Switchgrass Herbage and Seed Yield and Quality with Partial Spring Defoliation

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Abstract
Partial defoliation of switchgrass (Panicum virgatum L.) during early to mid‐June followed by a regrowth period can provide high yields of quality forage for midsummer grazing. No information is available on effects of partial spring defoliation on subsequent sameyear seed yields. The objective was to determine the effect of spring defoliation and N fertilization on spring herbage yield and quality and fall seed yield and quality of switchgrass cultivars. Established swards of ‘Cave‐in‐Rock’ (CR), ‘Pathfinder’ (PF), and ‘Blackwell’ (BW) switchgrass were grown in Ames, IA, on a Webster‐Nicollet soil complex (fine‐loamy, mixed, mesic Typic Haplaquoll—Aquic Hapindoll) with 0 or 90 kg N ha−1 for 2 yr. Spring herbage yield was highest for CR. Mean yield increased 0.11 and 0.16 Mg ha−1 d−1 between late May and mid‐Jane and reached 1.97 and 3.01 Mg ha−1, respectively. Herbage crude protein (CP) and in vitro dry matter digestibility (IVDMD) decreased linearly 3.36 and 3.57 g−1 d−1 respectively, daring the same period. Herbage CP and IVDMD concentrations were increased by N fertilization, whereas N reduced days to anthesis for PF and BW but had no effect on CR. Mean anthesis date was delayed 0.36 d for each day of delay in defoliation after late May; more so for CR (an earlier and more rapidly growing cnltivar) than for PR and BW. Cultivar seed yield was similar, and N more than doubled seed yield. Seed yield of the uncut check (521 kg ha−1) was highest. Defoliation reduced seed yields 27.7 and 11.5 kg ha−1 d−1 for 90 and 0 kg N ha−1, respectively. Reduction in seed yield caused by partial defoliation resulted primarily from removal of leaf blades rather than apical meristem removal. Defoliation did not affect seed germination or percentages of total viable seed. We conclude that spring top‐grazing of switchgrass would substantially reduce seed yields.