Core Ideas Defoliation management resulted in a wide range of plant responses that varied by year, with deleterious responses more evident in the second year. Greater leaf/stem ratios occurred with more frequent defoliation treatments. Frequent defoliations such as every 3 wk should maintain a stubble height of at least 40 cm, defoliation frequencies ≥6 wk to 20‐cm stubble height, and ≥9 wk to 10 cm stubble height are warranted to ensure stand persistence. ‘Performer’ switchgrass (Panicum virgatum L.) was released in 2006 by the USDA‐NCSU forage program because of its greater digestibility compared to standard cultivars grown in the southeastern United States. Forages with greater digestibility have potential to positively impact animal responses, however, defoliation management can influence its productivity and persistence. The objectives were to determine the effect of the factorial combination of four defoliation heights (DH) (clipped to 10, 20, 30, and 40‐cm) and four defoliation frequencies (DF) (clipped every 3, 6, 9, and 12 wk) on productivity, leaf/stem ratio, and tiller counts. The experiment was conducted in 2016 and 2017 at the Central Crops Research Station, Clayton, NC. The 16 treatments were allocated in a complete randomized block design replicated four times. In 2016, total dry matter (DM) yield ranged from 4.2 to 13.1 Mg ha−1 being greatest for the 9‐ and 12‐wk DF at 10‐ and 20‐cm DH and lowest for all DH at 3‐wk defoliation frequency. In 2017, the DM yield range was wider ranging from 0.5 to 14.0 Mg ha−1 with lower tiller counts for the 10‐ and 20‐cm DH, 3‐wk DF treatments. Leaf/stem ratio was greater for the more frequently defoliated treatments in both years. ‘Performer’ switchgrass is a productive forage and on the basis of total DM harvested, leaf/stem ratio, and tiller counts, frequent defoliations such as every 3 wk to 40‐cm stubble height, ≥6 wk to 20‐cm stubble height, and ≥ 9 wk to 10 cm stubble height, are warranted to ensure stand persistence.

Defoliation management can determine the presence or absence of desirable forage species. Canopy characteristics and light interception are two critical determinants of productivity and can influence weed pressure. The objectives of this study were to determine the effect of the factorial combination of four defoliation heights (clipped to 10, 20, 30, and 40 cm; DH) and four defoliation frequencies (clipped every 3, 6, 9, and 12 wk; DF) on light interception, canopy height, and weed canopy cover and frequency for ‘Performer’ (Panicum virgatum L.) switchgrass and to relate the abovementioned responses to previously reported productivity measurements. The experiment was conducted for 2 yr (2016 and 2017) at the Central Crops Research Station, Clayton, NC. The 16 treatments were allocated in a randomized complete block design replicated four times. Across years, light interception before harvest ranged from ∼45 to 88%, canopy height ranged from 35 to 97 cm, and weed cover and frequency ranged from 0 to 75 and 0 to 95%, respectively. Greater canopy height was associated with greater light interception and lower weed infestation. Weed infestation occurred mainly for treatments harvested every 3 and 6 wk and defoliated to lower stubble heights; however, there was no impact of DH for DF treatments harvested every 9 and 12 wk. Canopy light interception values of at least 70% obtained in our study were associated with defoliation thresholds previously reported in the literature for sustained yields of ‘Performer’ switchgrass. Switchgrass canopy light interception values of at least 70% were achieved with canopy heights before harvest of ∼60 cm tall.

Forage species with greater nutritive value have the potential to positively affect animal responses. 'Performer' switchgrass (Panicum virgatum L.) was released because of greater digestibility and lower lignin concentrations as compared to 'Alamo' and 'Cave-in-Rock.' However, the relationship between nutritive value, canopy characteristics, and dry matter yield for this species has not yet been established. The goal of this study was to determine in vitro true digestibility (IVTD), crude protein (CP), neutral detergent fiber (NDF), and acid detergent fiber (ADF) as a function of a wide range of defoliation management strategies to aim optimize production of nutritious forage. Treatments were the factorial combination (4 × 4) of defoliation height (DH; clipped to 10, 20, 30, and 40 cm) and defoliation frequency (DF; clipped every 3, 6, 9, and 12 wk). The range of digestibility values was greater due to DF (from 590-779 g kg −1 when averaged across DH treatments) than DH treatments (from 675-692 g kg −1 when averaged across DF treatments). In general, frequent defoliation resulted in greater IVTD and CP but lower yields; however, there were interaction effects of DF × DH for all response variables. With the exception of NDF, all response variables had strong correlations with dry matter yield, canopy height, and leaf/stem ratio. Although there are tradeoffs when managing for productivity and nutritive value, there is a wide range of defoliation management options for 'Performer' switchgrass that provide flexibility in terms of harvesting schedules to optimize productivity and persistence of nutritious forage.

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Constant supply of biomass from the field is limited by the seasonality of production of warm‐season grasses in the transition U.S. region. Delaying harvest after occurrence of freeze may be an alternative to extend the biomass supply period of switchgrass (Panicum virgatum L.) in North Carolina. The objectives of this study were to evaluate the effects of harvest frequency (HF) and harvest timing at the end of the growing season (HT) on switchgrass biomass yield, nutrient (N, P, and K) removal, and dry matter (DM) and ash concentrations. Treatments were the factorial combination of two HF (clipped once [1X] or twice [2X] per season) and three HT (before freeze in October, after first freeze in November, and late winter in February). Delaying harvest after occurrence of freeze did not affect total annual biomass yield for the 2X treatment (average of 15.5 Mg ha−1), whereas for 1X yield declined from 14.4 to 10.1 Mg ha−1 when harvest was delayed from October to February. Ash concentration declined from 29 g kg−1 in October to 14 g kg−1 in February. The DM concentration level reached in February was lowest (893 g kg−1) and it would be considered safe for storage of biomass. Nutrient removal was consistently greater for 2X than 1X (ranging from 43 to 137, 3.6 to 25.1, and 54 to 213 kg ha−1 for N, P, K, respectively). Delaying harvest of switchgrass after a freeze event is feasible when clipping twice a year to extend the window of biomass supply.