Using Cored Bale Samples to Assess Alfalfa Nutrient Needs: Possibilities and Limits.

Plant tissue tests are better predictors of nutrient requirements than soil tests but are rarely utilized by alfalfa growers. The long-standing UC recommendation is to sample the standing crop at 10% bloom, and divide the sample into thirds.  The top third is analyzed for boron, molybdenum and copper, the leaf portion of the middle third for sulfur (SO₄-S), and the mid stem portion for potassium and phosphorus (PO₄-P).  Research was conducted in Northern California to determine if cored bale samples, like those used for forage quality analysis, could be used to assess the nutrient status of alfalfa fields.  Samples were collected in 2006 and 2007 from 39 alfalfa fields (two harvests each year) to compare three plant tissue testing protocols: fractionated plant parts, whole-top grab samples and cored samples from hay bales.  Three different locations were sampled in each field.  There was a strong relationship between all three sampling methods.  There was also a strong correlation between SO₄-S and total S and PO₄-P and total P. Additional research was conducted in controlled field studies to quantify changes in phosphorus concentration with advancing alfalfa maturity.  Phosphorus fertilizer was applied at six rates (0, 33, 66, 99, 132, and 165 kg ha⁻¹ P₂O₅) to established alfalfa fields in two high elevation valleys in March of 2007 and 2008.  Whole tops and mid stem samples were collected at early bud, mid bud and 10% bloom and analyzed for phosphorus concentration.  Phosphorus concentration declined significantly with advancing maturity. Mid stem samples were more affected by maturity than were whole tops.  These results suggest that cored bale samples analyzed for K, total P and total S could be used in lieu of fractionated stem samples but proper interpretation of the results requires standardizing data to account for changes in alfalfa maturity.