PRIN OF WEED SCIENC
PRIN OF WEED SCIENC AGRON 317
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This 6 page Class Notes was uploaded by Daryl Gulgowski on Saturday September 26, 2015. The Class Notes belongs to AGRON 317 at Iowa State University taught by Robert Hartzler in Fall. Since its upload, it has received 198 views. For similar materials see /class/214420/agron-317-iowa-state-university in Agricultural & Resource Econ at Iowa State University.
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Date Created: 09/26/15
I D t 1 Agronomy 317 Principles of Weed Science Photosynthetic Pigment Inhibitors Light harvesting complex LHC A The LHC consists of pigments chlorophyll a and b carotenes and polypeptide chains that form the framework B LHC are wrapped around photosynthetic reaction centers PS I and PS II and funnel energy captured from solar radiation to the centers C Energy is transferred to center via resonance energy transfer at the reaction center electrons are excited to a higher energy level D Role of pigments l Chlorophyll a and b have slightly different absorption spectra b absorbs light between 400 and 500 nm wavelength more efficiently than chlorophyll a 2 Carotenes 7 long linear compounds a Absorb light in 400500 nm range b Second important function is suppressing photochemical reactions particularly those involving oxygen Carotenoid inhibitors A Site of action several enzymes in carotene pathway inhibited by different herbicides l Zcarotene desaturase amitrole 1954 2 Isopentylpyrophosphate clomozone Command 7 1980 s 3 phydroxyphenylpyruvate dioxygenase HPPD isoxa utole Balance 7 1990 s mesotrione Callisto 2001 topramezone Impact tembotrione Laudis 4 Phytoene desaturase nor urazon Zorial uridone Sonar B Mode of action 1 Carotenes protect chlorophyll from destruction by light photooxidation 2 Inhibition of carotenes results in destruction of chlorophyll and membranes 3 Initial symptoms are bleaching white of tissue C Behavior in plants Absorbed both by roots and shoots Translocated within plant via xylem Selectivity due to differential metabolism Drift to nontarget plants an issue a Herbicides active at fractions of label rates b Injury symptoms easily observed D Mesotrione 1 Corn 7 pre and postemergence control Callisto and several premixes 7 Lumax Camix etc a Primarily annual broadleaf control b Excellent margin of safety c Low rates of atrazine enhance activity 2 Turf Tenacity a Postemergence control of crabgrass creeping bentgrass b Only registered for use on golf courses and sod farms 3 Soil behavior a Decreased absorption with increasing pH b Decreased halflife with increasing pH 59 E c Occasional carryover to rotational crops Isoxa utole Balance 1 Preemergence applications in com a Annual broadleaf and some grass control b Marginal crop safety c Typically used in combination to allow lower rates and minimize injury risk 2 A new formulation Balance Flexx will be marketed in 2009 that includes a safener The safener will allow postemergence applications in corn Without safener corn unable to tolerate the herbicide due to rapid absorption 3 Selectivity a Differential metabolism b Corn hybrids vary in tolerance Differences related to rate of degradation 4 Soil relations a Isoxa utole is rapidly converted to DKN in soil DKN is the phytotoxic compound b DKN is relatively mobile in soil c Decreased absorption in high pH soils increasing activity d Label has several restrictionsprecautions concerning use on specific soils e Registration limited to certain states due to groundwater concerns f Halflife 45 days occasional carryover F Issues with carotene inhibitors 1 Drift a Although these herbicides do not drift anymore or less than other herbicides the symptoms are easily noticed when susceptible plants are contacted b Risk of injury varies with sensitivity of the plant and specific herbicide For example soybean are more sensitive to tembotrione than they are to mesotrione or topramezone Thus beans in fields adjacent to corn are more likely to be damaged by tembotrione than other carotene inhibitors used on corn 2 Groundwater a Initial degradation product of isoxa utole DKN is relatively mobile in profile Because of this EPA initially granted it a provisional label and required monitoring of water supplies to determine if it was reaching these areas b Several states in Combelt did not approve its registration due to these concerns including Minnesota Michigan and Wisconsin III PPO PROTOX inhibitors propoporphyrinogen oxidase A Site of action PROTOX 7 enzyme involved in synthesis of chlorophyll B DJ Mode of action 1 Inhibition of PROTOX allows accumulation of substrate for enzyme in chloroplast 2 As substrate concentration increases it diffuses into cytoplasm where it is oxidized to protoporphyrin IX 3 Protoporphyrin IX reacts with light and 0 creating singlet oxygen that disrupts cell membranes 4 Herbicides often are classified as membrane disrupters Herbicides l Diphenyl ether family aci ourfen Blazer lactofen Cobra fomesafen Re ex oxy ourfen Goal 2 Flumiclorac Resource 3 Sulfentrazone Authority 4 Carfentrazone Aim SpeedZone PowerZone 5 Flumioxazin Valor 0 0 U Behavior in plants 1 Can be absorbed both by foliage and roots The diphenyl ethers are primarily used as postemergence herbicides due to tight adsorption to soil colloid that reduces soil activity 2 Some translocation in xylem although it is limited 3 Selectivity is due to differential metabolism fomesafen Re ex FlexStar 1 Uses a Postemergence broadleaf control in soybeans dry beans cotton b Preemergence control in soybean Pre x 7 combination with metolachlor 2 Soil relations a Km 50 b Halflife 100 days c Some soil activity 3 Plant relations a Xylem translocated b Selectivity differential metabolism 4 Issues a Crop injury 1 High likelihood of foliar burning 2 Yields normally not affected especially when applied early in growing season b Consistency of control 1 Due to limited translocation timely application small weeds and good coverage is required for consistent control 2 Broadleaf weeds vary widely in sensitivity 7 scouting important to know what weeds are present c Resistance 1 PPO resistant waterhemp reported in IL MO and KS 2 Syngenta developed PPO resistant crops in the 1990 s but did not bring to market due to antiGMO sentiment in Europe Sulfentrazone Authority Spartan 1 Uses Pre broadleaf control in soybean tobacco sun ower strawberries vegetables 2 Soil relations a Halflife 180 days b Binding decreases with increasing pH c Microbial degradation 3 Plant behavior a Selectivity differential metabolism b Translocation xylem 4 Issues a Relatively high cost to synthesize chemical Primary market at this time is high value crops rather than soybean due to differing prices the markets will bear Crop injury risk 7 soybeans can be damaged if a heavy rain occurs when soybeans are first emerging The herbicide becomes more available in the ponding that occurs at the soil surface during the rain and is absorbed by the hypocotyl and plumule Usually only occurs when first rain following application occurs during soybean cracking Stems can be girdled or growing point killed resulting in need for replanting 2 Principles of Weed Science Agronomy 317 Competition De nition struggle between two organisms for a limited resource that is essential for growth Interference collective interactions that occur among plants growing close together Most studies that study interactions between plants assume that the response is due to competition A Competition B Allelopathy release of chemical that suppresses growth of adjacent plants C Parasitism parasitic plant acquires nutrients through direct association with host plant D Others light quality effects Nature of competition A Limiting resources 1 Water 2 Light 3 Nutrients 4 002 5 Space In most agronomic situations water and light are resources which weeds and crops compete for most C Growth habit of crop and weed determines which plant has an advantage for specific resource ie tall plants typically more competitive for light than low growing plants B Factors influencing cropweed competition A Species characteristics Yield loss varies widely among weed species Usually closely related to canopy size a Shading capacity b Water use Yield loss in soybean caused by one weed 4 ft of row a Sunflower 13 b Velvetleaf 6 0 Giant foxtail lt 1 B Weed density 1 Low densities have little impact on crop yields since there are enough resources to satisfy needs of both the crops and weeds MA A 2 Yield loss increases linearly with increasing weed density to a certain point and then rate of increase diminishes a C39 lnterspeci c competition competition between two species 1 occurs at low weed densities when weeds are spread out such that individual weeds don t compete with each other only compete with the crop 2 linear portion ofyield response curve lntraspecific competition competition among plant of same species 1 Occurs at high weed densities when weeds begin to compete with each other as well as with crop plants 2 Yield loss associated with each additional weed diminishes 3 Both inter and intraspeci c competition occur at high weed densities 4 At very high densities the yield curve plateaus due to high level of intraspecific competition C Environment 1 Yield loss associated with speci c weed infestation varies from field to field year to U ear 2 Weed more competitive if environment favors its growth over the crops 3 Crops generally most sensitive to competition during highyield environments Duration of competition 1 Plants that compete for the entire growing season cause greatest yield loss a b Plant that initially captures resources will have competitive advantage for remainder of the growing season Important to provide crop with even start with weeds plant into clean seedbed 2 Crops can tolerate competition for portion of season without suffering yield loss 3 Critical period length oftime weeds can be allowed to remain in field without yield loss A o Um O 39D Earlyseason competition lnvolves weeds that emerge at same time as crop Critical period specifies how long weeds can remain before need to implement control tactic cultivation postemergence herbicide etc Weeds generally can remain for three to four weeks after crop emergence without impacting yield Critical period varies widely with weed infestations production practices and environment The higher the weed population the shorter the critical period The critical period for earlyseason competition has become important with the adoption ofglyphosate resistant crops Roundup Ready since they allow the use of glyphosate to control large weeds very effectively Many farmers lose yield because they delay application of glyphosate until after the critical period has been reached 5 Lateseason competition a Involves weeds that emerge after crop establishment typically after control tactics have been implemented b Impact of weeds diminishes rapidly as their emergence is delayed due to head start provided to the crop c Critical period specifies how long weeds need to be controlled after planting d Weeds that emerge more than four weeks after crop typically will not impact yields e Lateemerging weeds that don t impact yield can produce significant numbers of weed seed E Economic thresholds for weed management 1 Weed infestation controlled only if value of yield lost due to competition exceeds cost of control 2 Reasons for little adoption ofthis practice a Fail to consider other costs of weeds i Seed production ii Harvest efficiency Variability in competition limits ability to predict the potential yield loss caused by a specific weed infestation be applied Aesthetics i Farmers take pride in their ability to produce weedfree fields ii High percentage of Iowa farm ground is rented landlords often demand weedfree fields 0quot O