Economic losses associated with grass tetany either of death or poor growth performance of livestock are a growing concern. Breeding of high magnesium (high-Mg) cool-season forage grass (C3) has been a challenging process to minimize the hazards of grass tetany. For appraising the breeding of high Mg-containing cultivars in Japanese Andisol, agronomic properties and grass tetany risk of high-Mg cultivars were compared with commercial cultivars. The high-Mg cultivars of Italian ryegrass (Lolium multiflorum L.), tall fescue (Festuca arundinacea Schreb.), and orchardgrass (Dactylis glomerata L.) were “Magnet”, “HiMag” and “Mgwell”, respectively. The commercial cultivars were viz., Ace, Tachiwase, and Waseyutaka of Italian ryegrass, Hokuryo, Kentucky-31 (Ky-31), and Fawn of tall fescue, and Akimidori and Okamidori of orchardgrass, respectively. Grasses were grown in temperate Andisol under field conditions with standard management practices and were harvested four times during the year. The average plant heights of the high-Mg containing cultivars were lower than the commercial cultivars with the relative range of −7.4%, −3.7%, and −1.5% for Italian ryegrass, tall fescue, and orchard grass species, respectively. The seasonal yield of high-Mg cultivars was ranked as Magnet > HiMag > Mgwell. The Mgwell orchard grass had lower potassium (K) content compared with their respective commercial cultivars with a relative range of −2.2%. Across four harvests, a significantly higher calcium (Ca) and magnesium (Mg) content, and lower grass tetany potential were recorded in high-Mg cultivars when compared to commercial cultivars, irrespective of species. Across four harvests, the lowest grass tetany index, [K/(Ca + Mg)] of 1.36 was recorded in Mgwell orchard grass. The high-Mg cultivars showed the lowest [K/(Ca + Mg)] across four harvests compared to commercial cultivars, promoting the effectiveness of breeding cool-season grass species to control grass tetany in temperate regions (Andisol) and climatic conditions.