2015 25.2 Assessing Productivity of Baby-Leaf Lettuce in the Open Field for W. Washington

Assessing Productivity of Baby-Leaf Lettuce in the Open Field for W. Washington:
A Comparison of Baby-Leaf Salad Mix Production Options

Charlene Grahn, Carol Miles, Chris Benedict, Washington State University (WSU)

Over the last two decades, annual supermarket sales of ready-to-eat salad mix have increased more than five-fold in the United States, from $197 million in 1993 to a $2.7 billion industry (Cook, 2008). These baby-leaf salad mixes are composed of many different salad crop types including lettuce, spinach, arugula, pak choi, kale, Asian and Indian mustards, and radicchio. Despite the diversity of colors, textures, and flavors in these mixes, the common thread uniting 96% of all salad crops sold in the United States is their point of origin: California and Arizona. The Southwestern United States is incredibly adept at producing baby-leaf salad crops on a very large scale thanks to its mild climate, sandy soils, large farm size, and water subsidies. Precision agriculture techniques such as laser levelling of fields, precision seeding, and mechanical harvesting are very heavily utilized to meet strict market standards for product uniformity.

Salad crop market in Washington State

Despite the Southwest’s dominance of the national baby-leaf salad market, Washington’s highly diversified vegetable growers feed a substantial and growing market for locally produced salad crops. According to the 2012 USDA Census of Agriculture, 222 Washington farms produced lettuce in 2012, 113 produced kale, 44 produced spinach, and 14 produced mustard greens. Availability of local baby-leaf greens is at its peak from mid-May to the end of September in Northwest Washington, but consumer demand for baby greens is strong year-round. Consumer demand is especially high for baby-leaf lettuce, which is still the primary component of baby-leaf salad mixes nationwide by a wide margin despite expanding consumer preference for more nontraditional salad crops.

The primary barrier to baby-leaf salad production for Washington’s growers who are organic, transitioning to organic, or do not use herbicides, is weed pressure. Baby-leaf salad crop production is very difficult in fields with even slight weed pressure. This is because weed management for baby-leaf salad green crops is more akin to weed management for lawns than for vegetables. The crop is produced in densely seeded beds and the only practical post-emergence weed management method is hand weeding before harvest, and manual removal of weeds from the crop after harvest. Both of these practices are very labor and time-intensive and greatly increase production costs. Direct market customers are often willing to pay higher prices for locally produced greens, however high production costs severely limit the opportunities of growers to expand into larger markets and attain wider distribution. Higher production costs cut into grower profits even under circumstances where customers are willing to pay higher prices.

Several trade names exist for head lettuces designed for salad mix production, one of which is Salanova™ from Johnny’s Selected Seeds. At the mature head stage, the individual leaves of these cultivars remain short (approximately 3 inches) and the head will develop an average of 200 leaves as opposed to the 60 leaves that are typical of conventional head lettuce. These salad mix cultivars were first available to the large-scale bagged salad mix industry for several years before becoming available to smaller growers for the first time in 2013 (Mefferd, 2013).

Seed companies tout several claims as to why ‘baby-leaf’ head lettuce production is better than traditional baby-leaf lettuce production, including better yield, flavor, texture, and shelf life. One major advantage from the perspective of Washington’s smallscale herbicide-free growers is that this lettuce is transplanted instead of direct-seeded, which means that it is compatible with plastic and biodegradable mulch applications. Mulch provides excellent weed suppression and prevents the head from contacting the soil so that heads are cleaner. Even if no mulch product is used the crop can still be weeded using hand-tools instead of bare hands.

Despite the apparent advantages of the head lettuce salad mix production method over traditional baby-leaf production, one drawback to growing Salanova™ lettuce is substantially higher cost per seed. Additionally, little information from unbiased sources is available on yield advantage, and the information available overlooks the fact that traditional baby-leaf lettuce can be reharvested up to two times after the initial harvest, whereas Salanova™ is harvested only once. What follows is a comparison of the yield, time to harvest, profitability and seed costs of the two salad mix lettuce production systems.

Study Methods

Two separate but adjacent studies were conducted at the Washington State University Northwest Washington Research and Extension Center in Mount Vernon, WA, one a traditional baby-leaf lettuce trial and one a Salanova™ lettuce production trial. The Salanova™ trial included eight cultivars: Red and Green Oakleaf, Red and Green Butterhead, Red and Green Sweet Crisp, and Red and Green Incised. Seed was provided by Johnny’s Selected Seeds of Winslow, ME. The traditional baby-leaf trial included the romaine cultivar ‘Flashy Trout’s Back.’ Both studies were completely randomized and managed organically. Poultrymanure based 8-2-4- pelletized fertilizer was broadcast applied to the center of each bed and incorporated when beds were shaped. Fertilizer application rate was 50 lb N/A for baby-leaf lettuce and 75 lb N/A for Salanova™, the standard application rate for each crop. Beds for all trials were shaped with a Model 2600 Raised Bed Mulch Layer (Rain-Flo Irrigation, East Pearl, PA) and were 2.5 feet wide, 7 inches high and spaced 7 feet center-to-center. Only Salanova™ lettuce beds were covered with 1 mil black embossed plastic mulch. Baby-leaf lettuce was seeded in six rows spaced 4 inches apart within the bed, and within each row seed was spaced 1/2 –inch apart, for a seeding density of 144 seeds/ bedfoot. Salanova™ transplants were spaced 8 inches center-to-center with three rows per bed. For both studies, drip irrigation was applied throughout the growing season via drip tape (8 mm wall thickness, 8 inch emitter spacing 0.670 gpm/100 feet) at a rate of 1 inch per week accounting for precipitation.

Traditional baby-leaf trials were direct-seeded into the field on April 14 and April 30 of 2014. Salanova™ 162-cell flats were seeded on April 9, grown in a greenhouse, transferred to an outdoor hoophouse to harden off on April 23, and transplanted into the field on May 8. Crops were harvested when they reached maturity, which for traditional baby-leaf lettuce was defined as a 4-inch leaf length, and for Salanova™ was when heads reached 8 inches in diameter. The traditional baby-leaf trial was harvested only once and yield was multiplied to estimate the yield for one and two reharvests. The regrowth interval between harvests was estimated to be 10 days. Yield and time for multiple harvests were based on experience of growers in the area. For Salanova™, yield was measured for cored heads. Seed costs and wholesale and retail value for 100 bed-feet were calculated for both crops.

Crop Comparison

Weather Conditions.

The low, high, and mean average field temperatures from the time of seeding the baby-leaf lettuce to the time of final harvest was 48 oF, 65 oF, and 56 oF, respectively. For Salanova™, low, high, and mean temperatures were 50 oF, 70 oF, and 59 oF, respectively, from the time the crop was transplanted into the field to the time of harvest. Baby-leaf lettuce was planted into the field either 8 or 24 days earlier in the spring 2014 season than Salanova™ was transplanted, therefore Salanova™ lettuce experienced slightly warmer field temperatures than baby-leaf lettuce. Additionally, Salanova™ lettuce was planted into black plastic mulch, which increases soil temperature near the root zone. Precipitation during the growing season was 6.3 inches for baby-leaf lettuce and 3.4 inches for Salanova™.

Lettuce Yield.

Mean yield of traditional baby-leaf lettuce was 0.33 pounds per bed-foot for the first harvest, and yield did not differ due to planting date (P = 0.27). Salanova™ yield averaged 1.6 pounds per bed-foot. There were no significant differences in yield among Salanova™ cultivars (Butterhead, Oakleaf, Sweet Crisp, and Incised) (P > 0.05), but green Salanova™ cultivars had a significantly higher yield than red cultivars (P < 0.0001) (Table 1).

Yield Comparison.

To compare yield between Salanova™ and traditional baby-leaf lettuce production, the cored Salanova™yield per bed-foot was compared to the baby-leaf yield per bed-foot. Even when yield for baby-leaf beds was calculated for three harvests, Salanova™ heads produced a significantly higher yield than traditional baby-leaf lettuce (P < 0.004) (Table 2). Yield of Salanova™ lettuce was 4.7 times greater than a single harvest of traditional baby-leaf lettuce, 2.4 times greater than two harvests of traditional baby-leaf lettuce, and 1.6 times greater than three harvests of traditional baby-leaf lettuce.

Baby-leaf lettuce was first harvested 48 days after seeding and a third harvest would be 68 days after seeding (Table 3). Salanova™ cultivars were harvested 75 days after seeding and 52 days after transplanting, both significantly later than baby-leaf lettuce (P = 0.0001 and P = 0.005, respectively).

The ratio of yield:days to harvest (lb. per day per bedfoot) was greater for Salanova™ cultivars (0.02 lb. per day from date of seeding, 0.03 lb. per day from date of transplant) than for baby-leaf lettuce even when babyleaf lettuce was harvested up to three times (0.007 lb. per day for one harvest, 0.01 lb. per day for two harvests, 0.016 lb. per day for three harvests) (Table 2). Yield of Salanova™ lettuce was 155 lb. per 100-foot bed, with a value of $317 wholesale ($2.04 per lb.) (USDA ER S 2014) and $1275 retail ($8.22 per lb.) (USDA ER S 2012) when grown organically. Traditional baby-leaf lettuce yield was 32.6 lb. for a single harvest with a value of $67 wholesale and $268 retail, 65.0 lb. for a double harvest with a value of $145 wholesale and $539 retail, and 97.7 for a triple harvest with a value of $199 wholesale and $801 retail. Yield of Salanova™ lettuce was 1.6 to 4.7 greater and value was 0.6 to 3.8 times greater than for traditional baby-leaf lettuce.

A 100-foot bed of Salanova™ lettuce with three rows requires approximately 300 seeds at an average cost of $0.064 per seed and a total cost of $19.20. A bed of traditional baby-leaf lettuce with the same dimensions requires approximately 14,400 seeds at an average cost of $0.0006 per seed and a total cost of $8.64 (all seed prices from Johnny’s Selected Seeds 2015 Catalog). The cost per seed is 100 times greater for a Salanova™ cultivar than for a common lettuce leaf cultivar, and the cost of seed per 100-foot bed is two times greater for a Salanova™ than for a common leaf lettuce.

Summary

Salad mix head lettuce shows promise for baby-leaf lettuce production in Washington. The difference in cost of seed for Salanova™ lettuce was insignificant considering its increased yield, and combined with the weed management advantages of growing Salanova™ on plastic-covered beds, complete economic comparison of the two production systems is warranted. For future research, several lettuce cultivars grown for traditional baby-leaf production should be planted in the same field study along with Salanova™ cultivars. Fertilizer and irrigation should be managed optimally for each crop type, and costs recorded. Baby-leaf lettuce seeding and Salanova™ transplanting should co-occur to expose each crop type to the same environmental conditions during field growth. Additionally, performance of Salanova™ and traditional baby-leaf lettuce outside of the main growing season should be compared as these trials took place during the beginning of the region’s main lettuce growing season. Baby-leaf lettuce should be harvested multiple times so as to measure yield that would be attained in this production system. Time for harvesting and time for removing weeds from the harvested crop should also be measured.

Charlene Grahn is a Masters candidate in the Department of Horticulture at Washington State University studying baby-leaf salad crops with Dr. Carol Miles. Her research focuses on the production of organic baby-leaf salad mix in Western Washington. She will attend the University of Wisconsin, Madison as a Ph.D. student in the Plant Breeding and Plant Genetics Department in Fall of 2015. charlene.grahn@wsu.edu. Carol Miles is a Professor in the Department of Horticulture at WSU, and is the Vegetable Specialist located at the WSU Mount Vernon Northwestern Washington Research and Extension Center. She specializes in vegetable crop production and has a strong interest in alternative crops and organic production. 360-848-6150, milesc@wsu.edu, vegetables.wsu.edu. Chris Benedict works for WSU Whatcom Extension, Bellingham. chrisbenedict@wsu.edu

References

Cook, R. 2008. Trends in the Marketing of Fresh Produce and Fresh-Cut Products. Presentation for the UC Davis Postharvest Technology Short Course. Davis, CA. Statistics from Information Resources, Inc.

Mefferd, A. 2013. New head lettuces are designed for salad mix. Growing for Market. 22:4. 1-5.

USDA. 2014. Washington State Market Value of Agricultural Products Sold. 2012 Census of Agriculture. 27 August 2014.

USDA ER S. 2014. Wholesale vegetable prices, organic and conventional, monthly and annual, 2012-13. Last updated 3/21/2014. Accessed 2/18/2015. www.ers.usda.gov/data-products/organic-prices.aspx.

USDA ER S. 2012. Monthly retail prices for organic and conventional milk (2004-2007), eggs, rice, carrots, salad mix, spinach, and strawberries, 2004-06. Last updated 5/21/2012. Accessed 2/18/2015. www.ers.usda.gov/ data-products/organic-prices.aspx.

Johnny’s Selected Seeds. 2015. 2015 Seed Catalog. Winslow, ME.

Tags: baby leaf, leaf lettuce, Production Trials, Research, salad crop, salad mix, WSU

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