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Open Access Publications from the University of California


California Agriculture is a quarterly peer-reviewed journal reporting research, reviews and news on California’s agricultural, natural and human resources.

Volume 75, Issue 2, 2021

Issue cover
Cover Caption: A robotic harvester manufactured by gently picks a strawberry from a plant on a ranch in Oxnard. Photo credit: Visionary Photography c/o
UC Agriculture and Natural Resources (UCANR), as an organization that has connected the people of California to UC research for over a century, is very well positioned to take advantage of emerging trends in scientist-community collaborations.

Research and Review Articles

Robotic strawberry harvest is promising but will need improved technology and higher wages to be economically viable

While the prospect of robotic harvest in strawberry production has received much attention within the strawberry industry and the popular press, there is little available information on the economic feasibility of this technology. It is not clear how close the industry is to being able to profitably adopt robotic harvest systems; also unclear is the relative importance of wage rates, robotic harvest efficiencies and machinery field speeds on the adoption threshold. This study aims to clarify these issues by estimating the net income to strawberry production under robotic harvest scenarios, and comparing the values to standard enterprise budgets for strawberry production in California under different wage rates for harvest labor. Results confirm that robotic harvest remains economically unviable under current wage rates and the field speeds and harvest efficiencies achieved by leading robotic harvest development teams. However, results indicate that with expected increases in wage rates in the coming years, and with modest improvements in the technical parameters, use of robotic systems will likely become profitable in some form.

The potential threat of branched broomrape for California processing tomato: A review

Branched broomrape (Phelipanche ramosa), a parasitic weed that was the focus of a $1.5 million eradication effort four decades ago in California, has recently re-emerged in tomato fields in several Central Valley counties. Processing tomatoes are important to the California agricultural economy; the state produced over 90% of the 12 million tons of tomatoes grown in the United States in 2018. Branched broomrape is listed as an “A” noxious weed by the California Department of Food and Agriculture (CDFA); discovery of broomrape in California tomato fields leads to quarantine and crop destruction without harvest, resulting in significant economic loss to growers. In countries where broomrape is common, yield reductions caused by this parasitic weed can range from moderate to 80%, depending upon the infestation level, host and environmental conditions. Developing a detailed understanding of the biology of this weed under local conditions is an important step towards developing effective management plans for California. In this review, we discuss branched broomrape in the context of California production systems, particularly of tomato. We also discuss the potential management practices that could help to prevent or reduce the impacts of branched broomrape in tomatoes and other host crops.

Point- and reach-scale measurements are important for determining accurate seepage rates in controlled flow channels

A critical component of water-resources management in the irrigated agriculture landscape, particularly those landscapes dependent on groundwater availability, is determining groundwater recharge rates from streams and other channels. In California, flows in many such channels are “controlled” by upstream reservoir releases to meet downstream urban, irrigation and environmental water requirements. Seepage volumes from these channels and how they might vary during controlled release periods is a key component of meeting downstream riparian and groundwater-pumping needs. Understanding annual seepage from streamflow channels is also important in developing water budgets as part of the management of groundwater resources under the Sustainable Groundwater Management Act (SGMA) in California. However, direct measurements of channel seepage rates are infrequent or unavailable, and these rates, or associated volumes, are most often only estimated. Here we describe direct point- and reach-scale field measurements of channel seepage rates in Lower Putah Creek (Solano County) and in distribution lateral channels of the Oakdale Irrigation District on the east side of the San Joaquin Valley (San Joaquin and Stanislaus counties). We measured overall average seepage rates of about 2 feet (610 mm) per day at both locations and determined how these rates varied spatially and temporally during the summer when channel flows are controlled for downstream requirements.

Integration of grazing and herbicide application improves management of barb goatgrass and medusahead in pasture and rangelands

The invasive annual grasses barb goatgrass (Aegilops triuncialis L.) and medusahead (Elymus caput-medusae L.) are widespread in western states and present management challenges on grasslands. To develop an integrated management strategy for these species, we treated sites in five pastures in Mendocino County, comparing combinations of intensive sheep grazing, glyphosate herbicide (low and high), and application timings (tillering, boot and heading stage). We found that grazing alone reduced barb goatgrass spikelet densities by 68% and the number of seeds per spikelet by 35%. Both rates of glyphosate application without grazing had similar effects on seed production. High and low glyphosate application at tillering resulted in almost complete control of both target species. Boot- and heading-stage applications reduced barb goatgrass density by 39% and 32%, respectively. Application at the boot stage also resulted in an 82% reduction in number of seeds per barb goatgrass spikelet. Our results suggest that intensive grazing may be a useful management strategy to reduce barb goatgrass and medusahead spikelet densities and barb goatgrass seed numbers, especially when integrated with a boot- or heading-stage glyphosate application.

Growers follow the label: An analysis of bee-toxic pesticide use in almond orchards during bloom

California almond orchards are most U.S. beekeepers' first stop on their pollination and honey production circuit, so the agrochemicals bees are exposed to in almonds can shape the vitality of their colony for the rest of the year. We explored the potential for honey bee exposure to bee-toxic agrochemicals during almond bloom by utilizing the California Department of Pesticide Regulations' Pesticide Use Report database from 1990 to 2016. We found that overall, growers are observing the pesticide labels and reducing their use of labeled bee-toxic pesticides during almond bloom. However, we also found that insect growth regulators, fungicides and organosilicone surfactants — agrochemicals often not labeled as toxic to bees — are commonly applied during almond bloom. These agrochemicals can be sublethally or synergistically toxic to adult honey bees and bee larvae, presenting potential harm to colonies during almond pollination. Our findings demonstrate the need for a shift in the U.S. Environmental Protection Agency's labeling requirements, as well as continued communication between almond growers, pesticide applicators and beekeepers to keep colonies at a low risk of bee-toxic agrochemical exposure.