Integrated Weed Management

Weeds compete with small grains (barley, oats, wheat, rye, and triticale) for nutrients, water, and light, reducing crop yield and quality. An integrated weed management program in small grains combines cultural, mechanical, and chemical weed control practices. Although several herbicides are available for use in small grains, a vigorous, competitive crop produced through proper cultural and agronomic practices is the best defense against weed competition.

The variety of environments and production systems in California requires several different strategies for weed control. Grain is grown under irrigated and dryland conditions in California. These different cultural and climatic conditions have considerable impacts on weed management.

The dryland grower's options for crop rotation are limited compared to those of the irrigated grower. Where annual rainfall is less than 18 inches, dryland management may include a rotation into summer fallow to conserve moisture and to reduce weed problems. In irrigated fields, the grower has more options to manage weed problems within a rotational system.

MONITORING

Weeds are controlled best in the seedling stage; therefore, knowing how to identify weed seedlings is important. A good pictorial reference, such as the Weed Photo Gallery, will help in identifying weeds. Books such as Weeds of the West and Weeds of California and Other Western States: Volumes I and II may also be useful. Plant identification apps can be used as well; however, use caution when using mobile apps for identifying weeds because each app has limitations. It is important to keep a record of summer, winter, and perennial weeds by field for a comprehensive, long-term management system.

Examine the grain field when the crop is in the 2- to 3-leaf growth stage so that weeds can be detected easily and the most effective weed management practices can be made prior to rapid crop canopy growth. The entire field should be examined to determine which species of weeds are present. Visualize an area of 1 square yard and count the number of weeds or estimate their abundance, identifying species and size or growth stage. Make this type of count frequently as you walk in a "W" pattern through the field to get a realistic picture of the problem. Weed distribution within a field is highly variable and often occurs in clumps. High-resolution, remote imagery from airplanes and drones can be used to observe agricultural fields and may assist in identifying areas of interest within a field for scouting weeds.

WEED MANAGEMENT BEFORE PLANTING

Manage weeds before planting by properly preparing the field; implementing the cultural practices described below; and keeping fence lines, ditches, rights of way, and other nearby noncultivated areas free of weeds to reduce sources of infestation. Thoroughly cleaning combines, forage choppers, and tillage equipment before entering or leaving a field is an important practice that can reduce the long-distance spread of weeds to other agricultural fields.

Certified Seed

Seed certified by the California Crop Improvement Association is slightly more expensive than noncertified seed but has the potential for higher yields, increased germination, and a reduced risk of introducing new weed species. Even a small number of weed seeds planted with grain seed can contaminate a sizable area within a few years. Wild oat, downy brome, Italian ryegrass, and other weed seeds are often spread as contaminants in noncertified seed.

Crop Rotation

Crop rotation helps manage weeds, such as johnsongrass, wild oat, downy brome, foxtails, and Italian ryegrass. Other types of tillage and cultivation can be used in different crops, and various herbicides can be used when crops are rotated. Crops other than small grains may be more competitive with certain weeds because their different canopies and root structures allow them to compete with weeds for water, nutrients, and light in ways that small grains do not compete. For example, corn or dry beans can cover and shade the rows during summer, competing with annual weeds. Cotton, corn, sorghum, alfalfa, potato, dry beans, tomato, and safflower are among the crops grown in rotation with grains. Carefully read herbicide labels when choosing a rotation crop to ensure that residues from herbicides applied in the grain crop are not harmful to the following rotation crop within the desired planting window.

In dryland small grain production, rotation between crops and fallow fields conserves moisture and has many additional benefits, such as controlling wild oat by curtailing its seed production and reducing the number of seeds in the soil. Fallow also encourages microbial activity, releasing essential elements from decomposing small grain stubble. Crop-fallow rotation may include crop-pasture-fallow or crop-hay-fallow rotations. A grower usually needs to till during fallow to control weeds. In conservation tillage farming or in fields severely infested with weeds, a grower will need to use herbicides during fallow, a practice called chemical fallow that helps maintain the desired benefits of conservation tillage, such as conserved soil moisture and preserved soil structure. Herbicides used for chemical fallow are listed in the Herbicide Treatment Table.

Preirrigation

In arid regions, such as the Imperial and San Joaquin valleys, small grain growers often preirrigate or wait for the first rain to germinate weed seeds and remove them by tilling or by applying postemergence herbicides such as glyphosate (Roundup) or paraquat (Gramoxone) before planting the crop. In spring-planted areas, fall preirrigation germinates wild oat and volunteer grains; however, in high rainfall areas or in heavy soils, preirrigated fields may remain too wet for timely planting of the crop.

Land Preparation

Adequate drainage is essential for fields planted to small grains. Excessive moisture in low areas creates and aggravates problems such as stand loss, loss of soil nutrients, reduced oxygen supply, and root diseases. Chiseling the soil in fields with marginal soil drainage before seedbed preparation greatly enhances drainage and root development.

Growing grain on beds in the Sacramento and San Joaquin valleys is a common practice that has a two-fold advantage: drainage in winter and furrow irrigation in spring. Where grain is grown on organic soils in the Delta, spud ditches (narrow, shallow trenches) are used for this same purpose. In areas where flooding and high water tables occur, small grains are grown on 30- to 60-inch raised beds.

Tillage

Tillage practices vary widely in California. Tillage is used to eliminate existing weeds; incorporate residues, fertilizers, and herbicides; reduce soil compaction; and prepare a seedbed. It includes chiseling, discing, plowing, and harrowing to firm the seedbed before planting. Plowing the soil 12 to 14 inches deep reduces the risk of herbicide carryover from the previous crop. Diluting possible residuals of herbicides applied to previous crops is important if grains are planted after cotton, sugarbeet, dry beans, tomato, alfalfa, corn, potato, lettuce (Imperial Valley), or oilseed crops.

Under dryland conditions, primary fall tillage with a disc, chisel plow, or harrow usually follows soon after the first autumn rainfall to eliminate germinating winter weed seedlings before a crop is planted.

Planting Methods and Seeding Rates

Small grains are either planted with a grain drill or broadcast and incorporated to a depth not to exceed 2 inches. Drilled grain generally produces a more uniform stand than broadcast planting. The sowing date can influence weed competition. Grains planted late are shorter, produce fewer tillers, and are less competitive with weeds than grains planted at the recommended time. Plant late-planted wheat at higher seeding rates to improve yield and weed competitiveness. Broadcast applications require about 15% more seed than drilling. In many areas, high seeding rates of wheat (175 lb per acre) are used as a form of weed control to compete with johnsongrass, pale smartweed, and wild oat. Excessive seed rates in barley and oats increase the risk of lodging and disease. In the intermountain region where fields have a history of high numbers of winter annual weeds, tilling or applying herbicides before planting may control emerged winter annuals, reducing competition with the grain.

Preplant Fertilizer

Proper amounts of nitrogen and phosphorus are critical in managing crop yield and its ability to compete with weeds. Grain fields low in nitrogen and phosphate are not vigorous, giving weeds the advantage. When fertilizer is broadcast, weeds can benefit more than the crop. Banding fertilizers in or near the crop seed row will make fertilizer more available to the crop during the seedling stage than to weeds. This practice is particularly advantageous with phosphate applications. Care should be used to avoid placing large amounts of potassium or ammonium-based nitrogen fertilizer in direct contact with seeds, or injury to seedlings may occur. Injecting fertilizer 2 inches away from seeds should prevent salt injury from most fertilizers.

Herbicides

Weeds that have germinated can be removed with paraquat (Gramoxone) or glyphosate (Roundup) before planting or before crop emergence. Pyraflufen-ethyl (ETX), carfentrazone (Shark), and saflufenacil (Sharpen) are also labeled for control of emerged broadleaves. These nonselective herbicides have no soil residual effects on germinating small grain plants as long as they are applied before the plants emerge through the soil. Glyphosate can also be used before planting or crop emergence to suppress perennial weeds such as johnsongrass, field bindweed, and Canada thistle.

Preemergence herbicides are not commonly used in small grains in California but can be effective in certain situations. Trifluralin (Treflan) is a preemergence herbicide used for wild oat and canarygrass control in wheat and barley. It is applied before or after sowing and must be incorporated no deeper than 2 inches. A double incorporation is more effective than a single incorporation. Small grains must be planted below the 2-inch herbicide zone (for semidwarf wheat, this depth is near the limit for successful emergence). Results can be erratic if the zone of treatment does not have adequate moisture, and crop safety may be negatively affected. Several crops commonly rotated with small grains are sensitive to trifluralin residue, so great care should be taken when considering an application of trifluralin when planning crop rotation after small grains.

Flumioxazin (Chateau) can be used as part of a preplant burndown program to aid residual control and increase emerged-weed burndown. However, the product must be incorporated following application with at least 1 inch of rainfall or irrigation to activate the residual activity, and a 30-day plantback restriction is required for wheat, so it cannot be seeded immediately. In weedy fields that are not planted for 1 to 2 months, including flumioxazin with the burndown application may prevent a further flush of weeds before planting.

WEED MANAGEMENT AFTER PLANTING

After grain is planted, weeds typically emerge along with the crop. Cultivation is not possible, and producers must rely on herbicides and good agronomic practices for effective control. Depending on the amount of rainfall after planting, one to three nitrogen top dressings may be required for wheat or triticale.

Herbicides

Postemergence herbicides are usually applied to the crop at the 2- to 3-leaf growth stage for grass weeds and the tillered stage for broadleaves. Fall-planted grains are treated between December and mid-April, depending on the time of planting, growing conditions, elevations, and local regulations. Generally, spring-planted grains at higher elevations are treated from April to June. Depending on weeds present, one or two herbicide applications or combinations may be required. Applications must be properly timed to maximize weed control and avoid unacceptable crop injury, which may reduce yield. To maximize effectiveness, weeds should be within the appropriate growth stage for control, and applications should be made before the crop begins rapid growth and canopy closure. Grass and broadleaf weeds germinate with the beginning of the rainy season. Depending on the species, they can sometimes be controlled at the same time, if tank-mixed herbicide combinations can be used, but often grass and broadleaf control need separate applications. A tank mix can provide an important savings of time and cost. Refer to herbicide labels for mixing recommendations.

Broadleaf Weed Control

Typically, only postemergence herbicides are applied after the crop has emerged. Fall-sown small grains usually are treated between December and mid-March, depending on the sowing date and growing conditions. In the intermountain area of northern California, spring-sown small grains are treated between April and June and fall-sown small grains are treated in March and April. Several postemergence herbicides are registered for use.

Group 4 herbicides, or auxin mimics, are one of the major mode-of-action groups used for broadleaf weed control in small grains. They can be broken down into subgroups with various characteristics (detailed below), including phenoxycarboxylates (2,4-D, MCPA), benzoates (dicamba), pyridyloxycarboxylates (fluroxypyr), and chloropicolinates (clopyralid), all of which can be used alone or in combination.

Phenoxy herbicides, including 2,4-D and MCPA, are commonly used in small grains alone or in combinations. Dicamba (Clarity), another hormone-type herbicide, is often included in the phenoxy herbicide group because of its similar mode of action. These herbicides are most effective when applied to small, actively growing weeds. Small grains have varied sensitivity to these herbicides, and cold weather conditions can increase crop injury. Oats are more tolerant to MCPA than to 2,4-D. Ester and amine formulations of 2,4-D and MCPA control many broadleaf weed species found in small grains. The ester form is usually more effective than the amine form. However, ester use is not permitted in most counties, or applications are limited to certain times of the year. Additionally, a choline salt formulation can be used that is even less volatile than the amine formulation; however, use restrictions may still apply.

Apply phenoxy herbicides after the small grains are well tillered but before they reach the boot stage (see Developmental Growth Stages) to avoid yield reductions caused by phytotoxicity. Best control is obtained when weeds are small and before the crop has reached the jointing stage. Late applications are sometimes ineffective because the crop canopy shields the weeds, preventing herbicide contact. Dense numbers of weeds require a more thorough application with a greater spray volume to ensure that the herbicide contacts the weeds. Using aircraft often helps with timely herbicide application, but be sure to make applications at the appropriate time to avoid injuring adjacent crops with drift or volatilization. MCPA does not control large weeds as well as 2,4-D amine or ester herbicides but has greater crop safety, especially when applied to small grains in early growth stages.

Dicamba (Clarity) is effective for broadleaf weed control; however, small grains generally are more sensitive to it than to 2,4-D. It is safer when applied at early growth stages (2- to 5-leaf stage). It controls small common chickweed and fiddleneck, which are not controlled by 2,4-D or MCPA. Dicamba is usually combined with bromoxynil (Maestro) and MCPA or carfentrazone (Shark). When applied early, this combination is very effective and increases the range of weeds controlled compared to either of the herbicides used alone.

Fluroxypyr is a very effective herbicide for controlling a variety of broadleaf weed species. In particular, it is useful for controlling kochia in the intermountain region. It can be applied from the 2-leaf stage to flag-leaf emergence. At this time, fluroxypyr is registered in California for wheat, barley, and oats production only as a premix with bromoxynil and MCPA (Cleansweep).

Clopyralid (Stinger), a picolinic acid, is registered for use in wheat, barley, and oats. It translocates systemically through weeds, similarly to phenoxy herbicides. It is persistent in soil longer than phenoxy herbicides, so many broadleaf crops cannot be planted until 12 to 18 months after application. It is effective on a different spectrum of weeds than 2,4-D, MCPA, or dicamba (Clarity). Clopyralid is especially effective for controlling legumes and composites (such as Canada and yellow starthistle). It does not control many common broadleaf weeds such as mustards, so it must be tank mixed for complete control of the wide range of broadleaf weeds found in small grains. The application timing in wheat is from the 3-leaf stage to the early boot stage (see Developmental Growth Stages), which complements the timing of 2,4-D and MCPA.

Bromoxynil (Maestro), a contact herbicide, is effective on young seedling weeds with no more than two to four leaves. It is less effective on older weeds and must be tank mixed with other herbicides when larger mustards are present. Bromoxynil is not translocated, or moved, from the site of absorption as are the phenoxy herbicides. Therefore, higher-volume applications and thorough coverage are more important with bromoxynil than with phenoxy herbicides. An advantage of bromoxynil is that it controls fiddleneck, which is toxic to livestock. Bromoxynil is also recommended in areas with phenoxy-sensitive crops (grapes, cotton, tree crops) and can be tank mixed with grass herbicides.

Chlorsulfuron (Glean) is registered for use in wheat, barley, oats, and triticale. It is intended for use in these crops when the land used is dedicated for growing only these crops. It is a sulfonylurea herbicide, which is an ALS inhibitor. It is not widely used in California because it has a long soil life (at least 18 months) that prevents its use in areas where many different crops are grown. This herbicide controls most broadleaf weeds, including fiddleneck and chickweed. However, recent findings have shown that some populations of chickweed in California are resistant to ALS inhibitors, including in the sulfonylurea chemical class. Apply it to small weeds when the small grain crop is in the 2- to 3-leaf stage to the boot stage (see Developmental Growth Stages), but be cautious when applying it on soils with a pH above 7.5 because the residual half life is significantly longer on basic soils, which greatly increases the replanting interval for small grains.

Carfentrazone (Shark) is a contact herbicide that controls weeds by disrupting cell membranes. It is effective at very low use rates on fiddleneck, malva sp., burning nettle, and other weeds that are difficult to control with other herbicides. Adding surfactants to carfentrazone often causes temporary crop burn. Applying carfentrazone when conditions favor dew on leaves can increase the chance and severity of crop injury. Tank mixing with a spray-grade nitrogen fertilizer may enhance weed control. Tank mixing with dicamba (Clarity) provides good control of chickweed. Combining carfentrazone with phenoxy herbicides broadens the weed spectrum controlled, lowers herbicide application rates, and can reduce the risk of increased herbicide resistance in weeds. Avoid air applications during inversions (e.g., foggy conditions) to prevent off-target movement to sensitive crops (e.g., almonds). Several San Joaquin Valley counties have additional restrictions for carfentrazone, along with most of the other small grain herbicides.

Pyraflufen-ethyl (ETX) is another contact herbicide that controls weeds by disrupting cell membranes. It is effective at very low use rates on Malva spp., burning nettle, and other weeds that are difficult to control with other herbicides. Pyraflufen-ethyl usually causes temporary spotting on crop leaves. Tank mixing with dicamba (Clarity) provides good control of chickweed. Combining pyraflufen-ethyl with phenoxy herbicides broadens the weed spectrum controlled, lowers herbicide application rates, and can reduce the risk of increased herbicide resistance in weeds.

Tribenuron-methyl (Express) is effective on several broadleaf weeds. It is a systemic herbicide that controls shepherd’s purse, coast fiddleneck, London rocket, mayweed chamomile, prickly lettuce, common groundsel, miner’s lettuce, mustards, pineappleweed, spiny sowthistle, and common chickweed, although some populations of chickweed in the San Joaquin Valley have been found to be resistant to tribenuron-methyl. It also provides partial control of other broadleaf weeds such as filarees, henbit, prostrate knotweed, wild radish, redmaids, and annual sowthistle. Applications during times of general crop stress, especially during prolonged cold or exceptionally wet or dry weather, can increase crop injury and reduce the efficacy of the herbicide. Control can take 3 to 4 weeks to observe with some weeds.

Mesosulfuron (Osprey) is effective on certain broadleaf weeds, including wild radish, mustards, and chickweed, although some populations of chickweed in the San Joaquin Valley have been found to be resistant to mesosulfuron. It also provides partial control of many other broadleaf weeds, including common groundsel, common malva, fiddleneck, yellow starthistle, and milkthistle. Mesosulfuron can be tank mixed with bromoxynil (Maestro) and MCPA and may be applied from the 1-leaf to 1-tiller wheat stage and up to the 2-tiller stage of grass weed development (see Developmental Growth Stages). A methylated seed oil or a nonionic surfactant is required; adding a spray-grade nitrogen fertilizer will enhance control of weeds that are difficult to remove. More restrictions on crop rotations are in place for mesosulfuron than for fenoxaprop (Puma).

Grass Weed Control

Fenoxaprop (Puma) controls canarygrass, wild oat, and several species of foxtails, including yellow and green foxtails. It also suppresses mustards. It has a wide window of application, providing effective control when applied between the 1- and 6-leaf growth stages (see Developmental Growth Stages). For best control of wild oat, delay application until most wild oat plants have emerged, which often requires applying this herbicide during the 5- to 6-leaf stage. A tank mixture with bromoxynil (Maestro) controls a wide range of weeds early in the season. Fenoxaprop cannot be tank mixed with phenoxy herbicides because this practice often reduces grass control.

Mesosulfuron (Osprey) controls most grassy weeds and many broadleaf weeds in wheat. It is especially effective at controlling Italian ryegrass, wild oat, littleseed and hood canarygrass, and annual bluegrass. It controls ripgut brome and other brome species depending on weed size at application. Most California wheat cultivars have good tolerance to the herbicide. However, wheat plants will turn a lighter green for a couple of weeks following application. If soil nitrogen levels are low, this symptom will persist longer and require an application of supplemental nitrogen. Applications made beyond the 1-tiller stage will temporarily suppress growth and stunt wheat plants. The crop will recover more quickly from these symptoms under good growing conditions.

Pyroxsulam (Simplicity) is an ALS inhibitor that controls various grass species, including bromes. It also suppresses many other grasses and controls Italian ryegrass that is not resistant to ALS inhibitors. Pyroxsulam can be applied in actively growing wheat from the 3-leaf stage until jointing (see Developmental Growth Stages). Some yellowing of the crop is common, but injury is typically outgrown within a couple of weeks. Pyroxsulam also provides good control of a variety of mustards and pigweeds, Russian thistle, and chickweed that are not resistant to ALS inhibitors. Most weeds are effectively controlled when small.

Pinoxaden (Axial) is an ACCase inhibitor that controls grasses. It can be applied from the 2-leaf stage to the boot stage (see Developmental Growth Stages) in spring wheat, winter wheat, and barley, but not in durum wheat. It provides control on young grass weeds with fewer than five leaves on the main stem, and before the third tiller. It provides good control for foxtail barley, wild oat, and Italian ryegrass (assuming no resistance) but does not control Bromus species.

Imazamox (Beyond) is an ALS inhibitor that can be used in imazamox-tolerant wheat (Clearfield Production System). It provides broad-spectrum control on a variety of small grassy and broadleaf weeds (see the product label for size restrictions). Larger weeds are not controlled. It can be applied in wheat that has begun to tiller but should be applied before the jointing stage (see Developmental Growth Stages) in both spring and winter wheat varieties that are tolerant. Including a nonionic surfactant or methylated seed oil in combination with a nitrogen fertilizer can provide better weed control.

Pendimethalin (Prowl) is a selective herbicide applied after the wheat has emerged but before the weeds have emerged. Pendimethalin controls most annual grasses and certain broadleaf weeds as they germinate. Adequate rainfall or irrigation after application but before weed seedling emergence is needed to provide effective weed control. Pay close attention to the product label to understand crop rotation restrictions, as several crops regularly rotated with small grains in California may not be planted immediately after harvesting the small grain crop.

Harvesting

Uncontrolled weeds delay maturing grains and create harvest problems. Field bindweed, Russian thistle, fivehook bassia, kochia, lambsquarters, mustards, smartweed, johnsongrass, and prostrate knotweed are examples. Heavy concentrations of green vines, stalks, or stems slow down combines and raise the moisture content of the harvested crop. Green debris and weed seeds mixed into grain cause heating in the grain bin; grain may be discolored and acquire off-flavors. Unclean storage also invites insect and mold damage.

A mature grain field contaminated with immature weeds may be harvested in one of two ways: (1) the crop may be swathed (cut and left in windrows) or (2) where permitted, formulations of 2,4-D and glyphosate (Roundup) that are registered as preharvest aids can be applied to mature grain to kill and dry out weeds. In either case, grain is harvested after weeds have dried. Windrows are harvested with a combine equipped with a pickup attachment. Dry weeds and their seeds usually can be separated from grain during combining. In the San Joaquin Valley, use of 2,4-D is restricted between March 15 and October 15.

Weed seed viability or weed competitiveness can be reduced mechanically in small grain fields with the use of specialized harvest equipment and precision agriculture techniques. These methods are referred to as Harvest Weed Seed Control.

Fallow

In rainfed production systems, fallow fields every other year to prevent increased weed seed numbers and conserve moisture to maximize small grain growth. Do not allow weeds to grow and produce seed during the fallow season. Summer fallowing is practiced in dryland areas of the state that generally receive less than 18 inches of annual rainfall. The fallow season starts in fall after the crop is harvested. Using herbicides to burn down weeds during fallow and reducing tillage can effectively conserve soil moisture. These practices prevent soil moisture from being depleted through plant evapotranspiration and being exposed to the atmosphere and evaporating after mixing during tillage.

Depending on local rainfall patterns and the availability of time and equipment, primary tillage is often performed in fall or winter when the following year's crop is being planted. Tillage tools of choice for primary tillage vary from plows in the Sacramento Valley to chisel plows along the Central Coast. If primary tillage cannot be accomplished before heavy winter rains begin, it is often delayed until spring to reduce erosion hazards. One or more traditional tillage trips, with discs or field cultivators, are performed during the fallow season, depending on additional weed growth. Often, one of the final tillage trips in late summer or early fall is used to inject nitrogen fertilizer into the soil.

In some areas, small grains are seeded in early fall to take advantage of the first rains and the ability to still move equipment across dry fields. The Herbicide Treatment Table lists herbicides used in small grain fallow.