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Researchers say water table critical to alfalfa irrigation

Tam Moore Oregon Staff Writer 9/9/05
  Forage researcher Steve Orloff finds little effect of irrigation cutback at Tulelake, Calif., where the perched water table is accessible to the deep-rooted alfalfa plant.

TULELAKE, Calif. – Steve Orloff is still looking for answers to the impact of deficit irrigation on alfalfa. The results he got in 2004, however, show there’s not that much of a drop in tonnage when no irrigation occurs after the second cutting.

Orloff, a University of California farm advisor in Siskiyou County and a prolific researcher of forage and grain crop performance, thinks the answer to yields is in the soil and the depth of water beneath it. That also confounds an easy way to put a value on forgoing irrigation of alfalfa.

A few miles north in the Klamath Basin, where Oregon State University established alfalfa variety trials at Klamath Experiment Station under both irrigated and non-irrigated conditions, there’s a marked decrease in yield for the dry plots.

Rich Roseburg, the KES agronomist and a soil scientist, said that’s to be expected because the heavier soils at his site don’t have a perched water table accessible to long-rooted alfalfa. Roseburg said he’s establishing plots to repeat the deficit irrigation work Orloff launched three years ago.

Driving the experiments is a need to gather data that farmers can use with U.S. Bureau of Reclamation to put a price on forgoing irrigation of alfalfa for part of a season.

BuRec is under mandate to line up seasonal water supplies to supplement natural flows on the Klamath River.

Called a Water Bank, the BuRec program spends money each year paying farmers not to irrigate for a full season, or to pump groundwater into ditches where it can be returned to the Klamath.

There’s no part-season water bank deal at present.

“We’ve still not answered the true water savings (of a cutoff),” said Orloff.

What he has answered on the plots is that the stand survives well if it has deficit irrigation and the next year is given normal irrigation. Plant counts were steady, and yields returned to normal in the year after deficit treatment.

The UC experiment on silt loam soils at Tulelake, where the 2004 water table was estimated to be 3 to 3 1/2 feet beneath the surface, showed 4 tons per acre yield under normal irrigation practice. When irrigation was cut off July 15, total yield was 3.5 tons per acre. With a cutoff after first cutting, yields were 3.4 tons per acre.

When the same experiment was run on sandy loam soil near Malin, Ore., normal irrigation had 2.8 tons per acre, July 15 cutoff dropped to just over 2 tons per acre, and the first-cutting cutoff was a bit over 1.7 tons per acre.

Orloff repeated the test in Scott Valley of Siskiyou County, where 2.8 tons came under normal irrigation, the July 15 cutoff dropped to just over 2 tons, and a first-cutting cutoff dropped total yield to about 1.4 tons. There’s no perched water table in the Scott Valley.

Tam Moore is based in Medford, Ore. His e-mail address is tmoore@capitalpress.com.



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