C o n s e r v a t i o n T i l l a g e S e r i e s Number Two

An Introduction to Management for Conservation Tillage Systems

Planning a Weed Management Vegetation Considerations Program To establish a new crop in Managing in reduced-tillage conservation tillage systems, systems requires a planned approach. it is necessary to kill or Through planning, successful suppress emerged vegetation conservation tillage producers anticipate before or at planting time. potential problems, find timely “Preplant” vegetation can include solutions, and make use of integrated a fall-seeded cover crop or an old pest management techniques (IPM), grass or alfalfa hay crop; or it can such as field scouting and crop rotation, simply be weeds that emerge prior to find alternative pest management to crop establishment. Managing such strategies. Flexibility and foresight also vegetation in no-till systems generally aid in success. means using a , but in reduced- This fact sheet discusses ways to till it can also involve chisel plowing, manage weeds and other vegetation field cultivating, or disking, depending such as hairy vetch (Vicia villosa) in conservation tillage systems using on the desired tillage system. Which or cereal rye (Secale cereale), can cultural, mechanical, and chemical control strategy to use will depend on provide excellent cover when properly control tactics. See individual several factors, including the emerged managed; they may even help to reduce commodity sections within the Penn species, timing of control, and how the presence of certain problem weeds. State Agronomy Guide or related much tillage the grower wants to do. Living like crown vetch publications for specific weed (Coronilla varia) can provide long-term management guidelines for corn, Cover Crops soil management benefits without the soybeans, small grains, and forages. Cover crops often require additional need for annual establishment. Several factors make weed attention in conservation tillage Mismanagement or neglect of management in conservation tillage systems. Winter annual cover crops, any cover crop, however, can lead unique. In reduced-tillage agriculture, to additional weed problems and growing vegetation may already be the potential for significant losses in present at planting time and must be crop yield. See the fact sheet entitled Weed Management Factors in killed or suppressed. Certain types “Cover Crops for Conservation Tillage Conservation Tillage of weeds are more common in no-till Systems” for more information on cover and may require special consideration. ■ Vegetation may be present at crop management. If weed pressure or plant residue is planting time intense, an adjustment in herbicide rate ■ Certain weeds are more common Weeds or alternative control strategies may be Certain traits help define what we call ■ Herbicide rate may require required. Occasional use of selective weeds. A characteristic of many weeds adjustment tillage may be necessary to keep is the ability to invade and succeed problem weeds in line. Finally, other ■ Selective tillage may be necessary under almost any environment. Certain management techniques, such as crop ■ Crop rotation is critical for habitats favor certain weeds, and rotation, are critical. managing problem weeds conservation tillage provides a specific habitat. 2

Annuals reproduction can allow a few plants to operations. Therefore, in limited tillage Annual weeds emerge from the soil, become a large community within a systems, there is greater dependence on produce seed, and die in a single season short time. cultural and chemical control options. or less than one year. There are two The simple perennials, such as types: winter and summer annuals. dandelion and the dock species (Table Cultural Winter annuals begin growth in late 1), have one main taproot and spread Cultural weed control may include summer or early fall and set seed in principally by seed. Creeping perennials altering cropping patterns or rotations; spring before dying. Table 1 includes spread by rhizomes, budding roots, row spacing; planting or harvesting examples of these and other types of stolons, tubers, bulbs, or corms; they date; variety selection; and fertility weeds. Summer annuals have a life include Canada thistle, quackgrass, practices. These strategies may be well- cycle that begins in late spring and ends and even woody perennials like wild planned events, such as crop rotation, in late summer or fall. Examples are the blackberry (Table 1). Some perennial or simply preventive techniques that foxtails, fall panicum, large crabgrass, species were once thought to be are more subtle and spontaneous. and small-seeded broadleaved weeds harmless or even aesthetically pleasing. Spot spraying or hand removal of an such as pigweed and lambsquarters An example is Japanese knotweed, isolated infestation can prevent further (Table 1). which now shows up as a weed in no- spread of problem weed species, Some annual weeds can germinate till environments. generally requiring little time or capital with little or no seed burial and are investment. In contrast, rotation to a common to no-till environments. In Options for Weed Management small grain or forage crop to interrupt general, the smaller the seed, the more An important benefit of tillage is weed life cycles or altering the row adapted the plant is to a reduced-tillage weed control. In conservation tillage spacing on a planter to increase crop environment. These problem annual agriculture, the grower relies on the competition over weeds can involve weed species tend to be prolific seed same weed management practices as more time and money. Regardless of producers; they germinate readily under in more conventional tillage systems the level of input, cultural weed control crop residue, and depending on species, but eliminates most or all of the tillage strategies reap long-term benefits. can emerge in the cool soils of April or May or during the warm summer rains of July. Their primary means of spread Table 1. Common weeds in no-till systems. is by seed production. Winter annuals Simple perennials Creeping perennials Biennials Common chickweed Common dandelion Canada thistle Other weeds that thrive under reduced- Henbit Dock spp. Hemp dogbane tillage conditions are the biennials. Red dead nettle Common pokeweed Bindweed spp. These weeds, which include burdock, Shepherdspurse Summer annuals Common milkweed musk, and plumeless thistle, have a Field pennycress Common lambsquarters Horsenettle two-year life cycle (Table 1). Biennials Wild mustard Pigweed spp. Quackgrass emerge from seed and exist as low- Yellow rocket Foxtail spp. Wirestem muhly growing rosettes the first year. The Wild radish Fall panicum Johnsongrass following spring or summer, biennial Pepperweed spp. Large crabgrass Yellow nutsedge plant stems elongate, flower, and produce Downy brome Common ragweed Wild garlic/onion seed, ending their two-year life cycle. Horseweed/marestail Velvetleaf Virginia creeper Biennials usually have a large taproot Prickly lettuce Annual smartweed Wild blackberry that extends deep into undisturbed soils Biennials Eastern black nightshade Sumac spp. of no-till fields. Like annuals, biennials Common burdock Jimsonweed Japanese knotweed reproduce and spread only by seed. Bull thistle Rough (daisy) fleabane Perennials Musk/plumeless thistle Perennial plants live for more than two Wild carrot years. Many reproduce and/or spread Wild parsnip through vegetative means as well as Poison hemlock by seed. Perennials thrive in soils with Common mullein little or no tillage where vegetative 3

Crop rotation may have the largest soil surface. Larger-seeded broadleafs chopping up the perennial root system single impact on weed control. No- (e.g., cocklebur, burcucumber) generally without effective follow-up measures till producers who raise continuous or can germinate from soil depths of 1½ may actually increase the weed multiyear corn, soybeans, or forages can inches or greater. For lasting benefit problem. But tillage combined with an expect to have greater weed problems in conservation tillage systems, tillage effective systemic herbicide application and to invest more time and money must be used in conjunction with other can have a greater impact on weeds than in weed control than their neighbors strategies such as effective herbicide either tillage or herbicide alone. who practice a more diverse rotation. programs and crop rotation. Disking or Regardless of tillage system, crop rotation can provide an opportunity to manage some problem weeds effectively. Table 2. Optimum and maximum seed emergence depth of several Fall-seeded cereal grains like wheat weed species. are very competitive against summer annual weeds that become common in Optimum Maximum corn or soybeans. Including a fall- or Weed (inches) (inches) spring-seeded small grain in the rotation Common lambsquarters 0.2–0.4 2.0 can allow timely herbicide applications Canada thistle 0.4 2.4 for perennial weed management Common chickweed 0.4 0.8 following the small grain harvest. Crabgrass 0.4 1.6 Rotation to forages where repeated Giant foxtail 0.2–0.6 1.2 mowing is part of the management Green foxtail 1.0 3.0 strategy will reduce the frequency of Burcucumber 1.0–2.0 6.0 certain annual and perennial weeds. Grass forage and hay crops can be Sources: King 1966 and Mann 1981. very competitive and can permit the application of several effective broadleaved . Crop rotation’s benefits are diverse and run well beyond the realm of weed management. Figure 1. Components of a conservation tillage cultivator. Mechanical Weed Control Mechanical weed control includes Down pressure tillage and mowing. Except in strict springs no-till situations, many producers still Parallel links Tool bar rely on some mechanical control within the rotation. -tillage systems often depend on tillage prior to planting. Although chisel plows, disks, and field cultivators generally are not as effective Shank as the moldboard plow, they can help Furrowing suppress weed populations and still wing maintain a certain level of surface plant residue. These “less aggressive” tillage tools kill emerged weed seedlings Depth wheel but are less effective in burying weed seeds beneath their maximum depth of emergence (see Table 2) or in disrupting perennial vegetative structures. In general, most small-seeded weeds Residue cut- (e.g., foxtail, pigweed) germinate and Wide sweep ting coulter Disks emerge within the upper half inch of the 4

Cultivation and Specialized band applications of herbicides, while dependable weed control tool in higher- Tillage Systems between the row weeds may be removed residue environments. Row Crop Cultivation with cultivation. Mowing Row crop cultivation is a good In general, two trips with a complement to chemical weed control. cultivator are necessary for successful Mowing can be effective for managing In fact, producers who wish to enhance control. Most current models of certain cover crops and some weed weed control or reduce herbicide use conservation tillage cultivators include species. Managing weeds with mowing may find an alternative management ridging tools along with standard is generally limited to forage or pasture tool in cultivation. Two or more passes high-residue sweeps (see Figure 1). situations. Frequent mowing (every 30 with the cultivator may be necessary, Because of their weight, high-residue to 60 days) can reduce perennial weed but cultivation can allow for reduced cultivators require a fair amount of competition and longevity as well as herbicide rates, especially when the lifting and pulling power (horsepower), prevent seed production for many types herbicide is banded. The first cultivation and most models come with guidance of weeds. It can also prevent the spread should be done early, so as not to prune systems that can help prevent damage of troublesome weeds into cultivated crop roots, and deep enough (4 to 6 to the crop. Regardless of the type of fields. inches) to kill weeds and loosen soil. cultivator, cultivation on highly erodible When properly timed, mowing can Weeds should be shorter than 4 inches land can be challenging even when a successfully control certain cover crops and the crop tall enough (greater than 6 guidance system is being used. before the primary crop is planted. Mow inches) to avoid being buried under soil legume cover such as hairy vetch after Mulch-Till Systems the first purple flowers appear. Mow or crop residue. Mechanical operations in mulch-till The second cultivation should be cereal grains after heading to ensure systems are designed to leave 30 percent adequate control. Mowing cover crops done when the crop is under 24 inches or more residue on the soil surface. tall to allow the cultivator to pass like hairy vetch or cereal rye prior to Generally, no more than two passes with flowering can fail to provide adequate through and avoid disturbing the root a chisel plow, disk, or field cultivator system. This single later cultivation control and could result in severe are needed before planting. For competition with the primary crop. may suffice if early weeds have been maximum weed control, one operation controlled with herbicides. Combining is performed early (fall or early spring) Chemical Weed Control mechanical and chemical methods is and the second is completed as late Chemical weed control remains an economical and effective. as possible (immediately before important pest management tactic in High-residue row crop cultivators planting). For greater effectiveness, the reduced-tillage agriculture. Regardless can be used to help manage small second operation should be delayed of how effective cultural control annual weeds in reduced-tillage until the soil warms and the initial strategies are, herbicides provide a way systems. Conservation tillage cultivators flush of germinating weed seeds has to manage weeds successfully with little are relatively heavy and made to pass emerged. With the less frequent and or no tillage. Which type of control through larger amounts of surface intense tillage of the mulch-till system, method to use depends on the tillage residue while dislodging emerged weed certain weed problems may increase. system. More options are generally seedlings. An effective herbicide program and/or available for mulch-till than for no-till Ridge-Till Systems cultivation must compensate for this. situations. Chemical approaches are High-residue cultivators are the Except for in-ridge tillage, post- based on timing of herbicide application backbone of weed control programs in planting tillage operations such as and include burndown, soil residual, and ridge-tillage systems. Weed control and rotary hoeing or row cultivation are postemergence treatments. conservation practices such as residue seldom used in conservation tillage management can be maintained by systems. The rotary hoe can be effective Burndown Herbicides cultivation and/or herbicide application. for controlling emerging weed seedlings Burndown herbicides kill or suppress In ridge-till, weed seeds and “trash” are in corn or soybeans in conventional live vegetation present at planting removed from the ridge and deposited systems. However, the rotary hoe time. The need for and selection of a between the rows. Ridge-till equipment was designed for use in low-residue burndown herbicide depends on the type can move as much as 70 percent of environments and encounters problems and quantity of vegetation. Burndown the previous year’s weed seed out of as surface residue increases. While herbicides are either systemic or the crop row. The remaining weeds the rotary hoe may have a place in contact-type. Systemic herbicides are within the row can be controlled by some mulch-tillage systems, it is not a absorbed by plants (roots or leaves) 5 and move throughout the plant with tillage systems are also used in reduced- Clarity, or Marksman) or 2,4-D water (“xylem-mobile”) or with sugars tillage systems. There is one exception— preemergence. (“phloem mobile”). Phloem-mobile herbicides that must be mechanically Where weed pressure is heavy and/ herbicides include glyphosate (Roundup incorporated are impractical in no-till or plant residues are high, you may or Touchdown), 2,4-D, and dicamba environments—but incorporation need to adjust the residual herbicide (Banvel). These chemicals are generally of products may still be possible in rate. Herbicide sprays may be initially used to kill larger annual and perennial reduced-tillage systems. Herbicide intercepted by plant residues and remain grasses or broadleafs like alfalfa or treatments can be mechanically bound to them until sufficient rainfall is other hay or pasture crops. incorporated in mulch-till systems, received. In addition, residue may act Contact herbicides including allowing for more herbicide options as a physical barrier during herbicide paraquat (Gramoxone Extra) are not than no-till preemergence treatments. application, making uniform spray mobile in the plant and kill only the Regardless of tillage system or timing, coverage difficult. tissue surface contacted by the spray. soil residual herbicides depend on There are few clearcut rules about A burndown application of paraquat is rainfall for mobilization and even herbicide rates under conditions of high well suited for smaller annual weeds distribution in the soil and subsequent plant residue, but several herbicide or younger cereal grain cover crops. uptake by germinating weeds. labels recommend an increase in In corn or soybeans, soil residual rates. In heavy crop or weed residue Application Timing for Soil-applied broadleaved herbicides may also situations, check the herbicide label Herbicides serve as all or part of the burndown for specific rate guidelines. In general, In conservation tillage systems, soil program if only certain annual weed residual herbicide rates for grasses residual herbicides are applied to seedlings are present at planting time. may need to be increased 10 to 25 crops early preplant, preplant, or Emerged weed seedlings not destroyed percent above rates for conventional preemergence. One way to avoid using a by tillage or a burndown application tillage systems if large amounts of burndown herbicide in corn or soybeans may subsequently be more difficult plant residue and/or heavy weed is to make early preplant applications to control because of greater size or pressure exist. Specifically, if severe (EPP). An EPP application is simply maturity. Specific burndown and other populations of foxtail or fall panicum applying the residual herbicide(s) 15 herbicide information can be found are anticipated, grass herbicide rates to 30 days ahead of planting. Ideally in more detail under the individual may need to be increased or the the herbicide is applied in the spring, commodity sections within the Penn application timing altered to include before new vegetation has emerged. State Agronomy Guide or related split applications or postemergence Care must be taken, however, to avoid publications on corn, soybeans, small control. Remember that herbicide rates applying the herbicide too early or grains, and forages. are not based solely on the amount of residual control may be inadequate. surface residue, however, but on other In general, an application about 10 to Soil-applied Herbicides factors as well. 15 days ahead of planting is plenty of Soil-applied herbicides are generally lead time. EPP herbicide application applied before the weed or crop Selecting Herbicide Rates rates may require a slight increase over emerges. The timing of herbicide Before selecting the correct soil-applied conventional application rates (10 to 25 applications can be classified by the herbicide rate, consider: percent). Check the herbicide label for ■ number of days ahead of planting as soil texture and organic matter—soils the appropriate EPP rates. early preplant, preplant, or preemergent. with higher percentages of clay and organic matter require higher Soil-applied products are mostly xylem- Effects of Increased Plant Residue herbicide rates. mobile systemics that are taken up by In reduced-tillage systems, greater ■ soil texture and proximity to the roots (or shoots) of germinating amounts of plant residue can affect the groundwater—coarse or sandy soils, seedlings. They are used primarily to performance of soil residual herbicides. high water tables, or wells close to prevent annual weed emergence or When planting into heavy residue, be fields may dictate that lower rates or growth in annual cropping systems by sure the seed furrow closes completely different products be used. Generally, maintaining “residual” activity over an and the germinating crop seed does low solubility, low persistence, extended period. not remain exposed to the subsequent or postemergence herbicides may “Soil residual” herbicides generally herbicide spray. This is especially be used to avoid groundwater are those supplying at least four weeks true if you are using herbicides like contamination. of weed control. Many of the same soil pendimethalin (Prowl) or applying a ■ surface residue—high amounts of residual herbicides used in conventional product that contains dicamba (Banvel, 6

surface plant residues may require In summary, managing weeds in higher herbicide rates. reduced-tillage systems may present ■ soil moisture—rainfall is needed to a challenge. But successful growers move the herbicide into germinating anticipate potential problems through weed seed zones; reduced rates are planning and field scouting, applying particularly subject to failure without timely solutions, and using crop rotation adequate soil moisture. to present alternative pest management ■ application timing—early preplant strategies. Remember, foresight and applications may require higher flexibility are keys to success. rates. ■ severity and types of weeds—larger References weeds and/or perennials may require Mann, R. K., C. E. Rieck, and W. W. higher herbicide rates. Witt. 1981. Germination and emergence of burcucumber (Sicyos angulatus). Postemergence Herbicides Weed Sci 29(1): 83–86. The performance of postemergence or King, L. J. 1996. Weeds of the World: foliar-applied herbicides is generally Biology and Control. New York: unaffected by tillage system, so Interscience. application rates are the same as for conventional tillage systems. In general, apply postemergence herbicides when annual weeds are young and most susceptible to the herbicide spray. As is true for soil-applied herbicides, the effectiveness of postemergence products depends on weather. Rainfall shortly after application or drought stress on weeds may reduce herbicide absorption by plant tissues. Postemergence applications offer some control of perennial weeds. Applications 3 to 6 weeks after corn or soybean planting will suppress weeds like hemp dogbane, horsenettle, and Canada thistle. Broadleaved perennials, however, are most susceptible to a systemic herbicide application as they approach their flowering period. Unfortunately, this period usually coincides with the time when corn is 4 to 6 feet tall and soybeans are in full bloom—not an appropriate time to apply a postemergence herbicide. For most perennial broadleaved weeds in spring-seeded crops, reducing the competitiveness of the weed in the current year’s crop is the most that can be expected from a timely herbicide application. Perennial weed management requires a diverse crop rotation along with the use of selective herbicides. Prepared by William S. Curran, professor of weed science, Dwight D. Lingenfelter, program development specialist, and Lyn Garling, program manager Penn State College of Agricultural Sciences research and extension programs are funded in part by Pennsylvania counties, the Commonwealth of Pennsylvania, and the U.S. Department of Agriculture.

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