PCC July-August 2020

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Weed Control in Lettuce A Comparison of Various Weed Management Strategies and Costs By RICHARD SMITH | UCCE Vegetable Crop and Weed Science Farm Advisor Economical and successful weed control in lettuce can be accomplished by utilizing key cultural practices, cultivation technologies and herbicides. Planting configurations vary from 40-inch wide beds with two seedlines to 80-inch wide beds with 5 to 6 seedlines. Recent studies of weeding costs for lettuce ranged from $454 to $623/A for 80- inch wide beds with 5 seedlines of head and 6 seedlines of romaine hearts lettuces, respectively (see coststudies.ucdavis.edu/ en/current/commodity/lettuce/). Weeding costs included the following: Herbicide applied in 4-inch wide bands over the seedlines, cultivation, auto thinning using a fertilizer to kill unwanted lettuce plants and hand weeding/double removal. The costs for auto thinning also include fertilizer costs, which can satisfy the need for the first fertilizer application. Significant weed control is accomplished by practices that occur before the crop is planted. For instance, weed pressure is affected by prior crop rotations and how much weed seed was produced in them. The weeding costs given above are rough averages. If weed pressure is light, weeding costs can be lower, but if weed pressure is high, weeding costs can be much higher. In the Salinas Valley, good management of weeds is possible with rotational crops such as baby vegetables (spinach, baby lettuce and spring mix) because they mature in 25 to 35 days and don’t allow weeds to set seed. Long-season crops such as pepper and annual artichokes allow multiple waves of weeds to germinate which are difficult to see and remove once the plants get bigger. Preirrigation is standard practice to prepare the beds for planting. It stimulates germination of a percentage of weed seeds in the seedbank, and they are subsequently killed by tillage operations. Studies have shown that preirrigation followed by tillage lowers weed pressure to the subsequent crop by about 50%. In organic production, pregermination is one of the most powerful practices for reducing weed pressure, and if time allows, it can be repeated to further reduce weed pressure. Preemergence Herbicides There are three pre-emergence herbicides available for use in lettuce production: Balan, Prefar and Kerb. Balan and Prefar provide good control of key warm season weeds such as lambsquarters, pigweed and purslane, as well as grasses (Table 1, see page 5). Kerb is better at controlling mustard and nightshade family weeds such as shepherd’s purse and nightshades. Balan is mechanically incorporated into the soil and Prefar and Kerb are commonly applied at or post planting and incorporated into the soil with germination water. Kerb is more mobile in water than Prefar which can lead to issues with its efficacy. Often 1.5 to 2.0 inches of water are applied with the first irrigation to germinate the crop which can cause Kerb to move below the zone of germinating weed seeds, especially on sandy soils. For instance, Kerb is capable of controlling purslane however, its efficacy can be low on sandy soils due to its movement below the zone of germinating weed seeds with the first germination water. Prefar does not leach as readily as Kerb and that is why these two herbicides are often mixed in the summer to control purslane (Figure 1). In the desert, the use of delayed applications of Kerb has been used for many years. Due to the large amounts of water that are applied to keep the seeds moist Drip germination in lettuce has resulted in fewer weeds than sprinkler irrigation (photo by Marni Katz.) and cool, Kerb is applied in the 2nd or 3rd germination water, approximately 3 to 5 days following the first water, just prior to the emergence of the lettuce seedlings. The amount of water applied in the second and third irrigation is less than the first application and therefore does not push the Kerb as deep in the soil. Although the Salinas Valley is cooler than the desert, evaluations here have also found delayed applications to improve the efficacy of Kerb (Figure 2, see page 6). These data illustrate the loss of control of purslane by Kerb when applied before the first germination water, as well as the improvement in efficacy that results when applied after the first germination water. It also illustrates the role that Prefar plays in the control of purslane when the efficacy of Kerb is reduced by being pushed too deep. Clearly, there is benefit from applying the Kerb in the 2nd or 3rd germination water because it helps to keep it in the zone where weed seeds are germinating. Figure 1. On left: Kerb at 3.5 pints/A applied at planting; On right Kerb at 3.5 pints/A + Prefar at 1.0 gallon/A applied at planting. The main weed is common purslane which was not controlled by Kerb because it was pushed below the zone of germinating weed seeds by the germination water (photo courtesy R. Smith.) 4 Progressive Crop Consultant July / August 2020
Table 1. Weed susceptibility to registered preemergent herbicides. The use of single use drip tape injected 3 inches deep in the soil has become popular in the Salinas Valley. The uniformity of using new tape with each crop has allowed growers to consider using drip irrigation to germinate lettuce stands. Although the same amount of water may be applied to germinate the stand with drip irrigation as with sprinklers, the water tends to move upward with drip irrigation. In drip germinated lettuce, Kerb is sprayed on the soil surface and is solubilized by the upward movement of the drip applied water which allows it to move just deep enough in the soil to control germinating weeds, but not too deep to reduce its efficacy (Table 2, see page 6). Interestingly, drip germination alone resulted in fewer weeds than sprinkler irrigation. Lettuce is typically planted with 4-5 times more seed than is needed in order to assure a good stand. At about 3 weeks after the first irrigation, lettuce is Continued on Page 6 Weed species Balan Prefar Kerb CHICKWEED GROUNDSEL HENBIT KNOTWEED LAMBSQUARTERS LONDON ROCKET MALVA MUSTARD NETTLE, BURNING NIGHTSHADE, BLACK NIGHTSHADE, HAIRY NETTLELEAF GOOSEFOOT PIGWEED PINEAPPLE WEED PRICKLY LETTUCE PUNCTURE VINE PURSLANE SHEPERDS PURSE SOW THISTLE BARNYARDGRASS BLUEGRASS, ANNUAL LOVEGRASS NUTSEDGE, YELLOW C N N C C N N N N N N C C N N P C N N C C C N C N N C C N N N N N N C C N N P C N N C C C N C — Control; P —Partial Control; N — No Control C N C C C C P C C C C C C N N N C C N C C C N Protecting your crops from start to harvest NEW ONE PRODUCT, TRIPLE CONTROL FUNGICIDE + INSECTICIDE + MITICIDE Proven broad-spectrum control for major insect pests and diseases MULTIPLE MODES OF ACTION: Works on contact or by ingestion, as antifeedant, insect repellent and growth inhibitor. SUPERIOR STABILITY: Better handling with premium suspension stability for over 48 hours. GROWER FLEXIBILITY: Use right up to day of harvest with 4-hour REI and 0-day PHI. For field trial data and more information visit tryrango.com or call your territory manager at 1.805.788.8167. Always read and follow label directions for use and application rates. RANGO is a trademark of Terramera, Inc. Try RANGO today. July / August 2020 www.progressivecrop.com 5
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Page 15 and 16: Worms, Thrips, Leafminers IN ONE PA
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PCC July-August 2020

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