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

Onion 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 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 wheat each year. Vaquero, Esteem, Barbaro, and Sedona varieties were planted heavily and thinned to five nominal populations of 222,000 to 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 bed instead of two tapes. The experiment had a randomized complete-block split-split plot design with 6 replicates. Irrigation systems were the main plots, varieties the split plots, and plant populations the split-split plots. Onion yield and grade responses to plant population for each variety and each planting system were determined by regression of yield and grade on the actual onion plant stands. In general there were few differences between irrigation systems or interactions between irrigations systems, varieties, and plant populations. Averaging over varieties, total and marketable bulb yield out of storage increased with plant population, while the bulb diameters decreased with plant population. Average marketable yield averaged 119 Mg/ha both years. Average marketable bulbs > 10.2 cm in diameter decreased with plant population.  In 2011 estimated economic return increased linearly with plant population.  In 2012 estimated economic return responded quadratically to plant population with maximum return at 419,000 plants/ha.Esteem