Quantifying root structure response to multiple defoliation events in a grazing situation is critical in developing management plans for warm‐season tallgrasses. A pasture experiment was conducted in 1999, 2000, and 2001 near Mead, NE. The objective of the experiment was to determine the effect of timing and frequency of grazing on big bluestem (Andropogon gerardii Vitman) etiolated tiller growth and root and rhizome structure. Paddocks were grazed at a stocking rate of 9.9 Animal Unit Month (AUM) ha−1 in two to four cycles from mid‐May to early‐September. In April 2002, five 6.6‐ × 132‐cm soil cores were extracted from each paddock. Soil cores were subsampled at 30‐cm depth increments for estimates of root mass, root surface area, and root volume. Etiolated tiller tents were used to estimate organic reserves of big bluestem in each paddock in spring 2002. Mean number and weight of etiolated tillers were reduced by up to 40% and 50%, respectively, in paddocks grazed in a sequence of June after internode elongation, early August, and early September. Root structure in the top 30 cm of the soil profile was affected most by multiple defoliation events with <40 d of recovery between grazing periods. Root mass decreased by 25%, while mean surface area and volume of roots declined 10 and 15%, respectively, in the upper 30 cm of the soil profile in paddocks grazed in the sequence of post‐internode elongation in June, early‐August, and early‐September. To maintain vigorous big bluestem pastures, grazing management should concentrate on the elongation and postelongation periods. Grazing at the elongation stage should be rotated among paddocks in successive years and the recovery period following grazing at internode elongation should be >40 d.

Big bluestem (Andropogon gerardii Vitman) has a rapid growth phase that begins in early to mid‐June in eastern Nebraska. During this rapid growth phase, rate of biomass accumulation exceeds intake rate of grazing livestock, resulting in low levels of harvest efficiency. To delay the rapid growth phase, big bluestem pasture can be grazed in mid‐ to late May without affecting herbage yields for the remainder of the growing season. A pasture experiment was conducted in 1999, 2000, and 2001 near Mead, NE. The objective was to determine the effect of timing and frequency of grazing big bluestem pasture, following a May grazing period, on cumulative pregrazing yields, cumulative herbage disappearance, resulting harvest efficiency, leaf/stem ratio, and stand persistence. Yield and morphological characteristics were obtained immediately before and after each grazing period, and basal cover of big bluestem was estimated annually. May grazing had no effect (P < 0.1) on cumulative pregrazing yields and resulted in an increase of cumulative herbage disappearance (3638 vs. 2673 kg ha−1) and leaf/stem ratio (2.02 vs. 2.83) compared with paddocks with no May grazing. Grazing at the vegetative stage in June compared with the elongation stage resulted in an increase in cumulative pregrazing yields (10774 vs. 9510 kg ha−1), cumulative herbage disappearance (4116 vs. 3194 kg ha−1), and leaf/stem ratios (2.57 vs. 1.98). Grazing at the elongation stage in June followed by a grazing period in early August is not an advisable management strategy.