As more effective herbicides have become available, the need for cultivation has decreased. Over the past two decades, many growers have successfully eliminated cultivation, reducing equipment and labor costs, moisture loss, root damage, and subsequent weed flushes. However, eliminating cultivation can lead to more dependence on herbicides, which may hasten the development of herbicide-resistant weeds.Field monitoring and early detection of glyphosate-resistant weeds are the first and most important steps for preventing and managing glyphosate resistance. Before assuming that weeds that survived glyphosate application are resistant, rule out other factors that might have affected herbicide performance, such as misapplication, unfavorable weather conditions, improper application timing, and weed flushes after application. If multiple weed species are present in the field after glyphosate application, then it is likely that other factors related to performance, and not resistance, are responsible. If a single weed species remains in the field, however, this may indicate possible glyphosate resistance. If a high level of resistance is occurring in a field, you will observe dead plants next to uninjured or nearly uninjured plants, with a few intermediate-responding plants. If a low level of resistance is occurring in the field, dead plants will be right next to plants that appear nearly normal, with the majority of plants having an intermediate response. In early development stages of glyphosate resistance, grow trays it is common to observe patches of single weed species scattered in no particular pattern throughout the field.To prevent weeds from growing and to keep glyphosate-resistant weeds under control, incorporate multiple IPM tools into your weed management plan.
The potential for developing glyphosate resistant weeds should receive serious and thoughtful attention when glyphosate-resistant crops are introduced. Resistance management must be an integral part of the production system. If selection pressure is maintained through the continuous use of the same herbicide, resistance is likely to soon render that herbicide ineffective. Glyphosate controls a wide range of grasses, sedges, and broad leaf weeds and can be highly effective on deep-rooted perennials. The question is whether we can have an effective weed control program that is comprised only of postemergence glyphosate applications. This program would certainly cut down on costs, and it may work initially under certain field conditions, but problems with weed species shifts and glyphosate resistance may develop rapidly. The success of total postemergence glyphosate programs depends on what weeds are present in a field, their severity, proper timing of the herbicide application, and crop competitiveness. Fields withlow weed pressure and few late-germinating weeds are most likely to have acceptable weed control with a total postemergence glyphosate program in the short term, but it is not sustainable and should be avoided. A combination of residual preemergence herbicides followed by postemergence tank mixtures of glyphosate with herbicides that have other modes of action is the ideal herbicide program. If glyphosate is used as a burn down treatment prior to tillage, consider a tank mix of glyphosate with an herbicide that has another mode of action, such as an auxin-type herbicide. If you have glyphosate-resistant weeds such as marestail or hairy fleabane, tank mixes of glyphosate and 2,4-D can give good control, but the treatment must be made when the weeds are small. However, when contact non-selective herbicides such as paraquat replace glyphosate as a base program, antagonism has been reported for some tank-mix partners. To maximize weed control, the timing of glyphosate application is vital in order to maximize weed control and minimize the chances of crop yield loss due to early-season weed competition.
Our research has shown that glyphosate-resistant weeds are less resistant to glyphosate when they are young . Glyphosate should be applied in combination with herbicides that have other modes of action when weeds are small , if glyphosate-resistant weeds are present. Controlling weeds at this stage reduces the chances for crop yield loss as well as the risk of weed control failures with larger weeds that may be under stressful conditions. Applying the appropriate glyphosate rate is another important consideration for consistent weed control. Proper glyphosate rates should be based on weed type, weed size, and spray volume. It is important to match the weed spectrum and size with the appropriate glyphosate rate to reduce chances of weed control failures under extreme conditions. In most cases, the appropriate rate to use for weed control in glyphosateresistant crops is 0.75 pounds acid equivalent per acre of glyphosate. However, larger and more difficult-to-control weeds may require higher rates . In addition, when glyphosate is tank mixed with other herbicides, select herbicide partners that are effective on weeds present in your field. For help in selecting proper herbicide partners, visit the University of California IPM Program’s website , consult the Pacific Northwest Weed Management Handbook , or contact your county extension office for local recommendations.Corn Under ideal conditions, preemergence programs may provide full-season weed control. This requires applying a combination of products at full label rates to extend control. Combining preemergence herbicides with postemergence herbicides or cultivation often provides more consistent weed control and protects crop yields better than one-pass preemergence or total postemergence programs. Application timing is critical to achieve the desired control with postemergence herbicides. Generally, weed tolerance to postemergence herbicides increases with plant height. Cotton The most effective weed management program includes combinations of soil-applied herbicides, cultivation, and postemergence herbicides. Glyphosate, however, is the primary herbicide used to control weeds in glyphosate-resistant cotton, and it provides excellent control in most situations. Two postemergence applications typically are required to achieve full-season control and protect yields. Using soil-applied herbicides in glyphosate-resistant cotton often improves the consistency of control and reduces early-season weed competition that can lead to crop yield loss. This practice also introduces herbicides with an alternative mode of action, reducing the selection pressure for glyphosate-resistant bio-types. Alfalfa Using glyphosate offers alfalfa growers an alternative weed management system that may enhance alfalfa growth and persistence by providing a wider spectrum of weed control. Glyphosate can be applied to glyphosate-resistant alfalfa at any stage of growth. Glyphosate-resistance management should always be implemented when using glyphosate-resistant alfalfa. Repeated applications of glyphosate may result in glyphosate-resistant weeds. Once glyphosate-resistant weeds become established within a field, glyphosate becomes ineffective against those weeds. Measures to reduce the potential of developing glyphosate-resistant weeds can include crop rotation, herbicide rotation, proper herbicide rate, proper herbicide timing, and alternative weed control methods. Canola Various options exist for preventing glyphosate resistance, depending on the weeds that are prevalent in a particular field. Rotate herbicides and use tank mixtures of glyphosate with herbicides that have other modes of action to reduce the chance of developing glyphosate resistance. Depending on the weed populations and pressure, growers should alter crop rotations and herbicide-tolerant canola system selections to ensure a diverse weed control program in the cropping system. Sugarbeet Using glyphosate for weed control is expected to make weed management simpler and more effective for sugarbeet growers. Conventional sugarbeet herbicides must contain many herbicidal ingredients in order to simultaneously manage all the different weed species, whereas glyphosate provides broad-spectrum weed control. Glyphosate may be used before sugarbeet is planted or as a postemergence herbicide after this crop is planted. It is critical to use a preemergence residual herbicide to achieve full-season control and protect yields. In addition, tank mix herbicides that have other modes of action with glyphosate to use prior to planting and during the growing season. Several postemergence herbicides available for sugarbeet have contact activity and may antagonize or reduce glyphosate activity, grow systems for weed especially when glyphosate is used at low rates.Most crops are sensitive to weed competition during the early seedling stage of growth, which can ultimately reduce crop yield and quality . Weeds can also host many pathogens and insects, which can infect crops if left uncontrolled . For example, an outbreak of wild mustard in a cabbage field can attract aphids. Large infestations of Johnson grass in a maize field can host maize dwarf mosaic virus, which allows it to overwinter in the weed rhizomes . Weeds can also compete with the crop by taking up plant nutrients and water by interfering with the crop roots by not allowing the crop plant grow to its desired size, leading to yield loss . High-density leafy green plantings can mitigate weed competition due to the extensive crop canopy cover, blocking sunlight from reaching the soil . In contrast, a lowdensity spinach crop can be severely infested by weeds because more sunlight can reach the soil . Greater light penetration to the soil can lead to more weeds germinating and becoming established creating complication and even difficulty in harvesting due to the weeds getting stuck on the harvester, resulting up to 20% crop loss . Identifying weeds in a field seedbank can help guide weed management plans by knowledge of weed population pressure, seasonal weed germination patterns, life span longevity, and survival to understand the competitive advantage weeds have on crop plants .
Uncontrolled weeds will reproduce and disperse their seed as much as they can before they die creating a seed bank . After seed maturity, seed dispersal occurs with the help of wind, water, animals, and humans . Eventually seeds end up in the soil, which allows the seeds to remain in a dormant state until favorable conditions allows seed germination. . Where annual weeds are allowed to grow and reproduce, they create a weed seed bank that varies by field and therefore can be difficult to control . Lati et al., observed that weeds could impede crop development in the first four weeks of production. A farmer should know the weed seed bank in fields because the critical period to weed control can be based on the timing of emergence and weed management strategies can vary by season to determine crop choice and herbicide selection . Weed management strategies can vary by crop but the ultimate goal is to reduce hand weeding and prevent loss of quality and weed contamination in salad mixes. For example, high value crops, such as lettuce and spinach are fast-growing; therefore, these crops can often be harvested before weed seeds can fully mature leaving little room of weeds to contaminate salad mixes . Weeds and weed seeds are very sensitive to their environment. In addition, fully mature weeds can detect short and long day lengths in order to set flowers and get ready to produce seed . For example, dandelion responds to short day lengths and can set large numbers of seed in the fall . Emergence of weeds like common purslane emergence peaks in July when it loses light sensitivity, whereas burning nettle comes up throughout the year. The time of emergence peaks can vary by species and is crucial to know in order to make an effective weed management plan and prepare for future crop plantings. With a few herbicides available for organic vegetable growers and few new herbicides being introduced for conventional crops, it is necessary to look for non-pesticide solutions. One area that appears promising is the non-chemical soil pasteurization method to manage weeds and pathogens . In the late nineteenth century, steam pasteurization was developed as a method of soil disinfestation. Steam is injected into the soil to disinfest the soil from pathogens, insects, and weed seeds by maintaining 70 °C for 15-20 minutes . Treating an entire field with steam is expensive but strategic steam placement in bands is a way to reduce costs . Band steaming can reduce the viability of weed seed in the bands that are treated where the crop plant will be planted. The effects of steam treatment can last for weeks or months. A study done by Runia showed that steam soil pasteurization can be part of an integrated program to manage some of the most troublesome pathogens like Pythium. spp. caused by pythium wilt and Sclerotinia minor caused by lettuce drop in lettuce production. Pythium wilt can be very destructive toward the end of the growing season affecting plant roots, and lettuce drop can cause total crop collapse affecting the lettuce crown. Soil disinfestation with steam appears to have potential, given the need for better control of pests in organic vegetable production in California. With continued research and further machinery improvement, it may be possible to reduce operating costs on a commercial scale by precision application of steam. This literature review will describe the latest advancements in steam to control pathogens and weeds for California vegetable production.