- Author: Daniel H Putnam
The 2013-2015 California drought has brought much public attention to the amount of water used in agriculture, and particularly which crops use the most water.
Although almonds have taken the hit lately, alfalfa is often one of the favorite whipping boys of agricultural critics due to its high water use on a state-wide basis.
But is alfalfa's water profile really so miserable?
Contrary to popular belief, alfalfa has several unique positive biological properties and advantages when it comes to water. Due to these properties, alfalfa is remarkably resilient when it comes to severe drought conditions.
Alfalfa can grow year-round, and yield over 14 tons/acre of high quality forage, the basis for milk production-such as this drip irrigated field in Imperial Valley, CA. An average CA alfalfa field can produce a potential milk yield of 2400 gallons/a/year. Due to high yields and the fact that 100% of the above-ground plant material is harvested, water use efficiencies are high (photo, D.Putnam).
First a clarification—Alfalfa does not really use more water than other crops. At full canopy (when the leaves cover the soil surface), alfalfa's water use is not much different than any other crop (think spinach, lettuce, tomato, wheat, almonds or corn) per unit time. The Evapotranspiration (ET) requirement (the amount of water a crop really needs to grow) is remarkably similar across crops at full canopy (see FAO tabulated values for the water requirements of crops).
Alfalfa's water use profile in California is primarily due to its high acreage and nearly year-round growth pattern in many regions. If spinach were continually grown on 850,000 to 1 million acres all year long, the water use would be about the same as alfalfa, perhaps more.
Further, it's not so much how much water is used, but how much crop is produced per unit water that is important – also known as water-use efficiency. In that category, alfalfa shines.
Contrary to popular belief, alfalfa has several unique valuable properties and advantages which would enable cropping systems greater resiliency under drought conditions.
UTILIZING BIOLOGICAL FEATURES OF ALFALFA IN A DROUGHT
Alfalfa has a range of biological characteristics that make it very useful when a farm or an irrigated region is faced with drought conditions and resulting water limitations. These are:
Deep-Rootedness—alfalfa roots are commonly 3-5 feet deep and can extend to 8-15 feet in some soils. Therefore this crop can utilize moisture residing deep in the profile when surface waters become scarce. It shares this property with crops such as orchards, vineyards, and sugarbeets and safflower, unlike crops such as onion, lettuce and corn, where it's easy to lose water past the root zone.
Alfalfa's deep roots are capable of extracting water from deep in the soil, thus much of the water applied is not wasted. Additionally, deep roots enable the crop to survive periodic droughts (Univ. of Minnsota illustr.).
Perenniality—The fact that the crop grows for 4-8 years, grows quickly with warm conditions in the spring is a major advantage of alfalfa—it can utilize residual winter rainfall before irrigation is necessary. This is unlike summer-grown annual crops that need to be replanted each year (water use efficacy is low during this time). In many areas, the first cutting of alfalfa of the year requires zero irrigation– supported only by rain and residual soil moisture.
Very High Yields—Alfalfa is a very high yielding crop, and can grow 365 days a year in warm regions (such as the Imperial Valley of California and southern Arizona). Its biomass yields are very high—we can get up to 12 cuttings per year in those regions, and growers with top management can obtain more than 14 tons/acre dry matter yields. High-yields create higher water use efficiencies.
High Harvest Index, High Water Use Efficiency—Alfalfa's Water Use Efficiency is not only due to high yields, but because nearly 100% of the above-ground plant material is harvested (known as the harvest index). In most seed-producing and fruiting crops, only a portion of the plant is harvested (typically 30-50% of the total plant biomass).
Salt Tolerance/Ability to Utilize Degraded Water—Recent data has shown that alfalfa has a high degree of salt tolerance. A recent trial in Fresno County, where EC 5.5 water was used for irrigation over 3 years, yields were normal (10-12 tons/acre). This is important in a drought, since degraded recycled water (municipal waste water, drain water, other waste water) could be used on this crop, while saline waters would injure less-tolerant crops.
Contribution to Wildlife Habitat—In a drought, all of nature suffers. Alfalfa has been shown to be a significant wildlife habitat due to its lush foliage and insect diversity. Biologists have determined that 28% of California's wildlife use alfalfa for nesting, feeding, or cover. Even partial-season irrigation during drought can assist wildlife in survive a drought period by using alfalfa as habitat. Don't believe it? Visit an alfalfa field and observe the egrets, curlew, hawks, eagles, snakes, deer, antelope, elk, insects, and many birds and mammals who are at home there.
Many types of creatures use alfalfa, especially during drought periods, when alfalfa is an green oasis in a dry landscape, such as these curlews and gulls in Yolo county. Hundreds of species use alfalfa routinely. (photo, J. Kuhn)
Ability to Survive a Drought—Alfalfa evolved in regions of the world with long hot dry summers and wet winters – exactly like California. Although yields are highest with full irrigation, alfalfa can survive periodic droughts. This is due both to deep roots as well as ability to go ‘summer dormant' under dry conditions. In 2014, Central Valley growers that were forced to stop watering their alfalfa fields generally found the crop recovered after rainfall or irrigation resumed later in the year.
Ability to Deficit irrigate: Obtain Partial yields—Typically 50-60% of full yields are obtained by mid to late June (Figure 1). If only partial water is available, irrigation water can be applied early (supplemented by winter rains and residual moisture), and the crop dried down during late summer periods. This is particularly important, since water is scarcer in late summer vs. early, and that water may be used for other crops or uses, or economically transferred to cities. Additionally, Water Use Efficiencies are greatest early in the year, and yield and quality tend to be higher early vs. late.
Yield Patterns from University of California variety trials over a 10 year period in an Intermountain site (3-4 cuts/year), and the San Joaquin Valley (8-9 cuts/year). Over 50% of full yield is frequently obtained by mid-June, creating partial harvests in the event water is needed to be transferred for other uses later in the year.
FLEXIBILITY IS THE KEY
It is this combination of deep roots, ability to utilize rainfall early in the year, high water use efficiency, ability to survive droughts, salinity tolerance, and ability to give partial yields with as much as half of the irrigation water that makes alfalfa particularly valuable in a drought.
University of California work done over the past 20 years has confirmed the ability for growers to stop alfalfa irrigation in mid-summer, allow the crop to dry down, and re-water successfully later when irrigation water becomes available. One cannot do this with many other crop species.
Summer-grown annual crops (corn, cotton, tomato, sunflower) also have a high degree of flexibility --in the sense that one could elect not to grow those crops during a drought, thereby reducing water used for these crops to zero. But yields are also reduced to zero.
High value vegetable growers are unlikely to sacrifice any water since the economic value of those crops is so important to their farms. Additionally, one cannot really deficit irrigate spinach, broccoli, onions, lettuce or strawberries too much and expect an economic product.
Winter grown annuals (wheat, oilseeds) and safflower also have major advantages during drought, since they can be grown with very little irrigation water, and can be eliminated during low water years.
Orchards have the least flexibility in a drought, due both to the significant yield reductions when less-than-full irrigations are applied, but also the significant economic risk to the farm because of the high investment cost.
Contrary to some popular views, alfalfa has a range of positive biological characteristics that should be quite useful when facing water-short conditions. These characteristics include a high degree of flexibility to ‘deficit irrigate' the crop, ability to survive drought periods, high water-use efficiency, deep rootedness, salinity tolerance, and the ability to utilize degraded water. It is additionally very valuable to wildlife, which also suffers during a drought. Oh, and by the way, it is also very valuable to the millions of consumers who depend upon the milk, cheese, yogurt, and yes ice cream, produced from alfalfa.
This alfalfa variety trial was irrigated with saline (EC5.5) water during 2010-2012, and then received zero irrigation water during 2013 and 2014. However, the crop nearly fully recovered after re-watering in the fall of 2014. The resiliency of the crop may be attributed to its deep rooting patterns and saline tolerance (Western Fresno County, October, 2014, D. Putnam, photo)./h2>/h2>/h3>