With the recent and very welcomed rains that we have received, many farmers may be recalculating how much irrigation water they will need for their alfalfa crop and when should they apply it? Studies in Montana have shown that the right combination of irrigation and fertilizer management practices can double or triple alfalfa yields. 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.

A good strategy for most irrigators is to begin the growing season with a soil profile, which is holding as much water as possible. 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 so high as 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.

Amount of water to apply: 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 alfalfa increases of 1/6 to 1/5 ton per acre per year for each inch of water applied until irrigation and precipitation water total 24 inches between late April and the latter part of August.

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.

In addition to the amount of water available to the crop, several irrigation related factors affect alfalfa production. These factors are:

  • Temperature of the irrigation water
  • Salinity (salt level) of the irrigation water
  • Salt builds up in the soil
  • Air temperature and growth stage of the alfalfa
  • Age of the alfalfa stand
  • Amount of water applied
  • Length of time the alfalfa field is flooded
  • Length of time flood irrigated: Length of flooding time and irrigation water temperature can have an effect on the growth and survival of alfalfa. Irrigators frequently question the effect of extended periods of flooding on alfalfa growth. It is common in irrigated graded or dead level basins or in graded panels or border dikes for the alfalfa in some parts of the field to be under water for an extended period of time during and after irrigation. Alfalfa stand longevity can be shortened considerably by prolonged flooding. Alfalfa is very sensitive to excess soil water. In addition, air temperature and temperature of the irrigation water appear to significantly affect alfalfa stand longevity.

Several studies have compared the growth of roots and forage of alfalfa flood irrigated for various lengths of time and at different temperatures. In one study, water was allowed to stand on an alfalfa crop for 0, 4, 8, 12, 16, and 20 days at temperatures of 60°, 70°, and 80° F. In a second study, alfalfa was flood irrigated for 3, 6, 9, or 12 days at a temperature of 90° F. Root and top growth were measured immediately after drainage and then again three weeks after drainage. This three week wait and cut period would be similar to irrigating three weeks before harvesting the alfalfa. The following observations concerning the temperature effects on alfalfa were made:

  • Root growth stopped during flooding three weeks following drainage, the rate of top growth of plants was reduced by 50% with 8 days of flooding at 60°F
  • Top growth was reduced by 50% with 4 days of flooding at 70° F
  • Top growth was reduced by 50% with 3 days of flooding at 80° F
  • Top growth was reduced by 50% with 2 days of flooding at 90° F
  • Plants stopped growing or died under the conditions of 14 days flooding at 60° F, 10 days flooding at 70° F, 8 days flooding at 80° F, 6 days flooding at 90° F.

Yields of alfalfa can be significantly reduced by wet or flooded soil conditions. Wet soil conditions can reduce stand vigor and promote fungal root infection. These reduced yields are the result of direct interference with normal plant physiological processes. Soil temperature appears to be more critical than air temperature, when irrigating during midsummer.

Irrigators should avoid prolonged flooding of irrigated alfalfa, when the temperature of the irrigation water is high. There are two reasons for this. First, colder water will hold more dissolved oxygen and the cool air temperatures will keep the alfalfa demand for oxygen relatively low. Secondly, the colder water will usually contain less dissolved salt.

If you flood irrigate and are concerned about proper irrigation water management practices, either too much water, to salty water, temperature too high, flooded too long, then look for the following symptoms: 1) reduced stand vigor and loss of alfalfa, replaced by grass, especially where water is turned into the field or in low spots in basins; 2) yellowing and loss of the lowest leaves on the plant, followed by progressive loss of leaves up the stem; 3) rotted taproot and/or development of many fine, short, lateral surface roots; 4) progressive reduction in alfalfa tonnage; 5) the development of fine, new leaves at the axis on the stem where leaves have fallen off.

Several things can be done to minimize the potential damage caused by long duration flooding. Irrigate early in the season, when both the water and soil are relatively cold and the plants are not growing vigorously. Move water across fields quickly and avoid prolonged ponding, both at low spots and at turnouts. Insure uniform water distribution and apply only the amount needed to fill the profile; remember that under even the driest conditions, few soils will be able to hold and store more than 6 to 8 inches of water. Obtain a detailed map of the soil in your field and manage water to minimize ponding on the fine textured, poorly drained soils, i.e., the silty clay and silt loams. At low spots, where water tends to accumulate, divert water away or off, to avoid ponding. If you must irrigate when the soil or water temperature is high, i.e., above 65 to 70° F, do it immediately after cutting, and put on only the necessary amount of water.

Although most of this research comes from Montana and South Dakota it’s basic principles are still relevant to our local area. Hopefully, this information will help producers make better decisions on when and how much to irrigate their alfalfa in order to maximize their yields.

Source: Ag Notes - Montana State University - IRRIGATED ALFALFA PRODUCTION, AGRONOMY NOTES NO. 103

Jim Sloan 2009, How Much Water Does Alfalfa Need?, Extension | University of Nevada, Reno

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