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Supplementation of cattle on pasture is a well-known viable strategy for increased performance per animal and per ha. Ethanol has been proclaimed as a sustainable alternative to fossil fuels, and the vast supply of dried distillers' grains with solubles (DDGS) generated from ethanol production provides opportunities for addition to feedlot rations and supplementation of grazing cattle. A large percent of the cattle in the United States are located in the Southeast, where bermudagrass is the primary introduced forage species. Since relatively few research studies have been conducted using DDGS as a supplementation on actively-growing warm-season pastures, this research represents a significant and critical knowledge database for the pasture-beef industry. The first objective of this research was to evaluate performance of stocker calves grazing 'Tifton 85' (TIF) bermudagrass with varying rates of DDGS supplementation. Steers (n = 112, 364 ± 3.5 kg BW) were stratified by BW and allocated randomly to each of 16 pastures (0.7 ± 0.01 ha) across 2 years (2014, 2015). The TIF pastures were allocated randomly to each of 4 rates of DDGS: 0, 0.25, 0.5, or 1% BW hd-1 d-1. Average daily gain was greatest (P < 0.05) from steers offered 1% BW (1.3 kg/d), and least from non-DDGS steers (0.8 kg/d), with 0.25 and 0.5% BW intermediate (1.1 and 1.2 kg/d, respectively). There were supplemental feed to additional gain ratios of 3.7, 6.0 and 9.0, respectively, for 0.25, 0.5 and 1% BW. The second objective was to evaluate performance of Bos indicus versus B. taurus stocker calves grazing 'Coastal' (COS) bermudagrass with varying rates of DDGS. Steers within a breed type (n = 117, 343 ± 4.5 kg BW) were stratified by BW and allocated randomly to each of 9 pastures (1.3 ± 0.17 ha) across 2 years (2014, 2015). Pastures were allocated randomly to each of 3 rates of DDGS: 0, 0.25, or 1% BW hd-1 d-1. Across breed types, ADG was greater (P < 0.05) from steers offered 1% BW (1.0 kg/d) than either from non-DDGS steers or 0.25% BW (0.7 kg/d each). There was no difference (P = 0.70) in ADG between breed types. The third objective of the study was to evaluate any carryover effects in long yearling stockers previously stocked on TIF or COS with varying rates of DDGS. Following the pasture x DDGS phase in the first 2 objectives, steers were shipped 693 km to a commercial feedlot for an approximate 140-d finishing phase. At finishing, animals were transported 59 km and harvested at a commercial abattoir. There was no effect (P ≥ 0.36) of DDGS for either TIF or COS during the feedlot phase. Likewise, there was no effect of DDGS on percent Low Choice for TIF (P = 0.74) or COS (P = 0.86). The fourth objective was to evaluate in vitro digestion kinetics and methane production potential of TIF or COS with varying rates of DDGS. Hand-plucked plant parts of TIF and COS were mixed independently with DDGS to represent the intake of cattle at various rates of DDGS (0, 0.25, 0.5 or 1% for TIF; 0, 0.25 or 1% for COS) in each of the first 2 objectives. Newly-generated laboratory samples were incubated in an in vitro anaerobic fermentation chamber. In vitro true degradation (IVTD) and in vitro NDF degradation (IVNDFD) were greater (P < 0.05) from TIF (69% IVTD, 57% IVNDFD) than from COS (62% IVTD, 44% IVNDFD). Treatment of DDGS did not differ (P > 0.05) within TIF for IVTD, but 1% DDGS was greater (P < 0.05) than 0%, with 0.25% DDGS intermediate to the 2 for COS. Concentration of CH4 was also greater (P < 0.05) TIF (14 mM) than COS (12 mM), and CH4:H2 ratio was less (P < 0.05) from TIF (3:1) than from COS (2:1). The fifth and final objective was to evaluate the ruminal in situ digestion kinetics of TIF as affected by forage seasonality and DDGS. Samples of TIF were harvested in June, August and October 2014. Ruminally-fistulated steers (n = 6) were stratified by BW to 3 pens, and pens were allocated randomly to 1 of 3 rates of DDGS: 0, 0.25, or 1% BW hd-1 d-1. Samples were incubated for 2, 4, 8, 12, 24, 72, and 96 h. A trend of decreasing degradation of DM, NDF, and ADF was observed with both increasing seasonality (P ≤ 0.01) and increasing DDGS (P ≤ 0.04). The undegraded fraction (UDG) of DM from TIF was least (P < 0.05) from June (19%), followed by August (34%), and greatest from October (41%). The UDG from TIF DM was not different (P = 0.47) based on DDGS, and averaged 31%. Low rates of DDGS may be feasible using TIF but not COS at our vegetational region. Rate of DDGS on pasture did not affect feedlot performance nor carcass characteristics. Increased rates of DDGS may increase forage digestibility and decrease CH4 production, but this was cultivar-dependent. Reduction of DM degradation was dependent upon seasonality more so than DDGS rate. Overall, DDGS represented a viable option for stocker production systems, and economic assessments for each specific operation-location will be required prior to implementation of a supplementation program.. [ABSTRACT FROM AUTHOR]