Dan Putnam: Drought Management of Alfalfa

[Dan Putnam, UC Cooperative Extension alfalfa and forage crop specialist in the Department of Plant Sciences at UC Davis, tells alfalfa growers to concentrate on their most productive fields during drought, and let marginal fields go.]

Dan Putnam Describes Drought Management of Alfalfa
by Bob Johnson
reprinted from AgAlert Issue Date: July 8, 2015

The best strategy for alfalfa deficit irrigation during the drought is to let some older fields go, if necessary, and fully irrigate the most productive fields early in the season.

It does not make economic sense, University of California specialists say, to spread the water cutback evenly throughout the year.

"We do not recommend under-irrigating for every cutting," said Dan Putnam, UC Cooperative Extension alfalfa and forage crop specialist at UC Davis. "It's far better to maximize yields early in the year, and then dry it down. Alfalfa has the ability to survive harsh conditions."

Putnam discussed deficit irrigation strategies with the growers and researchers who came to learn and talk about water use during the UC Davis Alfalfa and Forages Field Day.

He described the water strategies in a drought as triage, or choosing less productive fields to stop irrigating altogether; a starvation diet, or deficit irrigating all fields throughout the year; and cold turkey summer cutoffs, or fully irrigating early in the year and cutting off when the allotted water runs out.

"We recommend a combination of triage and summer cutoffs to deal with droughts," Putnam said. "When faced with low water years, the older, least productive fields must go, especially if water can be moved to more profitable crops or economically transferred for other uses."

The reason to hold off on terminating irrigation is both yields and quality are at their peak the first months of the year with alfalfa.

"Alfalfa exhibits superior yield patterns early in the year, and this is also the period for highest quality," Putnam said. "Calculate the seasonal water use and the amount of water available to you -- continue full irrigation through a mid-summertime point, and then cease irrigation. Monitor the stands in late summer and fall for survival."

As Putnam spoke, a few feet behind him researchers had set up a battery of instruments, including a lysimeter, to measure water as it moves down in the soil, in order to take a new look at how much water alfalfa can use.

"In this trial, we're trying to make sure water is not a limiting factor," Putnam said. "Growers tend to under-irrigate in the summer to fit the cutting schedule, so the amount of water alfalfa really uses is up for debate."

Researchers established six acres of alfalfa on the Davis campus in November 2013 and are carefully measuring water use with two lysimeters, devices 20 feet in diameter buried four feet deep that estimate crop water use by recording in real time changes in weight of the soil and plants.

The lysimeters also take water samples from various depths, which include most of the root zone.

The study has, so far, shown alfalfa to have a seasonal average crop coefficient of 0.95, with a substantial drop from cutting until irrigation and a slight increase from two or three days after irrigation until the next cutting.

"The crop coefficient is around 0.95," said Daniele Zaccaria, UCCE assistant agricultural water management specialist. "We will have another year of figures this year, then we'll go for a third year of measurements so we can confirm these figures."

This study is not an attempt to develop an irrigation strategy for the crop, but to learn the point at which the crop always has enough water.

"We just tried to avoid water stress as much as possible," said Rick Snyder, UCCE biometeorology specialist. "The old crop coefficient was a little higher at full canopy, and we're trying to figure out why."

Although alfalfa can consume substantial amounts of water over the course of the year, its reputation as unfit for a drought is undeserved.

"Conventional wisdom would suggest alfalfa would be one of the worst crops to have in a drought year," Putnam said. "However, this is not the case. Alfalfa has several unique and positive values when it comes to water, especially during drought periods."

The crop has deep roots to mine for water, can tolerate salty water, is perennial and can come back after a dry year, and produces high biomass yields per drop of water applied, he said.

"Alfalfa has characteristics that give growers alternatives in a drought," Putnam said. "It has roots that go down six to eight feet, so it can get water from deep in the soil. 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 half or less of the irrigation water that makes alfalfa particularly valuable in a drought."

Unlike most other crops, alfalfa can also bounce back after even relatively long periods of water stress.

"UC work done over the past 20 years has confirmed the ability for growers to stop alfalfa irrigations in midsummer, allow the crop to die down, and re-water successfully later when irrigation water becomes available," Putnam said.

Putnam advised considering a field less productive, for the purposes of triage, when stem counts are less than about 35 per square foot averaged over six to 10 observations.

Even the fields that receive no water, however, may come back in the future if water becomes available.

He recalled an extraordinary example when researchers irrigated a test alfalfa field with salty water for five years, then cut off water for two years, and then saw it come back to life when they irrigated it.

"This crop does best under full irrigation, there's no doubt about it," Putnam said. "But if you don't have enough water, or want to transfer that water to another crop, you can."

The combination of letting the least productive fields go without water, and terminating irrigation in the other fields in the summer, should at least give alfalfa growers a viable drought strategy.

"This does not mean that there won't be a yield penalty from late-season water cutoffs -- there will be," Putnam said. "However, the extent of the seasonal reduction will depend upon a range of factors -- particularly the ability of the soil to store moisture."

But because alfalfa does require a significant water commitment, he said, efforts to improve irrigation efficiency are essential.

"We need to up our game with technology on water application with this crop," Putnam said.

(Bob Johnson is a reporter in Santa Cruz. He may be contacted at bjohn11135@aol.com)