In Oklahoma, irrigating alfalfa is more art than science, but adhering to certain practices makes irrigation profitable. During peak production periods, alfalfa uses water at the rate of more than three-tenths of an inch per day. Many irrigation systems in Oklahoma are not designed to meet water demands fast enough to start irrigating alfalfa in the summer and catch up while plants are actively growing.

If a system can deliver two inches of water per week for three weeks, this amounts to six inches of water between harvests. Alfalfa needs about six inches of water available to its roots to produce a ton of dry matter. Based on this rule-of-thumb, two inches per week is only enough water to produce one ton per acre per harvest with minimal rainfall.

Table 9-1 shows the normal intake rate and storage capacity for typical alfalfa soils in Oklahoma. To estimate the total available water storage capacity of a given soil, multiply the number in the water storage capacity (right column) by the depth of the rooting zone. Many alfalfa soils have a water holding capacity of more than two inches per foot of depth. Soil with a rooting zone six feet deep can store 12 inches of water. Some good alfalfa soils in Oklahoma have greater water storage capacities because of greater depth. If the crop depletes the available water content, it will become stressed to such a degree that growth is stunted. Normally, irrigation should occur when no more than 60 percent of the available soil water has been depleted from the effective root zone. This means that the crop should be irrigated after 5-6 inches of water have been used. The net amount of water to be replaced at each irrigation should be equal to the amount that has been used.

The period of time that elapses before this amount of water is used varies during the season, according to weather conditions. If the peak daily water use during the growth cycle is 0.29 inches per day, five inches of water should be sufficient to supply alfalfa for the first 18-22 days after harvest and removal of the hay, without subjecting the crop to significant water stress. An additional 2.5 inches of irrigation water is normally needed to see the crop through to cutting time, unless some rainfall occurs. Smaller, more frequent irrigations may be applied; however, this normally leads to inefficient water use. This is because immediately after irrigation the water use rate is elevated due to surface soil and vegetation wetness. The more often irrigation occurs, the more significant this excess evaporation becomes.

Various estimates indicate that 10-25 percent of Oklahoma’s alfalfa is irrigated. That is a relatively small portion, but correctly irrigated fields produce much higher forage yields. Alfalfa irrigation in western parts of the state during summer may be profitable because of the low probability of heavy rains following irrigation. Irrigation during the fall and winter will almost always pay for itself in increased yields with little possibility of hurting stand life.

Irrigation systems may be sized to meet peak water use demand of the crop during the driest period of the growing season. Systems may be sized to supply the complete water needs of the crop, assuring maximum production even during sustained drought conditions. More commonly, they are sized to supplement normal growing-season rainfall and assure maximum potential production in approximately six years out of ten. An irrigation system designed to supplement normal rainfall has a smaller water supply and smaller equipment components than a system that can supply total water needs, and is normally less expensive.

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Alfalfa's total water demand peaks in July at an average daily demand of about three-tenths of an inch per day. For an irrigation system that operates 18 hours a day with a 75 percent application efficiency, a water supply of nearly 10 gallons per minute must be available for every acre to be irrigated. This means that to meet the total water requirement of 40 acres of alfalfa when there is no rainfall, 400 gallons per minute must be supplied by the system. Most irrigated farms are larger and require proportionately higher capacity.

It is important to fill the soil profile in late winter while alfalfa is dormant and before soil temperatures warm to above 60^oF. This is important because it is nearly impossible to catch up during the growing season. Watering for three weeks between cuttings for a two ton per acre yield requires over three inches of water per week and may result in standing water, which is potentially damaging to alfalfa plants during the summer. Saturating soils when the soil temperature is above 60^oF should be avoided. This is a condition favorable for the development of phytophthora root rot, which hastens stand decline.

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The economics of alfalfa irrigation are complex, and irrigation does not necessarily always mean increased hay production and profits. Table 9-2 contains the most important input and output factors dealing with irrigation economics and illustrates how some of the variables work together. Any particular farm may have a set of factors somewhat different from those shown and the costs and returns can be recalculated. Not surprisingly, the data show that benefits from irrigation increase as effective rainfall decreases. Situation five (wet) with 40 inches of rainfall lost yield with irrigation. The dry situations (one and four) increased yield greatly, and irrigation was profitable.

Table 9-2 shows that returns above the specified costs with irrigation may vary from $105 per acre to $765 per acre. Returns without irrigation may vary from $84 per acre to $533 per acre. Both ranges are wide because of the influence of many different cost and benefit factors.

Summer rains following irrigation cause problems, especially in eastern parts of the state. Once the soil is at field capacity from irrigation, rain makes water stand and delays harvests, leading to reduced forage quality. In addition, standing water when soil temperature is higher than 60^oF promotes root rot.

Improper summer irrigation can increase weed problems and reduces yields as well as stand life. Average rainfall for December, January, and February combined is only 3-6 inches (depending on the part of the state). If alfalfa fields are not irrigated during the dormant season, the next season begins in a water deficit. Irrigation during the late fall, winter, and early spring is easier than irrigation during the summer. When soil temperatures are cool, there is little danger of excess water causing root rot. There is also much less danger of scald during the cool seasons.

Producers irrigating seedling alfalfa should avoid standing water during hard freezes. Hard freezes, alternating with warming periods, can cause seedlings to heave out of the ground.

Obviously, one problem associated with winter irrigation is freezing pipes. Pipes should be drained before temperatures become dangerously low, but most systems automatically drain aboveground pipes to avoid damage.

Many alfalfa fields in Oklahoma are subirrigated naturally by high water tables. Subirrigation of alfalfa can be productive and profitable, furnishing ample supplies of water without wetting the upper rooting regions where root rotting organisms thrive. This is an important factor when selecting a site for a new stand

Soil type also affects the type of irrigation system that can be used to supply water, and the rate at which water is applied. Soil permeability or intake rate determines the rate at which water can be applied, which is largely determined by soil texture. Typical intake rates for various soil textures are given in Table 9-1. Specific values for a given soil type should be determined by infiltration tests or by consulting the USDA County Soil Survey.

With sprinkler systems, the precipitation rate of the system should not exceed the ultimate intake rate of the soil. If the system applies water too rapidly, water will pond on the soil surface and run off of sloping fields. The precipitation rate of a hand-moved or side-roll system is determined by the discharge of  individual sprinklers and the spacing between sprinklers. For a continuously moving system, such as a center-pivot or lateral-move system, the precipitation rate is determined by the individual sprinkler discharge, the sprinkler spacing, and the speed of movement of the system. Alfalfa is adaptable to virtually all types of sprinkler systems. Side-roll and center-pivot systems are the most commonly used systems, but hand-moved and high-volume gun systems may be used also.

Alfalfa can also be irrigated by surface irrigation methods. Border irrigation can be practiced on smooth, level or uniformly sloping fields. Small cross-section furrows, or corrugations, can be used to control the movement of water across less uniform surfaces. To achieve an acceptable level of application efficiency, the permeability of the soil must fall within fairly narrow limits. If the intake rate is too high, the water cannot be spread over any but the shortest length of run before it is completely absorbed. If the intake rate is too low, extremely low flow rates must be used with very long application times to prevent excessive runoff.

Subsurface drip irrigation is a system being used increasingly by alfalfa producers across the country. Because the tubing of a subsurface drip irrigation system is normally buried 12-15 inches below the ground, the soil surface is not wetted appreciably during normal operation. This dramatically reduces evaporation losses and improves irrigation application efficiency to the 90-95 percent range. The spacing of subsurface drip lateral lines depends largely on soil texture, but will usually be from 5-8 feet.

Adequate filtration and frequent system flushing is necessary to prevent blockage of emitters by sediment. Occasional treatment of the system with acid may be necessary to prevent mineral build-up in emitter outlets, and treatment with chlorine may be required to eliminate blockages by biological growths such as bacterial slime and algae. With proper maintenance, subsurface drip systems can work effectively for 10-20 years.

Precise control of the timing and amount of irrigation, improved water use efficiency and low labor requirement make them an attractive alfalfa irrigation option despite their relatively high initial cost. The high application efficiency of these systems make them especially viable in areas with limited irrigation water or where water and pumping energy are very expensive.

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As with most crops, yield increases realized when irrigating alfalfa can be significantly affected by the quality of the irrigation water. The presence of dissolved mineral salts in the water can result in an increase in the energy level required for plants to remove water from the soil pores. Some crops are more readily affected by salinity problems than others. Alfalfa is rated as being moderately sensitive to salinity effects. Alfalfa first experiences a yield reduction when the saturated soil extract measures an electrical conductivity of 2.0 mmho per cm (millimhos per centimeter). This generally corresponds to irrigating with water that has an electrical conductivity of 1.3 mmho per cm. Beyond this level, relative yield is reduced by approximately seven percent for each mmho per cm increase in conductivity, leading to a zero yield when conductivity of the extract reaches about 16 mmho per cm.

Certain mineral elements have toxic effects on crops. Alfalfa is tolerant of the toxic effects of sodium in soil and water. In nonsaline conditions, alfalfa suffers no direct adverse effects from sodium. Exchangeable sodium in excess of 15 percent causes deterioration of the physical condition of the soil. As a result, water infiltration and percolation are greatly reduced. Alfalfa is tolerant of boron content in irrigation water. No visible toxic effects are observed until the boron content of irrigation water reaches three milligrams per liter.

Normally, slightly saline water can be used for irrigating alfalfa without harmful effects because relatively large quantities of water are required and the application of large volumes tends to push the highly concentrated salts down in the soil profile. Irrigation water quality plays an important role in stand establishment. Saline water used to irrigate a new stand can cause serious problems. During germination and emergence, a large volume of water will wash out seeds, but repeated uses of salty water followed by evaporation tends to concentrate the salts in the zone of germination and root elongation. The high salt concentration in the germination zone causes more energy to be needed for imbibition by seeds, and the salts may be toxic to new roots.

Before considering development of an irrigation system or using a source of water of unknown salinity, it is advisable to have the water supply tested to determine its suitability for irrigation purposes. Irrigation water can be tested by the Soil, Water, and Forage Testing Laboratory at Oklahoma State University.