity at study sites. All applications were made with a tractor mounted sprayer applying 30 gpa at approximately 120-day intervals. Treatments were applied to 5 tree plots replicated 5 times in a randomized complete block design to a young grove on the ridge (Lake Garfield) and flatwoods (Indiantown). Conclusions Herbicide combinations which controlled both grasses and broadleaf increased percent weed control. Herbicide combinations that included bromacil or norflurazon generally provided higher levels of weed control. Weed intensities vary among the different locations within the state with higher weed pressure being in the south Florida flatwoods area. Herbicide rates must be adjusted to control the weeds present at a given site. Herbicide products and/or rates that work at one location may not provide effective control at other locations. A clear understanding of weed species present and knowledge of species that are controlled by a given herbicide are important. Weed control of approximately 80% at the end of the rating period (every 120 days after treatment) should provide acceptable control under most conditions. Complete weed control (100%) is not necessary, may not be cost effective and could pose environmental risks. Seasonal differences in weed control will occur, with lower levels of weed pressure occurring during the winter months. References Florida Citrus Pest Management Guide, SP-43, University of Florida, Florida Cooperative Extension Service. 1990-2000. Futch, S. H. 1997. Horticultural and Environmental Aspects of Weed Control in Florida Citrus. Ph.D. Dissertation, University of Florida. Tucker, D. P., R. L. Phillips, & T. W. Oswalt. 1969. Guide for Use of Herbicides in Florida Citrus. Agricultural Extension Service, University of Florida, Gainesville, Circular 303B. Tucker, D. P., R. L. Phillips, & T. W. Oswalt. 1971 & 1976. Weed Control Guide for Florida Citrus. Florida Cooperative Extension Service, University of Florida, Gainesville, Circular 355.
Weed Control in Florida Citrus 3 Table 1. Preemergence Herbicides Currently Used in Florida Citrus Common Name Year Introduced Current Recommendations Application Site Rate (lb ai/A)* Maximum Per Year (lb ai/A)* Bromacil** 1963 < 1 year 1.6-2.4 4.8 1-3 years 1.6-2.4 4.8 > 4 years 1.6-3.2 4.8 Diuron 1962 < 1 year 1.6 6.4 > 1 year 1.6-3.2 6.4 Norflurazon 1985 all 2.4-4.0 8.0 Oryzalin 1993 all 2.0-4.0 6.0 Oxyfluorfen*** 1989 non-bearing 0.5-2.0 4.0 Thiazopyr 1997 all 0.56-0.84 1.0 Simazine 1962 < 1 year 2.25 > 1 year 2.25-3.96 7.9 Azafenidin 2000? 1.0 2.0 Labeled but not frequently used: Napropamide (Devrinol), Pendimethalin (Prowl, Pendimax), Trifluralin (Treflan). *Rate is converted to pounds active ingredient per acre (lb ai/A) to simplify table where multiple products exist. **Prohibited from use in vulnerable, deep, sandy ridge type soils. ***Non-bearing only.
Weed Control in Florida Citrus 4 Table 2. Postemergence Herbicides Currently Used in Florida Citrus Common Name Year Introduced Current Recommendations Application Site Rate (lb ai/A) Maximum Per Year (lb ai/A) Glyphosate 1979 undertree annual weeds 1.0-2.0 10.5 undertree perennial 2.0-5.0 middle management 0.14-0.28 (bahiagrass) 0.125-0.25 (bermudagrass) 0.25-0.375 spot treatment 1-2% solution Sulfosate 1992 undertree 1.0-2.0 (annual weeds) 4.0 2.0-4.0 (perennial weeds) Paraquat 1969 all 0.625-1.0 Fluazifop-P-butyl 1987 non-bearing 0.25-0.375 Sethoxydim 1993 all 0.28-0.46 1.875 Table 3. Weed Control by Various Products 50% Grove Acreage 50% Grove Acreage Undertree Broadleaf UndertreeGrasses Row Middle PRE POST PRE POST Chemical Mow Chemical Bromacil* x limited Norflurazon limited x
Weed Control in Florida Citrus 5 Table 3. Weed Control by Various Products 50% Grove Acreage 50% Grove Acreage Undertree Broadleaf UndertreeGrasses Row Middle PRE POST PRE POST Chemical Mow Thiazopyr x Oxyfluorfen** x Simazine x limited annual grasses Diuron x limited annual grasses Oryzalin limited x Azafedin*** x x Glyphosate/Sulfosate x x x Paraquat x x Sethoxydim x Fluazifop-P-butyl** x Biological Phytophthora palmivora Milkweed vine only Mechanical discing / mowing limited control of grasses and broadleaf due to inability to get under tree control broadleaf and grasses PRE = Preemergence. POST = Postemergence. x = Control (rate and weed species dependent). *Prohibited from use on ridge citrus. See label. **Non-bearing only. ***Registration pending. Table 4. Preemergence Herbicide Treatments and Annual Overall Weed Control (%) for the Ridge and Flatwoods Citrus Production Regions Herbicide*** Ridge Flatwoods Rate (lb ai/A) Control (%)* Rate (lb ai/A) Control (%)* Bromacil**** 1.6 91a 2.13 81 b Diuron 1.6 74 bcd 2.13 59 cde Bromacil + Diuron**** 1.6 + 1.6 92a 2.13 + 2.13 91 a Diuron + Oryzalin 1.6 + 1.6 79 bc 2.13 + 2.0 63 cd Diuron + Thiazopyr 1.6 + 0.25 74 b-e 2.13 + 0.33 60 cd Diuron + Thiazopyr 1.6 + 0.33 76 bcd 2.13 + 0.49 66 c Norflurazon 1.6 70 b-e 2.4 81 b Norflurazon + Diuron 1.6 + 1.6 81 b 2.13 + 2.4 92 a Norflurazon + Simazine 1.6 + 1.8 73 b-e 2.4 + 2.64 89 ab Norflurazon + Oxyfluorfen** 1.6 + 0.8 81 b 2.4 + 1.2 88 ab Oryzalin 1.6 64 def 2.0 37 hi
Weed Control in Florida Citrus 6 Table 4. Preemergence Herbicide Treatments and Annual Overall Weed Control (%) for the Ridge and Flatwoods Citrus Production Regions Herbicide*** Ridge Flatwoods Rate (lb ai/A) Control (%)* Rate (lb ai/A) Control (%)* Oxyfluorfen** 0.8 69 cde 1.2 55 c-g Oxyfluorfen** + Thiazopyr 0.8 + 0.25 76 bcd 1.2 + 0.375 64 cd Oxyfluorfen** + Thiazopyr 0.8 + 0.33 78 bc 1.2 + 0.49 61 cd Simazine 1.8 53 f 2.64 47 e-h Simazine + Oryzalin 1.8 + 1.6 75 bcd 2.64 + 2.0 57 c-f Simazine + Thiazopyr 1.8 + 0.25 69 cde 2.64 + 0.375 64 cd Simazine + Thiazopyr 1.8 + 0.33 72 b-e 2.64 + 0.49 53 d-g Thiazopyr 0.25 62 ef 0.375 46 fgh Thiazopyr 0.33 68 cde 0.49 42 gh Control 0.0 39 g 0.0 29 i Means followed by same letter within a column do not significantly differ (P<=0.05 Waller-Duncan). Percent control data were transformed using arcsin square root percent and are reported in de-transformed units. *Annual weed control based upon the average of 3 treatments per year with ratings at 120 DAT after each treatment. **Non-bearing. ***All treatment, including control, included glyphosate at 2.0 to 3.0 lb ai/A. ****Prohibited from use in ridge citrus. See label.