Effect of deficit irrigation on yield, quality and grower returns of native spearmint and hops in Washington State
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This study analyzed the effects of water stress levels on the yield, quality, water use efficiency and grower returns of drip irrigated native spearmint and four subsurface drip irrigated hop cultivars: Mt. Hood, Columbus, Chinook and Willamette. For native spearmint, stress levels were applied at different times during the growing periods. For hops, each stress level was applied throughout the growing season. The field experiments were carried out for two years at the Washington State University Irrigated Agriculture Research and Extension Center near Prosser WA. Mint results showed that biomass production decreased with increasing water stress as is expected. However, mint oil yield and quality were not significantly different across treatments. This suggests that deficit irrigating native spearmint might enhance oil accumulation. This is further supported by the observed increase in oil concentration as water stress increased. A number of mint production costs were affected by the reduction in applied water thus reducing the total mint production costs. Since oil yields were similar across treatments, revenues were not significantly affected. This resulted into increase in net farm income. Also, water stress improved the water use efficiency in native spearmint. Hop results showed that the dried hop cone yield decreased with increasing water stress in all the cultivars. The hop cone quality however did not change significantly across treatments. In the full year of production, water stress improved water use efficiency in hop production across all the cultivars. In the Yakima Valley, reduction in amount of water applied only affects the cost of pumping of the irrigation water which is minimal when compared to the total cost of producing hops. The yield reductions due to applying less water to hops caused reductions in revenues that were higher than the savings in the cost of producing hops due to pumping less water. This decreased the net farm income. The study observed how the subsurface drip irrigation system coupled with scientific irrigation scheduling can substantially reduce hop water use. This study also provides production functions for irrigation optimization of native spearmint and four hop cultivars.