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Ruminal acidosis is discussed with reference to causes and economic and health implications. Distinction is made between the acute form which with proper adaptation to high energy diets is seldom encountered and the more problematic chronic or sub-acute form, commonly referred to as sub-acute ruminal acidosis (SARA). Apart from stepwise transition from roughage to concentrates, methods adopted to reduce SARA include grain treatment to reduce starch degradation, feed additives such as buffers to control ruminal pH, dicarboxylic acids to stimulate the growth of lactate utilisers, antibiotics such as virginiamycin and the ionophores which inhibit the growth of lactate producers, and direct-fed microbials (DFM’s), some of which are lactate utilisers but used more often as stimulants of the major ruminal lactate utilisers Megasphaera elsdenii and Selenomonas ruminantium . Some of the feed additives are expensive and their effects on SARA mostly inconclusive. With regard to the ruminal lactate utilising bacteria, the potential of M. elsdenii to control lactic acid has been recognized and some success with patented strains has been achieved. However, these strains have not been commercialised because of one or more reasons which include inadequate growth rate, inability to multiply at a low ruminal pH, non-preferential use of lactate as primary substrate, inability to survive in sub-optimal anaerobic conditions, inhibition by ionophores, inadequate delivery methods to the ruminant and inability to keep on producing acetate when fibre digesters become inhibited. Megasphaera elsdenii NCIMB 41125, selected from the concentrate-fed rumen through stringent screening and a pH-auxostat technique, proved to meet most criteria mentioned above. In addition, the strain is unaffected by most anthelmintics and in-feed antibiotics. Research results show that: a) strain 41125 is highly successful in preventing ruminal pH decline and lactic acid accumulation to SARA levels; b) volatile fatty acid (VFA) production is similar to current in-feed products and the proportional contribution depends on substrate, dilution rate and pH. Propionate as preferred VFA can be promoted further by the synergistic benefits of strain 41125 with some antibiotics; c) feed intake may be enhanced but not consistently, apparently depending upon adaptation procedure followed and dietary composition; d) because of effective ruminal acidosis control, less roughage can be used during adaptation which is of economic benefit; e) animal health associated with the SARA-compromised immune system is improved with administration of strain 41125; f) in feedlot cattle carcass gain and carcass feed conversion may benefit by about 2% and in dairy cattle high producers may produce more milk because strain 41125 enables management to challenge these cows with higher levels of concentrate in total mixed rations (TMR’s), and g) drenching sheep with strain 41125 before entering harvested maize fields prevents ruminal acidosis. Keywords: Megasphaera elsdenii , ruminal acidosis, dairy cattle, beef cattle, sheep, DFM, antibiotics

Ruminal acidosis: A review with detailed reference to the controlling agent Megasphaera elsdenii NCIMB 41125 (Review)