Additional information on Mg status of temperate-origin forages under cool temperatures is needed if economic losses from grass tetany are to decrease. A greenhouse study was conducted to determine temperature and Mg fertilizer effects on herbage Mg concentration of one or more cultivars of tall rescue (Festuca arundinacea Schreb.), timothy (Phleum pratense L.), reed canarygrass (Phalaris arundinacea L.), smooth bromegrass (Bromus inermis Leyss.), orchardgrass (Dactylis glomerata L.), redtop (Agrostis gigantea Roth.), perennial ryegrass (Lolium perenne L.), Kentucky bluegrass (Poa pratensis L.), alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), ladino and white clover (T. repens L.), alsike clover (T. hybridum L.), birdsfoot trefoil (Lotus corniculatus L.), and crownvetch (Coronilla varia L.).
Plants were grown in Hagerstown soil, which belongs to the mixed, mesic family of typic Hapludalfs, in 4-liter pots under natural daylength at cool (X̄ = 14 C), intermediate (X̄ = 18 C), and warm (X̄ = 26 C) ambient temperatures. Two levels of fertilizer Mg were used (0 and 672 kg Mg/ha as MgSO4). Grasses received 1,300 kg N/ha as NH4NO3 and both grasses and legumes were supplied K2SO4 to furnish 600 kg K/ha over a 13-month growing period. Herbage was harvested at 2 to 3-week intervals at a height of 5 cm.
Under cool temperatures, in both spring and autumn, and with added Mg, cultivars having high Mg levels were ‘Timfor’ timothy, ‘Nordstern’ orchardgrass, and ‘Viking’ birdsfoot trefoil. Species and cultivar rankings were similar for spring and autumn but Mg concentration was greater in autumn, indicating that cool temperatures, per se may be less limiting to Mg accumulation than previously thought.
Marked differences, five to seven-fold, in Mg concentration occurred between species and cultivars of both grasses and legumes in response to Mg fertilizer. Low K/(Ca + Mg) ratios were obtained when the grasses were harvested under cool autumn temperatures. Yields were unaffected by Mg fertilization.
The consistency of cultivar ranking for Mg levels over a wide temperature range and different soil Mg levels, suggest that cultivar selection for Mg accumulation would be rewarding. Cultivars inherently efficient in Mg uptake, especially at cool temperatures, may help prevent grass tetany.