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FORAGE & GRAZINGLANDS

Impact of Long-Term Phosphorus and Potassium Fertilization on Alfalfa Nutritive Value–Yield Relationships

^a Dep. of Agronomy, Purdue Univ., 915 West State St., West Lafayette, IN 47907-2054
^b FFR Cooperative, 4846 East 450 North, Lafayette, IN 47905

^* Corresponding author (jvolenec{at}purdue.edu).

Phosphorus (P) and potassium (K) increase alfalfa (Medicago sativa L.) yield, but little information is available on how forage nutritive value is affected by P and K fertilization. The objective of this study was to investigate the effect of long-term P and K fertilization on alfalfa yield–forage nutritive value relationships. A factorial experiment with four P and five K treatments was replicated four times. Beginning in 1998 and continuing through 2004, herbage samples were collected in May, June, July, and September. Samples were analyzed for crude protein (CP), neutral detergent fiber (NDF), acid detergent fiber (ADF), in vitro true dry matter disappearance (IVTDMD), and acid detergent lignin (ADL) using near-infrared reflectance spectroscopy. Soils were also analyzed for P and K concentrations. Concentrations of NDF, ADF, and ADL were reduced by low P and K fertility. Addition of P fertilizer increased CP concentrations, while addition of K fertilizer reduced CP concentrations. Concentrations of NDF, ADF, and ADL and dry matter yield decreased with stand age and harvest within year. The low forage dry matter yield of unfertilized alfalfa was associated with higher IVTDMD. Yield of digestible nutrients per hectare (yield x IVTDMD) was highly correlated with yield but not IVTDMD concentration. Fertilizing for high yield, despite the slight reduction in forage nutritive value, remains the most viable strategy for maximizing digestible nutrient production per hectare in alfalfa.

Abbreviations: ADF, acid detergent fiber • ADL, acid detergent lignin • CP, crude protein • CPY, crude protein yield • DNY, digestible nutrient yield • IVDMD, in vitro dry matter disappearance • IVTDMD, in vitro true dry matter disappearance • NDF, neutral detergent fiber • NIRS, near-infrared reflectance spectroscopy