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Phosphorus and Potassium Applications Influence on Alfalfa Hay Quality and Value

Agronomy & Pest Management

Steve Norberg
Regional Forage Specialist
Washington State University
Pasco

Abstract

In two field and laboratory experiments, research was conducted 2019-2020 to determine impacts of nutrient applications on alfalfa hay yield, quality and nutrient value at Irrigated Research and Extension Center near Prosser, WA. The phosphorus (P) study P soil tests levels were 6.7 and 5.7 ppm at the beginning of 2019 and 2020 (Olsen P test). The potassium (K) study was conducted on a soil test level of 85 K in 2018. Applying phosphorus increased protein, NDFD 48, and Net Energy for Lactation (NEL) concentration and decreased ash and lignin concentration. Over five seasonal alfalfa cuttings, phosphorous increased protein higher in the first and second cuttings. Applying 240 lb. P2O5 increased total nutrient value of hay by $25 and $42 ton-1 averaged over cuttings and first cutting, respectively.  The total nutrient value increases over control was 382, 519, 930 and 1,004 dollars acre-1 year-1 when 30, 60, 120, 240 lbs. P2O5 acre-1 was applied, respectively.  Applying 320 lb K2O acre-1 increased protein concentration 1% when averaged over cuttings. Although not statistically significant applying 320 lb K2O acre-1 increased nutrient value of hay by $12 ton-1. Total nutrient value in $ acre-1 year-1 increase over the control was 121, 372, 407, 534, 722 when 40, 80, 160, 240, 320 lbs. K2O acre-1 was applied, respectively. Nutrient values responded more to phosphorus at 8 ppm than potassium at 82 ppm in second- and third-year alfalfa. Relative Feed Value (RFV) is a poor indicator of alfalfa nutrient value because RFV failed to detect value increases of protein and NDFD 48. Both protein and NDFD 48 directly impacts dairy animal digestibility and fiber fill and are not included in RFV calculations. The impact of negative forage quality parameters, ash and lignin, in alfalfa hay economic value can be partially mitigated through nutrient applications of P and K and viewed on a land area basis.

Authors: Steve Norberg, Steve Fransen
  1. Steve Norberg Regional Forage Specialist / Professor, Washington State University, Washington, 99301
  2. Steve Fransen Associate Professor, Washington State University, Washington, 99350