The paper explores the impact of electromagnetic stimulation of Ulstar alfalfa seeds on fresh mass yield, the quantum efficiency of the plants’ photochemical reactions, and the content of photosynthetic pigments in leaves. Before sowing, seeds were subjected to electromagnetic stimulation in the following configurations: control (C), no stimulation; stimulation with He–Ne laser light with the wavelength of 632.8 nm, surface power density of 3 mW·cm−2 and exposition time of 1 min (L1) or 5 min (L5); stimulation with alternating magnetic field with the induction of 30 mT and exposition time of 1 min (F1) or 5 min (F5). It was established that the variability of alfalfa yields at the onset of anthesis was dependent on weather conditions as well as, above all, on the electromagnetic stimulation employed. At the same time, the values of fresh mass yield of alfalfa and photosynthetic efficiency were higher in the first year of cultivation (2014) compared to the last year (2016). In terms of photosynthetic efficiency, the best results were observed for L1 and F5, respectively: 0.801 and 0.800. The significantly highest values in terms of chlorophyll a and b and carotenoid content were observed in 2014 at the onset of budding in the combination involving alternating magnetic field stimulation (F5), and were, respectively, 30%, 28% and 73% relative to the control