Managing Alfalfa Under Pivot Irrigation

Published on Tue, 04/01/2014 - 9:21am

Thanks to its reputation for being a deep-rooted, drought-tolerant perennial with a long growing season, it might appear that alfalfa can get along just fine without irrigation. That would seem especially true when you consider that in deep, well-aerated soil, alfalfa roots can extend to depths of eight to 12 feet.

 

Truth be told, however, alfalfa requires more water than most other crops grown in the U.S., particularly in the Midwest. When soil water is sufficient, alfalfa grows in direct relation to the temperature and sunlight available.

 

According to Danny Rogers, Kansas State University Extension Specialist, Irrigation, the average seasonal water need for alfalfa is four to five acre-inches per ton of production.  During the summer months, the water use is six to seven acre-inches per ton. However, the fact that alfalfa is drought tolerant means that it can also use up to 70 percent of available soil moisture without undue stress or loss of production.

 

“If stressed beyond this limit, it will merely stop growing until soil water is available,” Rogers says. “There are limits, but plants recover from extremely dry periods. Production suffers, but the crop survives. Consequently, alfalfa can be a good crop for irrigators with limited water supplies.”

 

Rich Knight and his dad, Rick Knight, who farms around 1,000 acres in partnership near White Swan, Washington, certainly know the importance of irrigation on alfalfa. Although corn is an important cash crop for the Knights, their main commodity is alfalfa, which is marketed directly to a nearby dairy. Corn and wheat only serve as rotation crops for the 600 acres that are devoted to hay.

 

“Because we only get about seven inches of rain a year, everything is under irrigation,” Rich explains. “We have three electric mini-pivots and three T-L center pivots — one sweep that covers 80 acres; a regular circular pivot, and a corner pivot that covers 146 acres, while navigating around two houses. The rest of the farm is covered with wheel lines.”

 

Unfortunately, the lateral-moving wheel lines are much more labor intensive than the pivots; and fields watered with wheel lines can’t be rotated to corn, due to the lack of clearance.

 

“We generally rotate a field out of alfalfa every five to six years,” says Rich, noting that they normally get five cuttings per season off of each field. “At that point, we’ll rotate the fields with center pivots to one year of corn and one year of wheat before going back to alfalfa. On fields with wheel line irrigation, we’ll go with two years of wheat. That’s another reason we’d like to add at least three more center pivot units.”

 

Rich says center pivot irrigation is also preferable for applying fertilizer to both their corn and alfalfa crops. In fact, the Knights use chemigation on every field that has a pivot. 

 

“On our alfalfa, we’ll take soil samples in the spring and then put on about 20 gallons per hour of custom-blended fertilizer until the recommended levels have been met. The key is getting it spread evenly,” Rich continues. “Of course, that’s one of the benefits of the continuous movement,” he adds, comparing the non-stop movement of his hydraulically driven T-L pivots to those driven by electric motors. “You wouldn’t think of going through a field with a tractor and fertilizer spreader and stopping and starting every few feet; yet that’s what an electric pivot is doing.”

 

According to K-State’s Danny Rogers, the even distribution of irrigation water can be just as important. Water-logged soils from over-irrigation or too much rain can quickly injure alfalfa, and it can encourage weed growth, especially if it occurs right after harvest. Rogers notes, too, that only the top three to four feet of the root zone are considered when irrigating alfalfa. Water below this depth is used, but roots remove water where it is most abundant in the root zone. The upper half of the root zone contains more than half the roots, and about 80 percent of the water will come from this area. Research has shown that if water is readily available to at least half the roots, plants experience little or no stress. As a consequence, if water is available in the upper half of the root zone, little will be used in the lower half.

 

The challenge is timing water applications so they do the most good and do the least amount of damage. Rapid regrowth depends on adequate soil water. Hence, watering prior to harvest or immediately after is the best time to promote rapid growth. On the other hand, soil compaction can occur if the field is watered too close to the harvest date. 

 

“A firm, dry surface is best for traffic and field drying,” Rogers says. “Yet, watering immediately after harvest stimulates growth of existing weeds. Rapid regrowth depends on adequate soil water; and as regrowth occurs, the use rate increases rapidly toward that of a mature plant,” he continues. “A normal use rate for midseason in the Midwest is 0.35 inch per day, or about an inch of water every three days.”

 

If the irrigation system has limited capacity, as most center pivots do during midseason, or rainfall is limited as it is on the Knights’ farm, the options may be more limited. Watering may need to continue until just two or three days before hay harvest and restart as soon as the hay is removed from the field. While irrigation shouldn’t be started until regrowth has begun in order to prevent weeds from germinating, some producers have found it beneficial to make a light water application on the first pass after harvest to give the crop a good start.

 

Bert Weststeyn, who switched from flood irrigation to T-L linear irrigation systems on his Willows, California, farm a dozen years ago, insists it’s not only easier to establish a stand of alfalfa under a sprinkler system, but it doesn’t force him to make a choice between hay quality or volume like flood irrigation did.

 

“It’s a lot easier to control hay quality when irrigating with a linear system than it is when flood irrigating,” he points out. “A linear lets me be much more flexible.

 

“For example, say I want a little more tonnage in a cutting. If I attempt that by flood irrigating, I have to apply nine inches of water an acre, then wait an average of 14 days to harvest it for either hay or haylage.”

 

On the other hand, he’s learned he can cut back the linear application to a half-or three-quarter-inch an acre, and still begin harvesting two or three days later. Or, he can decide to put on another inch or so of water and cut four or five days later. All total, he applies one-and-one-quarter inches of water seven times after each cutting to produce an eight-plus tonnage annual yield of hay.

 

However, depending upon average rainfall, evapotranspiration rates and soil type, it may still be necessary to build up a moisture reserve in alfalfa fields before the season begins. Glenn Shewmaker, extension forage specialist with the University of Idaho Twin Falls Research and Extension Center, recommends alfalfa producers irrigate early to fill the root zone.

 

“Pivots should be slowed down to the point of a little runoff to maximize the depth per irrigation,” he explains. “This is important to have healthy roots in deep soil to take advantage of the soil’s water-holding capacity. A deep soil capacity can be used for alfalfa growth when irrigation is halted for harvest or when application rate does not keep up with evapotranspiration.” 

 

So how do you know when and how much water to apply? Rogers notes that for optimum growth, it’s important to maintain soil water content in the effective root zone between 50 to 70 percent of the available water holding capacity.  For effective irrigation management, monitor soil water status and couple this information with crop water needs and system capacity.

 

One method is to go by crop appearance and proportion of growth. When adequate water is available, alfalfa will normally be light green in color. As moisture stress develops, the color darkens to a dark green or blue-green. Apply water when the plant begins to turn color, before wilting occurs, otherwise yield and quality will be reduced. Wilting generally will occur when about 25 to 30 percent of the available water capacity remains in the root zone. 

 

Producers can also estimate the irrigation water requirement based on crop water requirements, soil characteristics and proportion of crop growth that has occurred. Also consider weather conditions and precipitation. For example, if a yield of one ton per acre is expected, about six inches of water will be required for the cutting. To estimate how much water is needed to refill the profile at any given time, estimate the proportion of growth that has been made. If 50 percent of the growth has occurred, 3.0 inches of water will have been consumed.  The net application amount will be 3.0 inches, which will require a gross application of 3.5 inches if the system efficiency is 85 percent. 

 

“With linear irrigation the greater uniformity of water application is readily evident to the eye,” says Weststeyn. 

 

Other producers rely on soil moisture sensors, a tensiometer, a “hand feel” method or simply the amount of resistance on a steel rod pushed into the soil. Whatever the method, it’s important that the crop receives the amount of water it needs.

 

The good news is that the deep-rooting habit and drought tolerance of alfalfa does not make the irrigation schedule as critical as it is for other crops. The bad news is that maximum production may be lost if the timing or amount of irrigation water aren’t adequate to match the crop’s potential. 

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