Economic losses due to weeds can be a serious problem in the production of lettuce. With planting taking place from late August through December, summer annual, winter annual and perennial weeds may be a problem. Weeds decrease crop yield and quality through competition for water, nutrients and sunlight. In addition, many weeds harbor destructive insect pests and serve as alternative hosts for other organisms which cause crop diseases.
Weeds commonly encountered in lettuce in Yuma, AZ in the late summer plantings include summer annual grasses such as watergrass or junglerice (Echinochloa crus-galli or E. colona) and sprangletop (Leptochola spp.); and small-seeded broadleaved weeds including purslane (Portulaca oleracea), pigweeds (Amaranthus spp.) And groundcherry (Physalis wrightii) In the transition period between hot and cooler temperatures, lambsquarters (Chenopodium spp.), nettleleaf goosefoot (Chenopodium murale), knotweed (Polygonum spp.) And cheeseweed (Malva spp.) become difficult to control. By late-October when temperatures cool, winter annual weeds are prevalent. Winter annual grasses include canarygrass (Phalaris spp.), wild oat (Avena fatua), annual bluegrass (Poa annua), wild barleys (Hordeum spp.) And volunteer small grains. Cruciferous weeds such as London rocket (Sisymbrium irio), shepherdspurse (Capsella bursapastoris) and black mustard (Brassica nigra) are commonly encountered. Lettuce-related weed species are also prevalent under cooler conditions, including prickly lettuce (Lactuca serriola) and sowthistle (Sonchus oleraceous).
Cultural and chemical technologies are utilized for weed control in lettuce. Cultural practices including mechanical cultivation and hand hoeing are utilized from the fallow period prior to seed bed formation until first layby.
Herbicides have been used for weed management in lettuce for more than 30 years. There are four distinct timings of application for herbicides in lettuce: fallow, preplant, preemergence and postemergence.
Before seedbed preparation, Roundup Ultra (glyphosate), applied at 0.375 to 5.0 lbs-a.i. (active ingredient) per acre and Gramoxone Extra (paraquat) applied at 0.5 to 1.0 lbs-a.i. per acre, are often used to reduce weed seed banks. Fields are laser leveled and then flushed with water to germinate weed seeds. These broadspectrum non-selective herbicides are applied after sufficient weeds have emerged.
After a field has been laser leveled and prior to listing the seedbeds, Balan (benefin), at 1.2 to 1.5 lbs-a.i. Per acre, is commonly applied and disked into the top 4 to 6 inches of the soil. Balan controls some grasses and some small-seeded broadleaved weeds (Table 1). Balan moves very little in the soil profile or within the plant tissue.
Table 1. Efficacy of preplant and preemergence herbicides against commonly occurring weeds in lettuce.
Kerb (pronamide), at 1.0 to 2.0 lbs-a.i. Per acre, can be used preplant, preemergence or postemergence. However, most applications are applied banded over the top of the seedbed just before or immediately following planting. These preplant and preemergence applications should immediately be followed by sprinkler irrigation of 1 to 2 inches. However, Kerb is sensitive to movement in the soil with water, and excessive sprinkler irrigation will leach the material out of the weed seed zone (top 1 inch of soil) and render the product ineffective. Thus, the effectiveness of Kerb is greatest if used on lettuce that is not sprinkle irrigated. For postemergence applications, apply before or after lettuce emerges, but prior to weed emergence. Kerb is most effective against cool season annuals, and best fits lettuce planted after September (Table 1).
Prefar (bensulide); at 5.0 to 6.0 lbs-ai per acre, is often used in preplant or preemergence for control of selected grasses and broadleaved weeds (Table 1). Prefar is most effective against summer annual weeds, and is most efficient in lettuce planted in August and September. Preplant applications should be mechanically incorporated 1 to 2 inches before planting. Preemergence applications should be immediately followed by irrigation. When incorporating using sprinkler irrigation, wet the soil to the depth of 2 to 4 inches. Furrow irrigation should thoroughly wet, or "blackened," the top of the bed. Prefar moves very little in the soil and should stay within the weed seed zone. Prefar can also be applied through sprinkler irrigation.
Maximum weed control using preplant or preemergence herbicide is best achieved if the herbicide can be concentrated within the weed seed germination zone (top 1 to 2 inches of soil). Additionally, a smooth, clod-free surface will maximize weed control when using Kerb or Prefar. Combinations of Balan and Kerb control most weeds. Use Balan preplant incorporated followed by Kerb applied preemergence and incorporated by irrigation , or apply both herbicides preplant and mechanically incorporate. Weeds related to lettuce such as prickly lettuce and sowthistle are not controlled. Other difficult to control weeds that may occur in lettuce include horseweed (Conyza canadensis), fleabane (Conyza bonariensis), cudweed (Gnaphalium palustre), groundsel (Senecio spp.), annual clovers (Leguminosae), filaree (Erodium spp.) And broadleaf perennials and nutsedges (Cyperus spp.).
Balan, Kerb and Prefar can cause injury to lettuce, especially under environmental conditions where lettuce emergence is slowed. Also, some lettuce varieties are more prone to injury than others. Refer to the herbicide label for details. For more information concerning herbicide damage to lettuce, refer "Quantitative Guidelines for Evaluating Herbicide Injury in Lettuce"
Poast (sethoxydim) used at 0.1 to 0.3 lbs-a.i. Per acre with a crop oil concentrate added will selectively control most annual and perennial grasses found infesting lettuce. Timing is critical for optimal control; apply Poast when grasses are small. Do not apply to grasses under stress. Thorough coverage is required. Do not tank mix with other pesticides as efficacy may be reduced. Do not cultivate within 5 days prior to application or within 7 days following application. See the label for specific directions.