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Spring Defoliation to Improve Summer Supply and Quality of Switchgrass

Dep. of Agronomy, Iowa State Univ., Ames, IA 50011.

^* Corresponding author.

Switchgrass (Panicum virgatum L.) normally is lower in forage quality by the time cool-season forage species are less productive in early summer. The objective of this study was to determine if the major supply of switchgrass herbage could be delayed by use of partial spring defoliation and still maintain adequate herbage quality. Switchgrass plots were fertilized with 0 or 120 kg N ha^–1 and defoliated at six time-intensity combinations. Yield and quality of initial spring and 6-wk regrowth were measured. Initial 1984 spring defoliation treatments were 1 June at 15 cm; a multiple defoliation at 15, 20, and 25 cm over 1-wk intervals beginning on 1 June; 15 June at 6 or 20 cm; and 29 June at 6 or 20 cm. All 1985 treatments were 9 d earlier. Fertilization with N increased 6-wk regrowth dry matter (DM) yields by 79 to 92% for 1 June treatments and by only 38 to 54% for 29 June treatments. Close defoliation on 29 June produced the greatest total seasonal DM yield of 10.5 Mg ha^–1 and IVDDM yield of 5.79 Mg ha^–1, but only 30% of this was harvested in midsummer after 6 wk of regrowth. Total season DM yield and IVDDM yield were about 20 to 35% less for both 1 June treatments when compared with close defoliation on 29 June, but the 6-wk regrowth yield accounted for about 75 to 85% of total season DM and IVDDM yields. Little or no difference in leaf or stem IVDDM of the 6-wk regrowth was observed among defoliation treatments, so partial spring defoliation can delay the major supply of switchgrass herbage with no serious reduction in herbage quality. We conclude that a short period of moderate defoliation of switchgrass in early to mid-June could benefit grazing programs by providing (i) some very high-quality switchgrass herbage during early to mid-June; (ii) a brief rest and regrowth period for the cool-season pasture; and (iii) relatively high summer yields of quality switchgrass when it is most needed.

Received for publication September 3, 1987.

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