Agricultural pest management
Integrated Weed Management
(Reviewed 2/07, updated 2/09, corrected 7/16)
Weeds compete with small grains (barley, oats, wheat, rye, and triticale) for nutrients, water, and light, reducing crop yields and grain quality. An integrated weed management program in small grains combines cultural, mechanical, and chemical weed control practices. A vigorous, competitive crop produced through good cultural practices is the best defense against weed competition. Several herbicides are available for use in grain; however, one should rely on an integrated approach to weed management. An integrated approach is all inclusive, using good cultural practices and crop rotation along with the appropriate herbicides.
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 impact on weed management.
Dryland grain yields less than irrigated grain, reducing the grower's incentive to invest in chemical weed control. 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.
Weeds are controlled best in the seedling stage; therefore, it is important to be able to identify weed seedlings. A good pictorial reference, such as the online weed gallery photos on the UC IPM website will help assist in identifying weeds. It is important to keep a log of summer and winter weeds by field for a comprehensive management system.
Examine the grain field when the crop is in the 2- to 3-leaf stage so weeds can be detected easily. The entire field should be examined to determine which species of weeds are present. Visualize a square yard area and count the number of weeds, identifying species and size. Make frequent counts as you walk through the field to get a realistic picture of the problem. Five or more competitive weeds (e.g., wild oats) per square yard or one or more large weeds (e.g., wild mustard) per square yard indicates that the weed population is at the threshold stage and should be removed by a herbicide application to avoid an economic yield loss.
Weed management before planting is primarily a matter of properly preparing the field as well as various other cultural practices and keeping fence lines, ditches, and wasteland areas free of weeds to reduce sources of infestation. Thoroughly cleaning combines and tillage equipment before entering or leaving a field is an important practice.
Certified seed. Seed certified by the California Crop Improvement Association is slightly more expensive than common seed, but is a good investment to ensure potential for higher yield, increased germination, and reduced risk of introducing a new weed species. A few weed seeds planted with grain seed can contaminate a sizable area within a few years. Wild oats, ripgut brome, and field bindweed are often spread as contaminants in common seed.
Crop rotation. Crop rotation helps manage weeds (johnsongrass, wild oats, ryegrass, etc.) in the following ways: different crops allow different types of tillage and cultivation, plus various herbicides can be used when crops are rotated. Crops other than grain may be more competitive with certain weeds. For example, corn or dry beans can cover and shade the rows during summer, helping to compete with annual weeds. Cotton, corn, alfalfa, potato, sugarbeet, dry beans, tomato, and safflower are among the crops grown in rotation with grains.
Crop/fallow rotation conserves moisture and has many additional benefits, such as controlling wild oats by curtailing its seed production and reducing the number of seeds in the soil. Fallow also encourages microbial activity, releasing essential elements from decomposing cereal 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. Herbicides used for this use are listed in the HERBICIDE TREATMENT TABLE.
Preirrigation. In arid regions, such as the Imperial and San Joaquin Valleys, cereal growers often preirrigate or wait for the first rain to germinate weed seeds and remove them by tilling before planting or by applying postemergent herbicides such as glyphosate (Roundup) or paraquat (Gramoxone). In spring-planted areas, fall preirrigation is practiced to germinate wild oats and volunteer grains; however, in high rainfall areas or in heavy soils, preirrigated fields 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 before seedbed preparation greatly enhances drainage and root development.
Growing grain on beds in the Sacramento and San Joaquin Valleys has a two-fold advantage: drainage in winter and furrow irrigation in spring. In the Delta where grain is grown on organic soils, spud ditches are used for this same purpose. In areas where flooding and high water tables occur, grow small grains on 30 to 60 inch raised beds.
Tillage. Tillage practices vary widely in California. Tillage is used to eliminate existing weeds, incorporate residues and fertilizer, reduce compaction, and prepare a seedbed. It includes chiseling, disking, plowing, and harrowing to firm the seedbed before planting. Plowing the soil 12 to 14 inches reduces the risk of herbicide carryover from the previous crop. Dilution of possible residuals of herbicides applied to previous crops is very important if grains are planted after cotton, sugarbeet, dry beans, tomato, alfalfa, corn, potato, lettuce (Imperial Valley), or oil seed crops.
Under dryland conditions, primary fall tillage with a disk, chisel plow, or moldboard plow, usually follows as soon after the first autumn rainfall to eliminate germinating winter weed seedlings.
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. 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, smartweed, and wild oats. Excessive seed rates in barley and oats increase lodging and diseases.
Mulch planting. An effective means of weed control used in the southern desert is mulch planting. Mulch planting places the seed beneath a layer of dry mulch, which inhibits weed seed germination. Before planting, the field is shallowly cultivated to destroy weeds that have germinated following a rainfall or irrigation. The crop is then sown into moist soil below the mulch layer of dry soil that resulted from the cultivation. Because the crop seed is placed into moist soil, it germinated quickly, ahead of weeds. In some cases this method eliminates the need for herbicides.
Preplant fertilizer. Proper amounts of nitrogen and phosphorous 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. Anytime a fertilizer is broadcast, weeds benefit more than the crop. Banding of fertilizers in or near the crop seed row will make fertilizer more available to the crop during the seedling stage than to weeds. This is particularly advantageous with phosphate applications.
Herbicides. Weeds that have germinated can be chemically removed with paraquat or glyphosate before planting or before crop emergence. Pyraflufen-ethyl (ET) is 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. If the herbicide comes into contact with wheat and barley plants, severe injury will occur. Glyphosate also can be a tool at this time to suppress perennial weeds such as johnsongrass, nutsedge, bermudagrass and dandelion.
Preemergent herbicides are not commonly used in small grains in California but can be effective in certain situations. Trifluralin (Treflan) is a preemergent 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 semi-dwarf wheat, this depth is near the limit for successful emergence). Results can be erratic if the zone of treatment does not have adequate moisture. Crop safety is marginal.
WEED MANAGEMENT AFTER PLANTING
Herbicides. Postemergent herbicides are applied to the crop, usually at the 2- to 3-leaf and tillered stages. Fall-planted grains are treated between December to mid-March, depending on time of planting and on growing conditions. 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. Application must be properly timed for maximum weed control and avoidance of crop injury. Grass and broadleaf weeds germinate with the beginning of the rainy season. Depending upon species, they can sometimes be controlled at the same time, permitting the use of tank-mixed herbicide combinations, but often grass and broadleaf control need separate applications. A tank mix can mean an important savings of time and cost. Refer to herbicide labels for mixing recommendations.
Broadleaf weed control. Typically only postemergent 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. Spring-sown small grains in the intermountain area of northern California are treated between April and June. Several postemergent herbicides are registered for use.
Phenoxy herbicides, including 2,4-D and MCPA, commonly are used in small grains alone or in combinations. Dicamba, another hormonal-type herbicide, often is included in the phenoxy herbicide group because of its similar mode of action. These herbicides are most effective when applied to small and succulent weeds. Small grains vary in their sensitivity to these herbicides; for example, oat is more tolerant to MCPA than to 2,4-D. Ester and amine formulations of 2,4-D and MCPA control many broadleaf weed species encountered in small grains. The ester form usually is 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.
Apply phenoxy herbicides after the small grains are well tillered but before they reach the boot stage (see DEVELOPMENTAL GROWTH STAGES) in order 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 weed populations require a more thorough application with a greater spray volume to ensure herbicide/weed contact. The use of aircraft often facilitates timely herbicide application, but care must be taken to make applications at the appropriate time to avoid injury to adjacent crops from drift or volatilization. MCPA does not control large weeds as well as 2,4-D amine and 2,4-D ester herbicides but has greater crop safety especially when applied to small grains in early growth stages.
Dicamba (Banvel, 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). Dicamba cannot be used on fall-sown barley. It controls small chickweed and fiddleneck, which are not controlled by 2,4-D or MCPA. Dicamba usually is combined with bromoxynil and MCPA or carfentrazone (Shark). When applied early, this combination is very effective and increases the weed spectrum controlled compared to either of the herbicides used alone.
Bromoxynil (Buctril), 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 like the phenoxy herbicides. Therefore, higher volume application and thorough coverage is more important with bromoxynil than with phenoxy herbicides. An advantage of bromoxynil is that it controls the toxic weed fiddleneck. Bromoxynil also is 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 on wheat in a wheat/fallow rotation. It is a sulfonyl urea herbicide with a very low use rate. 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. Most broadleaf weeds, including fiddleneck and chickweed, are controlled. Apply it to small weeds when the small grain crop is in the 2-3 leaf stage to boot stage(see DEVELOPMENTAL GROWTH STAGES) and should not be used on soils with pH above 7.5.
Clopyralid (Stinger), a picolinic acid, is registered for use on wheat, barley and oats. It translocates systemically through weeds, similar to phenoxy herbicides. It has a longer soil persistence than phenoxy herbicides, which limits planting of many broadleaf crops before 12 and 18 months. It is effective on a different spectrum of weeds than 2,4-D, MCPA or dicamba. Clopyralid is especially effective for control of legumes and composites (such as Canada and yellow starthistle). It does not control many common broadleaf weeds such as mustards, so must be tank-mixed for complete control of the wide range of broadleaf weeds found in small grains. The timing of application on wheat is from the 3-leaf stage to early boot stage (see DEVELOPMENTAL GROWTH STAGES), complimenting the timing of 2,4-D and MCPA.
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. Tank mixing with UN-32 may enhance weed control. Tank mixing with dicamba 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 weeds building up herbicide resistance. 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 cereal herbicides.
Pyraflufen-ethyl (ET) 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. Tank mixing with dicamba provides good control of chickweed. Combining ET with phenoxy herbicides broadens the weed spectrum controlled, lowers herbicide application rates, and can reduce the risk of weeds building up herbicide resistance.
Grass weed control. Fenoxaprop (Puma) controls canarygrass, wild oat and several Setaria spp., including yellow and green foxtails. It also suppresses mustards. It has a wide window of application, providing effective control when applied between the 1-leaf and 6th leaf grass stage. For best control of wild oat, delay application until most wild oat plants have emerged, which often pushes this to the 5- to 6-leaf stage. A tank mixture with bromoxynil allows for a wide range of weed control at an early timing. Fenoxaprop cannot be tank-mixed with phenoxy herbicides because it often reduces grass control when tank mixed.
Mesosulfuron (Osprey) controls most grassy weeds and many broadleaf weeds in wheat. It is especially effective on Italian ryegrass, wild oat, little seed 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 color for a couple weeks following application. If soil nitrogen levels are low, this symptom will persist longer and require an application of supplemental nitrogen. When treated beyond the one-tiller stage, temporary growth suppression and shortening of the wheat plant will occur. The crop will recover more quickly from these symptoms under good growing conditions.
Mesosulfuron is effective on certain broadleaf weeds including chickweed, wild radish, and mustards. It also provides partial control of many other broadleaf weeds including common groundsel, common malva, fiddleneck, yellowstar thistle and milk thistle. Mesosulfuron can be tank mixed with bromoxynil 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 non-ionic surfactant is required; ammonium sulfate or low rates of UN-32 added will enhance weed control on difficult to control weeds. Restrictions on crop rotations are greater than with Fenoxaprop.
Pendimethalin (Prowl H20) 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.
Harvesting. Uncontrolled weeds delay maturing grains and create harvest problems. Field bindweed, prostrate knotweed, Russian thistle, fivehook bassia, lambsquarter, smartweed, and johnsongrass 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 causes 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.
Fallow. For rainfed production systems, fields can be fallowed every other year to prevent weed seed build up and conserve moisture for maximum 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. Depending on local rainfall patterns and 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 disks or field cultivators, are performed through 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.
UC IPM Pest Management Guidelines: Small Grains
W. M. Canevari, UC Cooperative Extension, San Joaquin County
Acknowledgment for contributions to Weeds:C. E. Bell, UC Cooperative Extension, San Diego County
H. L. Carlson, UC Cooperative Extension, Siskiyou County
D. W. Cudney, Botany and Plant Sciences, UC Riverside
J. E. Hill, Agronomy, UC Davis
M. J. Smith, UC Cooperative Extension, San Luis Obispo County
R. N. Vargas, UC Cooperative Extension, Madera County