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Optimizing Phosphorus Fertilizer Rates in Alfalfa Hay Production by Using Tissue Testing

Agronomy & Pest Management

Steve Norberg
Regional Forage Specialist
Washington State University
Pasco

Abstract

Knowing critical alfalfa phosphorus level in-season based on hay and phosphorus fertilizer prices improves recommendations and applications.  Using hay quality samples for tissue testing saves producers time, expense and effort since many growers already pull these samples. To do this tissue testing needs to be refined for more precise critical levels. Research from 2018-2021 provided detail information on phosphorus yield, tissue concentration and optimum economic fertilizer rates at early-bud stage on an irrigated fine sandy loam soil near Prosser, WA. The experiment was designed as phosphorus (P) rate study with using monoammonium phosphate (MAP); including: 0, 30, 60, 120, 240 lb P2O5 acre-1 on a low testing P soil 5-10 ppm (Olsen P method). Fertilizer application of 240 lb of P2O5 resulted in increased yield alfalfa of 25.5, 18.1, 10.3, 7.0, 7.4 and 12.9 % of established first, second, third, fourth and fifth cuttings, and average of establishment year cuttings, respectively. Early spring P applications resulted in first cutting responsible for 52.6 and 43.0 % of the total yield increase for established, and established cutting, respectively.  Maximum yield phosphorus concentration was found to be 0.42, 0.49, and 0.38 % for established first, second, and third cuttings, respectively. When hay is $220 ton-1 and P2O5 price was 0.77 lb-1, optimum economic whole plant P concentration at early bud stage was determined to be 0.33 and 0.37 % for first and second cuttings, respectively. When P2O5 price is 0.77 lb-1 increasing hay price from 120 to 400 dollars ton-1 increased first cutting economic optimum whole plant P levels from 0.30 to 0.38 %. When hay price is $220 ton-1, increasing phosphorus fertilizer costs from $0.58 to $1.15 pound of P2O5 decreases first, cutting economic optimum whole plant P levels from 0.35 to 0.29 %. Knowing these P levels can allow adjustment of your P fertilizer rates for next year. Research results from this trial have been shared at the NW Hay Expo, Western Regional Nutrient Management Conference, North American Alfalfa Improvement Conference and the World Alfalfa Congress.

Authors: Steve Norberg, Don Llewllyn, Steve Fransen
  1. Steve Norberg Regional Forage Specialist, Washington State University, Washington, 99301
  2. Don Llewllyn State Livestock Specialist, Washington State University, Washington, 99122
  3. Steve Fransen Emeritus Professor, Washington State University, Washington, 99301