With the transition from spring to summer and warm days, irrigators with alfalfa crops are either already irrigating or thinking about irrigation. The right combination of irrigation and fertilizer management practices can double or triple alfalfa yields, 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 a result of a wide variety of management levels, from best to worst.
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 late fall (after killing frost) or early spring irrigation would be a benefit in some years, but spring rainfall this year most likely filled most of the irrigated soils around the state. 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 to1.0 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.
|
Table 1. Estimate of approximate monthly alfalfa water use in Montana based on long term climatic conditions. |
|
|
Month |
Average water use |
|
April |
1.7-1.8" |
|
May |
4.3-5.4" |
|
June |
5.8-6.5" |
|
July |
6.9-7.7" |
|
August |
6.1-7.3" |
|
September |
3.2-4.2" |
|
October |
0.5-1.3" |
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 build 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 60xF
§ top growth was reduced by 50% with 4 days of flooding at 70x F
§ top growth was reduced by 50% with 3 days of flooding at 80x F
§ top growth was reduced by 50% with 2 days of flooding at 90x F
§ plants stopped growing or died under the conditions of 14 days flooding at 60x F, 10 days flooding at 70x F, 8 days flooding at 80x F, 6 days flooding at 90x F
Yields of alfalfa can be significantly reduced by wet or flooded soil conditions. High water tables which accompany spring thaws and exceptionally heavy rains can inhibit alfalfa growth. Wet soil conditions can reduce stand vigor and promote fungal root infection. Periodic inundation can create short term flooded conditions which inhibit growth. 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 mid summer.
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.
Three factors include: flooding itself, duration of flooding, and temperature during flooding can significantly affect vigor and yield of alfalfa. Symptoms of flooding damage in this study were yellowing and subsequent death of leaves, beginning with leaves lowest on the stem and progressing up the plant. At 60 and 70xF the progress was gradual; at the higher temperatures, the whole plant turned yellow after only a few days. Yellowing continued to appear even after the soil was drained. In all cases, the plants had a drastically reduced future yield potential by the time yellowing appeared.
Flooding also caused death and rotting of the taproot. The taproot was generally intact immediately following drainage, but at 5 weeks after drainage, rotting of roots had occurred on all flooding treatments. In severely damaged plants, the taproot was completely gone rather than partially rotted, and the root system consisted of adventitious (surface, branching) roots sprouting from the crown.
Alfalfa yields 3 weeks after flooding showed that while top growth may have been normal for some time during the flooding treatment, the plants were being damaged by the flooding from the beginning. All flooded plants showed yield loss compared to non‑flooded plants at 3 weeks after drainage. The actual yields decreased as the period of flooding increased: the yield loss increased proportionally to the degree of flooding.
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 70x F, do it immediately after cutting, and put on only the necessary amount of water. Concentrate your irrigated alfalfa production on fields with a slight slope and good drainage, i.e., the find sandy loam soils.
Categories: Alfalfa, Irrigation Management
Date: 1997