Limited Irrigation Alfalfa
by Neil C. Hansen, associate professor, Department of Soil and Crop Sciences

Alfalfa has several traits that allow some flexibility in managing a limited irrigation water supply. First and foremost, an established alfalfa crop has a root system that is efficient at exploring deep in the soil for water. Like most crops, alfalfa roots will develop most where soil moisture is available. As a result, alfalfa root systems look different under furrow and sprinkler irrigation systems. Because alfalfa roots deeply, it can use water that would move below the root zone of many other crops.
A second trait of interest is the ability of alfalfa as a perennial crop to go dormant during drought conditions and remain viable so that it can resume growth when water becomes available. This trait comes from alfalfa’s origination in the semi-arid lands of Mesopotamia, where annual cycles of arid conditions are interrupted by seasonal precipitation. Finally, alfalfa hay quality is generally higher when yields are reduced from moisture stress. The higher quality can partially offset the loss in yield.
Beginning in 2005, Colorado State University and Northern Colorado Water Conservancy District (NCWCD) teamed up to study limited irrigation strategies for alfalfa. As part of the project, a careful study was made of past irrigated alfalfa research throughout the U.S. Great Plains region. Information was also combined from six individual studies conducted in an area from the Texas panhandle to North Dakota and a fairly consistent relationship was found between alfalfa yield and consumptive water use (Figure 1). The slope of the line relating yield to consumptive water use tells us it takes about 7 inches of consumptive water use to produce each ton of alfalfa dry matter. The consumptive water use is evapotranspiration of water obtained from irrigation, precipitation and stored soil moisture. There will be some deviation from this line based on local conditions, with Colorado likely requiring less water per ton than the average. Another lesson learned from past alfalfa research is that alfalfa uses water more efficiently during the cool conditions in spring and fall than during the hot summer months.

Based on the lessons of the past, we designed an experiment that evaluates alfalfa growth and yield with different irrigation regimes. The experiment is located at the NCWCD headquarters under a linear move sprinkler irrigation system on a two-year old alfalfa stand.
Our water management comparisons were:
1. Full irrigation reference
2. Terminate irrigation after first cutting
3. Terminate irrigation after second cutting
4. One additional irrigation after first cutting plus irrigation in late summer and fall

These reduced water approaches, called partial season irrigation, seek to get the most return per unit of applied water by irrigating during the cooler parts of the growing season and withholding irrigation during the hotter part of the summer. Our results for the 2006 growing season are shown in Figure 2. Applied irrigation amounts were 27 inches, 14 inches, 13 inches, and 5 inches for treatments 1 through 4 listed above, respectively. Total growing season precipitation between March and October was 7.1 inches. When irrigation water is limited, yields decline as expected. Notice that in the third treatment where irrigation was stopped and was later resumed, the fourth cutting yield was the same as that for the full irrigation treatment, illustrating alfalfa’s ability to resume growth when water becomes available.

An initial economic analysis suggests that the two intermediate irrigation levels in this study may remain profitable, while yields in the driest system were not adequate for a profitable system. Partial season irrigation may be a viable strategy for limited irrigation of alfalfa. This strategy of fully irrigating alfalfa for a partial season is a better approach than deficit irrigation throughout the full growing season because it focuses irrigation and growth during the cooler time periods when water use is more efficient. This study will be continued for several more growing seasons to evaluate profitability and alfalfa stand quality over time. However, our initial results suggest that alfalfa is a crop that provides flexibility for irrigation water management when supplies are limited.

Neil Hansen: Neil.Hansen@colostate.edu
Department of Soil and Crop Sciences, CSU