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Key irrigation management practices for alfalfa

Blake Sanden, Blaine Hanson and Khalid Bali Published on 04 March 2014
Blake Sanden
Assuming fertility is not limiting, and the alfalfa stand is not in decline, the difference between an 8-ton and 12-ton-per-acre crop boils down to water management: simply the right amount at the right time.

Practice 1: Know the expected crop water use
The fuel of forage production is carbon dioxide (CO2) assimilation through the stomata on the alfalfa leaves. This provides the carbon base for carbohydrate production powered by photosynthesis and root nutrient uptake.

The more open the stomata, the larger the CO2 uptake, the larger your hay tonnage and the larger your crop water use.

Climate determines your “potential or reference crop evapotranspiration” (ETo) – essentially maximum water use by unstressed pasture for your region.

Since most forage crops are planted dense and cover the ground like a pasture, then it’s natural to assume that their evapotranspiration (ET) would be the same as ETo, and as a first guess this isn’t too bad.

But there are developmental differences due to initial seedling growth and physiology of the particular forage compared to pasture and cutting schedules.

Basically, the crop coefficient (Kc) is the ratio of actual crop water use for a particular stage of growth compared to ETo. We have typical Kc values for the developmental stages of most crops. Crop ET is then calculated as follows:

ETcrop = ETo x Kc x Ef

  • ETo = Reference crop (tall grass) ET

  • Kc = Crop coefficient for a given stage of growth as a ratio of pasture grass water use. May be 0.3 to 1.3 for a given day depending on when the hay was cut. A value of 0.95 is the commonly used value for alfalfa.

  • Ef = An “environmental factor” to account for immature permanent crops, salinity, low fertility, poor stand, etc. Usually 1 for good ground and water.

“Normal year” and real-time ETo values found at CIMIS can be obtained for all California hay-growing regions.

Bottom line
Normal year ET tables are a good guideline for planning irrigations, but actual crop ET can be plus or minus 15 percent. Therefore, you must check soil moisture and irrigation uniformity over the season to maximize yield and efficiency.

Practice 2: Know your soil moisture storage and irrigate to avoid stress or waterlogging
Soil texture will determine how much water you can store in the root zone and, in conjunction with the chemistry of the irrigation water and soil salinity, will affect how fast water infiltrates when it moves over the field.

This “infiltration function” should be considered when you decide what slope you want to level the field to, and how fast and how often you want to run the water to achieve good uniformity of irrigation or recharge across the field.

Simplified soil texture categories
For normal field irrigation scheduling, it is usually sufficient for the producer to identify his soil by four basic types: coarse, sandy, medium and fine.

But the basic way to estimate available water-holding capacity using the length of the soil ribbon you make with your thumb and forefinger is: If the wet soil at least makes a ball but no ribbon, your available water-holding capacity is about 0.4 to 1 inch per foot depth of soil.

Then, for all soils that make a ribbon: Available water-holding capacity (in/ft soil) ~ length of ribbon.

Sum up the available water for the depth of the entire root zone (maybe 5 to 8 feet) and that’s your total available water between irrigations. However, depleting more than 50 percent of this water will usually cause stress to the crop.

Alfalfa flood irrigation interval

Table 1 sums up the estimated available water to a 5-foot depth at field capacity for the 12 official USDA soil textures, assuming that 50 percent is available with little or no stress, and divides that moisture storage by the average daily alfalfa ET for a given month (in the San Joaquin Valley) to estimate the optimal irrigation interval.

Practice 3 – Make your irrigation as uniform as possible
Stress from dry soil, disease and salinity can add up to an overall decrease in stomatal conductance and uptake of CO2. So it follows that you want to irrigate the field as uniformly as possible to avoid having some parts too dry while avoiding saturating other areas, which leads to disease.

That way, every part of the field can produce hay at the optimum rate. The usual measure of field uniformity is the distribution uniformity (DU): DU (percent) = 100 x “low quarter infiltration” / average whole field infiltration.

If you have 70 percent DU, and you infiltrate an average depth of 5 inches, that means the tail-end quarter of the run averaged 3.5 inches and the head end got 7.1 inches – a 50 percent difference from the dry to wet side.

Installing a tail-water return system and increasing on-flow rates can boost DU to 90 percent using optimal scheduling and a quarter-mile or shorter runs.

Practice 4 – Check your soil moisture on a real-time basis
Does it “pay” to check the water status of your crop and soil? Hopefully, the above discussion has convinced you that there is no single perfect irrigation schedule for any given flood alfalfa field.

Optimal irrigation and yield require checking infiltration after an irrigation and how quickly the crop extracts the moisture. But unlike insect and disease problems, a little too much water or even deficit irrigation almost never causes a crop failure in alfalfa.

Growers know good irrigation scheduling helps, but it’s usually not a “make or break” decision, and the hay keeps growing. By the end of the season, however, it is usually the difference between having made 10 tons per acre or just 8 tons per acre.

So you decide, “I want to monitor but where should I check in the field? The infiltration is bad on this side. What about the head versus the tail end? What about weather changes and salinity? What should I check it with? How often do I need to check? Should I be checking both the plant and the soil?”

Checking soil moisture for scheduling irrigations can be very useful in the spring and late summer when you may only need one irrigation between cuttings instead of two.

In cooler intermountain areas, and for growers with severe water cutbacks, having some kind of soil-moisture sensor can give you the confidence to cut back to one irrigation per cutting all season and still have sufficient moisture for decent tonnage.

Practice 5 – Changing irrigation systems to improve efficiency and yield
For the same 50 to 55 inches of water, simply increasing DU per water use efficiency from 70 to 90 percent can increase yield by about 2 tons per acre even before the potential advantages of fertigation and pest control offered by subsurface drip irrigation (SDI) and center pivots that you don’t have with flood.

The goal here is to get as much water going directly to crop transpiration as possible. So anything we can do to minimize evaporation, deep percolation or waterlogging, runoff and drought stress potentially channels that water to the crop and boosts production efficiency and tonnage.

In principle, SDI is the system that should best optimize all these factors. It is also the system that requires the most attention to maintenance and scheduling. Specific advantages and disadvantages of the various system categories are:

Flood

  • Advantages – Gopher control is least problematic, low to no energy cost, no filtration necessary, total infiltrated water depth varies over season, tail water return systems improve uniformity and provide better stand quality by draining check ends

  • Disadvantages – Land must be leveled, pushing in head ditches, waterlogging ends, stress between irrigations and cuttings

Sprinkler

  • Advantages – Better water application control for stand germination, depth of water controlled by run time, no land leveling, no borders needed, fertigation possible

  • Disadvantages – More gophers, significant capital cost – highest for solid-set, high energy and labor costs

Pivot

  • Advantages – Rapid field coverage, usually more uniform than hand-move and side-roll which makes pesticide applications as well as fertigation possible, reasonable capital cost, lower energy cost than other sprinklers, least labor cost

  • Disadvantages – Gophers, high instantaneous application rates, potentially higher evaporation losses, lose field corners, needs filtration

Subsurface Drip Irrigation

  • Advantages – High-frequency daily irrigation possible even when cutting, maximum crop transpiration possible, potentially superior application of P and K fertilizers, uniformity unaffected by wind.

  • Disadvantages – Sprinklers needed for establishment, salinity may be a problem, and gophers – causing extensive damage to system if not controlled – root intrusion or emitter clogging, cannot “see” water – pressure and soil moisture monitoring essential for good yields, quality filtration essential

Bottom line

The major benefit that really pays for an irrigation system change is increased yield, not water savings.  FG

A more complete version of this article can be found at 2012 California Alfalfa & Grains Symposium.

Blake Sanden is a cooperative extension specialist with the University of California. Blaine Hanson is an irrigation specialist emeritus with the University of California Cooperative Extension. Khalid Bali is an irrigation and soils adviser with the University of California Cooperative Extension.

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