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Influence of Nitrogen Fertilizer on Stands, Yields of Herbage and Protein, and Nitrogenous Fractions of Field-grown Alfalfa¹

Studies of fertilizing alfalfa (Medicago sativa L.) with N were initiated because of the possibility of increasing protein yields since there is current interest in the leaves or herbage as a sourc of a concentrated protein for human and animal consumption. Two N fertilization trials were conducted at Arlington, Wis., on established stands.

The first experiment was conducted during 1968 with a 2-year-old stand of ‘Vernal’ alfalfa. Six N treatments (0, 56, 112, 224, 448, and 896 kg/ha of N as NH₄NO₃) were initiated in April and repeated in separate areas after the first harvest and after the second harvest. Each harvest was made at first flower. N had no significant effect on stands, herbage yields, or herbage total nonstructural carbohydrate percentages. N significantly increased protein yields only in the third harvest. Percentages of total-, non-protein-, nitrate-, free -amino-, asparagine-, and glutamine-N in leaflets, stems, and total herbage were increased with each increase in N rate, but percentage of ammonium N was not affected. Protein percentages in leaflets were not affected by N rates, but they increased slightly in both the stems and total herbage as N rate increased.

A second experiment was conducted during 1969 and 1970, in the same field of alfalfa as in 1968, when all N treatments had 336 kg/ha of N applied annually in various ways. Treatments were (1) no N; (2) all N applied in the spring, (3) after the first harvest, or (4) after the second harvest; (5) half the N applied in the spring and half after the first harvest, or (6) after the first and second harvests. All harvests were made at first flower. Stands were not affected by the treatments. N treatments significantly increased the total seasonal herbage and protein yields in both years, with a few exceptions in 1969 for protein. However, there was no significant difference among the N treatments that significantly increased yields. The average yield increase over the control amounted to only 6.1% for herbage and 12.2% for protein in 1969, and 16.0% for both herbage and protein in 1970.

Key Words: Leaflets • Stems • Protein N • Ammonium N • Nitrate N • Asparagine N • Glutamine N

² Research Assistant and Professor of Agronomy, University of Wisconsin, Madison, Wis., 53706.

Received for publication November 11, 1971.