Birdsfoot trefoil seed production: II. Plant-water status on reproductive development and seed yield.

Forage legume seed crop responses to water stress differ for each species, so a single optimal water management strategy is not applicable. The objectives of this study were to determine the effects of irrigation timing and replenishment amount on birdsfoot trefoil (Lotus corniculatus L.) reproduction and seed yield in the Willamette Valley of western Oregon, USA. Six treatments varying in water depletion percentage and replenishment amount were applied in 1994 and 1995 on a Woodburn silt loam soil (fine-silty, mixed, mesic Aquultic Argixeroll) near Corvallis. In 1996, only a low stress (LS) that met the weekly crop evapotranspirative demand and a non-irrigated control (C) treatment were investigated. In the first year of production, maintaining plants under low-stress conditions sustained flowering longer than with limited or no irrigation. Flowering was not affected by irrigation in the subsequent two production years. Total above-ground phytomass was correlated with the amount of irrigation water (r = 0.92). The C and all single application treatments had greater seed yields (SY) than the LS treatment in 1994. In 1995, all single application treatments had greater SY than the LS treatment. There was no difference between LS and C in 1995 and 1996. Umbel density and the number of seeds per pod were the primary determinants of total seed yield (r = 0.77 and 0.92, respectively). Optimal total seed production was achieved without supplemental irrigation under the humid temperate marine climatic conditions found in western Oregon.