The amount of gas associated with bloat of varying degrees of severity was determined by inflating a latex weather balloon in the rumen of cows after they had been fed a fresh lucerne diet. Twelve cows, with permanent rumen fistulae were used to determine the relationships between the volume of gas given and intra-ruminal pressure, bloat score, increase in girth, quantity of digesta in the rumen, and also digesta pH before and after eating. In addition, the intra-ruminal pressure and change in girth were measured in five cows fed lucerne when bloat did and did not occur. The cows were inflated to a pressure equivalent to moderate/ severe bloat (Grade 3#x2013;4). Girth increased by 21 cm and pressure was increased to 51 cm of water when 68 litres of gas had been given. Intra-ruminal pressure increased in an exponential fashion, per unit of gas given, whereas girth increased at an approximately linear rate until 60 litres of gas were given. Rumen digesta volume was 51 litres, so that the total volume of the rumen content of a cow with moderate/severe bloat would be about 115 litres. The large volume of gas responsible for pasture bloat will arise from both fermentation of feedstuffs and acidification of rumen bicarbonate. The rate at which gas is produced from these sources may affect the likelihood of a stable foam formation, but the presence or absence of digesta components able to stabilise the foam and trap the gas will be the principal factor affecting the occurrence of bloat.
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