Edamame, an edible version of soybean (Glycine max), is emerging in popularity in North America. A nutritious and appetizing vegetable with a sweet, nutty flavor (Miles et al., 2000), edamame is harvested when pods are bright green and immature seeds are at their maximum size (Shanmugasundaram et al., 1991). The protein-rich seed is attractive to health-conscious consumers (Rao et al., 2002). Edamame seed also contains cholesterol-free fat (5.7%), phosphorus (158 mg per 100 g), calcium (78 mg per 100 g), vitamin B1 (0.4 mg per 100 g), vitamin B2 (0.17 mg per 100 g), isoflavones and tocopherols (Shanmugamsundaram and Yan, 2004).
The United States is second in global grain-type soybean production, accounting for 122.5 million metric tons in 2018 (USDA-NASS, 2019). However, an estimated 70% of the edamame consumed in the U.S. is imported from China (Barlow, 2018). Cultivar evaluation trials conducted across the United States—including the Mid-Atlantic, Mississippi delta, upper Plains, and Midwest—attest to the adaptability of edamame (Duppong and Hatterman-Valenti, 2005; Hunsberger et al., 2007; Zhang and Kyei-Boahen, 2007). An economic analysis revealed that edamame returns five times per hectare more than grain-type soybean grown in a 2-year rotation with corn (Karlen et al., 2004). To meet growing consumer demand for domestic product in the past decade, several U.S. vegetable processors are growing edamame (Crawford et al., 2018).
Several challenges to expanding commercial edamame production exist, including voids in best management practices for growing the crop. Recent research has reduced some barriers, including registration of certain herbicides (Williams and Nelson, 2014; Williams et al., 2017, 2019), developing nonchemical weed control tactics (Crawford and Williams, 2018; Crawford et al., 2018), and identifying optimal seeding depth (Crawford and Williams, 2019; Zhang et al., 2013). For other crop production practices, the vegetable industry relies heavily on established grain-type soybean management guidelines; however, research to confirm or deny their appropriateness is lacking.
One fundamental decision for growing a crop is to determine the target plant population density, hereafter called simply “plant density.” Selecting the appropriate plant density is an important crop management decision in commercial edamame production where plant architecture determines efficiency of mechanical harvest (Zandonadi et al., 2010). Studies on grain-type soybean report that plant density influences plant height (Matsuo et al., 2018; Mehmet, 2008), number of main stem branches (Agudamu and Shiraiwa, 2016; Board and Kahlon, 2013), and plant biomass (Matsuo et al., 2018). Companies selling edamame seed identify seeding rates ranging from 197,600 plants/ha (Wannamaker Seeds, Saluda, NC) to 344,200 plants/ha (Johnny’s Selected Seeds, Winslow, ME), which is similar to recommendations for grain-type soybean (Nafziger, 2009). However, peer-reviewed literature to develop such recommendations appears to be nonexistent. In addition, edamame seed is more expensive, ranging from $25 to $50 per kg for edamame (Fedco Seeds, Clinton, ME; Johnny’s Selected Seeds, Winslow, ME; Wannamaker Seeds, Saluda, NC) compared with $2.5 per kg for conventional grain soybean. Conventional grain soybean cost (per kilogram) were calculated based on grain soybean cost of $57.40 per acre (USDA-ERS, 2019) and seeding rate assumed 22.68 kg (50 lbs) per acre.
The vegetable industry would benefit from research-based information on edamame response to plant density, including horticultural performance and an economic analysis. We define the EOPD as the plant density that maximizes grower profits when accounting for seed cost. The objectives of this study were to a) evaluate edamame plant morphology and yield response to plant density and b) determine the EOPD of machine-harvested edamame.
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