Rumen acidosis results from an excessive acid load in the rumen not neutralized by salivary or feed buffers. Changes in physiology, metabolism and behavior of heat-stressed cows increase their susceptibility to both subacute (SARA) and acute acidosis. In 1970, researchers observed lower rumen pH when cows were fed 65 percent forage diets under warm, humid conditions (pH 6.1; 84.9ºF; 85 percent RH) compared to cooler, drier conditions (pH 6.4; 64.9ºF; 50 percent RH).
When forage in the diet was reduced to 35 percent, the differences in ruminal pH were higher (5.6 versus 6.1 for warmer and cooler conditions). It is also important to consider how long the rumen was subjected to this acidity (i.e., hours). A drop in rumen pH below 5.6 during at least 2.5 to five hours daily has been suggested as necessary for SARA to occur.
The main changes occurring during heat stress
- Feed intake and rumination are reduced, resulting in too little saliva to neutralize rumen acid production.
- Respiration rate increases (panting) to dissipate heat through the lungs with excess loss of carbon dioxide. Respiratory alkalosis and bicarbonate is then excreted to compensate.
- Saliva production is reduced during panting, an additional loss of buffer effect.
- Changes in feeding behavior also contribute to rumen acidosis. Cows tend to reduce the number of feeding bouts and increase the food consumed at each meal. Rumen pH declines are more pronounced with the increase in meal size. In addition, cows tend to select finer particles (concentrates) of the TMR (sorting) or reduce forage intake if feeds are offered separately.
Continue reading this article published in Progressive Dairyman.
