Research by University of California agronomists indicates the traditional timing of tissue sampling for phosphorus and potassium in non-dormant alfalfa at the first cutting does not fit present-day emphasis on dairy-hay quality.
That’s one of several tips on testing for nutrient deficiencies in alfalfa given by Jerry Schmierer, agronomy farm advisor for Colusa, Glenn, Sutter, and Yuba counties.
At a recent alfalfa and forage field day at the UC Kearney Agricultural Center at Parlier, Schmierer said timing of the phosphorus and potassium sampling was established in the 1960s when the practice was to harvest at 10 percent bloom for higher yield.
However, growers today rarely let their hay mature beyond the bud stage because of market incentives for higher quality.
Schmierer said testing at the first cutting does not reflect true nutrient phosphorus and potassium values in non-dormant alfalfa in California’s interior valleys because typically the crop still has irregular spring growth at that time.
However, he added, one exception is testing for sulfur, which, when adjustments are made for soil temperature and moisture that govern uptake of sulfur, is accurate in non-dormant areas at first cutting.
Another exception is when a non-dormant stand has been treated with paraquat, after which the re-growth will be even.
Alfalfa researchers 40 years ago based recommendations on data from dormant varieties in intermountain areas where first cutting sampling is accurate because the growth is even there at that time.
“But now we see dilution of the phosphorus and potassium as plants gain biomass in non-dormant areas, so to get a valid test we need good, uniform growth, not something with very immature stems mixed with the mature,” he said after trials in three locations in the Sacramento Valley.
Collaborating with him in the trials were UC colleagues Roland D. Meyer, Extension soils specialist, and Dan Putnam, Extension alfalfa specialist.
Pending additional research at additional locations, Schmierer said timing of sampling for phosphorus and potassium could be at a later cutting, according to the grower’s preference.
“Many alfalfa growers are realizing they can’t make dairy-quality hay in the middle of the summer, so they give the alfalfa a chance to rejuvenate, get some bloom on, and store some carbohydrates,” Schmierer said
“That’s also good for sampling when the phosphorus and potassium levels are pretty stable. We hope to get a better handle on timing after we have additional data.”
Schmierer, Putnam, and Steve Orloff, Siskiyou County farm advisor, have submitted a proposal for funding through the California Department of Food and Agriculture’s Fertilizer Research and Education Project of a two-year study to sample fields throughout the state, from the Imperial Valley to Tulelake.
More detailed investigations would also shed light on other essentials of alfalfa tissue sampling. For the moment, Schmierer said he’s using canopy height as a measurement of plant biomass.
He said he was surprised to learn that phosphate initially declines as much as 100+ ppm per day as the canopy height increases, and then the decline levels off.
He also found that total phosphorus and potassium decreased in a similar manner as biomass increased, while SO4-S levels were static depending on soil temperature and moisture content.
For the present, Schmierer has compiled some tentative guidelines for critical levels of phosphate, which should be adjusted upward from the 800 ppm concentrations when samples are taken earlier than at the 10 percent bloom stage.
At early flower, or one node and one open flower, the concentration should be 1,000 ppm, plus or minus 100 ppm. At late-bud, or three or more notes with buds and no flowers, it should be 1,500 ppm, plus or minus 200 ppm. At mid-bud, or one to two nodes with visible buds and no flowers, it should be 2,000 ppm, plus or minus 200 ppm.
He used the factional sample method in his trials. The sample is cut into thirds. The top third is analyzed for total phosphorus, along with boron, molybdenum and copper. With the middle third, stems and leaves are separated, the stems evaluated for phosphorus and potassium and the leaves tested for sulfur. The lower third is discarded.
Schmierer also said he hopes the research project will come up with a method to test bales with core samples.