All areas in Oklahoma are dry enough to irrigate alfalfa from time to time. So, if irrigation facilities are available, consider putting them to work.   

Alfalfa plants continue to grow a little even during winter. While they do not put on a lot of top growth in the winter, the roots are growing, and plants are producing and storing energy needed for winter and for next spring's growth.   

A good rule-of-thumb says alfalfa needs about 6" of water available to its roots to produce 1 ton of dry matter per acre. Many alfalfa soils have a water holding capacity of nearly 2" per foot of depth. If the rooting zone is 6' deep, 12" of water can be stored at any one time. Some good alfalfa soils in Oklahoma have greater storage capacities because of great depths.   

The long term average rainfall for December, January, and February combined is only 3" to 6" (depending on the part of the state). If alfalfa fields are not irrigated or we receive some unusual storms, we will start out next years growing season in a water deficit.   

The economics of alfalfa irrigation is complex, but irrigation does not necessarily always pay for itself in increased hay production. Improper summer irrigation can reduce yields and stand life.   

Irrigation during the late fall, winter, and early spring is easier than irrigation during the summer. When soil temperatures are cool (below 60⁰) there is little danger of excess water causing root rot. There is also much less danger of scald during the cool seasons.   

Producers irrigating small seedling alfalfa should avoid standing water during hard freezes. Hard freezes, alternating with warming periods, can cause seedling plants to heave out of the ground. This is particularly true for the small seedlings that most of us have this year. This should not be a problem with the dry soils that are common throughout most of the state.   

During alfalfa's peak production periods, it uses more than 0.3" per day. Most irrigation systems in Oklahoma are not designed to put out water fast enough to start irrigating alfalfa in the summer and catch up while plants are actively growing. Even if the system can deliver 2" per week for three weeks, this amounts to 6" total between harvests. That is only enough to produce 1 ton per acre per harvest with minimal rainfall.   

Other problems with summer irrigation (especially in the central and eastern parts of the state) are summer rains following irrigation. Once the soil is at field capacity from irrigation, a rain makes water stand and delays harvests. Standing water in summer kills alfalfa, and delayed harvests reduce forage quality.   

Because of the low probability of heavy rains following irrigation, alfalfa irrigation in western parts of the state during summer is easier than in the east.   

Irrigation during the fall and winter will almost always pay for itself in increased yields with a greatly reduced possibility of hurting stand life.   

Various estimates indicate that 10 to 25% of the alfalfa in Oklahoma is irrigated. That is a relatively small portion of the acreage but correctly irrigated areas produce much higher yields.   

Obviously, one problem associated with winter-time irrigation is freezing pipes. Pipes should be drained each time the temperature is going to get very low, but most systems automatically drain above ground pipes well enough to avoid damage.