PASTURE MANAGEMENT STRATEGIES FOR REDUCING THE RISK OF LEGUME BLOAT IN CATTLE

Results from two decades (1973-1963) of bloat research at Kamloops are reviewed. The trials were conducted with groups of ruminally fistulat ed cattle either grazing or fed daily fresh-cut alfalfa (Medicago sati va) herbage. Studies were conducted during the growing season (May to September) and in the fall (October and November). The alfalfa was usu ally in the vegetative to early bloom stages of growth. Visual assessm ents of bloat severity on a scale of 1 to 5 were made .5 to 2 h after feeding started, at which time. ruminal cannulas were opened to reliev e ruminal pressure. Every cultivar of alfalfa tested caused bloat, but sainfoin (Onobrychis viciifolia), birdsfoot trefoil (Lotus corniculat us), and cicer milkvetch (Astragalus cicer) did not. This confirms the bloat-safe features of these alternate legume forages. Bloat was posi tively associated with the level of Fraction 1 protein and total solub le protein in alfalfa, supporting the concept of a decreased probabili ty of bloat with advancing stages of plant maturity. There was no asso ciation between alfalfa saponins and bloat. Prefeeding ruminal chlorop hyll levels were higher and ruminal clearance rates were lower in catt le that were susceptible to bloat. Cattle that bloated on a given day consumed 18 to 25% less alfalfa immediately before bloat than non-bloa ters did in the same time period. Ruminal cations were associated with bloat incidence but cation manipulation through supplementation did n ot prevent bloat. Of all the feed additives tested, only poloxalene (B loat Guard(R)) completely prevented bloat. The occurrence of bloat was not associated with a simple, unique weather variable and it could pe rsist after a ''killing'' frost. The claim that the risk of bloat may be reduced by waiting until the dew is off the alfalfa before allowing cattle to graze was substantiated, Bloat was observed from 2 to 17 ti mes more often when cattle were fed between 0700 and 0800 than when th ey were fed 4 h later. The risk of bloat was also substantially lower when cattle grazed alfalfa continuously than when grazing was interrup ted and cattle were allowed to graze for only 6 h daily. Pasture manag ement systems that promote continuous and rapid ruminal clearance (mor e bypass, less gas) are most likely to reduce the incidence of bloat.