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

Abstract

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 and marketable bulb yield out of storage increased with plant population, whereas the bulb diameters decreased with plant population. Average marketable yield was 119 Mg⋅ha−1 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.