Harvest Frequency and Timing Effects on Dry Matter Yield of Two Switchgrass (Panicum virgatum L.) Cultivars.

Switchgrass is a C₄ perennial warm-season grass adapted to the southeastern U.S. Defoliation management regimes may provide opportunities to extend the season of available forage or biomass. The objectives of this study were to evaluate the effects of harvest frequency (one, 1x; or two, 2x, clippings per year) and harvest timing (before frost, BF; after frost, AF, and late-winter, LW) on dry matter yield (DMY), crude protein (CP), and digestibility (IVTD) of ‘Performer’ and ‘BoMaster’ cultivars. For the 2x clippings, the first harvest was done in June (2x-June) and the second harvest followed the above-mentioned harvest timings (2x-BF, 2x-AF, and 2x-LW). The research was conducted at the Central Crops Research Station in Clayton, NC. For ‘BoMaster’, there was no difference in total DMY between 1x and 2x for BF (~10 Mg ha^-1). The DMY of 2x remained constant across harvest timings; nevertheless, DMY of 1x was lower by 34% for AF and 52% for LW compared to BF. The CP and IVTD were greater for 2x (57.3 and 390.4 g kg^-1 for CP and IVTD, respectively) compared to 1x (38.3 and 300 g kg^-1 for CP and IVTD, respectively). For ‘Performer’, DMY BF was greater for 2x (10 Mg ha^-1) than 1x (7.9 Mg ha^-1). Delaying harvest reduced DMY of 1x by 34% for AF and by 48% for LW compared to BF. The CP and IVTD were greater for 2x (65.4 and 414 g kg^-1 for CP and IVTD, respectively) compared to 1x (44.8 and 357.7 g kg^-1 for CP and IVTD, respectively). The CP and IVTD for the 2x-June harvest for both cultivars was ~80 and ~660 g kg^-1for CP and IVTD, respectively. The 2x clipping strategy has potential to serve as dual forage-biomass system and to harvest biomass material during AF and LW without compromising DMY.