Agricultural pest management
Integrated Weed Management
(Reviewed 11/05, updated 1/10, corrected 9/16)
Sugarbeet is not a very competitive crop, thus weed control is mandatory, especially where sugarbeets are planted at final stand density and will not be thinned. Uncontrolled weeds can reduce sugarbeet yield by over 90%; even one barnyardgrass in 10 feet of row can cause about a 5 to 15% yield loss. Dense weeds make hoeing, the use of electronic thinners, cultivation, and harvest difficult.
In California, sugarbeets are planted from September through June. Weed populations in sugarbeet fields differ by season and location in the state. From October to February, during stand establishment until layby, winter annual weeds such as mustard species and annual bluegrass can be troublesome. Winter annual weeds die out in summer, but summer annuals begin germinating in March and continue throughout the summer growing season. Troublesome summer annual weeds include barnyardgrass, cocklebur, pigweed, velvetleaf, and knotweed. In some areas curly dock, a deep-rooted perennial, can be a problem. Overwintered beets can become infested with winter annuals again in fall.
Selection of the best weed management program is governed by several factors: (1) Geographic location, which determines planting date, weed spectrum, and irrigation or rainfall; (2) Date of planting, which determines weed spectrum and irrigation or rainfall; (3) Weed species present (or anticipated to be present), which determines choice of weed control method and choice of herbicides; (4) Availability and cost of hand labor for weeding, which determines if hand weeding can be considered within the program; (5) Availability of equipment, which determines how well cultivation can be conducted and if herbicides can be applied accurately and properly incorporated into soil, if required; and (6) Method of irrigation, which determines choice of herbicide and influences cultivation choices.
Economically acceptable weed control can only be achieved with a management program that integrates several methods as no currently available weed control practice provides complete weed control in a sugarbeet crop. Band applications of an herbicide in the crop row, combined with between row cultivation(s), form the mainstay of a sugarbeet weed management program. This combination reduces the amount of herbicide used and minimizes the need for labor, which results in lower production costs and less herbicide being placed in the environment.
Because sugarbeet is a long-season crop that requires many months to grow, season-long weed control is difficult because early-season weed control may not last until harvest. Overwintering of sugarbeets complicates this problem. A typical weed management program may include a preplant incorporated herbicide or a preemergence herbicide at planting, an early postemergence herbicide, possibly a layby herbicide application, and one to several cultivations coupled with hand hoeing. The actual sequence of herbicides used in the program and the timing of the applications will vary by region, and by planting and harvest date.
Several herbicides are registered for selective weed control in sugarbeet, but no single chemical will control all weeds that infest beet fields. Frequently two or more herbicides may have to be combined sequentially or as tank mixes to achieve adequate broad-spectrum weed control. The weed species present will to a large degree determine the choice of herbicides in such combinations. The necessity for correct weed identification cannot be overemphasized. The best weed management program can only be devised when knowledge of the weed species present is coupled with the herbicide activity type and weed susceptibility to the herbicide.
Most sugarbeet herbicides are applied as bands centered on the crop row. Width of the band applied depends to a considerable degree on the capability to conduct close cultivation. Narrower herbicide bands can be utilized if close cultivation can be achieved. This has advantages in cost reduction and also places less herbicide into the environment. It may be useful to increase the width of the band for herbicides used in late fall and winter plantings if cultivation is likely to be delayed because of wet soil conditions.
Poor or erratic weed control can occur with any herbicide used in sugarbeets. Unsatisfactory herbicide performance may be the result of several factors, such as poor land preparation, faulty herbicide timing and application, the presence of resistant weed species, wrong soil moisture conditions, or adverse weather before or after application. In addition, sugarbeet herbicides are not 100% selective and can, under certain conditions, cause stunting, death of leaf tissue (necrosis), or even kill sugarbeet seedlings. Some beet growth retardation can be tolerated provided the stand is not reduced, but it may lead to increased problems with seedling pests. Minimize herbicide injury to the crop as much as possible.
Ideally, sugarbeet fields should be monitored for weeds in the winter, spring, summer, and fall. If this is not feasible, monitor fields at least twice per year: in late winter to determine the cool season weed population and in late summer to determine the warm season weed population. While monitoring, it is particularly critical to note any weeds that have escaped control in the previous crop(s) and were able to set seed. Because seeds can remain viable in soil for years, monitoring done over a period of years, can provide the means to predict which species are likely to be present.
If no weed history is available for a field, take a soil sample from the field and germinate weed seeds to determine which species are present. It is critical to know this information before planting so that correct weed management decisions can be made, especially if postplant or preemergence herbicides are to be used.
WEED MANAGEMENT BEFORE PLANTING
Management of weeds in sugarbeets requires a combination of control strategies. Cultural, including rotation, and mechanical controls are considered the core of a weed management program. Reliance solely on herbicides for weed control is not sound management. Before planting a sugarbeet crop consider field selection, sanitation, crop rotation, land preparation, and preirrigation as they relate to weed management.
Choose fields known to be free of perennial weeds such as johnsongrass, field bindweed, and curly dock, annual weeds such as sunflower, cocklebur, velvetleaf, and wild beets, or other weeds that are difficult or impossible to control economically in the sugarbeet crop.
Strict adherence to plantback intervals is critical to follow because small amounts of selective herbicides used in a previous crop may remain (carryover) in the soil long enough to affect a sugarbeet crop planted the following season. Sugarbeets are very sensitive to substituted dinitroaniline herbicides such as trifluralin (Treflan) or pendimethalin (Prowl), which are used for weed control in cotton, safflower, beans, tomatoes, and alfalfa. Avoid planting sugarbeets in fields where these herbicides were used the previous year. Benefin (Balan) used in lettuce, napropamide (Devrinol) in tomatoes and peppers, or atrazine (Aatrex) in corn or sorghum may also carry over and injure sugarbeets if the interval between crops is too short. Do not plant sugarbeets in fields previously treated with halosulfuron (Sandea) for at least 36 months following treatment.
Many weeds and volunteer sugarbeets from previous crops may host diseases (e.g., beet yellows virus, curly top virus), insects (e.g., green peach aphid), and nematodes (e.g., sugarbeet cyst nematode), and thus act as sources of infestation for the sugarbeet crop. To reduce the risk of infestation, control weeds and escaped volunteer beets in or around sugarbeet fields. In addition, do not allow weeds to grow in irrigation ditches because seeds can float and be carried back onto the field.
Clean all field equipment before entering a field if the last field in which the equipment operated was weedy. Land planes and sugarbeet diggers have great potential to carry seeds, tubers, etc., from field to field. Prevention is often easier than controlling an established weed problem.
Do not plant beets in the same field more than once every 4 or 5 years to minimize disease, nematode, and weed problems. Weeds are less troublesome if beets are planted following tilled row crops and are more troublesome following pasture, alfalfa, broadcast-planted safflower, sorghum, or any other crop in which weeds were allowed to mature and set seed. Rotation allows reduction of weed populations that are difficult to control in sugarbeets, such as velvetleaf.
Uniform beds with accurate row spacing are essential for precision cultivation and permit application of narrower bands of postemergence herbicides. The degree to which precision cultivation can be performed is established at the time of initial bed preparation.
A well-prepared seedbed that is free of large clods permits precision planting with more rapid and uniform emergence of beet seedlings. Uniform seeding depth is critical when using preplant incorporated herbicides as increased depth of seeding can result in increased phytotoxicity to the seedlings. Well-prepared seedbeds also permit proper and accurate incorporation of preplant incorporated herbicides, leading to improved weed control. Soil that is too finely cultivated, however, may crust and inhibit germination.
Unless winter rains occurred, preirrigate before seedbed preparation. Preirrigation followed by cultivation improves the tilth of the seedbed and permits better mechanical incorporation of preplant herbicides. A preirrigation can also be applied following initial bed preparation if there is not enough rainfall to germinate weed or carryover crop seeds. Preirrigation is particularly useful following barley, wheat, oats, sorghum or safflower crops. After the weeds and volunteer seedlings emerge, shallowly cultivate the beds. Paraquat or glyphosate may be used in place of cultivation on preshaped beds. In sprinkler-irrigated fields where preemergence herbicides are used, preirrigation reduces the amount of water needed to germinate the crop. This can improve the activity and selectivity of herbicides because less water is needed and, thus, the herbicide is not leached too far into the soil.
Before the crop is planted, there are two major types of herbicides for weed control. The first group kills existing weeds that have emerged after the beds were formed; these herbicides are referred to as preplant foliar herbicides. The second group, preplant incorporated, controls weed seeds as they germinate. As the name implies, the latter herbicides must be incorporated into the soil soon after application to prevent volatization of the chemical and to move the herbicide into the soil zone where weed seeds germinate.
Postemergence herbicides such as paraquat (Gramoxone SL 2.0) and glyphosate (Roundup) are used to kill existing weeds on preformed beds before planting sugarbeets. Paraquat has contact action only and is thus most effective on young seedlings. Be careful that the chemical does not drift off the targetfield. Glyphosate has systemic action and is thus effective on established weeds. A few species, such as mallows (Malva spp.) and nettles (Urtica spp.), are tolerant to this herbicide and are not well controlled by it.
Preplant incorporated herbicides perform best when incorporated with a power driven rotary tiller with L-shaped tines. Observe label directions regarding depth of incorporation as not all of these herbicides require the same depth. Incorporating an herbicide like ethofumesate (Nortron) too deeply dilutes it and decreases weed control. Too shallow incorporation of cycloate (Ro-Neet) results in poor weed control because of volatization of the herbicide and lack of exposure of the seedling's underground shoot to the chemical. A 1- or 2-inch error in depth of incorporation can lead to substantial loss of performance. If beds have not been shaped accurately, precise depth of incorporation may not be possible and herbicide performance will be erratic. Preplant incorporation does not work well in cloddy soil and herbicide performance will usually be poor under such conditions. Excessive speed (over about 2 mph) with a power incorporator results in poor incorporation. The soil should be dry at incorporation in order to obtain optimum results.
Cycloate (Ro-Neet) must be incorporated immediately after application to reduce losses to volatility; this is particularly critical if the soil is moist. Disc incorporation of cycloate (Ro-Neet) can provide adequate control of grass weeds but often results in only partial control of broadleaved species; consequently, another herbicide is often required to obtain adequate broadleaf weed control. Disc incorporation also runs the risk of mixing the herbicide too deeply in the soil, thus increasing the risk of injury to the crop.
WEED MANAGEMENT AFTER PLANTING
Proper fertilization, irrigation, and insect and disease control measures promote good crop growth. A healthy, vigorous crop provides substantial competition that suppresses weed growth and acts as part of the weed control program. A healthy, vigorous beet is also better able to tolerate herbicides.
Final beet stands should be uniform with skips not over 18 inches. Weed-free beets closer than 18 inches apart will compensate for such skips. A uniform stand will help to compete with weeds, but even with 12- to 18-inch gaps, weeds can invade and become established in the space that exists before the crop canopy fills in.
No specific systems of biological control have been introduced for control of weeds in the sugarbeet crop. Many weeds are attacked, however, by endemic insects and pathogens. Such attack weakens the weeds and makes them less competitive with the crop, and reduces seed production. Examples of insects that attack weeds include leafminers on purslane, fleabeetles on groundcherry and nightshade, various lepidopteran larvae on pigweed, and carabid beetles eating weed seeds. Pathogens attack johnsongrass, barnyardgrass, and purslane among others. Insect and disease control practices should be used so that they minimize impacts on organisms providing natural biological control of weeds.
Cultivation is an effective method of weed control, especially in fields with low weed infestations. It is essential that bed shaping and planting be accurate in order to permit close, or precision, cultivation. Repeated shallow cultivations will dislodge small weed seedlings that emerge after each irrigation and can be performed until the beet leaf canopy closes over the furrow.
Weed control by cultivation must be coordinated with irrigation scheduling. Wet soil can prevent the use of cultivation equipment at the optimum stages of weed growth. Timing of irrigation following cultivation can also be critical. Irrigation too soon after cultivation can lead to rerooting of weeds. Wet soil in winter may delay, or even preclude, cultivation for weed control; this possibility should be considered when designing a weed management program for fall-planted beets.
Many different types of cultivation implements are available. Each should be adjusted to disturb only a shallow layer (ideally not over 2 to 3 inches) of soil to minimize pruning of the beet roots and bringing weed seeds up from deeper layers. Timeliness in cultivation is essential. Seedling weeds are much easier to kill than older established weeds. Random or synchronous thinning also reduces weed populations. Some tools that aid in removing weed seedlings in beets older than the 4-leaf stage are the Bezzerides row weeder, the Texas rod weeder, and various spyder wheels.
Include hand hoeing as part of a long-term weed management program, especially when weeds such as wild beets, sunflower, velvetleaf, and tolguacha, which are resistant to most of the currently registered herbicides, are present. Although hand hoeing is relatively expensive and has no long-term effect, it may be useful for situations where weeds have escaped control by other techniques or are too large to control with other methods. Hand hoeing is easiest when the weeds are small; large weeds are difficult to remove and their removal may damage the crop.
Following planting of the crop, there are three periods in which herbicides may need to be applied, depending on the weed species present. Preemergence treatments are applied after planting but before the first irrigation; postemergence treatments are applied when sugarbeets are in the seedling stage; and layby treatments may be useful after the crop has been thinned if continued emergence of weeds is anticipated.
Phenmedipham plus desmedipham (Betamix) gives erratic control when applied to weeds larger than cotyledon to 2-leaf stage of growth. Poor weed control has occurred when weeds were stressed for moisture at application, and low soil moisture also increases injury to sugarbeets. Temperatures above about 85°F on application day may lead to increased injury to the beets. When temperatures are high, or rising, applications made in the morning cause more injury; spraying after 3 p.m. will reduce injury to the beets. Injury is also less if temperatures are decreasing.
Split applications about 7 to 12 days apart (shorter split in spring and summer, longer split in late fall and winter) with the first application to cotyledon stage beets and weeds. This usually increases weed control and reduces injury to sugarbeets; the first application must not exceed 0.5 lb/acre. If pigweed is the predominant broadleaf weed present, application of desmedipham (Betanex) will provide slightly greater control than phenmedipham plus desmedipham (Betamix). The combination of phenmedipham, desmedipham, and ethofumesate (as a tank mix of Betamix plus Norton or a premix of Progress) can improve control of difficult-to-control weeds such as common knotweed.
Sethoxydim (Poast) must be applied with an oil concentrate adjuvant to obtain satisfactory activity. Repeat applications if perennials such as johnsongrass are present. This herbicide should not be mixed with any other herbicide; mixtures with phenmedipham plus desmedipham (Betamix) have resulted in decreased grass control. Soil moisture must be adequate at application in order to obtain high levels of grass control; low soil moisture reduces control substantially. High spray volume leads to reduced activity; follow label restrictions in relation to spray volume.
Neither trifluralin (Treflan) or EPTC (Eptam) have any activity against established weeds; it is thus essential that the field be weed free prior to application of these herbicides. Both herbicides must be physically mixed into the soil (incorporated) immediately after application, or they must be applied in the irrigation water.
UC IPM Pest Management Guidelines: Sugarbeet
K. J. Hembree, UC Cooperative Extension, Fresno County
Acknowledgment for contributions to Weeds:R. F. Norris, Vegetable Crops/Weed Science, UC Davis