1) SUMMARY
Sugarbeets are a relatively long-duration crop in comparison with many other row crops. In some regions of the Central Valley and in the intermountain production region, the crop may be harvested in five to six months after seeding; but in other regions, such as the Imperial Valley, the crop is grown through the winter and harvested in the spring after about eight to ten months' growth. In certain regions of the Central Valley, sugarbeets are planted in the late spring and then harvested the following spring (the beets are said to be overwintered) and are in the ground for about 12 months. This means that weeds must be controlled for relatively long periods, which may span two or three climatic seasons.
Problem Weeds: 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, chickweed, shepherdspurse, sowthistles, volunteer cereals, annual ryegrass, 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. Summer annual weeds that are often problems include barnyardgrass, johnsongrass, nutsedges, pigweeds, lambsquarters, common purslane, nightshade, velvetleaf, and knotweed. Again in fall, overwintered beets can become infested with winter annuals such as mustard species and wild oat.
Losses Caused by Weeds: The sugarbeet plant is a poor competitor against weeds. Uncontrolled weeds that emerge with the crop typically cause from 50 to 90 percent yield loss. Increasing weed density causes increasing magnitude of yield loss (Figure 1). As few as one or two weeds, such as barnyardgrass every five to ten yards of row, still cause economic loss. Little is known about the losses caused by other weed species under California conditions. Weeds present late in the season can hinder harvesting operations. High levels of weed control are essential for profitable sugarbeet production.
Many weeds and volunteer sugarbeets from previous crops may host diseases (e.g., beet yellows virus, curly top virus), insects (e.g., black bean and 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, weeds and escaped volunteer beets in or around sugarbeet fields must be controlled as part of an IPM program. Weeds known to be hosts of sugarbeet cyst nematode should be controlled in rotation crops.
Potential Impact of FQPA on Weed Management: Current weed management programs rely heavily on herbicides in the general class of carbamates. The herbicides EPTC (used at lay-by for late season control of many grass and broadleaf weeds), pebulate and cycloate (used preplant incorporated [PPI] for control of grass weeds and many broadleaf species) are all thiocarbamates. Economics have shown that the use of one of the latter two herbicides is essential to profitable production of the spring-sown crop. The herbicides phenmedipham and desmedipham, which are bis-carbamates, are used postemergence alone or in mixture for control of annual broadleaf weeds in all sugarbeet production regions of the state. There is no alternative postemergence herbicide that could replace these chemicals. With the currently available alternative herbicides it is thus likely that profitable sugarbeet production would not be possile in California, due to increased losses to weeds and increased costs of weed control, if the use of carbamate herbicides was canceled/suspended.
2) CURRENT PEST MANAGEMENT PRACTICES
The extended planting period, the long duration of the crop, the various growing regions in the state, differing weed spectra, and the lack of competitive ability mandate that an integrated program approach be taken for weed management. The current program integrates use of cultural, mechanical, and chemical controls. Cultural controls include varieties that are resistant to pathogens, and correct fertilizer and water use to ensure a vigorous crop can suppress late emerging weeds. Rotations are used to control perennial and other weeds that currently cannot be controlled in sugarbeets. Mechanical control includes use of between-row cultivation, mechanical thinners, and hand hoeing.
Selection of the best weed management program is governed by the following factors:
Biological Control: Biological control of weeds that infest sugarbeets is inadequate to provide economic weed suppression. Purslane, for example, is attacked by two leaf miners. Attack by these two insects can lead to complete defoliation of the weed, but in a study conducted by Dr. Robert Norris in 1997, this defoliation only resulted in reduction in competitive ability of the weed in one of four years. Although insects or pathogens attack other weed species to a limited extent, there is no evidence that such attack provides economic control of the weeds.
Cultural Controls: (A) Field selection/seedbed preparation: Most perennial weeds are difficult to control in sugarbeets. Avoid fields heavily infested with johnsongrass, nutsedges, and field bindweed. Some annual weeds, such as sunflower, cocklebur, velvetleaf, and wild beets, are difficult to control economically in the sugarbeet crop, and heavily infested fields should also be avoided.
It is critical to follow plantback intervals 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 trifiuralin (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 or peppers, atrazine (Aatrex®) in corn or sorghum, or imazethapyr (Pursuit®) in alfalfa can also carry over and injure sugarbeets if the interval between crops is too short.
Clean all field equipment before entering a new field if the previous field in which the equipment operated was weedy. Sugarbeet diggers, for example, have great potential to carry weed seeds, tubers, etc., from field to field.
Uniform beds with accurate row spacing are essential for precision cultivation and to 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.
(B) Planting date: Date of planting determines the weed spectrum that can be anticipated. Weed management programs must be adjusted to reflect the weed species that can be expected to grow. Climatic limitations are also important for certain planting dates. It may, for instance, be difficult to cultivate a fall-sown crop during the winter in the Sacramento Valley.
(C) Preirrigation: Unless winter rains occurred, beds should be preirrigated before seedbed preparation. Preirrigation followed by cultivation improves the tilth of the seedbed and permits better mechanical incorporation of preplant herbicides. Preirrigation is particularly useful following barley, wheat, oats, sorghum, or safflower crops to germinate the volunteer crop prior to seeding the sugarbeets. The beds should be shallow cultivated after the weeds and volunteer crop seedlings emerge. Paraquat or glyphosate may be used in place of cultivation on preshaped beds. In sprinkler-irrigated fields where pre-emergence 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 required to germinate the crop.
(D) Crop rotation: 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 populations of weeds that are difficult to control in sugarbeets, such as velvetleaf. Rotation also permits control of perennial weeds that can be troublesome in sugarbeets, such as field bindweed.
(E) Mechanical: Cultivation is an effective method for between-row weed control. It is essential that bed shaping and planting be accurate in order to permit close, or precision, cultivation. Repeated shallow cultivation will dislodge small weed seedlings that emerge after each irrigation. Timeliness in cultivation is essential; seedling weeds are much easier to kill than older established weeds. Cultivation 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, such as purslane. Wet soil in winter may delay, or even preclude, cultivation for weed control. This possibility must be considered when designing a weed management program for fall-planted beets.
None of the above can provide complete weed control, but all are part of an integrated weed management program.
Hand weeding (pulling or hoeing) is still necessary in many situations and should be included as part of a long-term weed management program. However, attempting to rely on hand labor without herbicides is not economically feasible. Studies between 1994 and 1997 at UC Davis for the spring plant/fall harvest crop have shown hand hoeing times without herbicides varied between 40 and 100 hours per acre. At $6.50 an hour, this translates to between $260 to over $650 per acre. Thus, it was not possible to produce an economically viable crop in the absence of herbicides for in-row weed control.
Chemical Controls: A typical weed management program may include a preplant incorporated herbicide or a pre-emergence herbicide at planting, an early postemergence herbicide, and possibly a layby herbicide application. Omitting any one of these applications often results in less than optimum weed control. Economic analyses of various combinations of herbicides have demonstrated that inclusion of a PPI herbicide (cycloate) in the program for spring planted sugarbeets is essential to avoid decreased net return or even net loss. Herbicides used in California sugarbeets are shown in Table 1. The actual combination of herbicide(s) used depends on the region of the state and the weed spectrum present. The attached herbicide/weed selectivity chart provides an indication concerning the potential impact of loss of particular herbicides to management of particular types of weeds. No attempt will be made here to prescribe actual herbicide programs.
Several herbicides are registered for selective weed control in sugarbeet (Table 1), but no single chemical controls all weeds that infest sugarbeet fields. Frequently, two or more herbicides must be combined sequentially or as tank mixes to achieve adequate broad spectrum weed control.
Herbicides used for sugarbeets are typically 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.
Herbicides available for weed management in sugarbeets have been grouped according to time of application.
(A) Preplant and pre-emergence herbicides: Preplant foliar: Postemergence herbicides such as paraquat (Gramoxone®) and glyphosate (Roundup®) are used to kill emerged weeds on preformed beds before planting sugarbeets. This has no direct bearing on the in-crop weed management. Paraquat has contact action only and is thus most effective on young seedlings. Glyphosate has systemic action and is thus effective on established weeds. A few species, such as mallows (Malva spp.), are tolerant to this herbicide and are not well controlled by it. Care should be taken to ensure that either chemical does not drift off the target field.
(B) Preplant incorporated: Preplant incorporated herbicides must be physically mixed (incorporated) into the soil soon after application to prevent volatilization of the chemical and to move the herbicide into the root zone. These herbicides perform best when incorporated with a power driven rotary tiller. All of these herbicides do not require the same depth of incorporation; herbicides such as pyrazon (Pyramin®) and ethofumesate (Nortron®) incorporated too deeply dilute them and decrease weed control. Too shallow incorporation of cycloate (Ro-Neet®) or pebulate (Tillam®) results in poor weed control because of volatilization of the herbicide and lack of exposure of the seedling's underground shoot to the chemical. A one- or two-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 incorporated herbicides do not work well in cloddy soil, and herbicide performance will usually be poor under such conditions. Excessive speed (over about two miles per hour) with a power incorporator results in poor incorporation. The soil should be dry at incorporation in order to obtain optimum results.
(C) Pre-emergence: Pre-emergence pyrazon (Pyramin®) or ethofumesate (Nortron®) treatments are not effective unless incorporated by light rainfall or sprinkler irrigation (less than 0.5 inch). Apply less than 0.75 inch of sprinkler irrigation per set following pre-emergence application of pyrazon until the beets have four true leaves as the herbicide may be leached into the seedling root zone and cause injury to the crop. Under furrow irrigation, physical incorporation of the herbicides is required.
Endothall (Herbicide 273®) is water soluble and not bound to soil particles. It thus moves readily with water and performs erratically when applied preplant or pre-emergence unless irrigation is absolutely uniform.
(D) Postemergence herbicides: Betamix® is the current standard treatment for post-emergence control of most annual broadleaf weeds. It is mixed with other herbicides to obtain broader spectrum control.
Endothall (Herbicide 273®) provides postemergence control of several weeds, such as fiddleneck, knotweed, smartweed, and volunteer cereals, that are difficult to control with other herbicides. Thorough coverage is essential. Endothall will usually have to be applied in combination with another herbicide, such as phenmedipham plus desmedipham (Betamix®) if broad spectrum weed control is required. Endothall responds to temperature; best results have occurred when volunteer cereals were treated during periods of occasional frost. Control has been erratic, and the risk of phytotoxicity to sugarbeets increases in warm weather.
Inadequate control following postemergence use of pyrazon (Pyramin®) occurs when weeds at application were beyond the two- to four-leaf stage or when applied in warm weather (over 70º to 76º F) and not irrigated within two or three days. Irrigation following application is essential for best results because the herbicide must be moved into the weed root zone. The likelihood of crop injury increases in warm weather.
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 (either as a tank mix of Betamix® plus Nortron® or as Progress® premix) can improve control of difficult to control weeds such as common knotweed.
Sethoxydim (Poast®) can be used for postemergence control of grass weeds. It must be applied with an oil concentrate adjuvant to obtain satisfactory activity. This herbicide should not be mixed with any other herbicide; mixtures with phenmedipham plus desmedipham (Betamix®) have resulted in decreased grass control. Control is often erratic because of lack of soil activity.
(E) Application timing: Young weeds are more readily controlled than older weeds. Norris (1991) estimated that for every day in delay of application of phenmedipham plus desmedipham (Betamix®) passed the optimum, a one to two percent reduction in control resulted. Phenmedipham plus desmedipham (Betamix®), or Betamix® plus ethofumesate (Progress®), give erratic control when applied to weeds larger than cotyledon to early two-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. Under spring or summer conditions, applications made in the morning can cause substantial injury; spraying after 3:00 p.m. reduces injury to the sugarbeets. Injury is also reduced if temperatures are decreasing following application.
(F) Multiple applications: Split applications of phenmedipham plus desmedipham (Betamix®), or Betamix® plus ethofumesate (Progress®), about seven 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. Multiple application increases weed control and reduces injury to sugarbeets; the first application must not exceed 0.5 pounds per acre. Multiple applications of triflusulfuron methyl (UpBeet®) are essential in order to control weeds like velvetleaf. Application must start at the cotyledon to one-leaf growth stage of the weed. Three applications may be required for satisfactory control.
(G) Layby herbicides: Trifluralin (Treflan®) or EPTC (Eptam/Genep®) can be applied to the furrows and bed shoulders just prior to canopy closure (layby) to provide seasonlong control until harvest. Neither herbicide has any activity against established weeds; it is thus essential that the field be weed-free at the time of 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.
3) REDUCED-RISK OPTIONS
(A) Development of an integrated crop establishment system based on the use of metham-sodium (Vapam®).
(B) The use of transgenic sugarbeets resistant to glyphosate (RoundUp®) or glufosinate (Liberty®).
4) CHALLENGES
(A) Significance of weeds in sugarbeet production: Except for barnyardgrass and purslane there are few accurate determinations of yield loss. This work would focus on obtaining a better understanding of the importance of different weed species in reducing sugarbeet yield. Weed species to be investigated would include redroot pigweed, lambsquarters, knotweed, mustard, mallow, prickly lettuce, and wild oats. We would attempt to develop competitive indices so that yield loss from mixtures of weeds could be predicted.
(B) Economic analysis of weed management programs: Continue to develop data on costs of weed management. Emphasis on fall plant/summer harvest area in both San Joaquin and Sacramento Valleys. Follow protocols similar to those already in use where chosen herbicide weed management regimes are compared with or without hand weeding and in comparison with an unweeded control.
(C) Evaluate possibility of using metham-sodium as a preplant
fumigant to kill weed seeds prior to sowing the crop. This should
probably be part of the systems used under (B) above. This herbicide
is a biocide and thus offers possibility of control of pests other than
weeds. The cost for stand-alone weed control has, to date, been too
high in comparison with currently available herbicides to warrant use of
this as an herbicide.
(D) Evaluate weed management possibilities using herbicide-resistant
transgenic sugarbeets; glyphosate in Roundup Ready® and glufosinate
in Liberty-Link® sugarbeets. Both herbicides are environmentally
and toxicologically benign and thus appear to offer alternatives to current
herbicides. Extensive reliance on either of these herbicides has
the drawback that, if also used in rotational crops, there will be development
of a weed flora that is resistant to them.
(E) Investigate sugarbeet production as part of a system
of no seed production coupled with hand weeding and cultivation.
These would be part of long-term weed management studies that I feel are
essential if we are to take a population dynamics approach to managing
weeds. The rotations would necessarily involve crops other than sugarbeets.
This is long-term research that will probably require about ten years to
complete.
5) INNOVATIVE RFEATURES IN REDUCED-RISK PROGRAM
The use of Vapam® to control weeds, nematodes, and other pathogens could serve as a multiple pest control. The use of transgenic varieties provide possibility of controlling perennial weeds using reduced risk herbicides.
6) BARRIERS TO ADOPTION OF REDUCED-RISK METHODS
None known at this time.
REFERENCES
Norris, R. F. 1991 Sugarbeet tolerance and weed control efficacy with split applications of phenmedipham and desmedipham. Weed Research 31:317-331.
Norris, R. F. 1992 Case history for weed competition/population ecology: barnyardgrass (Echinochloa crus-galli) in sugarbeets (Beta vulgaris). Weed Technology 6:220-227.
Norris, R. F. 1997. Impact of leaf mining on the growth of Portulaca oleracea (common purslane) and its competitive interaction with Beta vulgaris (sugarbeet). Journal of Applied Ecology 34:349-362.
Schweizer, E; E. and A. G. Dexter. 1987. Weed control in sugarbeets (Beta vulgaris) in North America. Reviews of Weed Science 3:113-133.
UCIPM. 1996. Pest Management Guidelines: Sugarbeet, UC 1PM Guideline Series, # 24. Univ. of California, Div. Agric. Nat. Resources. (see http:www.ipm.ucdavis.edu for further details)
Weed Management contributed by Robert F. Norris, PhD., Weed Science
Program, University of California, Davis.