Forage Characteristics Limiting Animal Performance on Warm-season Perennial Grasses

Chemical and in vitro characteristics of six perennial summer grasses {common, ‘Coastal’ and ‘Coastcross-1’ bermudagrasses [Cynodon dactylon (L.) Pers.]; ‘Pensacola’ bahiagrass [Paspalum notatum Flugge]; common weeping lovegrass [Eragrostis curvula (Schrad.) Nees.]; and ‘Kleingrass 75’ [Panicum coloratum L.]} were determined to identify factors limiting the performance of grazing animals. In vitro dry-matter digestibility (IVDMD), cell wall content (CWC), acid-detergent fiber (ADF), acid-detergent lignin (ADL), silica, and in vitro cell-wall digestibility (IVCWD) were determined for samples of available forage taken at 14-day intervals. Animal performance was obtained from yearling heifers grazing established stands of each grass. Available forage was estimated from strips harvested from each pasture at 2- to 4-week intervals. Animal performance was significantly correlated (r = 0.85**) with available forage when dry-matter digestibility was relatively uniform. As IVDMD of available forage decreased, more available forage was required to reach maximum average daily gain (ADG). Also, as IVDMD decreased, maximum ADG decreased. The CWC of the grasses ranged from 45 to 82% with cell wall digestibility ranging from 82 to 36%, respectively. CWC and IVDMD were significantly correlated with animal performance (r = ∗0.80**, and 0.78**, respectively). Cell wall digestibility calculated from Van Soest's “summative equation” significantly over-estimated IVCWD. Lignification was one factor limiting cell wall digestibility (r = ∗0.76**) and in Coastcross-1 bermudagrass and Pensacola bahiagrass silica was as important as lignin. CWC and IVCWD could be used to accurately predict IVDMD in Kleingrass 75 (r = ∗0.96** and 0.95** respectively). High lignin and/or silica content of available forage was associated with poor animal perfortnance on all grasses.