Alfalfa is a tough, drought-tolerant perennial crop with rooting depths down to eight feet or more. Depending on deep soil profile moisture during drought, some varieties can move into a dormancy and survive up to 45 days without rainfall. However, forage yields drop dramatically since alfalfa requires about 6 inches of root available water to produce 1 ton of forage per acre.
So, what irrigation strategy is recommended for alfalfa under extreme moisture stress prior to the first cutting? Remember that once perennial fields lack spring moisture, it is hard to catch up with irrigation during the hot summer season due to high ET (evapotranspiration) water needs.
Also, stressed alfalfa, which may be dropping leaves prematurely, may be cut before the recommended one-tenth bloom development stage to salvage yield. This timing disruption may impact target irrigation, which is usually applied, just prior to the first cutting to increase regrowth tonnage for the next cutting.
Moving forward, an advantage of early alfalfa cutting is that water use will drop sharply; because transpiration is lower when the leaf foliage is removed. Following the first cutting, irrigation might be delayed slightly in fields where weeds could respond quicker to the moisture than alfalfa regrowth.
Otherwise, full water irrigation or rainfall target amounts will be typically six to seven inches of water for each subsequent cutting. Spring water usage may only be one-quarter inch per day but will rapidly increase as summer temperatures rise. Peak water alfalfa usage in July and August can extend beyond the normal one-third inch per day (or two inches per week) to a half inch per day during extreme stress times. — Todd Whitney, Nebraska Extension
