Put Weeds Out To Pasture

Pasture weeds can be grouped into three distinct categories: grasses, sedges, and broadleaf weeds. Grasses have one seed leaf or cotyledon when they emerge from the soil. They have round hollow stems with nodes. Grass leaves are narrow with parallel veins. Sedges closely resemble grasses but have solid, triangular stems filled with pith. The edge can be felt when rubbed between fingers. Their leaves are often arranged in groups of threes. Like grasses, sedges will have one seed leaf when they first emerge. Sedges are often found in areas with poor drainage. Broadleaf weeds are also known as dicots and have two seed leaves when they germinate. They have net-like veins that cover the entire leaf surface. Broadleaf and grass weeds are adapted to a variety of soil moisture conditions.

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Annual weeds only live for one growing season. Warm season annuals germinate in the early spring, vegetatively grow in the summer and flower and produce seed and die in the fall. Cool season annuals germinate in the fall, grow throughout the winter, and set seed and die in the following spring. Common warm season annual weeds include showy crotalaria (Crotalaria spectabilis) and crabgrass (Digitaria spp). Wild radish (Raphanus raphanistrum) and fireweed (Urtica chamaedryoides) are common cool season weeds. Weeds that live for two years are known as biennials. Most biennials germinate in the fall and begin to flower and produce seed during the spring of the following year. Common biennials include thistles and cudweed (Gnaphalium spp.). Perennials are any weeds that live for 3 or more years. Many perennial weeds go dormant in the winter and re-sprout from rootstalks in the spring. Perennial weeds often seen in pastures include dogfenell (Eupatorium capillifolium), red sorrel (Rumex acetosella) and smutgrass (Sporobolus indicus).

The timing of herbicide application is important for optimum weed control. When applicable, annual weeds should be prevented from germinating by applying a preemergence herbicide. If a preemergence herbicide application is not feasible, annual weeds should be controlled as seedlings. Weeds are most susceptible to herbicides in this stage. As the weeds mature, higher herbicide rates and/or different herbicides will be required to control them. Herbicides will have little to no effect on annual weeds near the end of their life cycle. Herbicide application timing for perennials varies with the weed species. Most perennial weeds are more susceptible to fall herbicide applications. During this time the plant is actively transporting sugar manufactured in its leaves down to its roots to survive the winter dormancy. Herbicide application during this time will also travel downward and kill the roots. Perennial weeds are also susceptible to herbicides when flowering. Fall herbicide applications may not always be practicable for some perennial weeds such as dogfennel. Waiting for a fall herbicide application will result in the dogfennel competing with the pasture grass for the entire summer. IFAS research has shown that waiting to control dogfennel in August will result in a 75% reduction in bahiagrass yield.

Herbicide Basics

Just like pasture weeds, pasture herbicides can be divided into several categories. Preemergence herbicides are applied to weed seed germination. These herbicides need approximately an inch of rain or irrigation to activate them and move them in to the top layer of soil with the weeds. These herbicides will have no effect on weeds that have already germinated. Post emergence foliar herbicides are applied to the leaves after they have emerged. Post emergence herbicides are classified as either contact or systemic. Contact herbicides undergo no movement in the plant and kill only what they touch. Thorough coverage is essential to control weeds with contact herbicides. Contact herbicides are much more effective on annual broadleaves than grasses and other perennials. Contact herbicides will only control the top portion of weeds. The growing point of grasses is below ground and will be protected from contact herbicides. Perennials weeds will usually have enough root reserves to resprout after an application. Systemic herbicides circulate throughout the plant after they are applied. The herbicides can move in both the xylem and phloem plant tissue or only in the xylem. Sap in the phloem moves in a downward direction transporting the sugars manufactured in the leaves to the roots below. Water and nutrients are absorbed by the roots and transported upward to the rest of the plant by the xylem tissue. Foliar applications of xylem mobile herbicides will not transport down and kill the roots. Some systemic herbicides will also have soil activity. These herbicides are absorbed by the weed’s roots and transported to the rest of the plant. Selective herbicides can control weeds while causing little to no damage to the crop. Non-selective herbicides will damage or kill most plants.

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Herbicide Mode Of Action

Mode of action (MOA) is the biological process in which the herbicide kills the plant. There are 5 plant processes inhibited by herbicides labeled for pasture use.
1. Growth regulation
2. Amino acid and proteins
3. Photosynthesis
4. Growth inhibition
5. Cell membranes
6. Fatty acid synthesis

The Weed Science Society of America (WSSA) classifies herbicides according to their mechanism of action. Herbicides should be rotated with different mechanisms of actions to help limit herbicide resistance.

Amino Acid Inhibitors

Amino acids, the building blocks of proteins, are classified as essential or non-essential. Proteins are necessary for plant growth and development. There are three herbicide groups that inhibit amino acid synthesis.

Group 2 ALS Inhibitors

Acetolactate synthase (ALS) is the enzyme that helps plants manufacture the essential amino acids valine, leucine, and isoleucine. These herbicides bind to the enzyme and prevent it from functioning. The ALS enzyme is not found in animals. As a result these herbicides have very low toxicity. This herbicide group has two chemical families labeled for use in pastures. The sulfonylureas are a large chemical family. Some like metsulfuron control a wide range of weed species, while others such as sulfosulfuron target only specific weeds. Nicosulfuron and chlorsulfuron are also used in combinations with other herbicides. The other group 2 herbicide labeled for use in pastures is imazapic, in the imidazolinone chemical family. Imazapic is used to control crabgrass, sandspur, nutsedges, and vaseygrass. Metsulfuron and imazapic can cause severe injury to bahiagrass. The ALS inhibitors are not fast acting and can take up two weeks for herbicide injury to appear. Treated weeds will have stunted growth and will eventually turn yellow. ALS inhibitors are transported in both the phloem and xylem tissue of the plant. Some of the herbicides also have soil activity.

Group 9: EPSP Synthase Inhibitor

Glyphosate is the only member of this herbicide group. Glyphosate binds to the enzyme EPSP synthase and interrupts the Shikimate pathway, the metabolic process used to produce the amino acids phenylalanine, tyrosine, and tryptophan. The Shikimate pathway is not found in animals. Glyphosate is non-selective and is phloem and xylem mobile. It causes similar injury to the ALS inhibitors. It binds quickly to soil particles and does not have any soil activity.

Group 4: Growth Regulators

These are the most commonly used herbicides to control pasture weeds. These herbicides have the ability to selectively remove broadleaf weeds from most grass crops. Limpograss is the exception and can be damaged from several growth regulator herbicides. These herbicides mimic the plant hormone auxin. They cause rapid, uncontrolled growth in weeds. These herbicides are mobile in the xylem and phloem tissue. Injury symptoms include twisting and curling of the leaves and stems. Leaves can also become stretched out. It can take several days for injury to appear. They all have foliar activity while only the herbicide aminopyralid has significant soil activity. 2, 4-D, dicamba, and aminopyralid are effective on herbaceous broadleaf weeds. Triclopyr and fluroxypyr can control both woody and herbaceous broadleaf weeds.

Groups 5 & 7: Photosynthetic Inhibitors

These herbicides disrupt photosynthesis by attaching to the Q1 binding site in the chloroplast inside the plant cell. This blocks the energy flow in the photosynthesis process. As a result of this blockage, free radicals begin to form inside the chloroplast. The free radicals destroy the chloroplast membrane in a chain reaction. The free radicals can also attack and destroy the cell membrane. Photosynthetic inhibitors only move in the xylem tissue of the plant. Injury symptoms include rapid yellowing in the leaves, especially among the veins. Foliar applications of these herbicides will not move down into the roots. Hexazinone is part of Group 5 and will control broadleaf and grass weeds. However, it is mostly used to control smutgrass. Hexazinone is labeled for booth bahiagrass and bermudagrass. It will cause slight injury to bermudagrass. Diuron is a Group 7 herbicide used in preemergence applications to control grasses in only in bermudgrass establishment.

Group 22: Cell Membrane Disrupters

These herbicides affect photosynthesis but in a different manner than the photosynthetic inhibitors. Group 22 herbicides divert the energy flow of the photosynthesis reaction inside the chloroplast. The herbicides reactions with oxygen inside the chloroplast to form superoxide and hydroxyl free radicals. The free radicals then destroy the chloroplast membrane and the cell membrane is a cascading chain reaction. These herbicides are very fast acting. Treated weeds can show symptoms in only a few hours. Symptoms include bronzing of the leaves, followed by necrotic spots and leaf desiccation. Paraquat is the only Group 22 labeled for pasture use. It can only be applied to dormant pastures. Paraquat is a contact and non-selective. It binds tightly to soil particles and as a result has no soil activity.

Group 3: Cell Division Inhibitors

Pendimethalin is the only herbicide in dinitroaniline family labeled for pasture use. This herbicide only provides pre-emergent control and must be applied to dormant pastures. It prevents seeds from germinating stopping cell division in their roots. In order to activate the herbicide, rainfall or irrigation needs to take place within two weeks of application. It provides good control of annual grasses such as crabgrass, sandbur and goosegrass. It is the only herbicide that will provide selective control of sandbur in bahiagrass. Pendimethalin provides good residual control, but late season escapes will occur.

Group 1: ACCase Inhibitors

Clethodim is only approved for use in perennial peanut fields. Clethodim and other Group 1 herbicides inhibit the enzyme acetyl-CoA carboxylase (ACCase). This enzyme is responsible for the chemical reactions that produce fatty acids in plants. Fatty acids are needed by plant cells to create and maintain cell membranes. Clethodim is mobile in the xylem and phloem. These herbicides will only control grasses. They will not control nutsedges which are sometimes referred to as “nutgrasses.” Injury symptoms include new leaves turning brown. The top portion of the grass can be easily pulled apart from the lower half. It will take 1-3 weeks for the plant to die.

Problem Pasture Weeds

Prickly pear (Opuntia spp.) is found in dry sandy soils throughout Florida. It is a slow growing perennial that is capable of vegetative reproduction. Mowing prickly pear will exacerbate the problem. The cut up pads can quickly put down new roots. Prickly pear should be sprayed in the spring and summer when it is actively growing. Prickly pear can be controlled with Remedy Ultra or Vista XRT. Remedy should be applied as a spot treatment in a 20% mix with basal oil. The grass will be burned, but it will recover. The cacti will slowly shrivel up and die over a period of 6-8 months. For widespread infestations that require a broadcast application, apply Vista at 22 ounces per acre. Vista must be applied with a surfactant to absorb into the cacti pads. Vista can also be applied at a spot treatment at a rate of 0.5 ounces per gallon of water. Vista will not burn the grass. Control with Vista is very slow and can take more than one year.

Blackberry and dewberry are two closely related species in the Rubus genus. Blackberry grows in an upright bush, while dewberry grows as vine along the ground. To achieve optimum control, mow the plants and allow six months of active regrowth and then spray with an herbicide. This will allow better herbicide absorption into the roots. The next best option is to spray the briars when they are blooming. Blackberry can be controlled with the herbicide Remedy Ultra at 2 pints per acre or PastureGard H/L at 2 pints per acre, while dewberry can needs to be controlled with the herbicide PastureGard H/L at 2 pints per acre Some plants may need to be retreated the following year after a spring application of Remedy Ultra or PastureGard H/L. Spring or fall applications of metsulfuron at 0.5 ounces per acre and Chaparral at 2 ounces per acre provide good control of blackberry and dewberry. Both of these products will severely damage bahiagrass.

Smutgrass (Sporobolus indicus) is a bunch grass that can quickly take over pastures. Velpar L at 4 pints per acres is the recommended herbicide. Velpar is a costly herbicide and economic analysis has shown that fields should not be sprayed until 50% infestation. Velpar has little leaf absorption and must be applied in the summer where rain will move the herbicide into the smutgrass roots zone. Rainfall must take place with 2 weeks of Velpar application for successful control. Bahiagrass will have a slight yellowing after treatment. Bermudagrass will have more injury than bahiagrass but will recover in approximately one month. Velpar is highly toxic to oak trees. Stay at least 100 feet away from the drip line of any desirable oak tree. Apply glyphosate at a 3% solution for smutgrass stands underneath oak trees.

To simplify information available in this article, it is sometimes necessary to use trade names of herbicides. No endorsement of these fungicides is intended, nor is criticism implied of similar fungicides not mentioned.

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