By Bob Johnson
Alfalfa’s water resiliency may become even more important, with the changing climate expected to produce more extreme drought and flood years alike.
Many growers could reduce water use by 10% with no loss in yield with more precise irrigation scheduling. They may also still produce a viable crop if drought forces more draconian cutbacks.
During winter storms, the crop has shown an ability to absorb extraordinary amounts of water with no loss of yield the following season.
“Alfalfa has a key role to play in a water-uncertain future,” said Dan Putnam, University of California Cooperative Extension alfalfa and forage specialist.
That’s because the crop has a “high flexibility during times of insufficient and excess water due to important biological features,” he said.
These were among conclusions Putnam and fellow California researchers presented in Reno, Nevada, last week during the 2021 Western Alfalfa & Forage Symposium. The event was sponsored by the UCCE and similar agricultural institutes in nine other western states.
Alfalfa has unusually deep roots to mine for water in dry times. It has multiple harvests, which make economic yields possible even when not enough water is available for full season irrigation. It can also be flooded in winter storms to recharge groundwater.
“While droughts get most of the attention, variation in water supply in the West is probably a more important consideration,” Putnam said. “Periodic drought, disappearing groundwater and even flooding events have been a reality in western states for decades and are likely to be.”
Under extreme drought conditions, the most economical strategy is to use limited water supplies early in the season.
“In 2021, we flood-irrigated selected plots of an alfalfa study field that was full and deficit-irrigated in the previous two years,” Putnam said. “We found that early season, February and March, irrigations not only increased yields in the first three cuttings but also sustained stands and yields later in the year, even when deficits were applied in the summer.”
Putnam presented the findings along with fellow researchers, UCCE irrigation specialist Khaled Bali and UC Davis Ph.D. student Umair Gull.
Meanwhile, a team of Utah researchers presented study data gleaned from a dozen fields using center pivot sprinkler irrigation. The Utah study concluded that improved scheduling is the most effective and economical way to improve water efficiency.
“Results showed that new irrigation equipment did not consistently improve alfalfa yield at any of the fields in 2019 and 2020,” said agroclimate specialist Matt Yost, head of the Utah extension research team. “Five of 51 cuts across fields and years had an average alfalfa yield increase of 0.3 tons per acre, but these five cuts were not consistently at the same fields. Likewise, 10% reductions in irrigation rates rarely impacted alfalfa yield.”
The study showed that using a combination of evapotranspiration data, soil moisture measurements and software designed to predict crop response to irrigation saved significant amounts of water without reducing yields.
“The three advanced irrigation scheduling approaches did not consistently improve alfalfa yield compared to the yield produced by the cooperating grower’s irrigation schedule,” Yost said. “They did, however, often reduce water use by up to 15% in some cuts and in some fields. These results suggest that many growers could cut irrigation rates by 10% without adversely impacting alfalfa yield, and that worn sprinkler equipment may not be causing as much nonuniformity issues as expected.”
Some alfalfa growers invest in buried drip lines to improve irrigation efficiency in times of water shortages.
“The idea behind drip has always been to place the precise amount of water at or near the root,” said Doug Larson, pest control advisor and national sales manager at Ag Water Chemical. “Decreasing evaporation and soil erosion, while simultaneously allowing for the spoon feeding of water and other nutrients directly to the root zone, increases efficiencies and sustainability factors. In most (subsurface drip irrigation) applications, irrigation water never actually reaches the soil surface, further reducing evaporation, waste and weed pressure.”
Subsurface drip is efficient, but the system brings its own set of problems. They begin with upfront costs of $2,000 an acre and more.
“Germination of alfalfa seed via subsurface drip irrigation systems can be a challenge for producers, due in large part to the shallow depth of newly sewn alfalfa seed and the soft soil structure of the seedbed,” Larson cautioned. “Moving water from the buried drip lines to the soils surface is not always possible with subsurface drip irrigation.”
The greatest challenge is that gophers, which love alfalfa, can frequently chew holes in the buried drip lines.
Helen Dahlke, a UC Davis assistant professor of integrated hydrologic sciences, is studying another attribute of alfalfa that could become important as groundwater pumping restrictions take affect. She is focused on its ability to absorb substantial amounts of water in winter storms to recharge the aquifer without damaging the crop.
“A total of 5.4-acre feet and 11.9-acre feet of water were applied in addition to 2.32 inches of precipitation during 2019,” Dahlke reported from her flooding studies. “In 2020, a total of 4.9 and 8.1-acre feet in addition to 0.56 inches of rainfall were applied on the low frequency and high frequency treatment, respectively.”
Even these enormous amounts of water did not negatively affect alfalfa yields the following season.
“Results from our experiments indicate that high-frequency and low-frequency flooding of dormant and semi-nondormant alfalfa during late winter, early spring showed no significant effect on alfalfa yield,” Dahlke said.
The flooding did, however, have impacts on crop quality that remain to be explained.
Dahlke said, “The forage quality test results showed a significant difference in crude protein content and total insoluble fiber content but not in the least digestible fiber content between the control and flood treatments in 2019.”
She said that indicates “that the flooding for groundwater recharge has an effect on forage quality.”
