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Onion (Allium cepa) plant population is an important factor in total yield and bulb size, both of which can influence economic return to growers. Different onion bulb marketing opportunities influence the plant populations that growers should target.With the transition from furrow irrigation to a drip irrigation system, growers have doubts as to the onion population that should be planted to assure favorable economic outcomes. Onions were grown on silt loam at the Oregon State University Malheur Experiment Station, Ontario, OR in 2011 and 2012 following bread wheat (Triticum aestivum L.) each year. Long-day onion cultivars Vaquero, Esteem, Barbaro, and Sedona were planted heavily and thinned to nominal plant populations between 222,000 and 593,000 plants/ha under furrow irrigation, subsurface drip irrigation, and ‘‘intense bed’’ subsurface drip irrigation. The intense bed configuration had 50% more rows of onions with three drip tapes per 1.94-m bed instead of two tapes. The experiment had a randomized complete block split-split-plot design with six replicates. Irrigation systems were the main plots, cultivars the split plots, and plant populations the split-split plots. Onion yield and grade responses to plant population for each cultivar and each planting system were determined by regression of yield and grade on the actual onion plant stands. In general, there were few differences among irrigation systems or interactions among irrigations systems, cultivars, and plant populations. Averaging over cultivars, total andmarketable bulb yield out of storage increased with plant population, whereas the bulb diameters decreased with plant population. Average marketable yield was 119 Mg·ha over the 2 years. Average yield of colossal bulbs >102 mm in diameter decreased with increasing plant population. In 2011, estimated gross economic return increased linearly with plant population, offset in part by increasing seed cost. In 2012, estimated economic return responded quadratically to plant population with maximum return of $45,357/ha at 419,000 plants/ha. About 8000 ha of onions (Allium cepa) are grown in the Treasure Valley located in eastern Oregon and southwestern Idaho of the United States. These onions are mostly longday cultivars and are marketed from August from the field and continuing to April from storage (Shock et al., 2000b). Before the mid1990s, jumbo size onions (76 to 102 mm) were the most valuable size class in the Treasure Valley. Shock et al. (1990) demonstrated that marketable (>57 mm) and jumbo yields of long-day onions were maximized at 93 and 78 Mg·ha, respectively, by a plant population of 385,000 plants·ha in the Treasure Valley. In the mid-1990s, Treasure Valley packers started marketing a new onion bulb size class (super colossals, bulb diameter >108 mm). Research in 1999, 2000, and 2001 in the Treasure Valley showed that although marketable and jumbo bulb yield increased to 120, 70, and 110 Mg·ha, respectively, with increasing plant population up to the highest tested of 400,000 plants/ha, colossal and super colossal yields were highest at the lowest plant population tested of 150,000 plants/ha (Shock et al., 2004). Despite the price differential between bulb size classes remaining relatively unchanged, since the mid-2000s the market for jumbo onions has been greater, causing growers to again want to produce fewer colossal onions and more jumbo onions. Other research has shown that marketable yield can peak at plant populations higher than 400,000 plants/ha. Frappell (1973) found that, using intermediate-day onions in Tasmania, maximum yield of bulbs larger than 50 mmwas 55 to 75 Mg·ha with 450,000 to 700,000 plants/ha depending on year. In another study, using long-day onions in California, marketable yield (>55 mm) in dry weight was highest with 800,000 plants/ha compared with 400,000 and 1,000,000 plants/ha (Hatridge-Esh and Bennett, 1980). Some of the studies of onion yield responses to plant population have been based on the rectangularity, the ratio of interrow to intrarow spacing. Uniform rectangularities of 2 in Frappell (1973), of 1.8 in Hatridge-Esh and Bennett (1980), and of less than 6 in Rickard and Wickens (1979) were used across all plant populations by varying row spacings. Rumpel and Felzczynski (2000) used a row spacing of 0.27 m. Rogers (1978) used rectangularities varying from 1 to 26 as plant population increased. Both Frappell (1973) and Hatridge-Esh and Bennett (1980) found that increasing rectangularity was correlated with decreasing bulb yield. With the advent of new cultivars and adoption of drip irrigation, a reevaluation of onion plant populations for the Treasure Valley was encouraged by onion packers. The most common onion planting configuration used in the Treasure Valley, named locally ‘‘conventional bed’’ has 1.94-m or slightly narrower beds with two drip tapes and four double rows of onions. An alternative planting configuration, ‘‘intense bed,’’ has beds of similar width with three drip tapes and six double rows of onions, hence lower rectangularity. Growers in the Treasure Valley of Oregon and Idaho who deliver onions on a contract want to be able to plan their plantings for adequate return based on the size and price stipulations of the contract. The objective of the trials reported here was to evaluate the yield response of four onion cultivars to plant populations under furrow irrigation and with two planting configurations under drip irrigation. Materials and Methods Onions were grown in 2011 and 2012 on an Owyhee silt loam (coarse silty, mixed, mesic Xerollic Camborthid) at the Oregon State University Malheur Experiment Station, Ontario, OR. The fields had previously been planted to bread wheat (Triticum aestivum L.). The wheat stubble was shredded and the field deep-chiseled, disked, irrigated, moldboard plowed, roller harrowed, and bedded in the fall before spring planting. Each year, before fall plowing, fertilizer was broadcast based on soil analyses and according to extension guidelines (Sullivan et al., 2001). The soil had a pH of 8.0 and 1% organic matter in 2011 and a pH of 7.7 and 1.7% organic matter in 2012. In 2011, phosphorus (P) at 224 kg·ha, sulfur (S) at 90 kg·ha, manganese (Mn) at 8 kg·ha, and boron (B) at 1.12 kg·ha were broadcast. In 2012, P at 112 kg·ha, S at 224 kg·ha, gypsum at 1120 kg·ha, and B at 1.12 kg·ha were broadcast. In 2011, at fall bedding, the field was fumigated with 187 L·ha of Telone C-17 (77.9% 1,3-dichloropropene + 16.5% chloropicrin, Dow AgroSciences, Indianapolis, IN). In 2012 at fall bedding, the field was fumigated with 140 L·ha of Vapam (42% Received for publication 21 Apr. 2015. Accepted for publication 12 June 2015. This project was funded by the Idaho EasternOregon Onion Committee, Oregon State University, Formula Grant no. 2014-31100-06041 and Formula Grant no. 2014-31200-06041 from theUSDANational Institute of Food and Agriculture, and onion seed companies. Professor and Director. Senior Faculty Research Assistant. Corresponding author. E-mail: clinton.shock@

Response of Onion Yield, Grade, and Financial Return to Plant Population and Irrigation System