Get reliable rangeland science

It has been proposed that the high silica content of medusahead (Taeniatherum caput-medusae ssp. asperum) reduces digestibility and thus intake by ruminants, making this weed a successful competitor in grazed plant communities of the western US. The goals of this study were to measure the final digestibility and digestibility kinetics of medusahead relative to more palatable forages (alfalfa and tall fescue hay) at (1) different phenological stages (from late vegetative to beginning of senescence and thatch) and (2) at different particle sizes (1, 5, 10, and 20 mm). Samples were incubated at 2, 4, 6, 8, 12, 18, 24, 36, 48, and 72 h in 2 replicates using an in vitro gas production method. The experimental design was a factorial with replicates as the repeated measure. Silica content was greater in medusahead at different phenological stages (6.4 to 10.5% range) than in tall fescue (4.1%) or alfalfa hay (0.9%). Digestibility of medusahead at different phenological stages, except thatch (64.8 to 70.5% range), was similar (P > 0.05) to that of tall fescue hay (67.0%) and greater (P < 0.05) than digestibility of alfalfa hay (53.3%). Medusahead thatch had the lowest digestibility (45.9%; P < 0.05). Fermentation kinetics showed medusahead with similar fermentation rates to tall fescue hay but lower fermentation rates than those of alfalfa (P < 0.05). Greater particle sizes reduced digestibility and fermentation kinetics of medusahead to a greater extent than in the rest of the forages tested (P < 0.05). Thus, fermentation kinetics and particle size reduction, instead of just digestibility values, explain the low intake and palatability of medusahead at phenological stages other than thatch. The high silica content of the weed likely decreases the mechanical and enzymatic breakdown of the plant material in the rumen, decreasing passage rate and prolonging fill effects in the rumen.