Sweetpotato is a warm-season root crop that is widely grown across the world under a wide range of environments and cultural practices. In the United States, commercial sweetpotato production is predominantly in the southeastern states and in California. In 2012, about 52,800 ha of sweetpotato were planted in the United States with a farm gate value of $462 million (U.S. Department of Agriculture, 2014). The average yield of sweetpotato grown under rain-fed conditions in North Carolina, Mississippi, and Louisiana in 2012 was 22 Mg·ha−1, 18 Mg·ha−1, and 23 Mg·ha−1, respectively (USDA-NASS, 2014). The root yield was comparatively higher in the irrigated fields of California, which averaged 38 Mg·ha−1 in 2012 (USDA-NASS, 2014). The average yield for sweetpotato grown under furrow or drip irrigation conditions in California in 2012 (38 Mg·ha−1) was 45% greater than the average yield of the rain-fed crop in North Carolina, Mississippi, and Louisiana. The relatively lower yield for sweetpotato grown under rain-fed conditions in the southeastern region of the United States could be attributed to fluctuating soil moisture conditions (Gajanayake et al., 2013; Villordon et al., 2012).
Soil moisture stress is one of the major factors that limits sweetpotato growth and development and in turn affects storage root production and yield (Gajanayake et al., 2013). Critical factors for successful sweetpotato production include irrigation scheduling and the amount of water to be applied. Irrigation scheduling options rely on the measurement of soil water content or SWT. Precise irrigation scheduling that uses SWT criteria is a powerful method to optimize plant performance. Through the use of the ideal SWT and adjusting irrigation duration and amount, it is possible to simultaneously achieve high productivity and meet environmental stewardship goals for water use and reduce leaching of agricultural inputs (Shock and Wang, 2011).
The primary sweetpotato production area in California is in Stanislaus and Merced Counties where transplanting is done between late April and early June (Stoddard et al., 2013). About 95% of the sweetpotato area in California is drip irrigated, with the remainder using furrow irrigation. Smith et al. (2009) indicated that the season water use total in California is ≈1200 to 2300 mm·ha−1. The ideal daily maximum air temperatures for sweetpotato are between 30 and 35 °C, but temperatures >38 °C are not harmful as long as the crop is adequately irrigated and temperatures drop at night (Stoddard et al., 2013). These requirements closely match the weather conditions in the Treasure Valley of eastern Oregon and southwestern Idaho where the growing season is characterized by high evapotranspiration and low precipitation, making irrigation essential for production of most crops (Shock et al., 2000). The average daily high air temperature at the Oregon State University Malheur Experiment Station between 1 June and 30 Sept. 2012 was 30 °C, while the average daily low temperature was 13 °C. The corresponding average daily high and low temperature from 1 June to 30 Sept. 2011 was 31 and 13 °C, respectively. The 20-year average daily high and low temperatures in both years were 30 and 13 °C, respectively. These conditions suggest that there are >120 d of favorable weather for sweetpotato growth in the Treasure Valley of eastern Oregon.
Newly developed commercial sweetpotato cultivars will produce mature roots in 80 to 90 d (Schultheis et al., 1999; Yencho et al., 2008), suggesting that sweetpotato transplanted in late May or early June could produce mature roots for harvest by the end of September or early October, around the time of the first killing frost in eastern Oregon. Information on the responses of different current commercial sweetpotato cultivars to a wide range of soil moisture regimes is limited. Therefore, the main goal of this study was to assess the possibility of producing sweetpotato in eastern Oregon. The specific objectives were to evaluate cultivars and develop the irrigation criterion suitable for sweetpotato production in the Treasure Valley of eastern Oregon and southwestern Idaho.
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