Hay Crop and Pasture Weed Management
Hay crops and pastures are essential, highly valuable resources for Tennessee's beef cattle and dairy industries. Many ingredients must come together to insure optimum yields of high quality forage for beef and dairy animals. One of the key ingredients is a well-planned and well-executed weed management plan. Fortunately, compared to many agronomic and horticultural crops, forages are more competitive with weeds, so the likelihood of weed problems requiring yearly herbicide applications is lower.
Why, then, do weeds become troublesome in forage crops? Weeds can become troublesome because most grow rapidly, and many are prolific seed producers. Many of the seed produced can remain dormant for years. For example, research has shown that musk thistle, curly dock and pigweed can produce as many as 10,000, 40,000 and 120,000 seed, respectively, per plant in one growing season. Ultimately, weeds become troublesome in hay crops and pastures because they find room to grow. This room to grow may be due to a thin stand because of low seeding rate, poor internal drainage of the soil, droughty soils, low pH and/or poor fertility status or overgrazing.
Whether it is permanent grass pasture, alfalfa or other legumes, a bermudagrass hay field or sorghum-sudangrass hybrids, management of the crop for maximum competition is the first and most effective weed management input. In practically all cases, it is also the least expensive.
What are the weed life cycles of Tennessee?
Is there weed management for Alfalfa?
What is good pasture management?
Are herbicides in pastures needed and what is the proper spray timing and volume?
What are the Fall (Nov to early Dec) spray opportunities?
Is there a check list for Grazon P+D and Surmount use in Tennessee?
Can spot spraying be used to control weeds?
What is weed management proper stewardship?
Can drift be prevented and sprayer contamination be reduced with pasture herbicides?
Are there poisonous plants in hay crops and pastures?
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What are the weed life cycles of Tennessee?
Common weeds that occur in Tennessee forage crops can be divided into four categories, based on their life cycle. Knowledge of weed life cycles is important in timing of certain weed control inputs such as clipping or application of herbicides. The four categories of weeds based on life cycle are:
Summer annuals -plants that germinate from seed in the spring, flower and produce seed in the mid to late summer, and die in the fall.
Winter annuals - plants that germinate from seed mostly in the late summer through winter, flower and produce seed in the late winter to spring, and die in the late spring to early summer.
Biennials - plants that complete their life cycle over two growing seasons. They germinate and produce a vegetative rosette the first year. The second year, the plants bolt, flower, produce seed and die.
Perennials - plants that produce a vegetative structure (taproot, tuber, bulb, rhizome, etc.) which allows them to live for more than two years. Most perennials also reproduce from seed.
Examples of common forage crop weeds and their classification according to life cycle are provided in 
Table 1.
Is there weed management for Alfalfa?
Seedling Alfalfa
In Tennessee, alfalfa is planted either in late summer to early fall, or in the spring. In both cases, the alfalfa seedlings are small and grow relatively slowly. Most weeds are stronger competitors for light, water and nutrients. Because of this, weeds can quickly outgrow the young alfalfa and this can lead to retardation of growth or even death of alfalfa seedlings.
In late summer to early fall-planted alfalfa, winter annual weeds such as chickweed, henbit, deadnettle or ryegrass may be major problems because these weeds emerge at the same time, or shortly after, alfalfa seedlings. These weeds continue to grow after the alfalfa becomes dormant. They become well-established during the fall and winter and compete vigorously with alfalfa when it resumes growth in the spring. Summer annual weeds such as cocklebur, ragweed, morning glory, pigweed or lambs quarters rarely create problems in fall-seeded alfalfa because frost kills them.
Planting alfalfa in the spring can reduce winter annual weed problems, but may allow summer annual weeds to create problems. Weeds such as crabgrass and the broadleaf summer annuals previously mentioned can compete vigorously with spring- seeded alfalfa.
Established Alfalfa
As alfalfa becomes established, it is a stronger competitor with weeds. However, many weeds can create problems throughout the life of the stand. In the late fall to early winter, as alfalfa growth slows and it becomes dormant, chickweed, henbit, deadnettle and other winter annuals can flourish. In the spring to summer, particularly where the stand is less than optimum or where alfalfa weevil damage has been severe, summer annual broadleaf weeds, crabgrass and johnsongrass can become troublesome. As an alfalfa stand ages and becomes less dense, perennial broadleaf weeds such as curly dock and plantain usually become more numerous.
Cultural Control
As with any crop, cultural weed control measures are essential ingredients in an effective alfalfa weed management program. Cultural weed control measures either prevent weed problems or increase crop competition. Crop competition is the best and cheapest method of long-term alfalfa weed control.
Cultural weed control begins with field selection prior to planting alfalfa. Selecting fields with well-drained soils will promote more rapid seedling growth and greater success in establishing an optimum stand. Also, avoid fields that are infested with troublesome perennial weeds such as curly dock and buckhorn plantain. If these weeds cannot be avoided, they should be controlled prior to seeding alfalfa. For example, corn would be a good crop to grow while cleaning up a curly dock infestation. Likewise, a number of options (2,4-D, Weedmaster, etc.) exist in established grass pastures for control of annual and perennial broadleaf weeds. Be sure to follow label directions regarding length of time to wait prior to seeding alfalfa, as alfalfa is sensitive to many herbicides used in rotational crops such as corn and grass pastures.
Reduce weed problems by establishing a vigorous stand of seedling alfalfa. Stands that emerge and grow rapidly usually are not as weedy as less vigorous stands. Warm temperatures, adequate soil moisture, good seed inoculation and adequate soil fertility are essential to making alfalfa competitive with weeds. Be sure to plant weed free seed of a recommended alfalfa variety.
Once alfalfa is established, good management over the life of the stand will help reduce weed problems. Proper cutting, fertilizing and liming according to soil test recommendations, and control of the alfalfa weevil and other pests will increase the competitive ability of alfalfa. Do not use manure to fertilize alfalfa, because the nitrogen in manure will stimulate weed growth. Also, weed seed are often present in manure.
Chemical Control
Even with the best efforts toward cultural weed control, herbicides are often needed in alfalfa production. Chemical weed control should be used as a supplement to a good cultural program and not as a replacement for it.
Recommended herbicides for alfalfa and other hay crops are listed in 
Table 2, and expected weed response ratings are provided in 
Table 3. Herbicides differ in terms of weeds they control, whether they can be used in seedling or established alfalfa, whether they can be used in dormant or actively growing alfalfa, whether they are for alfalfa-grass mixtures or pure alfalfa only, and how long to wait after use before cutting hay. The herbicide label contains information that will help to safely and effectively use the chemical. Rates, proper application timings, precautions and restrictions are on the label. Be sure to study the label and follow the instructions carefully.
A summary of grazing and hay cutting restrictions for recommended alfalfa herbicides is provided in 
Table 4.
What is good pasture management?
Nowhere does the old adage "An ounce of prevention is worth a pound of cure" apply better than in pasture weed management. A well-managed, vigorously growing grass or grass-legume sod is a difficult situation for weeds to dominate. Good pasture management begins at establishment. Select well-adapted grass and/or legume species that will grow and establish rapidly. This will minimize the length of time for weeds to invade easily. Lime and fertilize by soil test results. Proper pH and nutrient status will help insure that the forage will grow rapidly and be more competitive with weeds.
Once the stand is established, manage grazing properly. Overgrazing is a common cause of weed problems. Heavy grazing pressure may favor weed growth over grass. Maintenance of favorable pH and fertility status continues to be as important in established stands as it is at planting. Broomsedge has long been associated with acid and/or low phosphate soils. Liming and fertilizing according to soil test, combined with intensive grazing, will reduce problems with broomsedge. If stocking rates are not sufficiently high enough to allow intensive grazing, supplement with clipping. Lastly, the value of timely clipping is often underestimated. Properly timed clipping will kill many annual broadleaf weeds if they are cut below the growing point. Clipping will prevent or reduce seed production by annual and perennial weeds and stimulate new forage growth, which is usually of higher quality than older, more mature forage.
Are herbicides in pastures needed and what is the proper spray timing and volume?
Herbicides sometimes are needed in spite of a producer's best efforts in pasture management. Weeds usually are more of a problem in pastures than in hay fields because of lower cutting frequency and generally lower fertility levels in pastures. Overall, the quality of many thousands of acres of pastures in Tennessee could be substantially improved with a well-planned, inexpensive herbicide program. For example, a properly-timed application of 2,4-D will give excellent control of musk thistle and buttercup, two of the most troublesome pasture weeds in Tennessee. Recommended herbicides for pastures in Tennessee are listed in 
Table 5 & 5-2.
Importance of Proper Spray Timing- Unfortunately, due to the diversity of weed species in most Tennessee pastures, one spray timing will not work for all weeds. For example a March application of 2,4-D, which gives excellent control of musk thistle and buttercup, will not control summer annual weeds such as common cocklebur and pigweed. Conversely, a summer application will not control winter annual weeds. It is, therefore, important to properly time herbicide applications to get the most for your money and time expenditures. Expected weed response ratings for weeds normally sprayed in the late winter to early spring are provided in 
Table 6, and in 
Table 7 for weeds normally sprayed in the late spring to summer.
Spray Volume - Over the past 20 years we have seen a substantial decrease in spray volumes particularly with row crop herbicides. Roundup WeatherMax, Select and several other herbicides actually perform better at low (10 gallons of water per acre) (GPA) volume than at high (20 GPA) volume. However, pasture herbicides such as 2,4-D, Grazon P+D, Weedmaster and others must be applied in high water volume (minimum of 20 GPA) for good performance. This is due to the nature of the chemicals, the leaf structure and orientation of many pasture weeds, and in many cases, the unevenness or rolling nature of pastures. Except for improper spray timing, insufficient water volume is the most frequent cause for lack of weed control with pasture herbicides.
What are the Fall (Nov to early Dec) spray opportunities?
Winter annual weeds such as chickweed, henbit and deadnettle; biennials such as musk thistle; and perennials such as buckhorn plantain and some of the buttercups, are normally sprayed in late-winter to early-spring. However, most of these weeds emerge during the fall and may actually be controlled just as effectively in November to early December. In the cases of henbit and chickweed, control with Weedmaster or Redeem R&P is much better in November than in March. This is particularly true during years when winters are mild allowing growth throughout much of January and February. Table 6 lists response ratings for selected weeds when sprayed in the fall.
In addition to equal or better control in many cases, the fall application may allow renovation (addition of clovers) the following February to March, after herbicide residues have diminished. Another benefit of fall applications is that the November timing allows safe application in certain drift sensitive pastures, such as those near tobacco transplant beds or float beds. Trade-offs or disadvantages of fall applications include no control of musk thistle, for example, seedlings that may germinate during intermittent, relatively warm periods during December, January and February following the spray application. Also, producers are cautioned to proceed with care on newly-seeded fall pastures. Grasses such as fescue, orchardgrass and timothy should not be sprayed with most herbicides until they have tillered. Also, if legumes are seeded in the fall, they will be killed by these herbicides.
Is there a check list for Grazon P+D and Surmount use in Tennessee?
Grazon P+D and Surmount are marketed in a limited number of counties in Tennessee. These counties were chosen because they have little or no acreage of cotton, tobacco, and certain other sensitive crops or because the counties have had a history of Grazon P+D use without non-target problem. The University of Tennessee does not recommend the use of and Dow AgroSciences does not sell Grazon P+D or Surmount outside of these counties. See figure on page 17 of this publication. NOTE: These are the counties approved at the time of printing. Contact your County Extension Agent for the latest list.
Grazon P+D and Surmount are safe on established cool- and warm-season grasses used for pasture and hay production. They provide good control of a number of broadleaf weeds. Both provide some residual control. The residual effect will depend on temperature, soil type, moisture and plant sensitivity. These products will kill all pasture legumes and re-seeding should not be attempted within one year of application.
Grazon P+D and Surmount are restricted use pesticides, requiring applicators to have a commercial pesticide applicator certification card. They are restricted use due to the risk of injury to susceptible, non-target plants. Broadleaf crops, such as cotton, tobacco, tomatoes and others, are very sensitive to both herbicides. Care must be taken in use of hay from fields treated with Grazon P+D or Surmount. Do not spread manure from animals which have grazed on, or have been fed hay, on fields where picloram sensitive crops will be grown. Due to this sensitivity, it is recommended to use a sprayer dedicated to pasture applications only. Read and understand the label restrictions before use of this product.
If all of the following are answered as “satisfactory,” then an application of Grazon P+D or Surmount may be recommended:
- Is the site located within one of the approved counties for this herbicide?
- Does the applicator have a restricted use applicator certification or use a custom applicator?
- Is the site properly buffered from sensitive crops and other off-target species, including ornamentals?
- Is there surface water (ponds or streams) on site? If so, does the applicator know to keep a 50 foot buffer?
- Has the required personal protective equipment been prepared? G Are the wind conditions calm enough to prevent drift?
- Is rain in the forecast? If so, the application should be delayed.
- Does the applicator / land manager understand the grazing, haying and manure restrictions (see label)?
- Is there a risk of surface runoff of the herbicide, including erosion? (e.g., does the site contain steep slopes with bare soil?) If so, the application is not recommended.
- Is the site a permanent pasture? (If there is intention to rotate to any field crops, ornamentals, tobacco, vegetables or other vegetation, application is not recommended.)
- Does the applicator understand the sprayer cleanout requirements?
Tennessee Counties Approved for Grazon P+D and Surmount Application*
Can spot spraying be used to control weeds?
Some particularly hard-to-control weeds, such as multiflora rose and brambles, can best be controlled with spot applications using hand-held equipment and high volume applications. If possible, control these weeds before they infest the entire field. Recommended spot application programs for specific weeds are provided in 
Table 8. All of these recommended herbicides are cleared for use in pastures. Remember that for control of weeds in fencerows to which cattle have access, you must use a herbicide that is registered for pasture use.
What is weed management proper stewardship?
Successful use of herbicides in weed management programs for pastures involves proper identification of the weed or weeds, selection of the appropriate herbicide(s), thorough familiarization with label directions and precautions, and proper, timely application.
The herbicide label not only contains a list of weeds controlled and directions regarding rate and application timing, but it also will give you directions regarding waiting periods before grazing, cutting hay or slaughtering animals following application. In general, waiting periods are more restrictive for lactating dairy animals than they are for beef cattle, dry cows or replacement heifers. Also, waiting periods are usually longer for hay cutting than for grazing. Waiting periods for recommended pasture herbicides are provided in 
Table 9 & 9-2.
Another important point to remember is that just because a herbicide is labeled for turfgrass does not mean that it is labeled for forage grass. In fact, the number of herbicides cleared for forage use is much smaller than the number cleared for turfgrass. Examples of herbicides cleared for turfgrass, but not for forage, include DSMA, MSMA, Princep, Surflan and many others. Use of non-registered herbicides is a violation of federal law, and it may result in illegal, detectable residues in milk or meat.
Can drift be prevented and sprayer contamination be reduced with pasture herbicides?
Movement of small amounts of commonly-used pasture herbicides (2,4-D, Banvel, Crossbow, Grazon P+D, Redeem R&P, Weedmaster, etc.) away from treated fields can cause serious damage to sensitive crops such as cotton, soybeans, tobacco, tomatoes and other vegetables, and ornamental nurseries. Two types of drift, physical and vapor, can occur. Physical drift is the movement of liquid spray droplets (usually the finer or smaller droplets) away from the target. Factors which increase the likelihood of drift include wind, high temperatures and a sprayer set-up that produces high pressures and low application volume (a large number of small spray droplets). Vapor drift is most influenced by air temperature. Some chemicals volatilize (change from a liquid to a gas or vapor) readily at warm (85 F) temperatures. The higher the temperature is above 85 F, the greater the risk of volatilization. While less obvious than physical drift at the time of application, vapor drift can be just as damaging.
Similarly, small amounts of herbicides such as 2,4-D in sprayers can create serious problems if the same sprayer is used to apply pesticides to crops such as tobacco, tomatoes, peppers, melons and other vegetables. It is strongly advisable to have a sprayer dedicated for pasture herbicides, since small amounts of herbicides are difficult to wash out of the sprayer. This sprayer should not be used to spray sensitive crops.
The following are suggestions to reduce the likelihood of drift from pasture herbicides:
-Know adjoining farms and other properties well. Most producers are familiar with their neighbors and know if they grow sensitive crops. Check on when your neighbor, for example, plans to set tobacco, and which field he or she plans to use this year. Information of this nature will allow you to plan accordingly for individual fields. Also, be familiar with locations of gardens, outdoor tobacco float beds, greenhouses and container nursery operations, as the numbers of these are rapidly increasing in Tennessee.
-Calibrate your sprayer for low pressure. High volume (20 to 30 gallons per acre), low pressure (20 psi or less) applications will reduce the number of "fines" or small spray droplets. One of the problems with low pressure in the past has been that flat fan nozzles would not develop patterns adequately at low pressure. Manufacturers have made great advances during the last several years in the development of low pressure air induction spray tips which will perform adequately at low pressure. If your farm supply store does not stock these tips, ask the dealer to order them for you.
-Try to spray at a time of year when sensitive crops are not growing. This is often difficult to accomplish, because the optimum time for weed control may occur when a sensitive crop is in the field. However, some weeds, such as musk thistle, may be treated after mid-October with 2,4-D. This would be a good approach for a field across the fence from your neighbor's tomatoes or tobacco, in that you could treat at a time of year when the crop has already been harvested. This should also be considered in areas where cole crops (broccoli, cauliflower, cabbage) are grown, as most producers in Tennessee grow spring crops rather than fall crops of these sensitive vegetables.
-Avoid spraying on windy days. Although this is common sense, it is one of the most effective ways to reduce physical drift. Unfortunately, windy conditions are common during March and April, the time of the year for best control of weeds such as buttercup, musk thistle, buckhorn plantain and dandelion. In general, calm conditions are more likely to be encountered either early or late in the day. With early morning sprays, a light wind blowing away from the sensitive crop may be better than calm conditions, because temperature inversions are less likely to be encountered. A temperature inversion is when the air near the surface is cooler than that above it. Inversions are more common in cooler months.
-If sensitive crops are nearby, use the amine formulation of 2,4-D rather than the low volatile ester formulation. This is particularly important with late spring to summer applications, when warm (85 F) temperatures are likely to be encountered at or shortly after spraying. The amine formulation is much less volatile than the low volatile ester formulation. This is very important to remember, in that vapor drift will be worse under warm conditions, and that it can occur even a few days after application. Other herbicides that are temperature sensitive include Banvel, Crossbow and Weedmaster. Drift reduction measures such as low pressure, special nozzles, drift retardants, etc. help control physical drift, but do not reduce vapor drift.
Are there poisonous plants in hay crops and pastures?
In general, most poisonous plants are not very palatable and if given a choice, animals will not consume them. Factors that increase the likelihood of animal poisoning from plants include:
Lack of good forage- This is the most common cause of poisoning. While this can occur at any time of year with overstocking or deficient feeding, it is most likely in very early spring, late fall or winter, or during drought.
Deficient rations- Lack of required minerals or an unbalanced diet (only one type of plant) may cause an animal to graze on a plant not normally eaten.
Dry or partially dry water holes- This gives animals access to poisonous roots not normally exposed, or in some cases, high concentrations of toxic algae.
Waste or trash dumped in pastures- Hedge clippings and old garden refuse should not be dumped in pastures where cattle have access. Clippings of toxic shrubs such as yew, and weeds such as jimsonweed can create serious problems. Cattle, which are naturally curious, will seek-out these dump piles and forage through them.
Drought or frost- The sorghums (johnsongrass and sorghum-sudangrass hybrids), which normally are non-toxic, can develop dangerously high levels of prussic acid under conditions of drought, frost or trampling. The greatest risk is with early, light frosts in the fall. Also in dry summers, the sorghums will accumulate toxic levels of nitrates.
Over fertilization with nitrogen- Excessive nitrogen will lead to high nitrate levels in weeds such as johnsongrass and pigweed, and forages such as sorghum-sudangrass hybrids and pearl millet. Please note over fertilization problems are compounded by drought.
Herbicide applications- Some poisonous plants may become more palatable following application of herbicides such as 2,4-D and they may be grazed more readily. If poisonous plants are present in large numbers, grazing animals should be removed from the pasture prior to herbicide application, and they should not be reintroduced until the plants have turned brown and dry.
Table 10 & 10-2 provides a list of some of the more common toxic pasture plants encountered in Tennessee.
Poisonous Plants of the South- Basic information on regional poisonous plants which can be harmful to livestock. Learn to recognize common poisonous plants and immediately contact a veterinarian if poisoning is suspected.
Precautionary Statement
To protect people and the environment, pesticides should be used safely. This is everyone's responsibility, especially the user. Read and follow label directions carefully before you buy, mix, apply, store or dispose of a pesticide. According to laws regulating pesticides, they must be used only as directed by the label.
Pesticides recommended in this publication were registered for the prescribed uses when printed. Pesticide registrations are continuously being reviewed. Should registration of a recommended pesticide be cancelled, it would no longer be recommended by the University of Tennessee.
Use of trade or brand names in this publication is for clarity and information; it does not imply approval of the product to the exclusion of others which may be of similar, suitable composition, nor does it guarantee or warrant the standard of the product.
HAY CROP AND PASTURE WEED MANAGEMENT
Gary Bates, Professor and Extension Coordinator, Forage Specialist, The University of Tennessee
G. Neil Rhodes, Jr., Professor and Extension Specialist, The University of Tennessee
Gregory K. Breeden, Extension Assistant, The University of Tennessee
Scott McElroy, Professor, Extension Specialist, The University of Tennessee
