Assessment of 3 sudangrass cultivars for summer stockering of beef steers1

Three sudangrass [Sorghum bicolor (L.) Moench ssp. drummondii (Nees ex Steud.) de Wet & Harlan] cultivars were evaluated for calf growth performance and nutritive value for 56 d during summer for 2 yr. Replicated (n = 3) 2.17-ha Piper (PIP), Monarch V (MONV), and HayKing/Pro-Max (HKPM) sudangrass pastures were rotationally stocked with British-Continental crossbred calves (initial BW = 233 ± 28.5 kg) using put-and-take management. Forage IVDMD (P = 0.14), herbage mass (P = 0.14), stocking rate (P = 0.65), forage allowance (P = 0.54), ADG (P = 0.83), and BW gain per hectare (P = 0.46) were comparable among cultivars. Year × grazing period interactions (P ≤ 0.03) were detected for IVDMD, NDF, ADF, CP, stocking rate, forage allowance, ADG, and BW gain per hectare. Forage IVDMD and CP concentrations were greater (P < 0.01) on d 0 than on d 56, and forage NDF and ADF concentrations were greater (P < 0.01) on d 56 than on d 0 in both 2005 and 2006, but the magnitude of these changes varied by year. Thus, as forage matured, nutritive value decreased. This led to decreased (P < 0.01) ADG and gain per hectare in the second half of the grazing season each year. Least squares means for ADG were 1.05 and 0.42 ± 0.04 kg/d in 2005 and 1.07 and 0.72 ± 0.04 kg/d in 2006 for the first and second 28-d grazing periods, respectively. Grazing management and diet modifications may be warranted to avoid decreases in animal performance on sudangrass pastures later in the grazing season.