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.
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