Bloat in sheep ( Ovis aries)

Abstract

1. 1. Most of the field studies on bloat are conducted with cattle and most of the laboratory experiments seeking to explain the various parameters associated with bloat are done with sheep. 2. 2. Based on grazing behaviour, it would be expected that sheep might bloat more severely than cattle because they selectively choose to eat leaves over stems and chew what they ingest more frequently than cattle. Furthermore, sheep appear to select legumes over grasses because the legumes can be eaten more rapidly. However, because they are selective, sheep eat more slowly than cattle. Despite a higher bloat expectation, bloating in sheep is reported to be less of a problem than in cattle. 3. 3. Although frothing of rumen ingesta was described earlier in cattle as the cause of acute legume bloat, experiments with frothy bloat in sheep preceded those in cattle. 4. 4. Anti-frothing agents were used in sheep before cattle to treat acute legume bloat. 5. 5. Experiments devoted to the study of eructation in ruminants were carried out on sheep, then cattle. 6. 6. Convincing evidence that rumen motility does not cease during acute legume bloat was gathered using sheep. 7. 7. Although the transected tracheal technique for the determination of the volume of eructated gas was developed with cattle, the pathway of eructated gas was confirmed with sheep. 8. 8. All the current evidence accumulated from experiments with sheep supports the hypothesis that death due to legume bloat is caused by acute neural, respiratory, and cardiovascular insult resulting from the effect of the distended rumen on thoracic viscera, diaphragm, intercostal muscles, and the abdominal vena cava. 9. 9. Experiments with sheep and cattle being fed scabrous and nonscabrous diets similar in chemical composition show that sheep are more resistant than cattle to the increase in intrarumen pressure, decline in rumen contraction amplitude, and decrease in rumen contraction frequency caused by nonscabrous deits. 10. 10. The sequence of events in the reticulorumen during primary and secondary contractions previously described following visual and palpation experiments with cattle was confirmed by the use of myoelectrodes implanted in the various sacs of the reticulorumen of sheep. 11. 11. Elevated intrarumen pressure is associated with an increase in the frequency of primary (mixing) and secondary (eructation) contractions (more secondaries than primaries). Recently, the primary-secondary contraction has been described in sheep. The latter replace all primary contractions during elevated intrarumen pressure and are associated with gas expulsion. Concomitantly, rudimentary secondary contractions are converted to regular secondaries. The primary-secondary contraction and the conversion of rudimentary secondary contractions into secondary contractions have not been reported in cattle. 12. 12. Following insufflation, sheep deflate more rapidly than cattle because of the increased frequency of secondary contractions and the conversion of primary to primary-secondary contractions. Therefore, sheep should recover from legume bloat more rapidly than cattle, following treatment with an anti-frothing agent. 13. 13. The evidence for the involvement of saliva in the etiology of bloat is not convincing. A proline-rich protein in the saliva of deer capable of forming an indigestible complex with tannin has been reported. A similar protein has been shown to be present in the saliva of sheep but at a lower concentration than in deer. The concentration of this salivary protein in cattle is very low. 14. 14. Certain physiological conditions, e.g. pregnancy, lactation and fattening, may enhance feed consumption and increase the severity of bloat in sheep. 15. 15. There may be a breed difference in the incidence of bloat in sheep. 16. 16. Legumes which are easily ruptured and macerated may be more bloat-provoking than those which resist rupture and maceration. 17. 17. Plant tannins may form indigestible complexes with plant proteins believed to be involved with froth formation and the etiology of bloat.