When dealing with fast maturing crops, the grower can prevent weeds from dispersing seeds . Since weeds adapt to continuous use of the same tools, implementations of other integrated management plans are crucial .Using more than one management strategy increases the probability of weed control success like implementing a form of physical control like hand weeding integrated with herbicides can control weeds . In the produce industry 42% of total weed control expense is due to labor costs . Organic vegetable farmers are extremely limited in the choice of registered herbicides they can use, making it necessary for growers to explore more options to manage weeds . California passed legislation in 2016 that increased the minimum wage by a $1.00/hr. per year until it reaches $15.00/hr. in 2022 . With this change in place, the agricultural industry will face higher labor costs . Costs associated with hand weeding, in general, are between $300-$700 per hectare, which has continued to increase in prices as much as 64% in the last ten years . Because there are few herbicides for lettuce production, hand weeding labor costs will get even higher . Tillage and cultivation Tillage combined with herbicides can be an effective integrated treatment method . Seedbeds that are firequently cultivated will generally have higher weed populations, leading to more weed outbreaks . When dealing with soil that is cloddy the weeds are protected by the clods of dirt and need to be broken down further when prepping a field for crop seeding . Using a mechanical weed cultivator in a high-density spinach field can cause crop injury, cannabis dryer which is why spinach must be hand-weeded . There are drawbacks when it comes to mechanical cultivation . Studies have shown that even the more advanced weeding machines have a hard time removing weeds close to crop plants .
Intelligent robotic weeders are another method currently being implemented, but these can be challenging to use in high density crops and wet conditions . Even with more advanced machines available, these robotic weeders are expensive and only the larger growers can adapt to the technology and afford high fixed costs while small growers struggle to invest . Some cultivators can only control small weeds, but it is challenging to take out older, deep-rooted weeds in lettuce production . In conclusion, cultural, physical, and chemical control methods cannot be separated and work best when used together in an integrated pest management plan.Organic agriculture is a sustainable production system that considers long-term effects involving preserving biodiversity, soil health, and the environment. There is a growing demand for organic vegetable crops for consumers who don’t want to be exposed to pesticide residues, especially on food for children . While the demand for organic produce continues to increase, it’s challenging to find effective ways to manage soil pathogens and weeds with few good pest control tools. Some challenges include high weeding costs because there are few organic-compliant herbicides, and effective management of soil pathogens is difficult. Research in weed and pathogen management for organic systems is not a high priority for many researchers and does not receive much attention, especially in production dealing with high yielding produce crops in an ever-growing marketplace . There is constant worry about persistent weed infestations in organic agriculture due to many weed escapes . In organic leafy crop production, there is a couple strategies they can use to control persistent weed infestations, one way to reduce surface seed bank is by deep burial.
This method is called plowing, which involves incorporating weed seeds into the ground, which prevents weed seeds from germinating . Plowing can create a problem because it involves burying weed seeds deep into the soil and will eventually be brought back to the soil surface during future field cultivations, so other organic production methods should be incorporated . The timing and depth of cultivation can have an effect on different weeds in a seedbank . Some weeds depend on light in order to germinate and is why weed seeds can be viable for so many years when buried deep into the soil where they are not exposed to light . Farmers have adapted to increased food production using intensive land cultivation during the green revolution and saw how productive and efficient it was to produce crops . Yet, extensive land cultivation can create more problems if not planned accordingly, so farmers learned how efficient it was to grow crops in rotation after harvest as another strategy to control pathogen inoculum and weed seeds in the soil as a common practice in organic production . Some organic practices that are used in lettuce production in coastal areas of California are crop rotations with mustard family species, into their integrated pest management program to help enhance soil fertility in an area of extreme soil disruption . Rotational crops are planned months before planting, and farmers need to have high returns to offset land and high production costs with greater risk of aphids and diseases in organic production .In spinach production, the most common herbicides used are phenmedipham and cycloate . These herbicides control grasses and broad leaf weeds, but they only control 2 out of 5 important weeds . Phenmedipham canbe used at the three true leaf stage of spinach but often cause crop injury for a short period and is labeled for processed spinach in California .
Therefore, growers need to know their weed seed bank to determine what methods would work best in their weed management plan. For preplant fumigation, which involves applying a product such as Metam sodium before the spinach is planted . However, Metam sodium is not preferred because it is restricted and has a 14-day wait before planting . Cycloate is the main preemergent herbicide used after seeding but before emergence and can also be pre-plant incorporated into the soil . In lettuce production, herbicides like Benefin is also used as a preplant herbicide applied with the help of machine incorporation on the tops of lettuce beds . Using this herbicide comes with precautions because it can persist in the soil for months and can affect rotational crops like spinach, onion, corn, and sugar beets . Incorporating the herbicide too deep can result in poor weed control . Bensulide is an important lettuce preemergence herbicide that controls annual grasses but is weak on pigweed . Pronamide is the most common preemergence herbicide for lettuce organically registered in 1969 . The herbicide does not need to be incorporated into the ground . Kerb provides excellent control of grasses and weeds in the mustard family . In addition, Kerb partially controls goosefoot and purslane in the nightshade family, malva and pigweed does not control weeds in the sunflower family like common sowhistle and common groundsel . Many growers have trouble with Kerb, especially in Yuma, Arizona because it leaches too deep in the soil profile when lettuce is irrigated, causing poor weed control . In addition, Kerb is a possible human carcinogen and has been found in groundwater . In today’s herbicide market, herbicides have been regulated and banned, leaving those that are more than 40 years old and subject to cancellation in the future . Cycloate and Dual Magnum are the only pre-application products in the market that can be used in spinach production, but Dual Magnum is not used much . Lati stated Cycloate controls broadleaf weeds well but can be difficult to control weeds in warm conditions due to increased volatility and loss. Growers continue to use herbicides because it benefits them by reducing the number of weeds in the field. Therefore, it requires less labor to hand weed in leafy crop production. Dual Magnum, an herbicide that was registered in 1977 and has a pre harvest interval timing of 50 days and is too long because spinach can be harvested in as little as 25 days . In addition, Fennimore argues that weeds like purslane can be problematic and thrive under heavily cultivated soils and in Salinas Valley’s Mediterranean climate. When herbicides like Pronamide and Bensulide were introduced in the late 1960s, there was an improvement in control . However, cannabis growing systems growers continue to have problems where reoccurring purslane infestations are persistent . Developing a new herbicide can be very difficult and expensive. The process takes 9 years and involves studying the environmental impacts of the chemical and proper development and rigorous field trials before it can be registered to be used. Sometimes chemicals going through this process do not make it successfully. Other alternatives to control weeds include using biotechnology to make herbicide-tolerant lettuce. Fennimore found that using glyphosate tolerance lettuce could control weeds better than Bensulide and Pronamide. In general, it can becostly to take this type of biotechnology to a commercial scale, but the lettuce industry soundly rejected roundup ready lettuce before it was introduced.Steam application to the soil is a method to disinfest soils by killing pathogens and weed seed in the soil. The concept of heating the soil started in greenhouse flower production dating back to the 1800s and even as far as ancient times .
Steam disinfestation first came into use in the United States in 1893 to treat greenhouse pots and eventually, as technology started evolving, different types of machine steamers were created . For example, mobile steamers come in tractor-towed or self-propelled models. Several different types of steaming techniques have been developed and evaluated in terms of costs and pathogen and weed suppression . The average cost of steaming was $4,883.24 an acre compared to the cost to use chemicals like methyl bromide or 1,3 chloropicrin application that would cost $7,324.86 per acre . Extensive research on the efficacy of steam opened new opportunities for improved steaming techniques in greenhouse production . Steam was used widely in the 1950s and 1960s, but in the 1970s, high fuel costs made steam more expensive than fumigants and so greenhouses switched to fumigants like hot gas methyl bromide . With increased chemical use regulations in greenhouse and field settings and the loss of methyl bromide, growers have renewed interest in steam. Chemical fumigants came into use in the 1960s when farmers soon found fumigants to be a cheaper and more efficient way of killing weeds and soil pests than steam . However today, it is difficult and expensive to use chemical fumigants in California agriculture due to strict regulation. Steam on the other hand, provides an alternative and is not regulated. Many argue steam is a viable alternative to methyl bromide, but better steam application methods and applicators are needed . One concern about using steam disinfestation is overheating the soil, killing beneficial microbes key in soil nitrification . Wherever steam is used, steam needs to reach a temperature of 60-70 °C for 20-30 mins dwell time to control soil pathogens and weeds effectively . Because romaine lettuce and spinach are shallow-rooted crops grown in rows, the depth of steam injection application using a banded steam technique is set to 19 cm or shallower in depth .Studies done by Baker and Van Loenen et al., evaluated the effects of various temperatures and exposure times on soil pathogens by soil type using dry steam. Dry steam is created by generating steam with one pass through the boiler followed by a second pass so that the temperature is higher and there are no water droplets . Baker found that when steaming soil in pots that were dry at a lower temperature, it took longer to reach the appropriate temperature to kill soil pathogens. In contrast, Baker also observed that when potted containers were moist just before steam application and steamed at a higher lethal temperature dose for 30 mins he observed higher kill rates of soil pathogens.Sheet steaming is one of the oldest methods of steam application and is still used in greenhouse production . The downfall of this technique is that the steam process takes 8 hours, and the performance of steam is better in clay soils, according to a study done in Italy by Gay et al. . To heat the deeper layers of the soil takes long to reach the appropriate temperature to kill soil pathogens but gave the best results in steaming the top 15 cm of the soil . Sandy soils were the most challenging soil type to steam with this method .