Weed Technology 2014 28:000–000

Evaluation and Economics of a Rotating Cultivator in Bok Choy, Celery,Lettuce, and Radicchio

Steven A. Fennimore, Richard F. Smith, Laura Tourte, Michelle LeStrange, and John S. Rachuy*

A commercial intrarow rotating cultivator was tested for weed removal and impact on hand-weeding times in bok choy,celery, lettuce, and radicchio. The rotating cultivator was tested as an automated crop thinner and weeder in direct-seededbok choy and lettuce as an alternative to hand-thinning and -weeding. The rotating cultivator utilized machine-visionguidance to align a rotating disk with the crop plant to be saved and to remove weeds and undesired crop plants. Therotating cultivator was compared to a standard interrow cultivator, which could not remove weeds from the plant line.Main plots were cultivator type, rotating, or standard, and subplots were herbicides: pronamide for lettuce or prometrynfor celery. Weed densities, hand-weeding times, crop stand, and yields were monitored. Economic analysis was performedon a subset of the data. The intrarow rotating cultivator was generally more effective than the standard interrow cultivatorfor reducing weed densities and hand-weeding times. However, the rotating cultivator reduced seeded lettuce stands by 22to 28% when compared to hand-thinning and standard cultivation, resulting in lower yields and net returns. Intransplanted celery, lettuce, and radicchio, the rotating cultivator removed more weeds than the standard cultivator, andreduced stands by just 6 to 9% when compared to the standard cultivator. In transplanted lettuce, the rotating cultivatorwas more precise and did less damage to the crop. Because transplanted crops were larger than the weeds, they were moreeasily differentiated using this technology. Net returns were therefore similar between the two cultivators. What is neededfor celery and leafy vegetables is an effective intrarow weed removal system that reduces or eliminates the need for hand-weeding yet does not reduce yields. The rotating cultivator was developed for transplanted crops, where it performsadequately, but it cannot be recommended in the seeded crops evaluated.Nomenclature: Prometryn; pronamide; bok choy, Brassica rapa L. subsp. chinensis; celery, Apium graveolens L. var. dulce(Mill.) Pers.; lettuce, Latuca sativa L.; radicchio, Cichorium intybus L.Key words: Hand-weeding, interrow cultivation, intrarow cultivation, lettuce thinning, net returns, production costs.

Se evaluo el uso de un cultivador comercial rotativo intra-lınea para la eliminacion de malezas y su impacto en la deshierbamanual en bok choy, cilantro, lechuga y radicchio. El cultivador rotativo fue evaluado para ralear el cultivo y paradeshierbar automaticamente en bok choy y lechuga de siembra directa como alternativa al raleo y a la deshierba manual. Elcultivador rotativo fue guiado con una maquina de vision para alinear un disco rotativo con la planta del cultivo que debıaser salvada y para remover las malezas y las plantas del cultivo no deseadas. El cultivador rotativo fue comparado con uncultivador inter-lınea estandar, el cual no podıa remover malezas de la lınea de siembra. Las parcelas principales fueron eltipo de cultivador, rotativo o estandar, y las sub-parcelas fueron herbicidas: pronamide para la lechuga o prometryn para elcilantro. Se determino las densidades de malezas, los tiempos de deshierba manual, y los rendimientos. Se realizo unanalisis economico en un subgrupo de los datos. El cultivador rotativo intra-lınea fue generalmente mas efectivo que elcultivador inter-lınea estandar para reducir las densidades de malezas y los tiempos de deshierba manual. Sin embargo, elcultivador rotativo redujo el numero de plantas de lechuga establecidas en siembra directa en 22 a 28% en comparacioncon el raleo manual y el cultivador estandar, lo que resulto en menores rendimientos e ingresos netos. En cilantro, lechugay radicchio de trasplante, el cultivador rotativo removio mas malezas que el cultivador estandar, y redujo el numero deplantas establecidas en solamente 6 a 9% cuando se comparo con el cultivador estandar. En la lechuga de trasplante elcultivador rotativo fue mas preciso y causo menos dano al cultivo. Los cultivos trasplantados fueron mas facilmentediferenciados usando esta tecnologıa porque fueron mas grandes que las malezas. Los ingresos netos fueron de esta forma,similares entre los cultivadores. Lo que se necesita para el cilantro y los vegetales de hoja es un sistema efectivo para laremocion de malezas intra-lınea que reduzca o elimine la necesidad de deshierba manual sin reducir los rendimientos. Elcultivador rotativo fue desarrollado para cultivos de trasplante, en los cuales se desempena adecuadamente, pero este nopuede ser recomendado para cultivos de siembra directa.

California vegetables are valuable commodities, but thesecrops are susceptible to weed competition and diseases hostedby weeds, weed interference with hand-harvesting, and weedcontamination in the harvested produce (Bell 1995; Haar andFennimore 2003). Weeds in celery and leafy vegetables aremanaged through integrated programs that include the use ofcultural, chemical, and physical methods such as croprotation, stale seedbeds, herbicides, cultivation, and hand-weeding (Haar and Fennimore 2003; Daugovish et al. 2007;Shem-Tov et al. 2006). Because of the high cosmetic

DOI: 10.1614/WT-D-13-00051.1* First and fifth authors: Extension Specialist and Staff Research Associate,

University of California, Davis, Department of Plant Sciences, 1636 EastAlisal, Salinas, CA 93905; second author: Farm Advisor, University ofCalifornia Cooperative Extension, Monterey County, 1432 Abbott St.,Salinas, CA 93901; third author: Farm Advisor, University of CaliforniaCooperative Extension, Santa Cruz County, 1432 Freedom Blvd.,Watsonville, CA 95076; fourth author: Farm Advisor, University ofCalifornia Cooperative Extension, Tulare County, 4437-B S. LaspinaSt.Tulare, CA 93274. Corresponding author’s E-mail: safennimore@ucdavis.edu

Fennimore et al.: Intrarow cultivation in vegetables � 0

standards of fresh market vegetable crops, there is no tolerancefor poor weed control that contaminates or in any waycompromises produce quality. The vegetable grower eithermanages the weeds with an integrated program, or in the caseof failed weed management, the crop is abandoned anddestroyed before harvest. Herbicides available to vegetablegrowers are few in number and those few do not control allweeds. When a herbicide registration is lost for a particularvegetable crop, as is the case for the recent cancellation ofpronamide on leaf lettuce (Mou 2011), the default replace-ment for the herbicide is more hand-weeding—the mostexpensive weed control method. Labor shortages have beenreported and labor costs are increasing (Martin 2007; Tayloret al. 2012); this in turn is increasing hand-weeding costs.One possible means of reducing dependency on hand-weeding and herbicides like pronamide is to use a morerobust cultivation system that can remove intrarow weeds.

Weeds are among the most common pests of vegetablecrops, and present a constant obstacle to profitable vegetableproduction. Celery and lettuce are very susceptible to weedcompetition. Weed cover of 25% resulted in 20 to 40% yieldloss in California lettuce, and greater than 25% weed coverresulted in complete yield loss (Lanini and LeStrange 1991).Celery transplants grow slowly and are very susceptible todamage from weed competition in the first weeks aftertransplanting (Ogbuchiekwe and McGiffen 2001; PMSPcelery 2004). Effective weed control at early crop growthstages allows celery to establish a dense root system and leafcanopy that shades weeds (Ogbuchiekwe and McGiffen2001). Stakeholders have stated that the development of‘‘effective economical management techniques for weeds’’ is a‘‘high’’ research priority for celery and lettuce (PMSP–celery2004; PMSP-lettuce 2003).

California growers produce lettuce on 117,400 ha, ofwhich about 90% is direct-seeded and 10% is transplanted(NASS 2012; Turini et al. 2011). Seeds are planted 4 to 7.5cm apart in rows on 1- or 2-m-wide beds. Lettuce is thinnedto 22- to 30-cm spacings within the rows by hand crews withhoes approximately 30 d following planting; weeds that haveemerged are also removed at the same time. The speed oflettuce thinning is influenced by the densities of weeds present(Haar and Fennimore 2003). In previous work the differencesin hand-weeding and thinning times in lettuce plots cultivatedwithin 10 cm from the plant line was 13% greater thanweeding and thinning times in plots cultivated within 2.5 cmof the plant line (Fennimore et al. 2010). After lettucethinning, an additional hand-weeding is carried out 2 to 3 wklater to remove remaining weeds (Turini et al. 2011).Thinning and hand-weeding costs for lettuce are approxi-mately $444 ha�1 (Tourte and Smith 2010). Prior to 2009,the majority of California lettuce was treated with pronamide(CADPR 2008), but from 2009 onward, pronamide can onlybe used on head lettuce due to the loss of the tolerance in leaflettuce (Agrian 2012; Mou 2011).

Approximately 11,400 ha of celery were grown in the UnitedStates in 2011, with most of the production in California(10,700 ha) and the remainder in Michigan (730 ha) (NASS2012). Celery is grown year-round in the coastal districts ofCalifornia with exception of celery-free periods, established to

break vector cycles of the western celery mosaic virus. Celery inCalifornia is exclusively established in the field from transplants(Daugovish et al. 2008). Linuron and prometryn are theprincipal celery herbicides and are applied posttransplant (Smith2009). Physical weed control tools used in celery includecultivation and hand-weeding. Up-to-date herbicide andweeding costs for California grown celery do not currentlyexist. Bok choy and radicchio are niche crops with annualplantings in California of generally , 1,000 ha (Anonymous2013). Herbicides registered for use in bok choy in Californiaare bensulide and DCPA, and in radicchio are bensulide,clethodim, pronamide, and trifluralin (CADPR 2012).

Vegetable growers and researchers have been searching fortechnology to assist in reducing weed control costs.Commercially available machine-vision–guided cultivatorssuch as the Eco-Dan use cameras and electronics to establisha centerline on the plant row, which in turn controls ahydraulic cylinder that shifts the cultivator to the right or leftto keep the cultivator properly lined up as it moves throughthe field. However, Eco-Dan can only be used for interrowcultivation—cultivation between the plant rows—and isprimarily used to allow faster and closer cultivation(Fennimore et al. 2010; Slaughter et al. 1999). Control ofintrarow weeds, i.e., weeds in the plant row, is currentlyaccomplished only with herbicides and hand-weeding (Haarand Fennimore 2003) and intrarow weeders such as fingerweeders and torsion weeders (Cloutier et al. 2007; van derSchans et al. 2006). These intrarow weeders can remove smallweeds from the plant line providing there is a size differentialbetween the crop and weeds. Torsion and finger weeders arelimited to removal of small weeds and can damage the crop.Recently a precision-guided cultivator capable of intrarowcultivation has become commercially available. An intrarowcultivator based on a rotating disc design was developed inEngland by Tillett and Hague Technology Ltd. and is beingmarketed as the Robocrop In Row cultivator (O’Dogherty etal. 2007; Tillett et al. 2008). The Robocrop In Row cultivatorhas a hydraulically driven disc module for each crop row. Asthe cultivator advances down the row, a rotating disc that iscontrolled by a vision system detects the crop plant locationand rotates to properly align the disc’s cut-away section withthe saved crop plant (Figure 1). Due to variability in cropplant spacing, the disc rotational phase is altered as needed bychanging the speed of the hydraulic drive to align the cut-away section with the crop plant (Tillett et al. 2008). TheRobocrop In Row cultivator will hereafter be referred to as the‘‘rotating cultivator.’’

The objectives of this work were to determine if therotating cultivator improves weed control and labor useefficiency in bok choy, celery, lettuce, and radicchio;determine if the rotating cultivator can be used to thindirect-seeded lettuce and bok choy; and measure the cost-effectiveness of the rotating cultivator for lettuce thinning andintrarow weed removal in celery and lettuce.

Materials and Methods

Field experiments were conducted during 2009 and 2010(trials 1 to 7), on the Spence U.S. Department of Agriculture

0 � Weed Technology 28, January–March 2014

(USDA) and Hartnell field stations near Salinas, CA, and oncommercial fields (trials 8 to 11; Table 1). Commercial trialswere conducted in vegetable fields in the Salinas, Santa Maria,and San Juan valleys on the central coast of California.

Field Station Evaluations. Lettuce experiments were con-ducted at the Spence USDA field station. The soil is classifiedas a Chualar loam (fine-loamy, mixed, thermic TypicArgixeroll) 79% sand, 14% silt, and 7% clay with a pH of7.2 and organic matter content of 1%. Celery experimentswere conducted at the Hartnell field station where the soil isan Antioch sandy loam, fine, smectitic, thermic TypicNatrixeralf (53% sand, 32% silt, and 15% clay) with a pHof 7.0 and organic matter content of 2.1%. The experimentswere established using a split-plot design arranged in arandomized complete block with four replicates. Main plotswere cultivator type and subplots were herbicide treatments.Plots consisted of two 1-m raised beds that were 15.2 m and13.7 m long for lettuce and celery, respectively. Plant lines,two per bed, were 30 cm apart. Direct-seeded lettuce wasplanted using a tractor-mounted Stanhay planter (StanhayWebb Ltd. Grantham, U.K.). Head lettuce varieties wereselected for most of the lettuce trials as they are commonlyplanted in two plant lines per 1-m-wide bed and werecompatible with the four-unit rotating cultivator that wasavailable for field-scale testing (Figure 2). Approximately 2-mo-old celery plants and 1-mo-old lettuce plants weretransplanted with a tractor-mounted transplanter (MechanicalTransplanter Co., Holland, MI 49423). Critical dates, such asplanting, cultivation, thinning, weeding, and harvest for theexperiments are listed in Table 1. The herbicide used in celerywas prometryn (Syngenta Crop Protection, Greensboro, NC27419) applied 2 wk after transplanting at 1.7 kg ha�1.Pronamide (Dow Agro Sciences, Indianapolis, IN 46268) wasapplied at 1.3 kg ha�1 POST-plant PRE in seeded lettuce orpretransplant in transplanted lettuce. Both herbicides were

Figure 1. A close-up view of the rotating cultivator disc with the cut-awaysection that aligns with the saved plant. The bolt shown in the cut-away section isused during the setup process to calibrate the cultivator and is removed prior tooperation.

Table 1. Crop, cultivar, planting method, planting dates, cultivation dates, crop thinning/weeding, and harvest dates for bok choy, celery, lettuce, and radicchiocultivator evaluations at Salinas, Gonzalez, Gilroy, Castroville, and Santa Maria, CA.

Trial no. Crop CultivarPlant

method Planting CultivationThinning/weeding Harvest Location

Field station evaluations:1 Head lettuce Corona Seeded May 20, 2009 June 18, 2009 July 6, 2009 July 29, 2009 Salinas, Spence2 Head lettuce

(transplant)Corona Transplant July 23, 2009 August 6, 2009 August 6, 2009;

August 20,2009

September 22,2009

Salinas, Spence

3 Celery (transplant) Sonora Transplant July 21, 2009 August 5, 2009 August 6, 2009 October 27, 2009 Salinas, Hartnell4 Head lettuce Hallmark W Seeded June 23, 2010 July 21, 2010 August 6, 2010 August 31, 2010 Salinas, Spence5 Head lettuce Hallmark W Seeded August 24,

2010September 16,

2010September 20,

2010November 10,

2010Salinas, Spence

6 Head lettuce(transplant)

Hallmark W Transplant September 3,2010

September 21,2010

September 27,2010

October 28, 2010 Salinas, Spence

7 Celery (transplant) Sonora Transplant July 15, 2010 August 5, 2010 August 11, 2010 November 2,2010

Salinas, Hartnell

Commercial evaluations:8 Bok choy (transplant) Joy Choy Transplant September 10,

2009September 24,

2009October 7, 2009 November 17,

2009Santa Maria

9 Romaine lettuce(seeded)

Sunbelt Seeded May 20, 2009 June 17, 2009 June 25, 2009 July 22, 2009 Gonzalez, R. Smith

10 Radicchio (transplant) Leonardo Transplant July 17, 2009 August 5, 2009 August 7, 2009 October 7, 2009 Gilroy, R.Smith11 Head lettuce (seeded) Steamboat Seeded May 20, 2010 June 16, 2010 June 25, 2010 July 22, 2010 Castroville, R. Smith

Fennimore et al.: Intrarow cultivation in vegetables � 0

applied to the bed top in 51-cm-wide bands in 374 L ha�1 at apressure of 207 kPa using a tractor-mounted CO2 pressurizedsprayer with two 8002E nozzles (Spraying Systems Co.,Wheaton, IL 60187)

mounted on a boom. Even nozzles were used as there wasno overlap of the nozzles and the row middles were notsprayed. Nontreated controls were included in all trials.Sprinkler irrigation, two to three times per week, was used inlettuce. Celery transplants were established using sprinklerirrigation; after 2 wk they were switched to drip irrigation forthe remainder of the season.

About 3 to 4 wk after seeding lettuce, cultivation wasperformed using the rotating cultivator (Garford FarmMachinery Ltd, Peterborough, U.K.) or a standard cultivatortool bar equipped with cultivator knives similar to thatdescribed in Fennimore et al. (2010). The standard cultivatorshanks on the tool bar were adjusted to leave a 10-cm-widenoncultivated band centered over the plant line as is typical inlettuce production. The standard interrow cultivator was notcapable of intrarow cultivation or lettuce thinning. Therotating cultivator was evaluated for intrarow cultivation andlettuce thinning.

Weed densities were measured on the bed top for the fulllength of the plot, 9.3 m2 for lettuce and 8.4 m2 for celery,before and after cultivation. After cultivation, the time for alaborer with a hoe to thin the lettuce to desired stand andremove weeds was measured with a stopwatch. About 2 to 3wk after transplanting celery or lettuce, the plots werecultivated with either the rotating or standard cultivator,and then a laborer was timed to hoe weeds in the plots. Cropstand counts were determined in a single plant line 15.2 mlong for lettuce and 13.7 m long for celery. Celery and lettucewere harvested at maturity (Table 1). Mature lettuce heads orcelery stalks were harvested, counted, and weighed. Yields arepresented in cartons ha�1, (19.1 kg for lettuce and 27.2 kg forcelery) as is typical for fresh vegetable crops in California(Lauritzen 2009, 2010).

Commercial Evaluations. On-farm trials were conducted inbok choy, lettuce, and radicchio with cooperating growers in2009 and 2010 (Table 1). The grower’s cultivator wasincluded as the standard cultivator and the rotating cultivatorwas operated in the same way as for the field station studiesdescribed above. The grower’s standard cultivator used knivesclose to the plant lines and cultivator sweeps between theplant lines. Each plot consisted of four 1-m-wide beds by the300- to 400-m length of the field, each treatment wasreplicated three times, and a randomized complete blockdesign was used. Weed densities were measured in a 15.2- by2.0-m section of each plot before and after cultivation; hand-weeding times were measured in the same sample area. Aftercultivation, crop stand counts were measured in 30.5-m-longsample areas. Commercial crews harvested the trials. Herbi-cide was not a factor in the commercial trials as the herbicideapplication (in conventional fields), or nonherbicide use (inorganic fields), was uniform across the trial. For lettuce,pronamide was applied at 1.3 kg ha�1 in a 15.2-cm-wide bandcentered over each plant line at planting. The bok choy andradicchio trials were conducted in organic fields and noherbicides were used.

Soils at the four commercial trials were as follows: trial 8 atSanta Maria was a Mocho silty clay loam fine-loamy, mixed,super active, thermic Typic Calciaquolls; trial 9 at Gonzalezwas a Cropley silty clay; trial 10 at Gilroy was a Pacheco siltloam; and trial 11 at Castroville was a Clear Lake clay (Table1).

Data Analysis. All data except weed densities were subjectedto factorial ANOVA, and mean separation was performedusing an LSD test (P ¼ 0.05). Two-way ANOVA wasconducted to determine the main effects of cultivator type(rotating, standard) and herbicide (herbicide, no herbicide) onweed densities, weeding labor time, crop stand, and yieldusing PROC GLM in SAS (version 9.3, SAS Institute Inc.,Cary, NC 27513). Analysis of covariance was performed usingPROC GLM to test the effects of cultivator type andherbicide and the interaction on postcultivation weeddensities. The weed densities prior to cultivation were usedas a covariate to analyze the postcultivation weed densities.Economic analyses were performed for the trials using BudgetPlanner Software version 2.0 (Karen Klonsky, Department ofAgricultural and Resource Economics, University of Califor-nia, Davis, Davis CA 95616). All data were collected andreported on a per-hectare basis, and the cost of equipment use(machine labor, fuel, lubrication, repairs), herbicides, hand-thinning and -weeding labor, and interest on operating capitalwere entered into Budget Planner for analysis. Budget Plannergenerated production cost (partial budget) tables for eachtreatment combination. Partial budgets were calculated usingthe following in both 2009 and 2010: lubrication and repairsby implement/machine and based on formulas from theAmerican Society of Agricultural Engineers (ASABE 2011);labor rates including a 34% benefits package($13.40 h�1

machine and $10.72 h�1 field); material costs in 2009 and2010 ($131.05 kg�1 of formulated pronamide for a total of$175.61 ha�1 and $9.26 L�1 of formulated prometryn for atotal cost of $16.20 ha�1); and fuel costs (diesel $0.54 L�1).Gross returns were calculated by multiplying the experimental

Figure 2. Thinning lettuce with the rotating cultivator on two commerciallettuce beds, each with two plant lines per bed.

0 � Weed Technology 28, January–March 2014

yield in cartons per hectare by the price per carton (Lauritzen2009, 2010). Net returns were calculated by subtractingproduction costs from gross returns.

Results and Discussion

Intrarow Weeding and Lettuce Thinning—Field StationTrials. The rotating cultivator removed more weeds than thestandard cultivator and hand-thinning and -weeding timeswere less using the rotating cultivator compared to thestandard cultivator (Table 2). The cultivator by herbicideinteraction term was significant for weed densities in trial 5and for thinning and weeding times in trial 4. In both trialsthe simple effects show that the standard cultivator withoutpronamide had higher weed densities (trial 5) or weedingtimes (trial 4) than standard cultivator with pronamide orrotating cultivator with or without pronamide. This suggeststhat intrarow weeding with the rotating cultivator withoutpronamide could achieve the same level of weed control as thestandard cultivator plus pronamide. The effect of herbicide onweed density was significant only in trial 5, but pronamidereduced lettuce thinning and weeding times in all three trials(Table 2).

The rotating cultivator reduced weed densities in trans-planted lettuce more than the standard cultivator in trials 2and 6 (Table 3). The rotating cultivator reduced hand-weeding times more than the standard cultivator in trial 2,although the hand-weeding time reduction was small inmagnitude. The main effect of pronamide on weed densities

and weeding time was not significant in these trials. Thecultivator by herbicide interaction was not significant for weeddensities in transplanted lettuce, but was significant for hand-weeding time in trial 6 (Table 3). The simple effects oftreatment on weeding time indicated that weeding time waslower in the rotating cultivator–no pronamide treatmentcompared to the rotating cultivator plus pronamide. Thereason for differences in weeding time between the tworotating cultivator treatments in trial 6 is not obvious. It ispossible that pronamide changed the weed spectrum byfavoring species such as common purslane (Portulaca oleraceaL.) that were harder for the rotating cultivator to control byuprooting (Table 3). In the no-pronamide treatmentscommon purslane would have been competing for space withburning nettle (Urtica urens L.) a weed that is easier thancommon purslane to kill by uprooting with the rotatingcultivator. Previous work has shown that common purslane istolerant of uprooting (Haar and Fennimore 2003).

The rotating cultivator reduced seeded lettuce stands by 22to 28% when compared to the standard hand-thinning andweeding operation (Table 4). Transplanted lettuce stand intrial 2 was 9% lower following cultivation with the rotatingcultivator. In trial 6 the 6% difference in stands between thetwo cultivators was not significant. Loss of stand intransplanted lettuce was much less severe than in seededlettuce, presumably due to the 22-cm spacings betweentransplanted lettuce plants in the row, which allowed betterplant detection for the guidance system than the 4-cmspacings among seeded lettuce plants. Also, the sizedifferential between the emerging weeds and transplanted

Table 2. Effects of cultivator type, rotating or standard, with or without herbicide on final weed densities prior to lettuce thinning. Time for a laborer to thin and weedseeded lettuce 100 m�1 of row.

Weed densitiesa,b Weeding and thinning timea

Pronamide

Seeded lettuce

Trial 1 Trial 4 Trial 5 Trial 1 Trial 4 Trial 5

kg ha�1 no. 100 m�2 s 100 m�1a

CultivatorRotating 1.3 — — 972 b — 474 b —Rotating 0 — — 1314 b — 476 b —Standard 1.3 — — 928 b — 500 b —Standard 0 — — 2223 a — 627 a —

Main effect of cultivatorRotating 1,629 b 469 b 1118 b 649 b 475 b 411 bStandard 3,467 a 912 a 1,599 a 936 a 563 a 639 a

Main effect of herbicidePronamide 1.3 2,516 a 725 a 1143 b 672 b 487 b 448 bNone 0.0 2,579 a 657 a 1575 a 914 a 551 a 607 a

Analysis of covariance P valuesCultivator type , 0.0001 0.0002 0.0013Herbicide 0.8827 0.5033 0.0223Cultivator 3 herbicide 0.90 0.6738 0.0021

ANOVA P valuesCultivator type 0.0002 0.0058 , 0.0001Herbicide 0.0013 0.0429 0.0005Cultivator 3 herbicide 0.2533 0.0483 0.16

a Means with the same letter within columns for a cultivator or herbicide are not significantly different according to Fisher’s Protected LSD at P � 0.05.b Weeds present in trials 1 and 4 were shepherd’s-purse, 42%; common purslane, 40%, miscellaneous species consisting of common groundsel (Senecio vulgaris L.),

annual bluegrass (Poa annua L.), hairy nightshade (Solanum physalifolium Rusby), and nettleleaf goosefoot (Chenopodium murale L.), 18%. The species composition oftrial 5 was common purslane, 60%; burning nettle (Urtica urens L.), 25%; and miscellaneous species consisting of shepherd’s-purse, hairy nightshade, commongroundsel, and nettleleaf goosefoot, 15%.

Fennimore et al.: Intrarow cultivation in vegetables � 0

lettuce was much greater than the size differential between

emerging weeds and direct-seeded lettuce. For example

shepherd’s-purse [Capsella bursa-pastoris (L.) Medik.] at the

time of cultivation would have been 2 to 3 cm in diam in the

seeded lettuce and 1 to 2 cm in transplanted lettuce, whereas

seeded or transplanted lettuce plant diameters would be 3 to 6

cm and 5 to 8 cm, respectively, at time of cultivation. The

rotating cultivator row pattern recognition algorithm is more

accurate when the size of the crop is greater than the weed

(Tillett et al. 2008). The effects of herbicide and the cultivator

by herbicide interaction on lettuce stand were not significantfor direct-seeded or transplanted lettuce (Table 4).

The rotating cultivator reduced yields in direct-seededlettuce and consequently reduced net returns compared to thestandard cultivator (Table 5). Overall production costs werelower when weeded and thinned with the rotating cultivatorcompared to standard practices, but these reductions were notenough to compensate for lower yield due to excessive standreduction. Trial 4 net returns for the rotating cultivator weremuch lower than for the standard cultivator due to standreduction that decreased yield. The stand loss in transplanted

Table 3. Effects of cultivator type, rotating or standard, with and without herbicide on postcultivation weed densities in transplanted lettuce. Time for a laborer to handweed lettuce 100 m�1 of row previously cultivated with the rotating cultivator or the standard cultivator with or without herbicide.

Weed densitiesa,b Weeding timea

Pronamide

Transplanted lettuce

Trial 2 Trial 6 Trial 2 Trial 6

kg ha�1 No. 100 m�2 s 100 m�1

CultivatorRotating 1.3 — — — 586 aRotating 0 — — — 388 bStandard 1.3 — — — 504 abStandard 0 — — — 516 ab

Main effect of cultivatorRotating 38 b 1,248 b 245 b 487 aStandard 266 a 2,288 a 276 a 510 a

Main effect of herbicidePronamide 1.3 150 a 1,863 a 259 a 545 aNone 0.0 154 a 1,673 a 261 a 453 a

Analysis of covariance P valuesCultivator , 0.0001 , 0.0001Herbicide 0.9325 0.4067Cultivator 3 herbicide 0.6723 0.0710

ANOVA P valuesCultivator 0.0369 0.67Herbicide 0.88 0.08Cultivator 3 herbicide 0.79 0.0451

a Means with the same letter within columns for a cultivator or herbicide are not significantly different according to Fisher’s Protected LSD at P � 0.05.b Weeds present in trial 2 were shepherd’s-purse, 42%; common purslane, 40%; and miscellaneous species consisting of common groundsel, annual bluegrass, hairy

nightshade, and nettleleaf goosefoot, 18%. Weeds present in trial 6 were burning nettle, 38%; common purslane, 35%; hairy nightshade, 8%; common chickweed[Stellaria media (L.) Vill.], 6%; shepherd’s-purse, 4%; annual bluegrass, 4%; and miscellaneous species consisting of nettleleaf goosefoot, little mallow (Malva parvifloraL.), and redroot pigweed (Amaranthus retroflexus L.), 5%.

Table 4. Postthinning crop stand 100 m�1 of row in seeded lettuce (trials 1, 4, and 5) and post-cultivation stand in transplanted lettuce (trials 2 and 6) after cultivationwith the rotating cultivator or the standard cultivator plus hand-thinning with or without pronamide. Listed also are the main effects of cultivator type, herbicide, andtheir interaction.

Pronamide

Seeded lettuce Transplanted lettuce

Trial 1 Trial 4 Trial 5 Trial 2 Trial 6

kg ha�1 Stand (no. 100 m�1)a

CultivatorMain effect of cultivator

Rotating 231 b 221 b 277 b 326 b 359 aStandard 293 a 309 a 360 a 357 a 383 a

Main effect of herbicidePronamide 1.3 264 a 252 a 320 a 336 a 376 aNone 0.0 259 a 271 a 325 a 347 a 367 a

ANOVA P valuesCultivator , 0.0001 0.0020 , 0.0001 0.0077 0.07Herbicide 0.7651 0.4627 0.68 0.3247 0.4510Cultivator 3 herbicide 0.5997 0.4627 0.30 0.2160 0.1126

a Means with the same letter within columns for a cultivator or herbicide are not significantly different according to Fisher’s Protected LSD at P � 0.05.

0 � Weed Technology 28, January–March 2014

lettuce from the rotating cultivator was modest compared tothe rotating cultivator in seeded lettuce and this translatedinto similar yields between cultivators (Table 4). Transplantedlettuce yields for trials 2 and 6, production costs, and netreturns were similar between the rotating and standardcultivators. Trial 2 yields (in cartons ha�1 followed by netreturns $ ha�1 in parentheses) for the rotating cultivator wereas follows: pronamide 1,304 ($13,587) and no pronamide1,173 ($12,513); and for the standard cultivator, pronamide1,366 ($14,403) and no pronamide 1,205 ($12,992) (P ¼0.61). Yields for trial 6 (cartons ha�1 followed by net returns $ha�1 in parentheses) for the rotating cultivator werepronamide 2,216 ($27,689) and no pronamide 2,011($25,537) and, for the standard cultivator, pronamide 2,035($25,569) and no pronamide 2,154 ($27,466) (P ¼ 0.80).

Intrarow Weeding and Thinning on Commercial Farms.Weed densities were lower where the rotating cultivator wasused compared to the standard cultivator in trial 8, but in trial

9 the weed densities, while light, were higher in the rotatingcultivator treatment than in the standard cultivator treatment(Table 6). Postcultivation weed counts were not taken for thestandard cultivator in trials 10 and 11. Hand-weeding and -thinning times were lower in the two commercial lettuce trialsand radicchio trial for the rotating cultivator compared to thestandard cultivator. Thinning and weeding times in bok choywere not different between the two cultivators. The rotatingcultivator had a greater lettuce stand than the standardcultivator in trial 9 but in the other three commercial trialsthere were no differences in stand between the rotating andstandard cultivators (Table 7). In commercial lettuce trialscultivator type did not influence yields, and production costswere similar for both cultivators. For trial 9 the yields for therotating and standard cultivators were 3,741 and 3,931cartons ha�1 (P ¼ 0.35), respectively, and net returns were$39,780 and $41,820, respectively. For trial 11 the yields forthe rotating and standard cultivators were 2,834 and 3,271cartons ha�1 (P¼ 0.0776), respectively, and net returns were

Table 5. Yields and net returns for seeded lettuce using the rotating or standard cultivator and pronamide in field station studies.

Pronamide

Seeded lettuce – trial 1 Seeded lettuce – trial 4

YieldaGross

returnsbProduction

costsc Net returnsd YieldaGross

returnsbProduction

costsc Net returnsd

kg ha�1e Cartons ha�1 $ ha�1 Cartons ha�1 $ ha�1

CultivatorRotating 1.3 2,329 b 25,853 983 24,870 1,422 b 18,347 936 17,411Rotating 0 1,934 b 21,467 887 20,580 1,573 ab 20,290 699 19,591Standard 1.3 2,638 a 29,282 1,183 28,099 1,862 ab 24,016 815 23,201Standard 0 2,497 a 27,718 1,028 26,691 2,143 a 27,647 706 26,941

ANOVA P valuesCultivator type 0.0026 0.0137Herbicide 0.0515 0.2691Cultivator 3 herbicide 0.3444 0.74

a Yield is in 19.05 kg cartons ha�1.b Gross returns¼ experimental yield in cartons per hectare3 $11.10 carton�1 (2009 price per carton for wrapped head lettuce from Monterey County Crop Report) or

$12.90 carton�1 (2010 price per carton).c Production costs include equipment, fuel, lubrication, repairs, herbicides, labor and interest on operating capital; they are rounded to the nearest dollar.d Net returns above production costs (gross returns� production costs).e Pronamide application rate 1.3 kg ha�1 for total cost of $176 ha�1 (rounded).

Table 6. Commercial field studies: postcultivation weed densities in direct-seeded bok choy (trial 8), lettuce (trials 9 and 11), transplanted radicchio (trial 10) and time100 m�1 of row to hand-thin or hand-weed the crop previously cultivated with the rotating cultivator or the standard cultivator.

Seededbok choy

Seededlettuce

Transplantedradicchio

Seededlettuce

Seededbok choy

Seededlettuce

Transplantedradicchio

Seededlettuce

Trial 8 Trial 9 Trial 10 Trial 11 Trial 8 Trial 9 Trial 10 Trial 11

Weed densities (no. 100 m�2)a,b,c Weeding and thinning time (s 100 m�1)a

CultivatorRotating 341 b 67 a 131 5 813 a 190 b 267 b 290 bStandard 691 a 21 b — — 766 a 523 a 380 a 598 a

Analysis of covariance P valuesTreatment probability (F) 0.0004 0.0059 — —

ANOVA P valuesTreatment probability (F) 0.47 , 0.0001 , 0.0001 0.0015

a Means with the same letter within columns are not significantly different according to Fisher’s Protected LSD at P � 0.05.b Weed counts not taken for the standard cultivator for trials 10 and 11.c Weed species present in trial 8 were shepherd’s-purse, 50%; common chickweed, 30%; and common purslane, 20%. Weeds present in trial 9 were primarily

shepherd’s-purse and nettleleaf goosefoot. Weeds present in trial 9 were hairy nightshade, 63%; little mallow, 21%; and annual sowthistle (Sonchus oleraceus L.), 10%.Weeds in trial 10 were hairy nightshade, 62%; common purslane, 20%; and common lambsquarters (Chenopodium album L.), 18%. Weeds in trial 11 were lesserswinecress [Coronopus didymus (L.) Sm.], 92% and wild mustard (Sinapis arvensis L.), 8%.

Fennimore et al.: Intrarow cultivation in vegetables � 0

$36,223 and $41,836, respectively. Likely the rotatingcultivator performed better in commercial fields where weeddensities were lower than in the research station trials. Incommercial trials 9 and 11 weed densities were 131 and 5weeds 100 m�2, respectively, compared to research stationtrial weed densities that were as high as 3,467 weeds 100 m�2

(Tables 2 and 6). The rotating cultivator guidance system isdependent upon detecting the pattern of the crop row. Whereweed densities are lower, the guidance system is more accuratethan where weed densities are high and the guidance systemoccasionally loses the position of the row, shifting thealignment to the right or left. When the rotating cultivatoris out of alignment with the row, damage to the crop standhas been observed (S Fennimore, unpublished data). Becausethe guidance system uses an algorithm to discern crop rowpatterns, high weed densities can overwhelm the system such

that it loses the row pattern (Tillett et al. 2008). Drift off ofthe row centerline likely increases the error rate and lettucestand loss where weed densities are high.

Cultivator Evaluations in Celery. In transplanted celery, therotating cultivator performed accurately by tracking the plantline and removing intrarow weeds, similar to its performancein transplanted lettuce. The cultivator by herbicide interactionfor weed densities was significant in trial 3 and the simpleeffect of treatment on weed densities indicated that therotating cultivator with or without pronamide and thestandard cultivator plus prometryn reduced weed densitiescompared to standard cultivation without prometryn (Table8). The main effect of cultivator on weed densities wassignificant in trial 3 where weed densities were lower followingcultivation with the rotating cultivator compared to thestandard cultivator. Prometryn reduced weed densities in trial7 but not trial 3. Cultivator type did not affect hand-weedingtimes, but prometryn did reduce hand-weeding timescompared to no prometryn. Celery stand was reduced 7%by the rotating cultivator compared to the standard cultivatorin trial 7. Cultivator type did not influence celery yields anddid not affect net returns (Table 9), similar to the results fortransplanted lettuce. However, in trial 7 celery yields in theprometryn treatment were 3,046 cartons ha�1 and in the noprometryn treatment were 2,532 cartons ha�1 (P ¼ 0.0004)using standard practices. Net returns were therefore higher inthe prometryn treatment.

Management Implications. Because vegetable crops havelimited herbicide options, vegetable growers are dependent onhand labor for intrarow weed removal. Projections ofdecreasing labor availability and increasing labor costs (Taylor

Table 7. Commercial field studies: crop stands in direct-seeded bok choy (trial8), direct-seeded lettuce (trials 9 and 11), and transplanted radicchio (trial 10)that were previously cultivated with the rotating cultivator or the standardcultivator.

Seededbok choy

Seededlettuce

Transplantedradicchio

Seededlettuce

Trial 8 Trial 9 Trial 10 Trial 11

Stand (no. 100 m�1)a

CultivatorRotating 341 a 386.1 a 386 a 386 aStandard 345 a 349.5 b 393 a 342 a

ANOVA P valuesTreatment probability (F) 0.7265 , 0.0001 0.3176 0.0766

a Means with the same letter within columns for a cultivator or herbicide arenot significantly different according to Fisher’s Protected LSD at P � 0.05.

Table 8. Effects of cultivator type, rotating or standard, with and without herbicide on postcultivation weed densities in transplanted celery. Time 100 m�1 of row for alaborer to hand-weed lettuce previously cultivated with the rotating cultivator or the standard cultivator with or without herbicide. Listed also are the main effects ofcultivator type, herbicide, and their interaction.

Prometryn

Transplanted celery

Trial 3 Trial 7 Trial 3 Trial 7 Trial 3 Trial 7

kg ha�1 Weed densities (no. 100 m�2)a,b Weeding time (s 100 m�1) Stand (no. 100 m�1)Cultivator

Rotating 1.7 46 b — — — — —Rotating 0 358 b — — — — —Standard 1.7 974 b — — — — —Standard 0 5439 a — — — — —

Main effect of cultivatorRotating 90 b 5,294 a 424 a 883 a 379 a 319 bStandard 2,992 a 4,537 a 459 a 896 a 376 a 344 a

Main effect of herbicidePrometryn 1.7 927 a 783 b 286 b 347 b 381 a 336 aNone 0.0 2,154 a 9,048 a 597 a 1433 a 374 a 325 a

Analysis of covariance P valuesCultivator , 0.0001 0.5005Herbicide 0.1112 , 0.0001Cultivator 3 herbicide 0.0009 0.2911

ANOVA P valuesCultivator 0.2988 0.89 0.6444 0.0011Herbicide , 0.0001 , 0.0001 0.3415 0.14Cultivator 0215 herbicide 0.5927 0.97 0.4424 0.3641

a Means with the same letter within columns are not significantly different according to Fisher’s Protected LSD at P � 0.05.b Weeds present in trial 3 were burning nettle, 38%; common purslane, 35%; hairy nightshade, 8%; common chickweed, 6%; shepherd’s-purse, 4%; annual bluegrass,

4%; and miscellaneous species, 5%. Trial 7 weeds consisted of common purslane, 59%; common chickweed, 16%; burning nettle, 9%; common groundsel, 4%; hairynightshade, 4%; and miscellaneous species, 8%.

0 � Weed Technology 28, January–March 2014

et al. 2012), require that alternatives to hand-weeding for theintrarow space be explored. However, mechanical weedremoval in the intrarow space of vegetable crops is a verychallenging task. Traditional intrarow cultivators such asfinger weeders and torsion weeders lack a guidance systemother than operator alignment of the cultivators with the plantline. Finger weeders can remove small weeds near the crop,but the crop must be well rooted and the operating speedmust be slow to avoid uprooting the crop (van der Schans etal. 2006). In other words, traditional intrarow cultivators areuseful for a short time period and can damage the crop.

The rotating cultivator tested here is an ‘‘intelligentweeder’’ (Cloutier et al. 2007) equipped with a guidancesystem designed to protect the crop while automaticallyremoving unwanted crop plants and weeds. The rotatingcultivator was slow, operating at 0.8 km h�1. A faster intrarowweeder design is the Sarl Radis from France, which operates at3 km h�1. The Radis intrarow weeder, like the rotatingcultivator, requires a size differential between the crop andweeds to operate accurately (van der Schans et al. 2006).Rapid and accurate weed detection and differentiation fromthe crop will be necessary if intelligent weeders are to replacehand-weeding.

The rotating cultivator was evaluated for two tasks: (1) cropthinning and intrarow weeding in seeded bok choy and lettuceand (2) intrarow weeding in transplanted celery, lettuce, andradicchio. Generally, the rotating cultivator removed moreintrarow weeds than the standard interrow cultivator andhelped reduce hand-weeding times (Tables 2, 3, and 6).However, the rotating cultivator did not have the level ofprecision necessary to thin direct-seeded lettuce, whichresulted in reduced stands, lower yields, and lower net returnswhen compared to hand-thinned lettuce (Table 5). Accuratedetection and alignment of the discs used by the rotatingcultivator is necessary to avoid damage to the crop (Tillett etal. 2008), a task that requires rapid return to the cut-awayposition in alignment with the saved crop plant (Figure 1).Excessive stand removal during lettuce thinning may have

been due to guidance system failure to differentiate similarlysized lettuce and weeds, resulting in poor alignment with theplant line. Another possibility is that cultivation too close tothe saved lettuce plants with the rotating disk damaged rootsand the plants were weakened or killed. An alternativeapproach to automatic lettuce thinning is to use machine-vision guidance to control a spray nozzle solenoid so that itintermittently applies herbicide to selectively remove lettuceplants from the plant line and produce the desired plantspacings (Siemens et al. 2012). These researchers thinnedlettuce with intermittent spray applications of paraquat,H2SO4 solutions, and fertilizer solutions. The advantage ofthis intermittent spray system approach to lettuce thinning isthat it uses a relatively simple on/off control system comparedto the more complex variable rate guidance system on therotating cultivator (M. Siemens, personal communication).

In transplanted crops such as celery (Table 9) and lettuce,the net returns for the rotating cultivator were similar tohand-weeding. If intrarow weeding is to improve net returnsthen crop yields must be maintained at the level of currentpractices while reducing herbicide input costs, and improvinglabor use efficiency through reduced hand-weeding. The goalof an interrow cultivation should be to achieve the accuracy ofhand-weeding while also retaining the advantage of a roboticdevice. An advantage of an intelligent intrarow cultivator isthat it does not slow after hours of operation whereas a hand-weeding labor crew tires and slows after hours in the field (S.Fennimore, unpublished data). Additionally an intrarowcultivator equipped with lights has the potential to operateat night—an option not available to a hand-weeding crew.

The research conducted here took place in two croppingsystems, conventional and organic. The effectiveness of therotating cultivation on intrarow weeds (Table 2) suggests apossible role in organic transplanted crops such as lettucewhere the expense of hand-weeding for intrarow weedremoval is over twice that of conventional lettuce (Tourte etal. 2009; Tourte and Smith 2010). Continuing research andrefinements to intrarow weeder design will be necessary to

Table 9. Net returns for celery using the rotating or standard cultivator, and prometryn.

Prometryn

Celery – trial 3 Celery – trial 7

Yielda Gross returnsbProduction

costsc Net returnsd Yielda Gross returnsbProduction

costsc Net returnsd

kg ha�1e Cartons ha�1 $ ha�1 Cartons ha�1 $ ha�1

CultivatorRotating 1.7 3,115 36,131 398 35,733 2,907 33,434 410 33,024Rotating 0 2,747 31,861 553 31,307 2,109 24,258 1,013 23,245Standard 1.7 2,811 32,606 240 32,366 3,046 35,025 264 34,761Standard 0 3,137 36,388 375 36,013 2,532 29,116 862 28,254

ANOVA P valuescultivator 0.80 0.51Herbicide 0.90 0.0004Cultivator 3 herbicide 0.0500 0.76

a Yield is in 27.2 kg cartons ha�1.b Gross returns¼ experimental yield in cartons per acre 3 $11.60 carton�1 (2009 price per carton for celery from Monterey County Crop Report) or $11.50 carton�1

(2010 price per carton).c Production costs include equipment, fuel, lubrication, repairs, herbicides, labor, and interest on operating capital; they are rounded to the nearest dollar.d Net returns above production costs (gross returns� production costs).e Prometryn formulation rate 1.7 L ha�1 applied for total cost of $16 ha�1 (rounded).

Fennimore et al.: Intrarow cultivation in vegetables � 0

reduce or eliminate hand-weeding expense in both conven-tional and organic crops. Given farm labor shortages andrestrictions on hand-weeding in California, the incentives areclearly there to mechanize intrarow weeding and cropthinning (Gianessi and Reigner 2007; Taylor et al. 2012).

Acknowledgments

We thank the California Department of Food andAgriculture Specialty Crop Block Grant ProgramSCB08007 and the California Celery Research AdvisoryBoard for financial support. We are grateful to DavidFountain of Solex, Pete Davey of Sutton Ag, and Nick Tillettof Tillett and Hague Technologies for advice, equipment, andtheir many hours in the field. Thanks to growers Ed Mora,John Romans, and Betteravia Farms for their cooperation,and to Alana Miller for data preparation.

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Received March 29, 2013, and approved August 16, 2013.

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