Red Clover Seed Production: III. Effect of Forage Removal Time under Varied Environments

Abstract

Red clover ( Trifolium pratense L.) grown for seed in western Oregon must have the forage removed in spring so that flowering and seed maturation will be more uniform. There is little available information that describes the effects of time of forage removal on red clover seed production. This study was conducted to determine (i) the effects of time of forage removal on seed production of red clover grown under a range 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 red clover seed production fields throughout a six‐county area in the Willamette Valley of Oregon. Forage was removed once per treatment and there 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 be adjusted for the estimated combined effects of factors that influence the relative productivity of each site. Relatively low yielding sites 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 removal as are Oregon‐adapted ecotypes. Forage removal time is a critical factor that needs to be considered to achieve maximal seed yields.