RED-CLOVER SEED PRODUCTION .3. EFFECT OF FORAGE REMOVAL TIME UNDER VARIED ENVIRONMENTS

Red clover (Trifolium pratense L.) grown for seed in western Oregon mu st have the forage removed in spring so that flowering and seed matura tion nili be more uniform. There is Little available information that describes the effects of time of forage removal on red clover seed pro duction. This study was conducted to determine (i) the effects of time of forage removal on seed production of red clover grown under a rang e of common environmental conditions found in western Oregon and (ii) whether any crop phenological or environmental indicators can be used to describe optimal forage removal time. Twelve different experimental locations per year were established in 1990 and 1991 in commercial re d clover seed production fields throughout a six-county area in the Wi llamette Valley of Oregon. Forage was removed once per treatment and t here were seven forage removal time treatments from early May to late June. Red clover seed yields are dependent on a relationship with the number of accumulated heat units (HUs) at the time of forage removal. The average HU optimum (199 HUs) is not exact for all sites, but can b e adjusted for the estimated combined effects of factors that influenc e the relative productivity of each site. Relatively low yielding site s should have forage removal time advanced relative to the HU optimum, and high yielding sites should have forage removal time delayed. Also , named improved cultivars were not as sensitive to time of forage rem oval as are Oregon-adapted ecotypes. Forage removal time is a critical factor that needs to be considered to achieve maximal seed yields.