Forage and Pasture
Forage Management Guides
Self-Study Guide Guide 9: Weed Control in Forages
What Makes a Plant a Weed?
Three important characteristics that make a plant a weed are competitiveness,
persistence and forage value.
Competitiveness
Weeds may reduce forage yield by as much as 50 percent through competition
for space, light, water, and nutrients. The rule of thumb is that for 1 pound of
weeds produced there is 1 pound of forage lost. Many annual weeds have very
rapid growth rates compared to forage species. The fast growth rate combined
with the tremendous number of weed seeds in the soil increases the competitive
advantage of weeds over forages. Many weeds are left ungrazed in pastures
because they are less palatable than forage plants, thus they become larger and
even more competitive.
In addition to competing for the elements needed for growth, some weeds are
allelopathic meaning they exude chemicals that inhibit the growth of other
plants. Allelopathy is the production of chemical compounds by plants that
inhibit the growth of other nearby plants. Black walnut is a good example of an
allelopathic plant. Tall fescue is also an allelopathic plant. When there is a
good fescue sod on a field to be sprigged with bermudagrass, the fescue residue
will provide a significant level of weed control during bermudagrass
establishment. Tall fescue growing at the base of small fruit trees stunts their
growth beyond the effects of competition for light, water, and nutrients.
Allelopathy occurs widely in natural plant communities and is thought to be one
mechanism by which many weed species interfere with crop growth.
Persistence
Some characteristics that make weeds persistent are:
(1) The ability to produce large numbers of seeds. A single pigweed plant can
produce up to 117,000 seeds in a single season. Common ragweed is capable of
producing up to 15,000 seeds per year. Sandbur may produce 1,000 seeds per
plant. If left uncontrolled, weeds build a tremendous seed reserve in the soil
that will remain viable for many years.
(2) The most persistent weeds have the ability to produce seeds under adverse
conditions. Some summer annual weeds germinating as late as September in
Arkansas may set viable seeds before frost. Many weeds, even if mowed late in
the season, will resprout and produce seed before the first killing frost.
3) Perennial weeds often produce extensive vegetative reproductive structures
that help them survive and spread. Wild garlic can produce hundreds of hard
shell bulblets that can remain viable for five years or more. Nutsedge produces
thousands of tubers that help it survive. Horsenettle sends out runners from the
mother plant that give rise to daughter plants. Johnsongrass has been shown to
produce up to 4.5 tons of roots per acre. Weeds arising from vegetative
reproductive structures, such as roots, bulbs and tubers, are faster growing and
harder to kill than those emerging from seed.
(4) Many weed seeds have dormancy mechanisms that stretch their germination
period over many years. Factors needed to induce weed seed germination include
light, exposure to cold temperatures, and scarification of hard seed coats.
Crabgrass is an example of a weed that requires light for germination. Tillage
can create a crabgrass infestation in a field where crabgrass was not previously
a problem by exposing the dormant seed to light. Horsenettle, a problem pasture
weed, has chemical germination inhibitors in the seed that must be leached by
rainfall before they will germinate. Other weeds have the ability to germinate
almost immediately after ripening. Most of the common thistles found in Arkansas
require very little ripening prior to germination. About 50% of the seed of bull
and musk thistle will germinate within two months of seed release. This figure
increases over time to 90% and remains high one year after release. Thistle
seeds may remain viable in the soil for as long as five years.
Effect of Burial on Germination of Some Common Pasture Weeds
Forage Value
The grazing value of a plant should be considered before control efforts are
made. The weed may be a valuable forage during critical growth periods or it may
be easily kept under control by grazing management. Many weeds compete with
forages reducing the overall grazing days per acre, but other weeds are readily
grazed by livestock and are becoming more accepted as good quality forages. Some
examples include crabgrass, johnsongrass, and dallisgrass. Winter-annual weeds
such as little barley and cheat may produce a considerable amount of spring
grazing, especially in southern Arkansas. Other weeds such as common ragweed are
readily grazed when immature and produce an abundance of seed that is an
important food source for quail and other wildlife species. Goals for both
livestock and wildlife production should be considered when developing a weed
management plan.
Additional Problems
Spiny plants such as prickly pear, horsenettle, sandbur, and thistle prevent
livestock from grazing infested areas and may cause injury. Honey locust (thorn
trees) is another common spiny pest in Arkansas pastures. Honey locust may
injure livestock and will certainly puncture tires. Weeds that cause physical
irritation reduce the number of acres of grazing land available to livestock.
Poisonous weeds may cause direct loss of animals and exert indirect effects
through loss of production. Slow rate of gain, low conception rates, and
abortion are potential indirect effects of toxic weeds. Perilla mint,
horsenettle, pokeweed, jimsonweed, buttercup, cocklebur, and bitter sneezeweed
are just a few of the pasture weeds that may be toxic to livestock to some
degree. Endophyte infected fescue is an example of a forage plant containing
toxins that causes loss of production.
Dairymen face an additional problem with weeds that affect the taste of milk.
Wild garlic, wild onion, and bitterweed are three widespread species that taint
milk.
At what point does some kind of weed control become necessary? A good rule of
thumb is when undesirable weeds equal 20 percent of the stand it is time to
begin a control program. When thorny weeds such as horsenettle, musk thistle, or
pricklypear appear, forget the 20 percent rule and take immediate action to
prevent these species from becoming widely established.
Approaches to Weed Control
Accurate weed identification is the first step to successful weed control
regardless of the control method used. Before beginning a weed control program,
it is important to have an understanding of different types of weeds.
Annual Weeds. Annual weeds germinate from seed and complete their life
cycle then die within one year. Annual weeds commonly are the first plants to
grow on bare or disturbed ground. Annual weeds are often further separated into
winter annuals and summer annuals. Summer annuals germinate in the spring, grow
during the summer, and die in the fall. Pigweed and crabgrass are summer
annuals. Winter annuals germinate in late summer or fall, grow during the winter
and early spring, and die in late spring to early summer. Henbit, chickweed, and
little barley are winter annuals.
Biennials. Biennials include plants that require two years to complete
their life cycle. During the first year, biennials produce a circular cluster of
basal leaves called a rosette and store food reserves in their roots. A flower
stalk is produced in the second year, seeds are produced, and the plant dies.
Biennials spread only by seed and are usually found in pastures or on other
sites where tillage is not used. Bull thistle is an example of a biennial plant.
Perennial Weeds. Perennial weeds are plants, which live for several
years. Summer and fall overgrazing set the stage for the establishment of
perennial weeds during the following spring. Periods of good rainfall in spring
and early summer provide the needed moisture for young perennials to become
established especially in areas where competition from the forage is poor. After
perennials become established, they are able to spread by seed or vegetative
means such as runners (horsenettle) and bulbs (wild garlic). Perennials that
reproduce only by seed are referred to as simple perennials. Some perennials may
not produce seed every year. Perennials that can also spread by roots and
runners are called creeping perennials. Bermudagrass is an excellent example of
a creeping perennial. Other pasture perennials include goldenrod, ironweed, late
eupatorium, curly dock, and red sorrel.
Weed type is important when choosing a control method. Weedy grasses include
winter annuals such as little barley, sandbur, foxtail, cheat, downy brome and,
depending on your view, warm-season perennials such as johnsongrass, dallisgrass
and broomsedge. Little barley, cheat and downy brome provide some grazing early
in the spring before they become unpalatable. Broadleaf weeds include pigweed,
dandelion, thistles, red sorrel and curly dock.
The mode of action of many selective herbicides generally works against
either grasses or broadleaves, but not both. Non-selective herbicides may be
used to control a mix of broadleaf and grassy weeds. Other non-chemical methods
have different effects on grasses compared to broadleaf weeds.
Goals in Weed Control
What is your goal? Prevention, Control, or Eradication?
Prevention is more effective than attempting to control weeds after large
populations have become established. The potential benefits of a good weed
prevention program warrant serious consideration in forage production due to the
potentially high cost of controlling difficult weed species. There is a tendency
to expect more from weed control technology than it can deliver. Most people
think that effective control can be quickly obtained after weeds have invaded a
new area. This is not the case. Once established, weeds will be there for a
while. A single application of a herbicide or other weed control method will not
control all the weeds present. Ten steps to preventing the spread of weeds are:
1. Do not let weeds reproduce.
2. Do not bury seed in weed free areas. Inadvertently tilling in weed
seed in a weed free area guarantees a future population of weeds.
3. Do not spread reproductive structures such as roots, tubers, rhizomes,
and bulbs.
4. Do not use weed infested crop seed.
5. Do not bring in weed infested soil, bedding, manure, or hay. When
compared to soil seed bank numbers, manure is not an important seed source
for many farms, however problems may arise with imported feeds or manure
heavily infested with noxious weed seed.
6. Do not feed seed screenings to livestock. Screenings from seed
cleaning operations are often loaded with weed seed.
7. Clean equipment before moving between fields.
8. Kill new weed species before they set seed.
9. Keep fencerows, ditches, and field borders clean.
10. Spot treat small weed infestations before they become a problem.
Control is an attempt to suppress weed populations to a point at which there
is little economic impact. This is the most commonly practiced approach to weed
control because it is a compromise between total eradication and accepting yield
losses due to heavy weed infestations.
Eradication is ridding the area of all weed species and weed parts. It is
usually only feasible with new or small infestations of a weed. If you notice a
few plants of a new weed getting started, it may well be worth your time to get
rid of all of them. An example would be chopping or spraying a few thistle
plants before they can spread seed all over the farm. Prickly pear is another
plant that would warrant an eradication attempt.
Weed Control Methods
The number of herbicides available for weed control in forages is gradually
decreasing. New products are slow to appear in the forage herbicide market
because of the high cost of bringing a new herbicide into the marketplace. That
cost is currently estimated to be 40 million dollars, an investment that
companies will not make unless the product may be used on crops that represent
large number of acres such as corn and soybeans. If a product is developed for a
row crop it may not be registered for forage use because additional feeding
studies are required to determine the potential for meat and milk residues in
animals consuming treated forage. While pasture and forages in Arkansas
represent more acres than all the row crops combined, less than 10 percent of
that acreage receives any herbicide application. Thus, manufacturers consider
the profit potential to be marginal.
These developments point out the need for forage producers to refine and
re-emphasize some of the non-chemical weed control methods that have been around
for hundreds of years and to look for alternatives to chemical weed control. To
some extent, the advent of modern herbicides has caused some farmers to abandon
preventative practices such as good fertility and grazing management in favor of
crisis management using chemical weed control.
It is important to remember that the most successful weed control programs
use an integrated approach combining all the weed control techniques available
since a single method rarely gives completely satisfactory weed control.
Cultural Weed Control Methods
Mowing
The greatest effect of mowing is to reduce seed production and dispersal. For
example, a timely mowing during bloom has been effective in reducing seed
production of weeds like biennial thistles, cheat, and downy brome. Mowing at
bloom may be too late for other weeds. Although it is not found in Arkansas,
Canada thistle seed is viable 8 days after pollination. Field bindweed requires
about 10 to 15 days after pollination to produce viable seed. Thus, mowing in
the bud stage instead of waiting until the bloom stage or later is important in
preventing seed production by many species.
Mowing has not been very effective as a means of reducing weed competition in
the short term. By the time the weeds are big enough to mow, they have already
done a lot of their competitive damage. One shortcoming of mowing is that many
weed species can grow and reproduce below cutting height. Another problem with
mowing is lack of selectivity. Cutting most weeds without mowing the desirable
forage species is nearly impossible. Many weeds send up new growth after the
tops are clipped.
Repeated mowing while they are in the bud stage of growth can reduce the
vigor of some perennials. Some perennial species, notably blackberries, do not
seem to be weakened by repeated mowing. Dogfennel (Eupatorium capillifolium) is
controlled very well by mowing. Studies conducted in Florida found that a late
summer mowing reduced dogfennel regrowth by 81%. Herbicides plus mowing reduced
regrowth by 94%.
Following a mowing schedule is necessary if mowing is to have any effect on
weeds, especially perennials. Research at the University of Missouri shows that
repeated mowing (three times per year for two years) reduces the population of
perennials such as goldenrod and western ironweed by as much as 80 percent. Some
species may require up to six mowings per season to prevent
seed production. Many farmers have commented that they seem to have fewer weed
problems in their hay fields. This is undoubtedly due to regular mowing and
fertilization of these areas.
Mowing also stimulates the production of tender new forage for livestock to
graze. Tall fescue and broomsedge are two species that mowing makes more
palatable.
Soil Fertility
Good weed control is rarely a function of a single activity. A combination
approach is needed, and a program of soil testing, liming, and fertilization are
essential to get the most out of other weed control practices. Well-fertilized
forages are vigorous enough to prevent many weeds from becoming serious
problems. Before embarking on a weed control program with herbicides, the field
should be limed if needed and phosphorous and potassium levels brought up to
soil test recommendation levels. A good fertility program will greatly increase
the amount of forage produced while reducing annual herbicide cost.
Broomsedge, also known as sagegrass, is a weed that can be controlled through
a combination of fertilization and other improvement practices. Researchers in
Missouri drilled tall fescue into a pasture infested with broomsedge and then
began a program of N, P,K fertilization. The broomsedge was eliminated over a
four-year period. In a separate study, P and K fertilization eliminated
broomsedge over five years.
Pasture weed control research from North Dakota State University showed that
weed control can increase forage output by 74 percent while fertilizer alone
increased production by 80 percent. Combined, weed control and fertilization
boosted yields by 138 percent.
Grazing
Grazing can be an economical method for control of some weeds. Grazing
livestock consume many weeds. Several species of weeds are high in protein and
very palatable when they are immature. Little barley is very palatable for a
time in the spring, but cattle tend to avoid it after seedheads emerge. The same
is true for the weedy brome species such as cheat and downy brome. Mowing at the
seedhead formation stage encourages cattle to eat these weeds since regrowth is
more palatable than the mature growth.
The best control of sandburs comes from using a good fertility program
combined with intensive grazing. This combination has been most effective in
bermudagrass pastures. Both weed growth and seed production are reduced in a
well managed grazing system.
Flash grazing can control weeds in new forage plantings. Flash grazing is the
practice of putting a large number of animals on a pasture to graze for a short
period of time then rotating them off to allow the forage plants to recover.
Cattle are forced to eat most of the weeds which reduces weed competition.
Obviously, sufficient cattle numbers and strategically located fencing are
important when using grazing for weed control. Electric fences, once cattle are
used to them, are the most efficient tool for managing intensive grazing.
Tillage
Tillage can be used when a seedbed is being prepared or if the soil is being
tilled before sprigging or seeding. Early seedbed preparation allows time for a
couple of crops of weeds to be destroyed before establishing the new forage
crop. While it is practically impossible to reduce the soil weed seed bank in
the short term, lightly disking a couple of flushes of weeds will minimize weed
competition later on.
This method is often referred to as the stale seedbed technique. Applications
of nonselective herbicides will accomplish the same as light cultivation with
less soil disturbance.
Rotation
Rotating a pasture to another crop is often an excellent way to control
unwanted plants. On tillable sites, it is possible to take advantage of crop
competition and seedbed preparation as a means of weed control. Crop
establishment through no-till practices is also effective. Rotating to another
crop such as sorghum sudan or small grains is part of our current
recommendations for eliminating endophyte infected tall fescue. Crop rotation
also allows the use of a wider variety of herbicides since different crops have
different tolerance levels to certain herbicides.
Burning
Properly timed spring burning can increase bermudagrass yields and help
control weeds. The black residue left by burning absorbs more heat in the
spring, getting the bermudagrass off to a little faster start.
Burn timing in spring is important to avoid injury to bermudagrass. Time
burns about one week before the average date of the last killing frost, which is
just before the bermudagrass begins to green-up. The average last frost dates
are March 11 for south Arkansas, March 22 for central Arkansas, and April 15 for
north Arkansas. Burn should be made a few days after a rain to avoid serious
damage to plant parts near the soil surface. Enough fuel is needed (about 2,000
pounds per acre) to ensure a fire hot enough to kill weeds. Avoid cutting hay or
heavy grazing after September 1 if you plan to burn in spring.
Fields with tall native warm-season grasses may have more residue, and the
extra fuel can result in a hotter fire than the shorter residue on bermudagrass
fields. Hot fires will help control cactus, red cedar, and oak. Never burn in
late summer or early fall or many winter annual weeds will germinate making the
weed situation worse.
Follow a burn prescription that includes proper fire line preparation,
optimum weather conditions and many other factors. The local Arkansas Forestry
Commission office can give advice in deciding whether to burn.
Biological Control
Biological weed control is the use of natural enemies to reduce weed
populations to economically acceptable levels. The common types of biocontrol
agents are insects, plant disease organisms such as fungi, and grazing animals.
One example of bio-control is the musk thistle flower head weevil
(Rhinocyllus conicus) used for control of musk thistle in pastures. These
weevils have been released at several locations in northwest Arkansas and it
will take several years for their populations to grow enough to be effective.
The flower head weevil larvae eat the developing seeds in the musk thistle
flower. Over time the weevil reduce the number of seed produced so the
population of thistles declines. A second weevil species, called the musk
thistle rosette weevil (Trichosirocalus horridus), is established in several
south-central Missouri counties and may spread to northern Arkansas. The rosette
weevil larvae feed on the growing point in the crown or rosette of the young
musk thistle plant. Infested plants are weakened and produce less seed or are
killed. The feeding of the rosette weevil compliments any control from the
flower head weevil.
Care should be taken when using herbicides or mowing to control musk thistles
if the weevils are present. To get the most effective control from the weevils,
herbicides should only be applied in the fall. Musk thistles are in the rosette
stage during fall and are much easier to control with herbicides than in spring
after seed stalks have begun to form. The weevils will feed on any remaining
plants.
Chemical Weed Control
Specific recommendations for chemical weed control can be found in the
publication “Recommended Chemicals for Weed and Brush Control” (MP 44). This
publication is available at your county Extension office.
Determine the following before deciding on a chemical weed control program:
1. Find out what weeds you have and the density of the population. Make a
map of each field and make a note of which weeds are most common. Record
whether it is a light, medium, or heavy population and how they are
distributed over the large fields. If you are unable to identify the weeds,
get some help from someone such as your county agent. The weeds that are
present in the fall have set seed and can be expected to be problems next
year. Mapping will help get you off to a quick start in a control program as
soon as weather permits in the spring.
2. Determine the soil type, especially the proportion of sand, silt, and
clay plus organic matter and pH. These are important factors when using soil
active herbicides.
3. Consider environmental factors such as soil erosion potential,
nontarget species (your neighbor's tomatoes), and nontarget sites such as
minnow ponds, chicken houses, etc. Failure to do so may result in hard
feelings, lawsuits, etc., later on.
4. Look at the equipment and manpower available. Check into custom
services. Be realistic. Don't expect more from the technology than it can
deliver. Just because you use an expensive herbicide, it doesn't mean that
all weeds will be eliminated.
5. Determine the availability of herbicides for controlling a particular
weed problem in a specific forage situation. Some herbicides may be labeled
for use in established bermudagrass or alfalfa, but cannot be used when the
crop is being established. Other products may control a weed species, but
cannot be used on a particular forage crop. It is illegal to use a herbicide
on a forage or weed that is not listed on the label of that product.
Information on recommended products for different forages is available at
the county Extension office.
6. Finally, look at cost versus return. What works for someone else may
not work for you. Each producer has to find the system that works best.
Herbicides for Broadleaf Weed Control in Grass Pastures
Herbicides are the most selective and often the most economical way of
controlling pasture weeds. Herbicide selection depends on weeds present and the
forage species growing in the pasture.
2,4-D is the most commonly used pasture herbicide in Arkansas. It is cheap,
effective, and readily available. Some weeds are equally susceptible to the
ester and amine formulations of 2,4-D, but the rule of thumb is that ester
formulations are about 20% more effective than amine types. Ester formulations
are generally preferred for pasture weed control because they penetrate the waxy
leaves of some weeds better than amine formulation and are more effective on
brush species. 2,4-D should not be used on perennial legume/grass pastures.
2,4-DB is used in legume pasture weed control. Its primary advantage is that
certain legumes have a fair degree of tolerance to this herbicide. It is sold
under trade names such as Butyrac and Butoxone. Tolerant legumes include
established red and white clover, lespedeza, and alfalfa.
Ally (metsulfuron) is a member of the sulfonylurea family of herbicides. This
is a relatively new group of herbicides registered for use in many crops.
Members of this family are very active at low application rates. For example,
the recommended application rates for Ally are from 0.1 to 0.3 ounce of product
per acre. Sulfonylurea herbicides kill plants slowly over two to three weeks.
Ally may persist in the soil for some time. Consult the label for crop rotation
restrictions. Ally will control many of the 2,4-D resistant weeds such as
smartweed and red sorrel. It is the most effective treatment for some tough
perennials such as groundsel (Senecio spp.), cancerweed (Salvia lyrata). Ally is
weak on ragweed, dogfennel and thistle.
Banvel (dicamba) is a member of the benzoic acid family of herbicides. Its
primary use is for the postemergence control of broadleaf weeds in grass crops.
Banvel readily leaches through the soil and may be applied as a soil spot
treatment for species such as multiflora rose and persimmon.
Weedmaster is a mixture of 2.87 pounds 2,4 D amine and 1 pound of dicamba
(Banvel) per gallon. It is used in much the same manner as 2,4 D, but it
controls a broader spectrum of weeds, including smartweed, red sorrel, and
dogfennel.
Grazon P+D is a premix of 2 pounds 2,4 D amine and 0.54 pound picloram
(Tordon) per gallon. The addition of picloram to 2,4 D increases the range of
control. Grazon P+D controls most of the 2,4 D susceptible weeds and other
species such as red sorrel, smartweed, and dogfennel. It also does better on
horsenettle control than other broadleaf herbicides. It is the only option for
prickly pear control at 2 to 4 quarts per acre plus surfactant at 0.5 percent.
Grazon P+D is a restricted use herbicide. Picloram is a persistent herbicide
that may remain active in the soil for years. It is also quite water-soluble
which makes it a potential groundwater contaminant. Users are advised not to
apply picloram where soils have a rapid to very rapid permeability. These soils
include sandy and loamy sand soil types. Do not apply to areas with where the
underlying aquifer is shallow. Clarksville, Doniphan, and Woolper soil series in
Arkansas may have channels near the soil surface which would allow infiltrating
water to directly enter an underlying aquifer. Check with your county agent for
information on soil types in your area.
Weed Control during Establishment
If a seedbed is to be prepared for forage establishment, early preparation
allows time to let one or two flushes of weeds emerge and be controlled with
tillage before planting. Using herbicides to control perennial weeds before
planting is another option. Planning ahead is essential because it takes an
entire growing season to achieve appreciable control of perennials such as
common bermudagrass or tall fescue with herbicides.
Flash grazing can be used to control weedy grasses and some broadleaf weeds.
This is best done after the desirable forage species is firmly rooted and before
weeds become too large and unpalatable. Mowing provides some control of upright
growing broadleaf weeds. Selective herbicides are excellent for weed control
during establishment if the correct combination of forage species tolerance and
weed susceptibility can be achieved.
Bermudagrass Weed Control
Bermudagrass possesses characteristics that make weed control methods in this
grass a little different than in a forage grass such as tall fescue. These
include:
1. Good herbicide tolerance and the ability to spread by runners.
2. A dormant season that allows the use of nonselective herbicides such as
Roundup Ultra and Gramoxone Extra for weed control without injury to the grass.
3. Hybrids are established from sprigs rather than by seed presenting more
opportunity for chemical weed control during establishment since sprigs are less
sensitive to hebicides than are seedlings.
Bermudagrass Sprigging Weed Control
Sprigging as a means of bermudagrass establishment offers more weed control
options than seeding because sprigs have more tolerance for herbicides than
young seedlings.
Controlling weeds during sprigging reduces the time required to establish a
solid stand of bermudagrass. Weeds compete for water, sunlight and nutrients.
Grasses are more competitive for water and nutrients but broadleaf weeds will
seriously inhibit bermudagrass through shading. Weeds begin to compete when they
are very small, thus they must be controlled early. Some recent data from Auburn
University shows that, if left uncontrolled, crabgrass can cause up to a 68
percent reduction in dry weight production of newly sprigged bermudagrass. In
the same study, sandbur caused as much as 40 percent reduction in bermudagrass
growth. When all of the expenses of sprigging are considered, weed control costs
are minor relative to the benefits that they provide.
Weed Control in Dormant Bermudagrass
The winter dormancy period gives bermudagrass growers an excellent
opportunity to safely control unwanted cool-season plants without interrupting
grazing or harvesting. Perennial species such as tall fescue, dock, dandelion,
and wild garlic can be controlled during the winter. Bermudagrass growers may
also choose to control the winter annual weed complex that includes species such
as little barley, cheat, buttercup, Virginia pepperweed, cutleaf evening
primrose, Carolina geranium, henbit, and chickweed. These weeds usually
disappear after the first cutting. If little barley is the only winter grazing
available, it may not be wise to control it. The nutritive quality of
cool-season weeds can be very high while they are immature can provide high
quality and palatable feed for grazing animals before bermudagrass growth
begins. On the other hand, if you are growing high quality hay (such as for the
horse market), it is important to know that the effect of winter annuals is more
than cosmetic. They compete with bermudagrass and slow its greenup and early
growth. The presence of mature, cool-season weeds in the first hay cutting may
lower its quality.
Bermudagrass Release
This is a fairly simple, inexpensive procedure whose only prerequisite is a
partial stand of bermudagrass mixed with endophyte fescue. Bermudagrass release
takes advantage of the fact that nonselective herbicides can be used to control
fescue while bermudagrass is dormant. A good rule of thumb is that at least 20
percent of the stand should be bermuda at the beginning of the program.
Obviously, the sparser the bermuda stand, the longer it will take to get
complete bermudagrass cover.
Alfalfa Weed Control
Weeds can reduce the yield and quality of alfalfa. Weeds compete with alfalfa
for light, water, and nutrients. Since alfalfa seedlings grow slowly, the crop
is most vulnerable to weed competition during establishment. Once a stand is
well established, weed control becomes less important until the stand begins to
thin due to age or other problems.
The economic benefits of controlling weeds in alfalfa are obvious. Because
establishment costs are high, stand failure due to weed competition is
unacceptable. In addition, stands, which receive good weed control, yield more
and better quality alfalfa.
Research shows the yield of spring-seeded alfalfa to triple by controlling
weeds. Long-term studies show weed control to be of maximum benefit during the
first growing season.
Cultural Alfalfa Weed Control Practices
Using management to control alfalfa weeds is a matter of applying good common
sense. Key cultural practices include (1) good seedbed preparation, (2) using
high-quality, well-adapted varieties, (3) planting weed-free seed, (4)
fertilizing according to soil tests, and (5) proper cutting management.
A good seedbed should be well tilled, firm enough to retain moisture, yet
loose enough to allow root penetration. A vigorous, healthy alfalfa stand
competes with weeds and reduces the need for other control practices.
Weeds are usually a bigger problem in spring-seeded alfalfa. Warm
temperatures and frequent rainfall create a favorable environment for weed seed
germination. For this reason, fall has been the choice season for planting
because the seedlings get a head start on summer weeds. One problem with fall
seeding is that rainfall is often limited.
Mowing
Mowing suppresses weeds in seedling alfalfa. Wait at least six weeks after
alfalfa emergence before mowing. Mow at the normal height. Forage and weeds can
be chopped and blown back if the hay is not harvested.
Vigorous, mature alfalfa stands compete very well with weeds. Mowing alfalfa
at the right time gets rid of many weed species. Be careful not to weaken the
stand through repeated cuttings of immature plants. Cuttings made when one-tenth
of the alfalfa plants are blooming ensure maximum vigor and yield.
Time early cuttings to prevent seed production by winter annuals. Be sure the
alfalfa is at least 10 inches tall before an early cutting is done for weed
control purposes. Frequent harvests of alfalfa inhibit johnsongrass in the
stand.
Pasture Brush Control
Killing unwanted trees and brush or preventing stumps from sprouting is a
problem for many grassland farmers. A wide range of herbicides and application
methods are available to control undesirable trees or prevent stumps from
re-sprouting. The first step is identifying the target plant. Persimmon,
sassafras, and (greenbrier) sawbrier are very tough species to control with the
herbicides approved for pastures. Some of the herbicides provide only
suppression or partial control. Complete brush control from a single herbicide
application rarely occurs in chemical brush control.
Chemical brush control means carefully considering the environmental impact
of the herbicide and application method. Herbicides used for woody plant control
vary in environmental stability, leachability, flashback potential, selectivity,
and handling requirements. Herbicides may damage surrounding vegetation,
contaminate groundwater, and prevent desirable vegetation from becoming
established for several years. The herbicide label lists hazards that may make
it unsuitable in certain situations. Read and follow the requirements on the
herbicide label closely.
While mechanical brush control is usually more expensive, time consuming,
erosion prone (if done with a dozer) and less likely to achieve root kill, it
avoids the problems of standing dead brush and handling and applying herbicides.
Herbicide killed trees eventually decay, but this may take several years.
Options to avoid standing dead brush are to (1) cut the brush with an axe or
chain saw and then treat the stumps to prevent resprouting, or (2) treat the
brush and then cut the trees after they have died. With most herbicides, the
brush can be safely burned as firewood.
Important Considerations for Brush Control
Consider the following factors carefully before choosing a control method.
Each factor may affect the success of your brush control program.
Suberization. Plants use this natural healing process to prevent
insects or diseases from infesting tissues after cuts or wounds occur. The
plants develop a layer of protective, corky cells over the damaged tissue.
Suberization can reduce herbicide effectiveness by preventing absorption. When
you use hack and squirt or cut stump methods of application, apply the
herbicides immediately to achieve maximum absorption. Delaying application of
water-soluble herbicides for as little as one hour can reduce absorption and
subsequent control.
Formulations. The herbicide formulation may affect its performance
characteristics. Match the formulation and application method. For example,
water soluble amine formulations of 2,4-D and triclopyr are preferred for hack
and squirt applications. For basal applications, use oil soluble ester
formulations such as Crossbow or Remedy. Other herbicide formulations include
wettable powders, dry flowables, water dispersible granules, or flowables. These
soil applied formulations need rainfall to move them into the soil and thus the
root zone of the plants so they may be taken up by the trees.
Dyes. Adding a dye to the herbicide solution greatly improves
applicator accuracy. The dye helps the applicator keep track of treated trees,
which prevents skips and repeat applications. Dye also makes it easier to see if
any herbicide has splashed or spilled on the applicator. These agricultural dyes
are water soluble and wash off readily.
Dripline. This refers to the area directly under the spread of the tree
limbs or canopy. Herbicide labels often caution against applying soil active
herbicides within the dripline to avoid damaging desirable trees. Tree roots
often extend well beyond the dripline. One rule-of-thumb is that tree roots
extend a distance from the trunk equal to the height of the tree. It is better
to err on the side of caution when using soil active herbicides such as Spike or
Velpar around desirable trees.
Application Methods
There are several approaches to applying herbicides for brush control. They
range from very simple hand methods to elaborate mechanical means.
Hack and Squirt. This is a very simple method of killing trees with
herbicides. It is best suited to trees at least 4 to 5 inches in diameter. Use a
hatchet to make a series of downward cuts in the bark. Immediately apply the
herbicide into the cuts. Apply herbicides registered for this purpose undiluted
or in dilution ratios from one-half to one quarter strength. It takes about one
cut for every two inches of trunk diameter for most species. Amine formulations
of triclopyr, picloram, and 2,4 D are generally more effective than esters.
Grazon P+D undiluted or is excellent for hack and squirt applications.
Stump Treatment. This is the cutting down of a tree and treating the
freshly cut surface with a herbicide. Cut the top of the stump level to allow
uniform herbicide coverage. Thoroughly wet the cambium layer next to the bark so
the conducting tissue carries the herbicide to the roots. On larger trees, treat
only the outer 2 to 3 inches of the stump. The internal heartwood of the tree is
already dead. On trees three inches or less in diameter, treat the entire cut
surface. Apply treatment immediately after cutting for maximum effectiveness.
While there is some reduction in effectiveness after one hour, you have about 4
hours before any serious drop-off in penetration occurs. If application is
delayed after cutting, re-cut the stump and apply the herbicide to the live
tissue. Delaying herbicide application to freshly cut trees can result in
prolific sprouting from the tree collar and roots. Moisture stress may affect
control during the summer and early fall. Applications during the spring upward
sap flow are not as successful as late spring and early summer treatments.
Remedy and Crossbow are good stump treatment products that mix readily with
diesel fuel. Water soluble formulations are very effective if mixed with diesel
by using an emulsifier.
Basal Bark Treatments. Apply the herbicide to the lower 12 to 18
inches of the trunk from early spring to mid-fall. Some species can be treated
during the winter. Use herbicide mixed with oil until the bark is saturated. The
low volatile ester formulations are the only oil soluble products registered for
this use. This method is effective on trees of all sizes, but it is most
commonly used on small brush.
Foliar Treatment. Foliar treatment is a common method of treating
brush up to 15 feet tall by hand and all sizes by airplane or helicopter. Small
brush can be sprayed with broadcast equipment such as a boom sprayer or a
cluster nozzle. Timing varies from early summer to late September, depending on
the herbicide used. Foliar treatments are least effective during periods of
drought stress. Adding a surfactant improves the performance of most foliar
applied herbicides. Drift control additives are also available to reduce the
number of fine droplets produced. Do not use diesel as an additive when applying
herbicides to foliage. Diesel kills the leaves before herbicide can be
translocated by the plant. Spraying until runoff is not necessary. Just spray to
wet the leaves, and keep moving.
Soil Treatment. Herbicides applied to the soil surface move into the
root zone of the target plants with rainfall. Commonly used soil applied
herbicides include Spike and Velpar. Spike and Velpar may be applied in narrow
bands. Banding or lacing applies concentrated solution to the soil in a line or
band spaced every 4 to 6 feet. This type of application is used to kill large
numbers of trees as in the case of a fencerow. Soil active herbicides may also
be applied on a spot or individual tree basis with an exact delivery handgun
applicator, also known as a spotgun.
- Complete brush control from a single herbicide application rarely occurs
in chemical brush control. Consider these suggestions to improve the chances
of success with control of woody plants with herbicides:
- Identify the dominant brush species on the site and choose the herbicide and
rate of application that best fits the situation.
- Apply at the recommended time of year and do not treat drought stressed
or dusty plants.
- Add a surfactant and use sufficient spray volume to get good coverage.
- Do not use diesel fuel as a carrier for foliar herbicides.
- Plan on making a follow-up application the next growing season. When making
follow-up treatments, use a different herbicide because the escapes are probably
tolerant of the product used for the first application.
Herbicide Safety
Herbicides are formulated to kill plants and some are more toxic to humans
and the environment than others. Manufacturers provide safety information for
each chemical on the product label. The following list of safety practices
should be observed whenever pesticides are used.
1. Read the label for safety instructions.
2. Consult the herbicide label for worker protection precautions. Wear chemical
proof gloves
when mixing and handling herbicides, cleaning nozzles, etc. Neoprene gloves are
good for this purpose. About 90 percent of pesticide exposure is through hands.
3. Wear eye protection, especially when mixing and handling concentrates.
4. Rubber boots will prevent exposure and also avoid ruining leather footwear.
Once leather is soaked with herbicide it is released as you sweat while wearing
the boots, etc.
5. Have plenty of soap and water available for emergency washing. If you spill
chemicals on
yourself, wash or shower immediately and change clothes. Do not wait until you
are finished spraying.
6. Wear long-sleeved shirt and long pants or coveralls. Disposable Tyvek® suits
can be
purchased at reasonable cost.
7. Wash clothes daily separate from other clothing. Wash with hot water on long
cycle.
8. Wear a respirator approved for chemicals when handling products in enclosed
areas.
9. Keep emergency phone numbers handy.
10. Follow label guidelines for pesticide cleanup and disposal to reduce
environmental contamination.
Keep Records of Herbicides Applications
Federal law mandates that private applicators keep records of all applications
of “restricted use” pesticides. Application records are also good management
tools that can be used to improve a long-term weed management plan. The
following information should be included in the pesticide application records.
- Record product name, formulation, manufacturer, and the EPA product
registration number on the label.
- Record application date, amount of herbicide applied (both total and rate
per acre), and the number of acres treated. Note pressure, nozzle size, and
gallons spray mix applied per acre and surfactant if any.
- Record field name or number treated.
- Note the weather conditions at the time of application and time of day.
Calibrate the sprayer accurately. Surveys indicate that many farmers are not
within 20 percent of their target rate. Calibrate carefully to save money and
reduce application errors to a minimum.
Use a sprayer with good agitation. Bypass agitation is often insufficient,
especially if you are operating near the limit of your pump’s capacity. Since
the suction is located in the bottom of the tank, it is possible to apply most
of some herbicides on the first couple of rounds and then under apply on the
rest of the field. The heavy rate applied on the first few rounds will kill or
injure the crop.
Flag or stake the field or use a foam marking system to avoid overlaps. Any
overlapped strips will receive twice the target rate. Do not dress the ends of
the field where turning took place. At best, this practice will result in
over-application and, depending on conditions, crop injury. Equip your sprayer
with an electric solenoid control valve so that the boom can be turned on and
off with a toggle switch mounted next to you on the tractor.
For more information about forage management, contact your county Extension
office or refer to one of our
publications.
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