James W. Bauder

Alfalfa is a relatively drought tolerant, deep rooted perennial crop with relatively high water use requirement. These facts offer some significant information about alfalfa irrigation management strategy. Because alfalfa is a deep-rooting  perennial, it has the ability to survive long periods between irrigations. Maximum production of high-quality alfalfa can be obtained, however, only by timely application of proper amounts of water. Improper irrigation (too much or too little water) is the primary cause of low yields of alfalfa, aside from cold weather.

Water Use Characteristics

Alfalfa is a high water use crop because it has a long growing season, a deep root system, and a dense mass of vegetation. The amount of water needed is governed by temperature, wind, humidity, and the amount and intensity of light. However, the irrigation requirement for alfalfa is governed by rainfall and the water holding capacity of the soils growing the crop. Commonly cited ranges in water requirements for alfalfa are 18 to 36 inches of water per season.

Alfalfa is quite tolerant of drought or extended periods without highly available water. As much as 50 to 65% of the available soil moisture can be removed between irrigations early and late in the season, but only 35 to 50% removal will give better results during high water-use periods. Maintaining higher levels, especially with slow-draining soils, may cause loss of stand and competition from grass invasion. Texture and depth of soil have little influence on water requirements of alfalfa, but these factors affect the amount of moisture which can be stored in the soil. Naturally, the amount of moisture stored affects frequency of irrigation and the amount to apply at each irrigation.

Root Characteristics

Individual alfalfa taproots may exceed 20 feet in length but the active feeder roots are located closer to the soil surface. The effective rooting depth is about 6 feet where no restrictive layers or conditions exist. When provided with a satisfactory level of moisture, an alfalfa plant will extract moisture as follows:

 
Depth of Roots
Percent of Total Water Extracted Coming From that Depth
0-1.5'
40%
1.5-3.0'
30%
3.0-4.5'
20%
4.5-6.0
10%

Irrigation Scheduling

One of the most practical methods of scheduling irrigations involves estimating plant water use by measuring evaporation from a standard evaporation pan. Practical experience has shown that on-farm scheduling can be approximated by measuring the water use from a wash tub. Alfalfa water use and evaporation is approximately on a 1:1 ratio (one inch is used by the crop while one inch evaporates from the pan). Under surface irrigation the water-holding capacity of the soil must be known to use this method. This system works particularly well with well-designed sprinkler irrigation systems since it is easy to compute the amount of water applied.

On low water holding soils, hay cutting periods and irrigation scheduling take extra planning. Quick hay removal is needed because most sprinkler systems are not designed to allow many days of shutdown. Water use is somewhat less immediatel after cutting but quickly reaches normal levels as regrowth progresses.

Under most conditions, an irrigation just prior to cutting will increase tonnage for the succeeding crop by allowing faster regrowth. Under surface irrigation methods, good furrows are needed for efficient irrigation when applying water at this stage. Remember that late fall irrigation may be desirable on low water holding soils because dry soils during winter often cause stand declines.

Source: EM3418, Washington State University , 1978.