- Red clover (Trifolium pratense L.) seed yield can be affected by plant growth regulators (PGR) and irrigation; however, the effects of these factors on physiological maturity (PM), harvest maturity (HM), and seed quality are unknown. The objectives of this study were to: 1) determine how irrigation and trinexapac-ethyl (TE, a PGR) affect PM, HM, seed viability, and seed vigor of red clover at different stages of maturity, 2) evaluate the effect of irrigation, TE and their interaction on seed yield, its components, and the quality of red clover seeds at harvest, 3) investigate changes in gibberellic and abscisic acid contents in red clover during seed development and maturation, and 4) determine the potential of red clover seed storability under different storage conditions over two years. A field study was conducted over a two-year period at Hyslop Research Farm, Corvallis, Oregon. A single irrigation was applied at first flowering stage (BBCH 55). Five rates of TE, ranging from 0 to 700 g a.i. ha⁻¹, were applied at stem elongation and bud emergence stages (BBCH 32 and BBCH 51, respectively). Seed viability and vigor tests were conducted at Oregon State University Seed Laboratory to measure the effects of treatments on seed quality.Irrigation delayed PM by four days compared to the non-irrigated treatment. The TE applications did not alter seed maturation. At PM, the flower heads contained light brown petals with brownish-green sepals and seeds were pale green to pale yellow. Heads at HM contained dark brown petals and sepals, whereas seeds turned to yellow or yellow-dark grayish purple. Seed dry weight did not change significantly from PM to HM. Seed moisture content at maximum seed dry weight (PM) ranged from 340 to 540 g kg⁻¹ and decreased to below 140 g kg⁻¹ at HM. Seed quality as determined by tetrazolium (TZT), standard germination (SGT), and cold tests (CT) were gradually increased during seed development and maturation. The accelerated aging test (AAT) was not a reliable indicator for evaluating vigor of young seeds. At HM, seeds reached maximum quality for all treatments, with 92 - 98% viability by TZT and SGT, and 90 - 94% vigor by CT. Seed yield was increased by irrigation and TE application, but the interaction between these two treatments was not significant. Irrigation increased seed yield in both years by 10% due to the greater seed weight. However, TE increased seed yield by up to 18% only when applied at stem elongation stage in the second year. The increase in seed yield by TE was attributed to greater number of heads per stem. Neither irrigation nor TE had significant effect on above-ground biomass or stems m⁻². Seed viability and vigor were slightly correlated with thousand-seed weight and stems m⁻², respectively. However, none of them significantly affected seed quality. The study revealed that seed yield can be increased by: 1) a single irrigation application during first flowering stage (BBCH 55) in both years; and 2) TE application at a rate of 280 g a.i. ha⁻¹ at the stem elongation stage (BBCH 32) in the second-year stand of red clover. Gibberellic acid (GA₃) and abscisic acid (ABA) are two major phytohormones that affect seed germination. Changes in the contents of GA₃ and ABA from seed development to maturation was conducted using seeds from untreated, TE-treated, irrigated, and TE plus irrigated plots. The GA₃ and ABA were extracted from seeds using the solid phase method and were quantified by the liquid chromatography-tandem mass spectrometry (LC-MS MS). The ABA content was high (1242 pg g⁻¹ DW) at the early stage of seed development, and then gradually decreased to 388 pg g-1 DW at HM. The GA₃ content did not change significantly during seed development until HM, ranging from 173 to 187 pg g⁻¹ DW. Irrigation and TE application did not significantly affect the endogenous production of GA₃ and ABA in the seeds. The ABA:GA₃ ratio was high (6.7) at the early stage of seed development, but seed germination was low (24%). When seeds reached HM, the ABA:GA₃ ratio dropped to 2.2 and seed germination increased to 93%. These results suggest that physiological dormancy is not a substantial concern in red clover seeds. However, before scarification, seed with hard seed coat at HM was approximately 34%. Hard seeds were scarified before conducting the germination tests. Maintaining seed quality during storage is essential to ensure value until the time of planting. Two red clover seed lots, untreated and field treated with TE, were stored for 24 months in three conditions: 1) uncontrolled environment of open warehouse (WH), 2) controlled room temperature (RT) at 20°C, and 3) controlled cold storage (CS) at 10°C. Seed quality, i.e., viability and vigor, was determined at 6-month intervals to measure the rate of deterioration after each storage period. Relative humidity (RH) was observed as 55% in RT and 90% in CS. Average seed viability of both seed lots stored in WH and RT and were 96% and 95%, respectively, throughout the 24-month storage period. Seeds stored at RT for 24 months maintained high vigor of 87% as determined by the AAT, whereas seeds stored at WH maintained vigor of 81% for 18 months and then dropped to 67% at the end of the 24-month storage period. In CS, seed viability and vigor gradually dropped, reaching 0% at the end of the 24-month storage period due to the adverse effect of the high RH (90%) in the CS. Seed maintained acceptable viability and vigor standards of above 80% when seed moisture content was less than 10%. This study suggests that red clover seeds from untreated and TE-treated plots can be stored safely under similar WH conditions used in this study for 18 months and in RT for 24 months when the initial seed moisture content is under 10%. The results of this study improved our understanding of the potential storability of the red clover seed in response to TE application.