Effect of Irrigation Method on Herbicide Efficacy in Lettuce Production

University of California Cooperative Extension, Monterey County

Author: Richard Smith, Farm Advisor 

In 2020 we compared the efficacy of lettuce preemergent herbicides in trials with fields split between season-long drip vs sprinkler irrigation. The trials were conducted on sites with sandy soils along the Salinas River. In the first trials shepherd's purse was the dominant weed and in the second trial purslane was dominant. In both trials there were fewer weeds in the drip irrigated side of the field (Tables 1&2). This observation is consistent with a prior evaluation at the Spence Research Station in which we observed 85% fewer weeds in drip irrigated plots than in sprinkler irrigated.  It is not entirely clear why there tends to be fewer weeds in fields that are germinated with drip irrigation, but it may be because the soil surface may be drier in drip irrigated than in sprinkler irrigated fields. 

The use of drip irrigation also affects the activation of the preemergent herbicides used in lettuce production. Prefar provides excellent control of purslane, however, in Trial No. 2 it only controlled 12% of the purslane in the drip irrigated plots compared to 89% in the sprinkler irrigated plots. The mobility of both Balan (Koc 10,000 mL/g) and Prefar (Koc 1433-4326 mL/g) are quite low in soil and once wetted by the upward moving drip water they quickly adhere to organic matter or clay and before moving far enough downward to get to the zone where weed seeds are germinating. Kerb on the other hand is more mobile in soil (Koc 548-1340 mL/g) than Balan or Prefar and controlled 60-74% of purslane in the drip irrigated plots and 97-98% in the sprinkler irrigated plots. Interestingly, in these trials Kerb provided greater control of purslane than Prefar in the sprinkler irrigated trials. This result is surprising given that on sandy soils during the summer, Kerb is easily leached deep enough with the first sprinkler germination irrigation, making it less effective in controlling small seeded weeds such as purslane and shepherd's purse (Figure 1). The reason for this is, if enough water is applied in the first germination water, Kerb is mobile enough to move below the top 0.5 inch of soil which is where most small-seeded weeds germinate. As a result, Kerb applied in the 2^nd or 3^rd germination water, which is typically a smaller quantity of water, keeps the Kerb in the zone of germinating weed seeds. But in these trials, Kerb worked better in the sprinkler irrigated evaluations which indicates that the ranch manager was extremely careful with his germination water applications and kept the Kerb in the area where it could provide good control of purslane.

The results in these trials give some insight into the activation of the lettuce preemergent herbicides with drip and sprinkler irrigation. These trials are specific to the soil type, condition and management at this site and may vary at other sites with different soil types and management. It is all about placement of the herbicides and it is important to understand the capacity of the irrigation system to activate and move the herbicide where it is needed. From these trials, materials such as Prefar and Balan were not as effectively moved into the soil and activated with drip irrigation. Kerb was effectively activated with drip, and unexpectedly, very well with sprinkler irrigation.

1 – percent control of total weeds relative to the untreated

1 – percent control of total weeds relative to the untreated

Figure 1. Number of weeds in Kerb and Prefar treatments irrigated with the first germination

Water (July 1) or on the third germination water (July 4, 2018)

Author(s): Elizabeth Mosqueda, Richard Smith and Steve Fennimore

Assistant Professor CSU, Monterey Bay, Farm Advisor and UC Extension Weed Specialist

Background: Automated weeder technology has evolved  significantly over the past decade. The technology used by auto weeders is similar to that used by the auto thinners: cameras detect plants, a computer processes the image and makes decisions about which plants to keep and which to remove and then activates the kill mechanism. Automated weeders remove weeds from inside the uncultivated band (3-5 inches wide) left around the seedline and unreachable by standard cultivation.  The kill mechanism used by the currently available machines is either a split blade that opens around keeper plants (e.g. Robovator and Steketee IC) or a spinning blade that avoids the keeper plants by placing them in a notch in the blade (e.g. Garford Robocrop).  In 2015, evaluations found that Robovator and Steketee IC autoweeders removed 51% of the weeds in the seedlines and reduced follow up hand weeding time by 37%. From these studies we observed that auto weeders were not miracle workers, in that they required a relatively low to moderate population of weeds in order to operate effectively. As such good weed control in prior rotations or a good preemergent weed control program was needed to keep weeds at a moderate level. However, new developments in crop/weed detection may improve this issue. In addition, auto weeders do not remove all the weeds in the seedline because they cannot remove weeds that are too close to the crop plants without risking damaging them. And finally, the automated weeders are currently not capable of removing lettuce doubles in direct seeded lettuce fields, and as a result, it is still necessary to have a crew pass through the field following the passage of the auto weeder, if for no other reason than to remove double lettuce plants. The  main impact of the auto weeders is to reduce the amount of time that follow up hand weeding/double removal takes. This then brings up the hard question for a grower – does the reduced amount of time that follow up weeding/double removal takes, make up for the cost of running the automated weeder through the field. What is the economic threshold to run an autoweeder?

In 2020 we evaluated two new autonomous weeders.  These machines are designed to run without a driver and are intended to be set up to weed a field on their own. In these studies, the  machines always had someone accompany them through the fields, as auto weeding lettuce fields is still in the research and development phase. We conducted, evaluations of the Naio Dino platform (Photo 1) and the FarmWise Titan (Photo 2). We evaluated initial weed populations and subsequent follow-up hand weeding to better understand the relationship between weed pressure and the time savings for subsequent hand removal of weeds and doubles.

Methods: Two trials were conducted with the Naio Dino autonomous robotic platform equipped with finger weeders and five trials were conducted with the FarmWise Titan autonomous weeder which used a split knife that closes between crop plants, thereby taking out weeds in the seedline, and opens around the keeper plants. Auto cultivation was carried out following thinning (except Dino Trial No. 2 was cultivated prior to thinning) and were compared with standard cultivation which leaves a 4-5 inch wide band around the seedline. Pre and post cultivation weed and stand counts were made of a 6-inch wide band around the seedline to determine the efficacy of standard and auto cultivation. Weeding time of the treatments was evaluated by measuring the time it took members from a commercial hand weeding crew to pass through the treatment rows. Weeding time was then converted to hours per acre. Stand counts and harvest evaluations were conducted to determine if the auto weeders caused damage to the stand or to crop plants.  See Table 1 for trial details.

Results: Naio Dino evaluations: This cultivator used finger weeders and removed more weeds from the seedline than standard cultivation (Table 2). It reduced weeding time in trial No. 2 and did not reduce the stand or mean head weight of the lettuce. FarmWise Titan evaluations: Five trials were conducted with this implement. The FarmWise Titan removed a higher percent of weeds from the seedline in all trials and reduced subsequent hand weeding time in three of four evaluations. More time was required to hand weed fields with higher initial weed populations (Figure 1). According to the data in Figure 1, at high weed densities, subsequent weeding time was reduced using an auto weeder by 12% for each increase in weed density of 10/m². The FarmWise Titan did not significantly reduce the stand of lettuce or reduce the mean head weight of lettuce.

Overall, auto weeders removed about twice the number of weeds than standard cultivation from the 6-inch band around the seedline and reduced subsequent hand weeding/double removal by 4 hours/acre (Table 3). They did not reduce the stands of lettuce or affect mean head weight of lettuce and were therefore, did not damage lettuce plants to a significant degree. In general, the use of auto weeders appears to be clearly justified in fields with higher weed densities. However, other pressures may also spur the move to automated weeders such as increasing labor costs and lower labor availability.

Table 1. Details on the auto weeder cultivation trials

1 – SL = seedlines; 2 – The Dino cultivation was made prior to thinning and post cultivation stand counts were not made at this site

Table 2. Weed and harvest evaluations of the auto weeder cultivation studies.

Figure 1. Relationship between initial weed population and the reduction in subsequent hand weeding time of lettuce by auto weeder.

Table 3. Overall weed and harvest evaluations.

Photo 1. Naio Dino autonomous platform equipped with finger weeders.

Photo 2. FarmWise Titan autonomous tractor equipped with split knives

Author/s: Richard Smith, Joji Muramoto and Patricia Love

UCCE Farm Advisor, Extension Specialist and Staff Research Associate

In the fall, following the crop production season, residual soil nitrate-nitrogen (N) levels increase when N-rich residues from crops such as broccoli are incorporated into the soil. Soil temperatures in the fall and early winter are adequate to allow decomposition of crop residues, as well as continued mineralization of soil organic matter.  The resulting pool of residual soil nitrate-N is vulnerable to leaching by winter rains.  Winter-grown cover crops trap a significant portion of this nitrate in their biomass, thereby providing a useful practice to reduce nitrate leaching during the winter fallow period; however, although growers may want to include cover crops in their rotations for the benefits that they provide, conflicts with spring planting schedules and economic hurdles such as high land rents often preclude their use. As an alternative practice,  we conducted trials to evaluate the potential of using high carbon:nitrogen (C:N) organic amendments during the winter fallow period to immobilize soil nitrate-N and reduce leaching.  Immobilization is a process whereby the soil microbes utilize a readily available source of C and available soil nitrate to stimulate rapid growth. As a result of this microbial activity, the pool of nitrate in the soil is reduced which in turn reduces the risk of nitrate loss by leaching.  On the Central Coast, it is a common practice for grower to apply compost in the fall to improve soil tilth and health. Compost made from yard waste is the most commonly used material used on the coast and it has a C:N ratio that typically ranges from 13 to 20. By contrast high carbon compost is made from materials such as tree limbs and trunks that contain little N and has a C:N ratio of  >40 and is capable of immobilizing larger quantities of nitrate. If the use of high carbon compost could be substituted for normal compost applications, it could potentially provide a best management practice (BMP) to immobilize soil nitrate-N during the winter fallow and help growers comply with water quality regulations enforced by the Regional Water Quality Control Board.

Studies were conducted each winter from 2016 to 2020 to evaluate reductions in nitrate leaching with the use of high carbon compost amendments. Each trial was instructive in helping us understand this practice and evaluate if it was an effective and economically viable practice that growers could incorporate into their operations. The following are the highlights of evaluations of potential materials:

• One of the most effective materials was almond shells ground to particles 0.5 mm in size (Photo 1). The C:N ratio of this material varied from 59 to 70. It reduced the load of nitrate-N in the top three feet of soil by as much as 34 to 47%, at 5 and 10 tons/A, respectively. Almond shells are readily available in the Central Valley, but transportation and grinding costs make the expense of this material an issue.
• Glycerol is source of highly labile carbon which soil microbes are capable of utilizing immediately (Photo 2). It reduced the load of nitrate-N in the soil by as much as 48%. This material is highly effective, but high cost at the rates found to be effective (1.25 – 2.5 tons/A, equivalent to 240 and 479 gallons/A, respectively) is the main issue with this material.
• Locally sourced compost made from tree limbs and trunks was obtained from the Marina Landfill that had C:N ratios that varied from 185 to 215. The main issue with this material getting it ground fine enough so that it could serve as a rapid source of C to allow the immobilization process to proceed during the winter fallow. We obtained a material called Forest Mulch Compost which is triple screened and, in one study, had sufficient fines to effectively immobilize nitrate (Photo 3). However, the material also contains coarse material that is not active in immobilization during the winter fallow. This material costs about the same as typical yard waste compost that is commonly used and therefore provides an affordable option. Forest Mulch Compost is the most practical option for a high carbon amendment that we have found. If a market for this material is stimulated, there could be an incentive to grind it to a smaller size which would increase its effectiveness.

High carbon amendments ideally immobilize nitrate during the winter fallow period and then stop immobilizing immediately when the subsequent cash crop is planted. In actual practice however, that is not always achievable. We observed issues with the high carbon compost tying up nitrate in the subsequent crop and the following are some details from these observations:

• The high carbon compost works best when thoroughly incorporated into the winter bed in order to place it in the proximity of the nitrate in the  soil (e.g. in the top foot of soil). In our trials, the material was applied to the land planed field. The soil was then chiseled, and the beds listed (Photo 4). That process seemed to incorporate the material adequately for 40-inch beds, but less well for 80-inch beds. There is a need to explore ways to get more thorough incorporation into 80-inch beds. 
• In one trial, 10 tons/A of almond shells caused continued immobilization in the subsequent lettuce crop and caused yield reduction. The 5 tons/A rate did not reduce lettuce yield.  There is a need to continue to explore appropriate rates and incorporation methods.
• We conducted a trial in which we intentionally put too much high carbon compost onto beds and then applied various rates of a starter fertilizer to overcome the effect of immobilization. Higher rates of starter fertilizer were able to reduce the negative impact of immobilization.

The bottom line is that high carbon amendments can provide a useful practice to reduce the load of nitrate in the soil during the winter fallow. The use of a material like Forest Mulch Compost is affordable and has been shown to be effective. Like all new practices, grower will have to start off treating small areas and build up their expertise and confidence when employing this practice. It is our hope that the Regional Water Quality Control Board will consider high carbon amendments on the “R” (removal) side of the proposed “A (applied) – R” equation which is the metric that is currently proposed to measure compliance with the limits on the quantity of N applied to crops.

Photo 1. Finely ground almond shells. Particle size <0.5 millimeter

Photo 2. Glycerol being applied to 40 inch beds.

Photo 3. Forest mulch compost (made from tree branches and trunks and triple screened).

Photo 4. Chiseling almond shells to incorporate into the soil. 

Agro Gold WS was found  adulterated with glyphosate and diquat and CDFGA has issue a stop order for use on organic farms in the state of California. The press release is shown below:

CDFA ISSUES STOP USE NOTICE AND STATEWIDE QUARANTINE ON ORGANIC HERBICIDE AGRO GOLD WS

SACRAMENTO, December 4, 2020 – The California Department of Food and Agriculture (CDFA) today announced that a Stop Use notice and statewide quarantine have been issued for the organic fertilizer product AGRO GOLD WS to all organic operations registered in California. CDFA lab analysis of the product detected the presence of Diquat and Glyphosate, which are substances prohibited by the U.S. Department of Agriculture (USDA) National Organic Program for use in organic production. Continued use of this product in organic production may jeopardize an operation's organic status.
 
Pursuant to authority under the California Food and Agricultural Code (FAC), Division 17, Chapter 10, CDFA's State Organic Program (SOP) in coordination with the Fertilizer Materials Inspection Program (FMIP) issued a Stop Use notice today for AGRO GOLD WS to all organic operations in California registered with the SOP. CDFA's FMIP also announced today that all California operations registered as organic in possession of AGRO GOLD WS must hold the product and contact CDFA for quarantine instructions on how to handle it.
 
AGRO GOLD WS is manufactured by Agro Research International, LLC. It has been distributed in a co-packaged box that also contains the product WEED SLAYER. CDFA continues to provide follow up to this investigation and is working with state and federal agencies. CDFA received a complaint about the AGRO GOLD WS product and program staff collected product samples from various locations to conduct lab analysis in CDFA's Center for Analytical Chemistry. FMIP is an industry-funded program that ensures consumers receive fertilizing materials that meet the quality and quantity guaranteed on the product label. Investigators located throughout the state conduct routine sampling and inspections, respond to consumer complaints, and enforce the laws and regulations that govern the manufacturing and distribution of fertilizing materials in California. CDFA's State Organic Program protects the organic label through enforcement, education and outreach.
 
If you are in possession of AGRO GOLD WS and seek additional information, please contact the Fertilizing Materials Inspection Program at FMIP@cdfa.ca.gov. Any appeal of the determination that this product violates the Food and Agricultural Code must be filed with the Fertilizing Materials Inspection Program no later than 15 days from receipt of the Stop Use notice and statewide quarantine. See Food and Agricultural Code section 14659.