Objective To study the effects of two plant growth regulators sprayed on alfalfa on seed yield and yield components, endogenous hormones and carbon assimilates, with the aim of providing theoretical basis for the study of the physiological mechanism of alfalfa pod abscission as well as the cultivation of alfalfa with high and stable yield.

Methods Medicago sativa cv. Xinmu No. 4 was used as the material, and four concentrations of Uniconazole (S₃₃₀₇) and Diethyl aminoethyl hexanoate (DA-6) were sprayed at the bud stage of plants to determine the agronomic traits, seed yield, endogenous hormones and carbon assimilates of alfalfa. The agronomic traits of alfalfa, seed yield and the content of endogenous hormones and carbon assimilates were determined, and the effects of S₃₃₀₇ and DA-6 on the physiological metabolism and seed yield of alfalfa were clarified.

Results Compared with the control, spraying S₃₃₀₇ and DA-6 significantly increased the number of alfalfa florets, number of pods, seed yield, and significantly reduced the rate of alfalfa pod shedding. And the seed yield was the highest at spraying concentrations of 100 mg/L S₃₃₀₇ and 60 mg/L DA-6, which were 419.37 and 474.24 kg/hm², respectively; and the lowest rates of flower pod shedding were 28.67% and 28.00 %, respectively. The content of endogenous hormones (ABA, GA₃) and carbon assimilates (starch, soluble sugar) in alfalfa leaves and flower buds increased with the increase of spraying concentration.

Conclusion Spraying DA-6 to improve alfalfa seed yield was superior to S₃₃₀₇, with a 9.67% reduction in flower pod shedding and an 88.31 kg/hm² increase in yield.