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SEED PHYSIOLOGY, PRODUCTION & TECHNOLOGY

Birdsfoot Trefoil Seed Production

III. Seed Shatter and Optimal Harvest Time

^a Instituto Nacional de Investigaciones Forestales y Agropecuarias, CE Pabellon, Carretera Ags.-Zac. km. 32.5, Apdo. Postal 20, Pabellon de Arteaga, Ags. C.P. 20660, Mexico
^b National Forage Seed Production Research Center, USDA-ARS, 3450 SW Campus Way, Corvallis, OR 97331 USA

steinerj{at}ucs.orst.edu

Seed shattering is a major problem in birdsfoot trefoil (Lotus corniculatus L.) seed production and limited information is available describing the effects of agronomic practices on shatter losses. The objectives of this research were to: (i) quantify the effects of soil-water availability on seed shatter and (ii) determine optimal harvest time on the basis of a heat unit method to minimize birdsfoot trefoil seed losses under western Oregon climatic conditions. Six treatments varying in water depletion percentage and replenishment amount were applied in 1994 and 1995 and two treatments in 1996 on a Woodburn silt loam soil (fine-silty, mixed, mesic Aquultic Argixeroll) near Corvallis, OR. The total amount of shattered seeds was correlated with total harvested seed yield (r = 0.93). Crop water stress index (CWSI) was inversely related to the percentage of seeds shattered (r = -0.76). Increasing amounts of applied water increased the potential of seed yield shattered (r = 0.65). Seed shatter losses fluctuated during the late-reproductive period, but were not influenced by irrigation or fluctuating climatic conditions. A total of 109 heat units (approximately 11 d), which were determined on the basis of a 10°C base temperature, were accumulated from the time of initial pod dehiscence until rapid seed shattering. The average seed yield losses due to shattering was 3 to 5.3 kg ha^- d^-1. The non-irrigated control treatment generally produced more seeds than irrigated treatments. It is, thus, best not to irrigate birdsfoot trefoil grown for seed in western Oregon because increasing amounts of irrigation water increased seed shattering.

Abbreviations: CWSI, crop water stress index • FAWU, fraction of available water used • FC, field capacity • HU, heat units • LS, low-stress treatment • C, non-irrigated control treatment • RH, relative humidity • T, average daily temperature • VPD, vapor pressure deficit

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