Bloat in ruminants: the cell rupture hypothesis

An alfalfa (Medicago sativa L.) cultivar with a low initial rate of digestion (LIRD), AC Grazeland B-r, was developed to test the hypothesis that the rate of cell rupture was a cause of frothy bloat in cattle or sheep. The amount of cell wall increases and digestibility declines as plants mature. Thus the cell rupture theory also predicts that the bloat potential of alfalfa is related to maturity. This study compared the digestibility of cell walls (NDF) in LIRD cultivars with that of a standard, Beaver, in relation to maturity and bloat incidence. Fresh alfalfa was fed to wethers during two experiments. NDF was greater (Pless than or equal to.05) in Beaver (43.9%) than the less mature LIRD cultivar (41.1%) during Experiment 1. The digestibility of NDF was greater (Pless than or equal to.05) in the LIRD cultivar (48.4%; Beaver=44.5%) during Experiment 2, when both cultivars were in vegetative stages of development but NDF digestibility was lower (Pless than or equal to.05) when the cultivars had matured to the bud stage (LIRD=37.1%; Beaver=48.4%). Bloat incidence did not differ between cultivars at the same stage of development. The number of bloats declined as the alfalfa matured, an effect which was attributed to the barrier to microbial invasion created by increasing amounts of cell wall and reduced cell wall digestibility. The cell rupture hypothesis of frothy bloat was supported by these results demonstrating that alfalfa maturity plays a key role in bloat etiology.