The California Chapter, American Society of Agronomy and Cooperative Extension, UC Agriculture and Natural Resources are hosting the annual Plant and Soil Conference on Tuesday, February 1 - Thursday, February 3.  This will be a virtual event using zoom. The Plant and Soil Conference is a great opportunity to increase your knowledge of crop production, soil and water management, and pest management as well as receive CCA and DPR educational hours.  The full agenda can be found here.   Registration is currently $90, but will increase to $115 on January 25th.  You can register for the conference at https://calasa.ucdavis.edu/Conference_Registration/.

Summary:Automated weeders are capable of selectively removing weeds from the uncultivated band (3-5 inches wide) left around the seedline by standard cultivation. A camera detects the image, a computer processes the image and then activates a kill mechanism (e.g. a split blade). Twelve evaluations of autoweeders were conducted in 2020-21. Autoweeders removed from 31.7 to 98.7% of the weeds (average = 69.5%). The percent of weeds removed by the autoweeders was affected by the width of the uncultivated band and by the use of transplants; autoweeders removed fewer weeds if the uncultivated band was narrower (e.g. 3 inches vs 4 inches); in addition, automated weeders worked more efficiently with transplanted lettuce because the crop and weeds were smaller when weeded. Autoweeders did not reduce the lettuce stand or mean head weight in any of these evaluations. Autoweeded fields generally require follow up hand weeding to remove double lettuce plants (in direct seeded lettuce) and any remaining weeds. The question that comes up for growers is, does the reduction in follow-up hand weeding pay for the cost of running the autoweeder the field? We were not able to provide a simple answer to this question, however, we did see greater reduction in the time to subsequently hand weed fields where an autoweeder was used when the initial weed population was higher. However, given the labor situation, autoweeders may be justified regardless, if hand labor is simply not available to weed fields.

Introduction: 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 Stout) or a spinning blade that avoids the keeper plants by placing them in a notch in the blade (e.g. Garford Robocrop).  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. In addition, automated weeders are currently not capable of removing double lettuce plants 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 to remove to remove double lettuce plants and any missed weeds. Therefore, there are two costs when weeding with an autoweeder: the cost of the automated weeder and subsequent hand weeding/double lettuce plant removal. If the auto weeders can reduce the amount of subsequent hand weeding to cover the cost of running the autoweeder, then it is a net gain for a grower. In these evaluations we examined the initial weed population and the reduction in subsequent hand weeding following the use of an autoweeder in order to better understand the impact of the use of autoweeders on weed control in lettuce production.

Methods: Twelve trials were conducted in 2020 and 2021. Table 1 shows the details about each trial: machine used, planting configuration, lettuce types and dates of operations. The FarmWise and Stout autoweeders used a split-knife kill mechanism and the Naio machine used finger weeders. Autoweeding was carried out after 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. In 2020, 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, but in 2021, pre and post cultivation weed counts were made following the passing of the standard cultivation. Weeding time measurements were made of the time it took a commercial hand weeding crew to pass through the treatment rows which 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.

Results: The number of weeds in the seedlines varied from as low a 1.6 to as high 41.5 plants/m² (Table 2).  Autoweeders removed from 31.7 to 98.7% and averaged 69.5%. The percent of weeds removed by the autoweeders was affected by the width of the uncultivated band. For instance, in first 2021 FarmWise evaluation 69.0% of the weeds were removed; however, in the second FarmWise evaluation only 31.7% of the weeds were removed. We think the difference between these two trials is that the first evaluation had a 5-inch wide uncultivated band which include many weeds that were easier for the autoweeder to remove a large portion. However, in the second FarmWise evaluation the uncultivated band was 3-inches wide and many of the easy to remove weeds were already gone, thereby reducing the number of weeds available for the autoweeder to remove. In the first 2021 Stout evaluation 97.7% of the weeds were removed. This field was a transplanted field established with PlantTape. The lettuce plants and weeds were smaller at the time of weeding and the machine was able to operate extremely efficiently. Autoweeders were did not damage the lettuce stand or mean head weight in any of these evaluations (Table 3). The percent reduction in the amount of time to weed the field varied from 2.5 to 60.0% with the use of the autoweeders in comparison with standard cultivation. Autoweeders significantly reduced subsequent hand weeding in 2021 (Table 3). Figure 1 correlates initial weed populations with the amount of reduction in subsequent hand weeding. The correlation indicates that there is a greater impact on subsequent hand weeding with greater weed populations. As mentioned above, this correlation may be impacted by the width of the uncultivated band and by the use of transplants. In spite of these influences, it appears that automated weeders may have a greater economic impact on total weeding costs when there are higher weed populations.

FarmWise Titan

Stout Smart Cultivator

Naio Dino Weeding Platform

We published a blog post last year stressing the need to control winter weeds and felt it necessary to issue this information again this winter as we cannot let down our guard regarding reducing host plants for impatiens necrotic spot virus (INSV). In 2021, INSV continued to be a significant production issue for lettuce in the Salinas Valley. Although it appears that infestations were not as severe as 2020, the disease continues to be an ever-present threat to the lettuce industry and there are no guarantees that 2022 will be a light infection year. Controlling winter weed hosts is a first step in reducing levels of infection during the production season. In 2020 INSV was most significant north of Gonzales. However, due to the diligent work of the Grower Shipper INSV task force which kept track of where infections were occurring, it appears that during the 2021 season, there was more infection further south, in the Soledad area, than we observed in 2020.

INSV is a tospovirus that is spread by western flower thrips (Frankliniella occidentalis). Lettuce fields are infected by INSV by thrips migrating in from infected host plants in the early spring. During the production season, infected lettuce fields can be the principal source of INSV. However, starting in November, at the end of the lettuce production season, INSV in weeds and other host plants (e.g. ornaments) becomes the principle over-wintering habitat. These plants serve as the bridge for the virus to survive during the winter fallow months and then as the source of the virus for the coming lettuce production season (Figure 1). Therefore, it is important to direct efforts towards weed management during the winter months to reduce the overall levels of INSV inoculum that is available for thrips to acquire and transmit to lettuce crops. In 2021, thrips populations were also generally lower, which may have contributed to lower incidence of INSV in the Salinas Valley (Figure 2).

A list of key winter weed INSV hosts is shown in Table 1. These species can have a high level of infection and need to be controlled in critical areas such as cropped and non-crop areas, fallow fields, roadsides, waste areas, banks, equipment yards and ditches. The first storms have occurred early in the Salinas Valley, and thus, winter weeds have germinated and are developing quickly. Specific attention needs to be paid to controlling them until March to reduce the number of potential virus reservoirs.

The Agricultural Commissioner is notifying growers of their responsibilities to control weeds and has authority to enforce removal of weeds deemed a nuisance. Given the continued risk posed by INSV to the lettuce industry it is critical that growers make a special effort to reduce weed populations in all the usual areas as well as areas that may not have received as much attention in the past. It is hoped that these efforts may prove successful in reducing the source of INSV, similar to the successful efforts of the lettuce free period to reduce the incidence of Lettuce Mosaic Virus (LMV) in years past.

Figure 1. Yearly cycle of INSV in the Salinas Valley

Figure 2. Thrip populations over the 2021 cropping season

The Zoom link will be the same each week. No preregistration required.

Meeting link: https://ucanr.zoom.us/j/97511217168?pwd=ZWVhVXorcFEzRHUwQ0Q0L1lqaHNWdz09
*The presentation (not the Q and A discussion) will be recorded and made available.