by James W. Bauder

With the transition from the cool spring weather to warm summer days, irrigators with alfalfa crops are either already irrigating or at least thinking about it. The right combination of irrigation and fertilizer management practices can double or triple alfalfa yields when compared to dryland conditions in Montana. Progressive alfalfa growers and research center studies report yields as high as five tons per acre in a three-cut program of early bloom and after frost harvest. And, average responses are somewhat conservative, since they are the result of a wide range of management levels.


Start with a Full Soil Profile
A good strategy for most irrigators is to begin the growing season with a soil profile which is holding as much water as possible. Some years spring rainfall will provide adequate moisture for most of the irrigated soils around the state, but in dry years, a late fall (after killing frost) or early spring irrigation will serve to fill the soil profile. A full profile at the beginning of the growing season will allow the crop to take full advantage of available water and put on rapid, early season growth when soil and water temperatures are not likely to be high enough to stress the crop and limit production. Harvest management studies indicate that alfalfa produces anywhere from 30 to 60% of its total yield for the year during the first harvest, depending on harvest strategy. Consequently, the ability of the crop to get off to a good start can have a significant effect on the annual yield.


Apply the Right Amount of Water
On the average, irrigated alfalfa will use approximately 4 to 5 inches of water for every ton of hay produced. A well-fertilized, healthy alfalfa crop may use as much as 8 inches of water from the soil in addition to 4 to 6 inches of water supplied by irrigation and rainfall before the first cutting. A North Dakota study showed that irrigators can expect yield increases of 1/2 to 1/5 ton/acre per year for each inch of water applied between late April and late August until irrigation and precipitation water total 24 inches.


Know How Much Water the Crop is Using
Irrigators may not appreciate just how much water can be used by a healthy alfalfa crop. During late April and early May, water use may be only 0.5 to 1.0 inches per week. By early June this can increase to 1.5 inches per week, while during the warmest part of the irrigation season, water use might be as high as 2.0 inches per week. So, when irrigation starts, the best approach is to fill the profile with each irrigation. This will help leach salts downward while also reducing the percent of water lost to surface evaporation. The following table shows estimated monthly alfalfa water use in Montana.

Average Water Use