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Phosphorus Fertilizer Response in Experimental Alfalfas selected for Different Phosphorus Concentrations¹

Six alfalfa, Medicago sativa L., synthetics from parents selected for high P concentration (HP synthetics) and four from parents selected for low P concentration (LP synthetics) were evaluated for first harvest forage yield and mineral concentration at two soil P levels at each of two locations. The experiment also included four synthetics from parents selected for low lignin concentration and four check cultivars. Seed of the experimental synthetics were produced by intermating a sample of plants from full-sib families that had been selected for forage P or lignin concentration in a previous generation. The experiment was conducted to determine if the two-parent synthetics from the selected full-sib families would maintain their relative ranking for P concentrations and if an interaction with soil P level could be detected. Yields of all entries in the experiment were less than normally observed for first harvest alfalfa in the region. The synthetics differed significantly for yield, but average yield of the HP synthetics was not different from that of the LP synthetics. Average forage P concentration of the HP synthetics was significantly greater than that of the LP synthetics at both soil fertility levels. Differences between the HP and LP synthetics were less than between their respective parental groups. Interactions between synthetics and P soil fertility level were not detected. The HP synthetics had a forage P concentration much less than that recommended for the diet of a moderately producing dairy cow. Thus successful selection for increased P concentration in the forage may not result in forage (hat meets animal requiremnts for P.

Key Words: Medicago sativa L. • Forage quality • Chemical composition

² Research agronomist, USDA-ARS (also adjunct professor of plant breeding), and assistant professor of soil fertility, respectively.

Received for publication January 21, 1983.