Integrated Weed Management

Successful weed management in the production of California rice is based on a combination of cultural and chemical control methods, including prevention, land preparation, crop rotation, tillage, fertilizer management, water management, and proper use of herbicides. Herbicide resistance is an increasing problem that has added considerable complexity to weed management. The components of an integrated weed program for a specific field are determined by the weed species present and their levels of infestation, weed resistance to herbicides, the water management system, and the capability to rotate to other crops or fallow.

Prevention is an important part of rice weed control. Use certified seed, clean farm implements when moving from field to field, and eliminate rice weeds growing on levees and roadsides.

High and low areas and steep slopes within a flooded basin make it impossible to achieve a uniform water depth, resulting in uneven weed and rice plant growth. Leveling to more gradual slopes and eliminating unevenness within each basin greatly improve weed control. Laser leveling is an excellent tool for this purpose because of its high accuracy and precision. Global positioning system (GPS) guided land leveling is a newer technology that can create field maps in three dimensions, making land leveling even easier and more precise than laser leveling.

Several factors influence choice of field grade and levee spacing. To create a desirable grade for optimum weed control and rice growth, allow variation of no more than 2 to 3 inches between shallow and deep areas of a basin and a maximum depth of 5 inches. This can be improved with a slope between levees of 0.25 feet or less. Although fields with near zero grades may improve weed control and rice stand establishment, they are more difficult to drain for postemergence herbicide applications.

In fields that can be rotated to other crops, use rotation to greatly reduce the numbers and species of weeds that cannot be selectively controlled with herbicides and cultural practices. Rotation crops to consider include tomato, safflower, sunflower, beans, or cereal crops. Non-flooded conditions, seedbank decay, and alternative herbicides that can be used in these rotation crops all contribute to reducing future weed infestations in rice.

In fields with severe infestations of perennial weeds with tubers, rhizomes, or large rootstocks (such as cattail, pondweed, Gregg’s arrowhead, bulrush, or spikerush), consider a dry fallow rotation out of rice. Plow the rice field to a depth of 8 to 12 inches during the fallow season to expose underground stems of cattails and Gregg arrowhead, tubers of river bulrush, and winter buds of American pondweed; this can reduce the numbers of these perennial weeds as long as sufficient drying of the soil and reproductive plant parts is achieved in spring. Plowing to a depth of 8 to 12 inches combined with rotating to a non-irrigated crop (such as safflower) improves soil desiccation further. Avoid transfer of stems, tubers, and buds to clean fields by tillage equipment.

In fields that are suitable only for a rice crop, rotate water management methods to help control weed species resistant to herbicides normally used in rice production. For example, flood the field one year and in the next, use dry seeding or stale seedbed techniques coupled with nonselective, preplant herbicides.

With the advent of rice straw incorporation and winter flooding, the objectives of tillage have changed. Because the soil is wetter for longer periods, it is not possible to use tillage in fall to expose and dehydrate the rhizomes, tubers, and corms of perennial weeds unless a particularly heavily infested field is specifically targeted for dry tillage. Straw incorporation by wet rolling, and especially discing and plowing in fall, also incorporates weed seeds into the soil. This protects weed seeds from depredation by bird and small mammals. The exception to this is fall or spring plowing 6 to 8 inches deep, which buries weed seed below the germination zone and may substantially reduce weed numbers as well as the vigor of seedlings that do emerge. Although buried weed seed eventually declines in vigor, which reduces its ability to germinate, some species survive for a long time and may be brought to the surface by subsequent tillage operations.

Spring tillage destroys weed seedlings that have germinated before seedbed preparation. However, unless spring temperatures have been warm, only a small percentage of the total seed bank will have germinated. Nonetheless, it is essential to kill these weeds by working dry seedbeds and allowing the soil to dry out before flooding. Tilling wet soils may only transplant seedling weeds, which become severe competitors with the later-planted rice and are very difficult to control with herbicides.

Groove the rice seedbed with a heavy, ridged roller to produce a uniform corrugated seedbed that will protect young rice seedlings from wind and wave drift. While heavy rollers provide a more uniform seedbed, they may bring moisture to the surface in wet soils, thus increasing early weed establishment. If a rice roller is not used, make sure the final seedbed is free of large clods. Large clods exposed above the water are havens for the germination and emergence of grass weeds that later will be too large to control with herbicides. Use land-leveling equipment to eliminate high areas before rolling or other methods of seedbed preparation; this will assist in reducing clod size.

Inject or soil-incorporate nitrogen and phosphate fertilizers 2 to 4 inches deep to increase their availability to the growing rice plant, reduce their availability to weed seedlings that germinate near the soil surface, and prevent nitrogen loss. Submerged aquatic weeds such as southern naiad (Najas guadalupensis), chara (Chara sp.), and algae grow more vigorously and may become well established when high rates of nitrogen and phosphorous are left on the soil surface. Avoid topdressing with nitrogen or phosphorous into the water before the rice canopy has covered the field surface, as this also encourages rapid growth of weeds.

Surface-applied triple superphosphate (TSP), a type of calcium phosphate fertilizer, can increase the number of sedge and broadleaf weeds, including smallflower umbrella sedge, blue-flowered ducksalad, redstem, ricefield bulrush, waterhyssop, and California arrowhead. Calcium alone can stimulate smallflower umbrella sedge germination but has no effect on ricefield bulrush germination, whereas phosphorus has no effect on stimulating either smallflower umbrella sedge or ricefield bulrush germination. Therefore, when applying calcium phosphate, consider incorporating preplant applications into the soil profile to reduce the emergence of certain (largely small-seeded) rice weeds.

Water management is the most important cultural factor for the successful control of many important rice weeds. Water that is too deep may impede stand establishment or result in water spillage; water that is too shallow may expose the soil surface and allow weed seed germination. During long water-holding periods, start with relatively deep water and allow it to recede to a desired depth (5 inches) rather than beginning at this level and adding water as needed.

Water management practices are an integral part of any rice weed management program and greatly influence the efficacy of many herbicides. In the past, fields were flooded continuously at a depth 4 to 8 inches to suppress weeds (e.g., grasses and smallflower umbrella sedge) and herbicides were applied into the water. However, as weeds developed resistance to many of the into-the-water herbicides, it has become necessary to use more foliar-active or contact herbicides that require the fields to be drained early in the season so the herbicides can adequately cover the weed seedlings. These fields must then be rapidly reflooded to prevent a new flush of weed seed germination.

Water Management and Herbicide Applications

The introduction of foliar-applied herbicides in rice has necessitated water depth management in relation to herbicide applications. Commonly used water management regimes in California include delayed pin-point flood (or just pin-point flood), the Leathers' method, and permanent flood. Less commonly used methods include the use of a stale seedbed, and drill- or dry-seeding.

Pin-point flood

Drain the field 2 to 4 weeks after sowing to facilitate early application of foliar herbicides. Water may be shut off and allowed to subside rather than drain. After the herbicide application, fields are reflooded to 4 to 6 inches and maintained. Another version of this practice is to lower water during the early tillering stage of rice to expose weeds to foliar herbicides. Quick removal of water at this time and replacement after spray application is important for good weed control. A prolonged drain period promotes weed growth; delayed reflooding reduces herbicide efficacy.

Leathers' method

Drain the field rapidly and completely immediately after sowing. Then leave the water off the field until the rice radicle penetrates the soil and anchors the seedling. Fields are usually left drained for 3 to 5 days, depending on temperature and growth of roots, and then rapidly reflooded after the rice seed has anchored in the soil. This method is generally used to promote early establishment of rice and where early-season, wind-caused wave action tends to dislodge and move germinating rice seed. This early drain can activate germination of a number of weed species at the same time. Certain herbicides with long water-holding periods may make this a difficult choice to use, as water cannot be drained off the field after application.

Permanent flood

A water depth of 4 to 5 inches is established as soon as possible after sowing and maintained for the rest of the season to maintain steady pressure on weeds and optimize rice growth. Fields are drained approximately one month before harvest, to allow for combines to drive onto the field.

Stale seedbed

Prior to planting in the spring, flush the field from March through early May. The field can be tilled or not tilled prior to flushing, but weed emergence may be less in a no-till field. Weeds will generally take 1 to 2 weeks to emerge, and the typical species that emerge are grasses (Echinochloa spp. and Leptochloa spp.) and smallflower umbrella sedge (Cyperus difformis). The easiest way to flush is to flood the field to 4 to 6 inches, board up the drain boxes, and allow the water to subside into the soil. After flushing, and when weeds have emerged to the 1- to 2-leaf stage or beyond, a non-selective herbicide can be sprayed, or shallow tillage can be used to control the emerged weeds. Warmer temperatures will speed up the emergence process, and cooler temperatures may delay it. This can cause planting to be postponed by a few weeks, depending on how long it takes the field to dry out enough for a tractor or spray equipment to be able to move onto the field. The method is good for herbicide resistance management.

Drill- or dry-seeding

The dry-seeding method is one in which the field is prepared as usual in the spring, but instead of flying the seed onto a flooded field, the seed is flown onto a dry field. The seed is then harrowed in or covered, which allows for the use of pendimethalin, an herbicide with an alternative mode of action which is not available for use in flooded fields. Typically, several flushes of water are put across the field, until the rice is emerged and established, with foliar herbicide applications made in between flushes. Fields are typically flooded around the 3- to 4-leaf stage of rice, and then a permanent flood is utilized until approximately one month before harvest.

Drill-seeding is typically utilized in the San Joaquin rice-growing area and is used by some growers in the southern Sacramento Valley as well, where crop rotation is more common. After planting rice seeds, preemergence herbicides such as pendimethalin can be used. If rice is planted deeply enough, non-selective herbicides can also be used, and applied to the emerging weeds before rice emerges. Several flushes of water are put across the field, allowing for foliar applications in between flushes. A permanent flood is typically established soon after the 3- to 4-leaf stage of rice.

Effect of Water Depth on Weeds

Exceptionally shallow water (up to 2 inches) promotes the growth of all rice weeds and, in addition, may promote weeds normally found only in other annual crops. Intermittent draining, particularly early in the season, may allow weed seedlings to establish that would not have survived a continuous flood. Soil exposure to air as a result of draining increases the diffusion of oxygen into the soil profile, especially if the soil is allowed to dry. Increases in oxygen concentration initiates the germination of weed seeds and favors establishment and growth of most weeds. A permanent flood restricts light penetration and oxygen diffusion into the soil, thereby decreasing weed germination and growth.

Grasses

Watergrass (Echinochloa spp.), also known as barnyardgrass, is the most serious weed in continuously flooded California rice. It is variable in form, and three distinct species occur in California rice fields: terrestrial barnyardgrass, early watergrass, and late watergrass.

Terrestrial barnyardgrass is the most widespread and easiest to control by floodwater. In fact, the California system of water-seeding rice was established to control this species. Maintaining water 4 inches (10 cm) deep still provides good control.

Early and late watergrasses will grow through 4-inch-deep water, and thus, deeper flooding (7 inches) is required to control them. ("Early" and "late" refer to flowering times. Early watergrass flowers about 40 days after flooding, well ahead of rice. Late watergrass flowers about 90 days after flooding, which is at or near the same time as early rice varieties.) Both early and late watergrass are larger seeded than typical barnyardgrass and more successful in emerging through continuous floodwater.

Field draining encourages watergrass germination. Exposing the soil to air for long enough to allow secondary root development in watergrass seedlings (3–5 days) greatly reduces the effectiveness of currently used watergrass herbicides. Very severe infestations of watergrass may require rotation to another crop.

Sprangletop (Leptochloa spp.) usually does not germinate and emerge through a depth of more than 4 inches of water. If the field is drained, however, sprangletop seeds germinate rapidly and subsequent plant growth is virtually uncontrolled when reflooding occurs.

Broadleaf weeds and sedges

Broadleaf weeds and sedges vary in their response to water depth and are much less susceptible to drowning than the grass weeds. In addition, some broadleaf weeds and sedges are favored by extremely shallow water depths or field draining. These weeds usually appear earlier than the grasses and are of a wider age-range in drained fields. The lack of uniform weed seed germination and development may reduce the effectiveness of broadleaf herbicides and timing of these herbicides for optimum weed control. However, short periods of drought can make rice more competitive against ricefield bulrush, so use these temporary periods of drought as a tool to help suppress ricefield bulrush.

Monitoring and accurate identification of weed species are necessary for choosing the best weed control program. Monitoring is especially important because herbicides used in rice are selective and control few weed species in the field. Also, where weed resistance exists, monitoring is crucial in making correct management decisions.

Keep good records of your monitoring to help select an herbicide, herbicide combination, or herbicide sequence. For assistance in weed identification, consult the color photos of weeds linked in Common and Scientific Names of Weeds.

There are two major monitoring periods for weeds in rice: (1) from flooding until 45 days after seeding, and (2) between panicle initiation and heading.

• The first monitoring period determines weed control needs for the current season.
• Monitor every 3 to 4 days after flooding for 21 days and once a week thereafter until the postemergence herbicide period is over (about 45 days after seeding).
• Identify and record weed species and their distribution throughout each basin as well as the growth stage of the rice plant so that the proper herbicide(s) can be selected and applied at the appropriate time. The period from 21 to 45 days after seeding is especially critical for monitoring and treating weeds showing herbicide resistance before they go to seed.
• During the second monitoring period, record the weed species that have escaped control and their distribution in the field. This information is important for knowing what weeds to expect the following year. Also note species that appear to be resistant to herbicides; this will help to implement strategies that prevent further spread of resistant weeds. However, failure to achieve expected weed control levels does not usually mean there is resistance.

For more information on indicators of herbicide resistance as well as other reasons for herbicide failures, see the section on Herbicide Resistance below.

With the onset of widespread weed resistance, many new herbicides have been registered. Most of the newly registered herbicides are limited in the spectrum of weeds controlled. While they may be applied alone for weed control, they are more frequently used in combinations or in sequence with another herbicide. To select the most effective herbicide(s) for a specific situation, several characteristics of the herbicide should be considered. These characteristics for herbicides currently registered for use in California rice fields are summarized in the table below:

Herbicide Resistance

Herbicide resistance is the ability of certain biotypes within a weed species to survive an herbicide treatment that would normally have killed it. Herbicide-resistant biotypes are present within a weed species' population as a part of normal genetic variation. Repeated use of the same herbicide or herbicides with the same mode of action will select for herbicide-resistant biotypes. In addition, the same weed can be resistant to more than one type of herbicide.

Factors that contribute to the development of herbicide resistance

In addition to the excessive reliance on chemical control and repeated sequential use of the same mode of action, herbicide resistance is promoted by:

• a monoculture of continuous rice production,
• weeds that produce lots of seeds with little dormancy and short longevity,
• an herbicide that has high efficacy on a specific weed species, and
• an herbicide with prolonged residual activity.

Detecting herbicide resistance

Weed resistance may be occurring if monitoring indicates any of the following:

• after treatment, healthy-looking plants are present alongside dead plants of the same species,
• one species that is normally controlled by the herbicide is poorly controlled, but other adjacent susceptible species are well controlled,
• a gradual decline in control has been noticed over time for a species that was previously well controlled by the same herbicide and rate,
• discrete patches of the target weed persistently survive treatment with a given herbicide or herbicides (escapes), and resistance in the same weed species and herbicide occurs in neighboring fields.

Resistance needs to be ultimately confirmed by a specific test. Failure to control weeds can be caused by several factors besides resistance; these include:

• faulty spraying,
• incorrect dose or timing,
• unfavorable environmental conditions,
• weeds too large,
• subsequent weed germination after treatment,
• dense infestations, and
• poor coverage.

Also, keep in mind that weeds not on a label may tolerate the herbicide but are not resistant biotypes.

Types of Weed Resistance

In California, there are two types of weed resistance:

1. Target site resistance refers to resistance to group 2—ALS inhibitors such as penoxsulam (Granite), sulfonylurea (Londax), and bispyribac (Regiment)— and group 7—propanil (Stam or SUPERWHAM!)— mode-of-action herbicides. It can develop when mutations occur in one or more sites. The most common site mutation results in resistance to a sulfonylurea (Londax). If there is resistance to Londax, there is a greater probability for cross-resistance to other sulfonylureas and triazolopyrimidines, such as penoxsulam (Granite). If the mutation occurs at a different site, it may cause resistance to the pyrimidinylthiobenzoates, such as bispyribac (Regiment), but not to the sulfonylureas or triazolopyrimidines. Finally, if the mutation occurs at a third site, resistance will be to all classes of ALS inhibitors.
   
   Because the actual site where the mutation occurred will probably not be known, it is not possible to know when choosing an herbicide what cross-resistant patterns may be present for a resistant population. In these situations, choose an herbicide with a different mode of action or if there is no other choice available, use the ALS inhibitor in mixture or in sequence with a different mode-of-action herbicide that is active on the target weeds.
   
2. Enhanced metabolic degradation resistanceis the second most common mechanism for resistance to rice herbicides; it affects primarily biotypes of barnyardgrass and the watergrasses. Herbicides with known enhanced metabolic degradation resistance include mode-of-action group 1—ACCase inhibitors: cyhalofop-butyl (Clincher)—; group 2—ALS inhibitors: penoxsulam (Granite), sulfonylurea (Londax), and bispyribac (Regiment)—; group 8—lipid inhibitors: thiobencarb (Abolish, Bolero)—; and group 13—pigment inhibitor: clomazone (Cerano). Note: group 2 ALS inhibitors have exhibited both target site and enhanced metabolic resistances.
   
   Barnyardgrass already has the ability to metabolize most rice herbicides, but very slowly, so the weed is killed before it can detoxify the herbicide. However, with continuous use, particularly at low rates, you can select for biotypes that have enhanced ability to detoxify the herbicide. With tank mixes, however, the weed’s ability to metabolize the herbicide can be influenced. For example, by mixing thiobencarb (Abolish) with bispyribac (Regiment), thiobencarb inhibits the metabolizing enzyme for bispyribac in early watergrass and prevents the weed from metabolizing bispyribac so that the weed is killed, but the phytotoxicity of bispyribac on rice is not increased.

Resistance Management

Use measures to prevent resistant weeds from going to seed and manage weeds that have exhibited resistance so that their spread is prevented. Several cultural practices that suppress weed growth and reduce weed seed carryover are described below. Most importantly, use herbicides with different mode-of-action group numbers as alternates to, in combination with, or in sequential applications with the herbicide to which resistance has been developed.

Other management methods that help manage herbicide resistance include: the stale-seedbed technique; alternating stand establishment systems; monitoring field for weeds that have escaped treatment and controlling them along with late-season flushes; using certified seed; and maintaining a suppressive water depth. When in doubt, assume resistance and use appropriate resistance management strategies.

For more information on herbicide resistance by mode of action, see the California Rice Weed Herbicide Susceptibility Chart at UC Rice Online.

Cultural practices

Cultural practices that suppress weed growth and reduce weed seed carryover include:

1. Fallow and flood the field during the summer to germinate aquatic weeds (control these weeds mechanically or use nonselective herbicides).
2. Rotate to another crop to reduce the weed seed bank in the soil.
3. Suppress weed growth by using as many of the following methods as practical:
   • level fields to avoid unevenness,
   • use wind checks to prevent seedling drift,
   • cultivate and dry the seedbed to a 3- to 4-inch depth to destroy weed seedlings,
   • groove or crease the soil across the prevailing wind direction,
   • flood fields as rapidly as possible,
   • maintain a flood depth of 4 to 5 inches; deeper water (6 to 7 inches) suppresses certain weeds and may be useful in their control, and
   • avoid draining until the end of the season.
4. Where herbicide resistance has occurred, avoid moving resistant seed from one field to another in water or on tillage and harvesting equipment. To avoid this
   • clean equipment before moving out of a field with known resistant weeds,
   • till, plant, or harvest these fields last,
   • hold water on the field to prevent the spread of resistant weed seeds, and
   • use water management practices that best complement alternative herbicides used to control resistant weeds.

Herbicide use

Where possible, avoid using the herbicide to which weeds have become resistant in fields known to have resistant weeds. Use herbicides with different mode-of-action group numbers (along with the cultural practices described above) in combinations or sequences at the labeled rate and at the correct stages. Do not use group 1 —ACCase inhibitors: cyhalofop-butyl (Clincher)— or group 2 —ALS inhibitors: penoxsulam (Granite), sulfonylurea (Londax), bispyribac (Regiment), orthosulfamuron (Strada) and halosulfuron (Sandea) herbicides only or repeatedly in the same season.

To successfully manage herbicide resistance, an effective tank mix or sequential program (within a single season as well as over multiple cropping years) will include:

• herbicides with different mode-of-action group numbers, and
• herbicides with mode-of-action group numbers 4, 7, and 14 [e.g., 4 = triclopyr (Grandstand), 7 = propanil (Stam, SUPERWHAM!), and 14 = carfentrazone (Shark)], which do not have enhanced metabolic degradation resistance.

Herbicide Combinations

Tank mixtures may be used when two or more herbicides are compatible, and the best management practices for their application such as timing and water depth are the same. Tank mix combinations reduce the cost of application and often reduce the rates of one or more herbicides. The purpose of combinations is to broaden the spectrum of weed control such that each herbicide in the mix will control weeds not controlled by the other. Use the Susceptibility of Weeds to Herbicide Control table to see the weed spectrum controlled by some of the common herbicides used in California rice.

Because almost all the tank mixes in use in California have problems with resistant species of weeds, it is imperative to accurately identify weed species and the presence of herbicide resistance among the weeds. If resistance to barnyardgrass or the watergrasses is suspected in a field, avoid tank mixes of herbicides with mode-of-action group numbers 1, 2, 8, and 13 (1 = cyhalofop [Clincher]; 2 = penoxsulam [Granite], bensulfuron [Londax], bispyribac [Regiment], halosulfuron [Sandea]; 8= thiobencarb [Abolish, Bolero]; and 13 = clomazone [Cerano]).

Herbicide Sequences

To achieve good broad-spectrum weed control, most herbicides must be used in sequence rather than in tank mixes because of differences in the herbicides with respect to timing, water management, antagonism, translocation, and other factors. Another particularly important aspect of herbicide sequences is to protect against the buildup of weed resistance by using herbicides with different modes of action. For example, a sequence of cyhalofop, clomazone, penoxsulam, or bispyribac followed by propanil will generally control watergrass that is resistant to the first herbicide. Use the Susceptibility of Weeds to Herbicide Control table to see the weed spectrum controlled by some of the common herbicides used in California rice.