Alfalfa is undoubtedly the most important forage crop grown in the U.S. It is easy to see why it gets the title “Queen of the Forages,” with hundreds of thousands of acres planted across the country. In many states, alfalfa ranks as one of the top three agricultural commodities in terms of value. Although it is important to the agricultural economy of many states, alfalfa often gets a bad rap due to its high-water use, especially in the western U.S., where it is most often irrigated.

The high water use associated with alfalfa is easy to understand once you look at its growth characteristics. It is a cool-season plant that breaks dormancy in early spring and grows well into the fall, as long as it has adequate soil moisture. Compared to many other crops, this results in a long growing season in which it is utilizing water. Depending on your environment, alfalfa can be harvested two to 11 times per season.

Origins of water use

For maximum seasonal yields, alfalfa requires a lot of water. Although variable, alfalfa requires about 5.6 inches of water per ton, when averaged across multiple environments. However, if one looks at the history of alfalfa and where it originated from, you soon come to realize it is actually a very drought-tolerant plant. Alfalfa is thought to have originated in Iran and surrounding countries, the Caucasus mountains and other areas in Asia Minor. The factor that ties much of this part of the world together is that it is dry. The fact that alfalfa evolved under dry, droughty conditions can be used to our advantage when we think about how to scale back on its water use.

In some parts of the western U.S., consumptive water use by alfalfa exceeds well over 40 inches per year. Although this can be viewed as a negative, it also provides a lot of opportunities for reducing water use. In other words, there is a lot of water to play with, unlike some other crops.

For example, if you are growing wheat or corn for grain, those crops must have water at appropriate times to set seed and obtain maximum yields. Typically, the main goal with alfalfa is biomass yield, not seed production. Even if yields are reduced due to less water being applied, the biomass that is produced still has value and can be sold. In addition, when alfalfa growth is stunted due to reduced water, it will generally be higher in forage quality, which can result in price incentives when sold.

Water trials

Our research group first started to look at ways to cut back on irrigation water use by alfalfa as an alternative to the traditional buy-and-dry method of freeing up water for municipal uses. We extended our initial findings to the Western Slope of Colorado, where there was interest in forming a Western Slope water bank that could be used to meet compact obligations to lower basin states in the Colorado River Basin, as well as free up water for municipal, industrial and higher-value agricultural crops. With the increased incidence of drought in the western U.S. in recent years, the strategies that we have been investigating can also be incorporated into drought-mitigation plans.

Strategy implementation

There are two basic strategies that have been considered. The first is generally referred to as limited irrigation in which water is applied over the whole growing season but at amounts less than required to meet full evapotranspiration (ET) needs. For example, a producer might apply 75% of estimated annual ET, which will result in reduced yields and plants being stressed to some degree over the entire season.

The second is referred to as seasonal-deficit or partial-season irrigation in which full water is applied during the spring and limited or withheld during the hotter periods of the growing season into the fall. This strategy typically improves water use efficiency (WUE) more than limiting irrigation uniformly throughout the growing season and is the one that our group settled on as the best alternative.

In Colorado, where four cuttings is the norm, the two alternatives that have worked best are to stop irrigating either after the first or second cutting of alfalfa and to allow the plants to go semi- or fully dormant for the rest of the season. Fields are irrigated to meet full ET until those points in time.

In a four-cut system, the first two cuttings generally account for about 60% of annual yield. When irrigation is stopped after second cutting, you can generally get at least a short third cutting as the plants draw on stored soil moisture and any precipitation that falls. Our numbers show between 70% and 80% of annual yield can be harvested in these three cuttings while applying only 50% to 55% of annual irrigation water. To save even more water, the stop after first cutting is a viable option. Only two harvests are generally feasible with this option, with annual yields being reduced by 42% to 62%, based on our trials. However, irrigation water applied was reduced by about 85%.

In most of our trials, we have applied these treatments for two years in a row and then gone back to full irrigation. The stop after first cutting appears to put a lot of stress on the plants (see photo above), but in our experience, plants in both the first and second stop cuttings have responded very well when full irrigation resumed, with even higher yields than plots that were fully irrigated throughout.

In fact, the more severe the water stress (i.e., stop after first), the higher the yield when irrigation was resumed. We have recorded higher plant densities in severe water treatments, and there appears to be less pressure from disease, weeds and alfalfa stem nematodes. The caveat one must keep in mind is that all our work has been done on heavier-textured soils. Results from California suggest that these strategies do not work on coarser, sandy-textured soils.

Trial results

In summary, alfalfa is a good candidate crop for being able to reduce irrigation water applied while still maintaining acceptable yields and stand life. Even though yields are reduced, there is the potential for price incentives due to higher forage quality with stressed plants. Each environment is different, and what we have presented must be adapted to fit your resources and goals.