Soybean [Glycine max (L.) Merr.] is an important crop grown worldwide for the provision of vegetable oil for human consumption and protein meal for animal feeds. Edamame, a Japanese term, is a type of specialty soybeans, also called vegetable soybean and maodou in China. In the United States, soybean is a major field crop, second only to corn in terms of total acreage and economic value. However, vegetable soybean is relatively new to North America, although it was once grown in the United States during WWII, compared with the history of edamame grown in Asia for many centuries (Shurtleff and Aoyagi, 2009). Edamame is harvested at the R6 growth stage, when the pods and seeds are still green (Fehr et al., 1971), different from general-purpose soybeans, which are harvested after full maturity (R8 stage).
Comparatively speaking, edamame is grown on a much smaller scale but has greater market and nutrition values than general-purpose soybean (Liu, 1999). It has been reported that the net return per acre for growing edamame was $259 for the wholesale fresh market in Kentucky (Ernst and Woods, 2001), $600 to $750 for farmers’ markets in Ohio (Bernick, 2009), and overall ranged from $400 to $1300 in the United States (Binder, 2010) compared with $350 to $600 of the gross income per acre for growing general-purpose soybean. Soy food provides complete protein with all essential amino acids and benefits human health (Messina, 1999). Although soybean historically has not been viewed as an edible crop in the United States, more people have become aware of soy foods like tofu and edamame, and more people are adopting plant-based diets (CBS News, 2013; Kelley and Sanchez, 2005). The increase of consumption of soy food is not only because soybean is an important source of complete protein, but also soy food helps to reduce health risks such as cardiovascular disease, osteoporosis, and cancers (Messina, 1999; Messina and Wu, 2009). Edamame consumption improves human health by providing high levels of protein and all essential amino acids in well-balanced profiles, along with other nutritional compounds like isoflavones.
The food-type soybean industry has spent more than 20 years growing from door-to-door sales to an integrated supply chain throughout the United States (Soyfoods Association of North America, 2011). The market demand for vegetable soybeans has significantly increased. In Arkansas, edamame is commercially grown and an edamame-processing plant is in operation (McBryde, 2012). Edamame (fresh or frozen pods and/or seeds) is now available in supermarkets (e.g., Walmart), grocery stores (e.g., Kroger), wholesale stores (Sam’s Club and Costco), farmers markets, and/or restaurants. The increased demand has resulted in a steady increase in land acreage under vegetable soybeans (Binder, 2010). Edamame provides a good option of crop farming, especially for small-scale farmers and urban agriculture growers who want to increase income by growing such a high-value niche crop. However, edamame products sold in markets across the United States are mainly imported from China and other Asian countries or regions and are mostly marketed frozen. One of the important reasons for this status is that there is a lack of domestically developed or improved edamame cultivars in the United States and also lack of affordable seed. Therefore, growing edamame faces challenges and/or problems. From the point of plant breeding, the major problems include limited genetic resources, lodging, inferior plant structure or type, susceptibility to seed diseases, low yield potential, and shattering, which is important for edamame seed production.
Correspondingly, research on edamame, in particular plant breeding and genetics studies, has rarely been reported in the United States. Rao et al. (2002) analyzed fresh green pod and seed yields in 12 vegetable or large-seeded soybean cultivars/genotypes from Japan and China and two adapted U.S. cultivars. Zhang and Kyei-Boahen (2007) evaluated five traits including fresh pod weight in 23 edamame cultivars from MG III to VII in Mississippi. Mebrahtu and Mohamed (2006) analyzed genetic variation for green pod yield and quality in 31 vegetable soybean genotypes from MG III to VI. More recently, Ogles et al. (2016) evaluated 11 selected edamame cultivars of four MGs for adaptability and production in central Alabama. By comparing 136 entries from 22 resources with 14 grain-type cultivars representing a range of MGs, Williams (2015) characterized the vegetable soybean germplasm lines for commercial production. In these studies, differential maturities were involved and the materials mostly originated from out of the United States. To some extent, the results might be affected by the poor adaptation. Use of adapted genotypes with similar maturities in research would reduce such impacts and provide more useful information for production. Since the 1990s, the Virginia State University soybean program has been dedicated to the development of edamame cultivars adapted to the United States, in particular Virginia and similar environments (Mebrahtu et al., 2005). The objective of this study was to evaluate the fresh edamame yield and related agronomic traits in locally bred soybean cultivars and/or breeding lines of MGs V and VI and to analyze the trait correlations, to help the identification and development of adapted superior edamame cultivars with desired traits for the United States.
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