Results and Discussion Maturity Stage.

1. Sampling Technique and Maturity Effects on Nutrient Concentrations in Alfalfa Steve B. Orloff and Daniel H. Putnam University of California Cooperative Extension, Siskiyou County Department of Plant Sciences, University of California, Davis, CA Introduction Alfalfa is the most critical feed for California’s dairy industry, the #1 agricultural enterprise in the state. Occupying around 400,000 hectares, alfalfa represents an important component of California’s fertilizer and agricultural footprint. Analytical methods have been developed to assess the nutritional status of alfalfa fields for fertilizer management purposes. Soil tests are effective to detect some nutrient deficiencies but plant tissue tests are believed to be more accurate overall. Plant samples often better reflect nutrient availability and uptake vs. soil samples. Unfortunately, most alfalfa growers do not tissue test and will fertilize based upon past practice with little idea of the actual nutrient status of the field. Tissue testing techniques vary from state-to-state. Simplified and standardized methods of analysis could promote wider adoption of tissue testing for nutrient monitoring purposes. Currently all guidelines are based on sampling alfalfa at one-tenth bloom growth stage, but alfalfa is frequently harvested before this stage. Research was needed to compare different sampling methods and to evaluate the change in nutrient concentration with advancing maturity. Results and Discussion Maturity Stage. Maturity stage had a large effect on the concentration of P and K (Figure 1). The change due to maturity was relatively constant across sampling methods for P but not for K. Mid-stem K concentration was more affected by maturity than were whole tops or top 15 cm. Abstract Although plant tissue analysis is generally believed to be more accurate than soil analysis, it is less frequently used by alfalfa growers in the West to predict crop fertilization needs. A simple, standardized and accurate sampling protocol is needed to encourage adoption. Research was conducted to compare the P, K, S, B, and Mo concentration of alfalfa using three different sampling protocols. Alfalfa was sampled at early bud, late bud and 10 percent bloom for all three cuttings to determine the effect of plant maturity and time of year on nutrient concentration. Nutrient concentration declined with advancing maturity for all sampling methods and nutrients, necessitating staging protocols for sampling. The concentration of the micronutrients boron and molybdenum decreased slightly with advancing maturity, but the degree of decline was not considered to be sufficient to warrant adjusting critical values. 1 The effect of maturity stage and sampling method on phosphorus and potassium concentrations (mid stem value is PO4-P while top 15 cm and whole tops are total P). Each data point is the mean of 5 grower fields, ranging from low to high fertility , averaged over cuttings. • Correlations between methods. • Nutrient concentrations were strongly correlated between the different sampling methods for P and K (Figures 2 and 3). The R2 values for the relationship between whole tops and mid-stems was 0.79 and 0.91for P and K content, respectively. Similarly, the whole tops and top 15 cm were closely related (R2values of 0.85 and 0.94 for P and K content , respectively). For the remaining nutrients, the correlation between sampling methods was stronger for some nutrients than for others. The strongest relationship between sampling methods occurred with molybdenum with a weaker relationship for sulfur and boron. We did not find a consistent difference in nutrient concentration between cuttings. • The strong relationship observed between sampling methods suggests that a whole-top sample, which is similar to a cored-bale sample, could be used for nutrient analysis. This would enable growers to use the same sample they currently use for forage quality analysis for nutrient analysis as well. This may encourage adoption of plant tissue testing to guide fertilization practices. 2 Whole tops (P, K, S, B, Mo) Top 15 cm (P, K, S, B, Mo) The relationship between whole top and top 15 cm sampling protocols for phosphorus and potassium concentration. Each data point represents a separate field/cutting. Alfalfa plant parts sampled at each sampling date (early bud, late bud and 10% bloom for all 3 cuttings) and analyses performed on respective plant parts. Top 1/3 (B, Mo) • Summary • Nutrient concentration declined with advancing alfalfa maturity for all three sampling protocols, necessitating standardization of maturity for sampling. • There appeared to be no advantage to sampling portions vs. whole plants suggesting adaptation of the most practical method (whole plants) for sampling. • Nutrient concentrations were strongly correlated between the different sampling methods for P and K. The correlation was stronger for some nutrients than for others. • Cored bale sampling (similar to whole tops) may be a recommended procedure due to ease of use and the ability to combine with normal sampling for forage quality analysis. Mid 1/3 stems (PO4-P, K) Mid 1/3 leaves (SO4-S) 3 • Methods • Plant tissue samples were collected in 2009 from five different alfalfa fields in distinct agricultural valleys in the intermountain area of Northern California. • Collected from all three harvests. • Taken at early bud, late bud and one-tenth bloom. • Protocols Compared: • whole plant samples (similar to cored-bale samples), • top 15 cm samples (used in some states) • fractionated plant samples (current UC protocol ). • The P, K, S, B and Mo concentrations determined using standard laboratory techniques. The relationship between whole top and mid-stem sampling protocols for phosphorus and potassium concentration. Each data point represents a separate field/cutting.