As this research trial photo shows, a fully irrigated field, at right, contrasts in late summer with a deficit-irrigated field, at left. From July on, the deficit-irrigated fields were not watered.
Not having enough water to fully irrigate an alfalfa crop is a tough reality for many growers.
But by utilizing deficit irrigation – strategies in which water is cut off or reduced – they may be able to get through periods of low water while still producing a portion of full yields.
It’s no secret that alfalfa is California’s largest ag-water user and a prime target for water transfers. In this drought year, urban and ag users in some cases have paid for water transfers at rates well over $1,000/acre foot.
But alfalfa has an advantage over many fruiting and grain crops; the deep-rooted legume can realize a portion of full yield even when the crop is deficit-irrigated.
The three deficit-irrigation strategies for alfalfa include:
• Starvation diet – partial irrigations spread over the season; and
The word triage comes from the practice of battlefield-sorting wounded soldiers who had a good chance of living and those who should be “let go.”
Older fields with poor stands or weeds should be the first candidates for irrigation stoppage. Water resources should be concentrated on fields with the best stands and the most chance of high yields and quality.
Yield will be reduced over the entire acreage. This has the advantage of keeping all fields in production. But lower yields will result in increased costs per ton, since harvesting and pest management costs may be almost the same as they would be for fully irrigated fields.
In a three- to four-cut system, 64% of yield is realized in the first two cuts, and in an eight-cut system, 50% and 75% of full yield are realized by mid-June and late July, respectively. In addition, the early ETc requirement is far less than during mid- to late summer. Springtime harvests are not only the highest yielding, but also often the highest quality, offering the highest water-use efficiency.
Alfalfa goes through a summer slump producing significantly lower late-summer yields of poor-quality crop, and water-use efficiency is low. It’s far better to use water early in the season when production and quality are highest, and then dry down those fields until rains return or the water situation improves.
The University of California has conducted deficit-irrigation studies on alfalfa over the past 25 years. In almost all cases, alfalfa that goes through a period of summer drydown is able to revive when rewatered in fall. There are exceptions, such as alfalfa grown in the Imperial Valley’s cracking clays. But the legume is well-suited to a period of drought, often of several months in duration.
Farmers are often caught by surprise during droughts. Some do not irrigate early enough to recharge the soil profile during alfalfa’s early growth period, from February through May. That compromises early growth.
It’s important to build moisture early in the season to maximize yields. It’s also important to use soil moisture sensors to see how much residual moisture is contained in your soil.