Sweetpotatoes are grown in the United States primarily for the fresh market, which demands attractive medium-sized roots of uniform shapes that are free from blemishes. This class brings the highest price. In contrast, the processing sector can use storage roots of all sizes, but in the sweetpotato fry industry, they prefer large roots because the recovery is greater (longer and more consistent fry length than with medium-sized roots). In addition, shape is not as critical as for fresh market, and total yield within their quality expectations is more important than the proportion of U.S. no.1 (medium size). The U.S standards for grades of sweetpotato are U.S. Extra no.1 with roots 1-3/4 to 3-1/4 inches diameter, 3 to 9 inches long that are less than 18 oz; U.S. no.1 with roots 1-3/4 to 3-1/2 inches diameter, 3 to 9 inches long that are less than 20 oz; U.S. no.1 Petite (or canner) with roots 1-1/2 to 2-1/4 inches diameter and 3 to 7 inches long; and U.S. no.2 consisting of roots larger than 1-1/2 inches diameter, unless otherwise specified, with a maximum weight of 32 oz [U.S. Department of Agriculture (USDA), 2005]. The commercial grade known as jumbo is larger than U.S. no.1 in diameter and/or length, and falls in the U.S. no.2 category. The market for jumbo and canner grades is primarily the processing industry. Because of the differences in size and quality standards for the processing industry vs. the fresh market, diverse production strategies may be necessary to optimize returns.
In the United States, sweetpotato is produced mainly in the southern states and the west coast with over 130,000 acres planted in 2012 (USDA, 2013a). Mississippi and Louisiana ranked second and fourth in the nation for sweetpotato acreage with 24,000 acres and 10,000 acres planted in 2012, respectively. In addition, Mississippi and Louisiana ranked third and fourth, respectively, in the nation for sweetpotato crop value with over $62.6 million and $33.3 million in gross farm value in 2012 (USDA, 2013b). The acreage and production of sweetpotato could increase even further because demand and per capita consumption are increasing (46% in the past decade) mainly due to two factors: the perceived nutritional and health attributes, and the availability and convenience of value-added processed products (Nalley et al., 2005; USDA, 2011; Woolfe, 1992).
Sweetpotato growers are quickly adjusting to processor’s demands to make the crop more widely available. The main objective of fresh market growers is U.S. no.1, but they supply processors with large roots for processed products to improve profitability. Therefore, an approach to increase root yield must be examined to satisfy the greater processing demand while maintaining yield of U.S. no.1 grade for fresh market. In contrast, some producers are targeting the processing sector exclusively with field-run bulk-harvest contracts; however, to be profitable, yield must increase and production costs must be reduced considerably in comparison with production intended for the fresh market. Achieving a higher total yield is critical given the lower price paid by the processors for field-run harvest. Therefore, this study focused on determining yield increases and changes in grade proportions in response to plant spacing and extension of the growing period to improve profitability of the production system intended for either fresh market or the processing industry.
Sweetpotato producers targeting fresh market venues strive to achieve a large percentage of U.S. no.1 grade roots, while minimizing small and jumbo grade roots. Growers are challenged to time-harvesting operations to reduce the number of unwanted roots and maximize marketing potential and profit. Growing roots to increase larger roots and tonnage while maintaining U.S. no.1 yield can create a larger window of success considering that the roots essentially can grow as long as environmental conditions are favorable. In addition, our assumption that a delay in harvest reduces U.S. no.1 yield may be incorrect and producers targeting the fresh market may realize a yield gain by increasing jumbo yield. There is a need to examine different planting and harvest dates, plant spacing, and row widths to determine the best combination to increase the number of large roots and overall tonnage while maintaining U.S. no.1 yield.
It is generally thought that high plant densities result in small storage roots, and as plant density decreases, the number of large roots increases (Rubatzky and Yamaguchi, 1997). Most growers use row widths ranging from 38 to 42 inches to produce sweetpotato in the United States. However, the difference, if any, between various row widths has not been evaluated with current commercial varieties. Extension publications from the main sweetpotato production states have no specific row width recommendations except California where a double-row system on 80-inch beds is used to accommodate the drip irrigation system (Boudreaux, 2009; Schultheis et al., 2005; Stoddard et al., 2013; Thompson et al., 2002). Row width of 3-1/5 to 4 ft is recommended in Alabama (Kemble et al., 2006). In-row plant spacing recommendations are available in all states, but vary somewhat among states and varieties, and range from 8 to 16 inches. In-row plant spacing has been reported to influence yield of U.S. no.1; however, varieties responded differently to spacing and planting-harvest time (Mulkey et al., 1994; Schultheis et al., 1999). In addition, the study in North Carolina reported that ‘Beauregard’ planted at 15 or 23 cm increased net economic value compared with wider in-row spacing (Schultheis et al., 1999). To our knowledge, in-row plant spacing has not been evaluated at different row widths with current commercial varieties.
The objective of this study is to determine the importance of planting and harvest dates, and plant spacing in ‘Beauregard’ and ‘Evangeline’ sweetpotato to improve yields and economic benefit. Most of the research in the United States with varieties destined for food consumption has been directed to optimize U.S. no.1 root grade toward the fresh market sector. Consequently, studies with the objective of increasing total tonnage of root grades destined for processing are lacking. Furthermore, no information is available in the United States about the response to in-row plant spacing and row width combinations at different planting-harvest dates to increase tonnage and determine changes in the proportion of root grades. Storage root grades differ in value and therefore influence gross income and profitability.
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