As agricultural water supplies slowly evaporate in the West amid increasing drought pressures, sprouting population growth, and other factors, Western farmers face the impending reality of growing crops with less water. Adding to the daunting task are the growers’ needs to boost yields and income.

“Regulated deficit irrigation (RDI) may be the holy grail of water management for some citrus growers,” said David Goldhamer, University of California (UC) water management specialist based at the Kearney Agricultural Center (KAC), Parlier, Calif.

He believes this innovative management tool may decrease the amount of water that an orchard consumes, and at the same time increase the value of the crop.

Fieldwork conducted by Goldhamer and technician Mario Salinas has furthered RDI research on major tree crops in California including pistachios, olives, prunes, and citrus. Numerous species, conducive to saving water without reducing production or profit, have been identified. However, the additional benefit of improved crop quality in citrus makes RDI an even more promising strategy.

While limiting water use in many herbaceous crops (row and field) usually results in production losses and reduced grower income (except in cotton), that’s not the case with RDI use in some trees and vines.

“The growth dynamics of trees are very different than in most herbaceous crops. With trees, the commodity produced includes a quality aspect that influences the product’s value. In citrus, the quality aspects are fruit load and size, plus peel appearance which determines fancy, choice, or juice grading.” Goldhamer detailed his RDI research with citrus growers during a June workshop at the KAC.

“Successful RDI is about planned water deficits at specific crop developmental stages that control vegetative growth without reducing production. The goals are to solve horticultural problems, reduce water usage, and increase farm income.”

The idea is to intentionally stress plants to reduce the consumptive use of the orchard by reducing leaf transpiration, and thus, a precious resource, said Goldhamer. The RDI concept came from Australia and New Zealand about 15 years ago.

The UC conducted the research in grower-cooperator fields. Researchers selected fairly uniform orchard locations. Methods of experimentation included mature trees, micro sprinkler irrigation, six replications of each treatment, and a minimum of eight trees monitored in each replicate.“We either modified the existing irrigation system to apply what and when we wanted, or we installed an extra line to the tree to deficit irrigate as planned. In all experiments, fertilizer management was identical for all trees,” noted Goldhamer.

Applied water amounts were recorded while tree water status was monitored with a pressure chamber. Fruit size measurements were registered to log the impact of RDI on growth. Fruit was randomly sampled at harvest and gross weights were measured in the field. Fruit samples were sorted into sizes. An independent professional graded the fruit as fancy, choice, or juice. At the KAC workshop, Goldhamer expounded on the RDI results compiled on three Navel varieties — Frost Nucellar, Lane Late (both studies completed) and Parent Washington (in progress).

Frost Nucellar

Goldhamer’s group imposed 10 RDI regimes with different stress timing, magnitude, and duration on Frost Nucellar from 1997 to 2000. Results indicated the most productive regime involved stressing trees from May 16 to July 15, which reduced water use by about eight inches. It also substantially reduced puff and creasing in mature fruit in all of the study years.

The RDI regime, irrigated at 25 percent of full citrus ETc (evapotranspiration from crops) from mid-May to mid-July, reduced the mid-day stem water potential by -20 bars (pressure measurement units). Soil depth and type, plus other factors can impact the actual time to reach this stress magnitude. Fully irrigated trees have values of -8 to -10 bars. “While the relationship between gross fruit yield (the mean of three years) and applied water was fairly linear, we found the relationship between gross revenue (dollars per acre) and applied water was completely different,” said Goldhamer.

The stress level and the duration of the stress period are important in saving water and gaining the desired effect on puff and crease. Uncontrolled stress can increase fruit drop or decrease fruit size. Stressed trees showed cupped leaves, leaf yellowing, and initially some smaller fruit sizes. Yet when irrigation resumed, the trees and fruit recovered.

Attempts to apply this research at the field-scale should be approached carefully, Goldhamer said, adding that monitoring tree stress with a pressure chamber is necessary.

The research was conducted at Paramount Citrus in McFarland, Calif.

Lane Late variety

RDI research on Lane Late, conducted from 2002 to 2005 at Griffith Farms, Visalia, Calif., focused on reducing two horticultural problems — excessively large fruit and granulation.

RDI regimes included: early season stress (T1) – delayed irrigation, water deficit through June; mid-season stress (T2) – deficit from July through September; late season stress (T3) – September through November; season long stress (T4) – interactive deficit; and the control (T5) – full irrigation. “The most effective RDI stress treatments were late season stress, irrigating at 50 percent ET from mid-August on, and reduced irrigation evenly throughout the season,” Goldhamer said.

The treatments had more fruit in the size 88 to 56 ranges at harvest, and less fruit in the categories of size 48 or fewer per carton. Since smaller fruit is worth more, Goldhamer found that RDI increased grower profits by $1,200 to $1,400 per acre, excluding the water cost savings.

Growers should calculate normal citrus evapotranspiration (ET), usually estimated by multiplying the citrus crop coefficient of about .65 by the reference ET. Information on estimating crop water irrigation is available from the California Irrigation Management Information System (CIMIS) on the Internet at wwwcimis.water.ca.gov.

Goldhamer believes a good way to try RDI is to experiment on smaller acreages.

The California Citrus Research Board in Visalia, Calif., funded the Lane Late and Frost Nucellar RDI research.

Parent Washington variety

While RDI proved useful in improving grower profit in Frost Nucellar and Lane Late, the researchers wondered what would happen in an orchard with no identifiable horticultural problem. They located such an orchard of the Parent Washington variety, and conducted an RDI trial using treatments similar to those with Lane Late.

“If we reduced consumptive use and have no negative impact, that would be a successful trial,” Goldhamer said.

Results so far indicate two new observations — a significant reduction in fruit load during water stress from mid-May to mid-July, and all season long. There was no impact on fallen fruit numbers.

“Interestingly, there has been no negative impact on irrigation delayed until the end of May. The only significant reduction in fruit size was season long stress with only 12 inches of applied water,” Goldhamer said.

The Parent Washington research, now in its third year of a five-year study, is underway at Griffith Farms. The California Department of Water Resources’ Office of Water Use Efficiency is funding the research.

Citrus RDI summary

Peel creasing can be reduced through early season RDI with Frost Nucellar. Growers should be careful with early season stress, and avoid extending it too far into July if the shaded leaf water potential has reached -20 bars, stated Goldhamer. Three years of results with Parent Washington have shown extended stress can reduce fruit load.

“With late harvest varieties like Lane Late, RDI can positively influence fruit size distribution and grower profits, with no negative impact on tree health including insects or disease. Season long stress in Lane Late, irrigated with about 16 inches, can improve production, but also destroy production with Parent Washington.”

It hinges on whether decreasing fruit size is a desirable outcome, he said. Stress can be positive or negative depending on the timing of the stress and the cultivar grown. While growers should manage stress carefully, some stress can be beneficial.

The California Central Valley research on RDI is applicable to the citrus growing areas in Arizona because evaporative demand (weather) conditions are similar. The research is not completely applicable along the California coast in Ventura, where evaporative demand and the rate of stress development for given RDI regimes is much different.

Other researchers who contributed to these findings included Neil O’Connell, UC Cooperative Extension advisor, Tulare County, and Elias Fereres, University of Cordoba, Spain.

e-mail: cblake@farmpress.com