WEED MANAGEMENT IN PERENNIAL PASTURES AND HAY FIELDS

Transcription

1 Forages Leaflet 17 Rev. May 1992 WEED MANAGEMENT IN PERENNIAL PASTURES AND HAY FIELDS B.W. Pinkerton and E.C. Murdock Extension Agronomists Department of Agronomy and Soils The first step in managing pasture weeds is understanding why weeds invade pastures and hay fields. Biological systems, such as pastures, are complex interplays of climate, soil, plants, animals, and people. Given the dynamic nature of these systems, few rules of thumb are always true, but they can be helpful. One general rule can be stated: weeds invade and colonize bare spots in pastures and hay fields. Weeds easily become established in the absence of desired forage species or in areas where the forages are uncompetitive due to poor growth, inadequate plant populations, and overgrazing. This means that weeds will tend to occupy bare ground in a pasture or hay field. Bare ground can occur in pastures and hay fields for many reasons. One of the most obvious is drought; unless you can irrigate, you cannot directly manage drought occurrence. However, other aspects that you can manage to reduce the occurrence of bare ground may also reduce the impact of drought on pastures. Insect and disease pressure can thin pastures and hay fields, but in general do not represent a major problem except in alfalfa (which will not be addressed in this leaflet). Improper ph can lead to stand thinning. A poor fertility program, including both amounts of added nutrients and the balance between added nutrients, will always lead to stand thinning. For example, large amounts of nitrogen on bermudagrass with little or no potassium will make the field susceptible to winter injury and thinning. Lastly, poor grazing and cutting management are, perhaps, the most prevalent causes of bare ground. Healthy pasture forage plants are fairly resistant to weedy plant invasion. Drought and improper ph, fertility, and defoliation practices decrease forage plant vigor and health, and make the pasture susceptible to weed invasion. Invading weeds tend to be rapidly growing, less palatable, more drought tolerant, and more insect and disease resistant. Weeds often possess some mechanism for dealing with defoliation. Some species change growth habit to become more prostrate under hay field mowing, or may be so unpalatable that they are not grazed in a pasture. Thus, we often see a different set of weeds in hay fields than in pastures that are grazed but seldom mowed. As the discussion above suggests, weeds in pastures are symptoms of underlying problems, rather than the base problem in and of themselves. Proper management methods for liming, fertilizing, cutting, and grazing are discussed in other related forage leaflets available from local county extension offices and will not be treated further at this point. However, the underlying cause of the weed problem should be corrected as the weed problem itself is addressed. If the cause of the weed problem is not corrected, any solution or action taken to control the weeds will be temporary, at best. Weed Identification Weed control, which is only one aspect of an overall weed management program, should always begin with identification of the weed or weeds that you wish to control. County extension staff, SCS personnel, chemi-

2 cal company representatives, and farm supply store personnel are all potential sources for assistance with weed identification. You can help a lot yourself. Take a specimen of the weed when seeking help and keep it in good shape; this may mean wrapping it in a damp paper towel, or even digging a sample and placing it, complete with soil and roots, in an old jar or pot. For the first level of identification determine if the plant is a grass or a broadleaf plant. Keep in mind that most identification keys work best with a flower or seedhead. Try to narrow the possibilities. Is it a warm season or cool season plant? Warm season weeds emerge in the spring. If they are annuals, the seeds germinate as the soil warms, and if they are perennials, they break dormancy and begin to grow. They grow, flower and set seed through the summer and then die or go dormant in the fall. Cool season plants generally emerge in the fall, then die or go dormant in the spring to early summer. They may be annuals or perennials. There are some biennial weeds. The thistles are good examples of biennials. They emerge from seed and remain small rosettes for a year. In the second year they flower and set seed. Weeds are also classified as herbaceous or woody. Woody plants, as the name implies, tend to have woody stems and grow fairly large. Examples include the dewberry/blackberry complex, multiflora rose, and various tree sprouts, such as persimmon. Herbaceous plants are more low growing and have soft stems, such as henbit and horsenettle. The characteristics examined above can be easily observed and are very helpful in weed identification. For example, characterization of a specimen as a woody, broadleaf, and warm season perennial narrows the possibilities considerably. In order to effectively control weeds, it is also important to know the history of the field and to note how the weed has responded to management. How extensive was the weed infestation last year? Do livestock consume the weed at all? If the weed is a perennial, has the field been mowed much? This can be very important since some weeds will have a tremendous root system but the amount of top growth may appear to be insignificant because the field has been mowed several times. Has the particular weed problem been treated with a herbicide; if so, which one, at what rate, and when? Before actual methods of weed control are examined, one final point needs to be addressed. You should have a set of overall management goals for your forage-livestock system firmly in mind, and preferably on paper, before you plan and implement a weed management program. Establishing goals will help prevent a hit-or-miss approach and in the long term will improve profitability. 2 Cultural Methods of Weed Control Proper ph, fertility practices, and defoliation management can prevent or slow down many weed invasions. However, once most weeds are established, correcting management problems in cultural practices seldom eliminates the weed. In some cases, such as with broomsedge, good management of cultural practices over several years may reduce the weed pressure, but only over the long term. This should not be automatically dismissed; if your overall management goals, and current operating system allow, consider the long-term cultural approach. Most of the more common weeds in South Carolina pastures are not controlled by mowing. Many may even spread and cover a greater area under a mowing program. However, mowing can be a useful tool when used to prevent seedhead formation of weeds that are not otherwise being controlled. This can be somewhat effective on annuals, unless there is already a large supply of seed in the soil, which is often the case. Biological Biological pasture weed control should also be considered. Goats offer the opportunity to turn weeds into feed. They will seek out and consume the dewberry-blackberry briars, McCartney rose, tree sprouts, and many others. Sheep will consume many of our common small herbaceous weeds. (Sheep and goat production, in terms of weed management, will be discussed in a future leaflet.) There are also cases of beneficial insects or diseases that may just affect target weeds. This area of weed control is being explored. Chemical In South Carolina, several herbicides are labeled for pasture and hay field use. As in all cases you should read and follow labeled instructions. The section below provides guidelines for use of herbicides for weed control. A major point to remember is that regardless of the degree of top kill, if the roots are not killed the weed is alive and regrowth will occur. Herbicide Selection. This should follow the weed identification, as various weeds respond differently to various herbicides. Refer to the weed response chart provided at the end of this leaflet to assist in proper herbicide selection (Table 1). Label Restrictions and Instructions. Read and follow all label restrictions and directions for use. These can save a lot of time, frustration, and money. For example, a label warning says not to use Banvel on seedling tall fescue because of potential injury; one pint Banvel per acre can severely damage or eliminate a newly seeded stand of tall fescue. The other type of restriction or warning on pasture herbicides deals with

3 the length of time after treatment before the forage can be utilized. Table 2 outlines these deferment periods. However, these time periods may change as labels are revised and should be confirmed on the label when the herbicide is purchased. When looking for information concerning weeds, herbicides, and application, keep in mind that the label on the herbicide container represents several million dollars worth of work; instructions and information on the label should be abided by. An extremely important management point needs to be stressed. Neither the livestock nor the hay may require a deferment period, but the pasture may seriously need a rest. The pasture may need time to recover and to fill the spots where the weeds have died. In some cases where there were extensive weed infestations, you may have to re-establish the desired forage species in areas formerly occupied by the weeds. Timing. Timing of application is often critical in achieving successful weed control. Most weeds are controlled best when very young and small. This is almost always the case with annual weeds. However, woody perennial plants generally need to have enough size and leaf area to ensure adequate herbicide absorption and translocation, which is necessary to obtain good, if not complete, root kill. Rate. Herbicide usage rates vary greatly. Using too little herbicide will provide incomplete and nonuniform, if any, control at all. And, although using too high a rate will generally give good control of pasture weeds, there are three problems associated with excessive rates: 1. In some cases the high rate is too effective and top kills the weed before the herbicide is translocated to the root, giving poor control. 2. Excessive rates substantially increase weed management costs. 3. Application of excessive rates may cause severe environmental problems. Environmental Conditions. Any herbicide can drift with the wind! Do not spray in winds greater than 8 to 10 mph, and always be aware of susceptible crops and plants on the downwind side of the pasture you are treating. This caution applies to yard and ornamental plants and trees. Consider temperature at both extremes. While some herbicide/weed combinations are relatively temperature independent, others are not. For example, the activity of 2,4-D is reduced at temperatures below 50 F; and at temperatures above 88 F 2,4-D is subject to volatilization. This process changes a liquid into a gas, and then the gas can float in and through the air. Volatilization occurs even with no wind. Most pasture herbicides require a six-to-eight hour rain-free period to achieve maximum weed control. These times vary some for each herbicide. Do not apply herbicides when heavy rainfall is expected since off-site movement and subsequent damage to non-target species may occur. Another aspect of rain, soil moisture, should always be considered. Most plants are less susceptible to herbicides when they are under drought stress. Mowing can injure drought-stressed weeds, and keep them set-back while waiting for soil moisture conditions to improve. However, the mowing may also injure the forage plant. In these cases mowing works best if there is a height differential with the weeds taller than the forage plant. The decision to spray weeds at the correct stage of growth, while under some level of drought stress or to wait for rain, even though the plants will be more mature than wanted, is not an easy one, and has to be made in light of the overall management plan and economics. Summary The presence of weedy species in pastures or hay fields is often an indication that the field may be suffering from poor fertility, overgrazing or too frequent cutting, or some other management or environmental factor that caused the forage species to be less competitive. Proper forage crop management is an essential component of a weed control system and can help prevent the establishment of weeds. Herbicides can be used effectively to control weeds if needed. Consider your forage management goals, then plan and implement an effective weed and crop management program. Additives. Some herbicides perform better if an additive is used. The best example is Ally, which for most applications is ineffective without the proper amount of a nonionic surfactant. Additives usually enhance absorption of herbicide into the plant. A crop oil can be used to increase the toxicity of some herbicides, but is not recommended for any of the labeled pasture herbicides. 3

4 Table 1. Common pastsure and hay field weeds response to labeled herbicides with broadcast applications.* Herbicide Ally Banvel Crossbow 2,4-D Weedmaster Bahiagrass 1 E N N N N Bitter sneezeweed E G - E E G - E G - E Blue vervain P P - F P P - F Bracken fern G P G Buttercup G - E G E F - G G Cherry, black P P F P P - F Chickweed E G G P F Cocklebur E E E E E Common mullein G F - G G P F - G Crotalaria G G P G Dandelion G E G F - G F - G Dewberry/blackberry 2 F G E P G Dock, Curly E G G F E Dogbane, Hemp P G G - E P - F F Dogfennel F F - G F P F - G Garlic, Wild E P - F F P - F P - F Geranium, Carolina G - E G - E G G G - E Greenbriar P F F P F Henbit E F - G G P F - G Honeysuckle P F - G G P - F F - G Horsenettle, Carolina F - G F F - G P F - G Horseweed G - E G G F F - G Jimsonweed F - G G - E P G Kudzu P F - G G - E P F - G Lettuce, Wild G G G - E F G - E Mayweed chamomile G F - G G F F - G Mustards E G - E G - E G - E G - E Nightshade, Black F F - G G F F - G Nightshade, Silverleaf F - G G G - E P - F G - E Onion, Wild E P - F F P - F P - F Persimmon P F F P F Prickly pear P P P P P Privet P P - F G P P - F Purslane G - E G G P - F G Radish, Wild G - E G G G - E G - E Ragweed F - G G - E E F G - E Rose 3 F G E P G Sorrel spp. G G F - G P - F G Sowthistle G F - G G P - F G Spiny amaranth E E E E E (Pigweed) Sunflower F - G G G F G Sweetgum P P F - G P P - F Thistle 4 G F - G G - E P - F G Trumpetcreeper P G G P G * Please see top of next page for explanation of table. 4

5 Ratings: Not suited None N 0 to 50% control Poor P 51 to 79% control Fair F 80 to 89% control Good G 90+% control Excellent E 1 Bahiagrass is the only grass weed listed. The labeled herbicides basically have no activity on the grasses, except Ally on bahiagrass. 2 The Rubus spp. complex of dewberry, blackberry, and raspberry respond similarly to the labeled herbicides. 3 The Rosa spp. complex of Cherokee, McCartney, and multiflora rose respond similarly to the labeled herbicides. 4 These include the annual thistles; bull, musk, and plumeless thistles, but not the pasture thistle that has been identified in Fairfield County. Table 2. Livestock withdrawal periods for pasture herbicides. Animal Use Herbicide (rate) Grazing Hay Slaughter Milking Days Ally Weedmaster Crossbow < 1.5 gal/ac > 1.5 gal/ac Banvel < 1 pt/ac pt to 1 qt/ac qt to 2 qt/ac qt to 8 qt/ac ,4-D Many companies manufacture 2,4-D and labels vary widely between companies. Follow the label on the band purchased. Not all 2,4-D's have pasture labels. The Clemson University Cooperative Extension Service offers its programs to people of all ages, regardless of race, color, sex, religion, national origin, or disability and is an equal opportunity employer. Clemson University Cooperating with U.S. Department of Agriculture, South Carolina Counties, Extension Service, B.K. Webb, Director, Clemson,S.C. Issued in Furtherance of Cooperative Extension Work in Agriculture and Home Economics, Acts of May 8 and June 30,