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Herbage Dry Matter Yields of Switchgrass, Big Bluestem, and Indiangrass with N Fertilization¹

Herbage dry matter (DM) yield potential and N requirement of native perennial, warm-season grasses must be known before these species can be effectively used in forage-livestock systems. This study was initiated to determine the herbage yield response of ‘Blackwell’ switchgrass (Panicum virgatum L.), ‘Kaw’ big bluestem (Andropogon gerardi Vitman), and ‘Nebraska 54’ indiangrass (Sorghastrum nutans L. Nash) to N fertilization.

Four replications of a randomized complete block field experiment were established on a Clarion loam (Typic Hapludoll) soil in 1971. A four-cut management system was used in 1974; a two-cut system was used during 1975–1976. Applications of ammonium nitrate (34-0-0) were made on 21 May 1974 and 13 May 1975 at 0, 75, and 150 kg N/ha levels. Levels of 0, 75, and 150 kg were applied in 1976 as urea (45-0-0) on 22 May and again on 3 July after the first harvest. Two of the four replications received 50 kg P/ha as triple superphosphate (0-46-0) and 100 kg K/ha as K₂SO₄ . MgSO₄ (0-0-22) on 7 May 1976. The K source included 60 kg Mg/ha and 118 kg S/ha.

Switchgrass, big bluestem, and indiangrass produced herbage dry matter (DM) yields of 6.14, 6.29, and 5.59 metric tons/ha, respectively, when averaged over N levels and years. In 1974, big bluestem out-yielded the rest, and in 2 out of 3 years indiangrass yielded significantly less than one or both of the other species. All species generally responded positively to N through 75, and often through 150 kg N/ha.

No herbage yield response was observed in 1976 to applied P and K fertilizers, the latter containing both S and Mg.

Herbage DM yields generally increased with each of three successive harvests in 1974, whereas second harvest yields decreased with a two-cut management system used in 1975 and 1976. Relatively unfavorable July–August precipitation seemed primarily responsible for lower second harvest DM yields.

A 5 Oct. 1974 harvest of aftermath herbage cut at a 5-cm height suggests that 2.2 to 2.4 metric tons/ha of herbage would be available for use by nonlactating beef cows. No species difference in DM yield was observed, suggesting equal carrying capacity/ha, for the aftermath.

Key Words: Warm-season grasses • Panicum virgatum L. • Andropogon gerardi Vitman • Sorghastrum nutans L. Nash • Cutting management • Harvest date • P • K • S • Mg • Moisture stress

² Area crop production extension specialist, professor of agronomy, and graduate student in agronomy, respectively, Iowa State Univ., Ames, IA 50011.

Received for publication October 22, 1979.

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