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 sub-acute (SARA) and acute acidosis. Mishra et al. (1970) observed lower rumen pH when cows were fed 65% forage diets under warm, humid conditions (pH 6.1; 84.9F; 85% RH) compared to cooler, drier conditions (pH 6.4; 64.9F; 50 % RH). When forage in the diet was reduced to 35% the differences in ruminal pH were higher (5.6 vs. 6.1 for warmer and cooler conditions, respectively).
In addition to ruminal pH measurement, 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 5 hours daily has been suggested as necessary for SARA to occur (AlZahal et al. 2007).
The main changes occurring during heat stress:
- Feed intake and subsequently 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. The result is 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.
The main herd problems observed with SARA during warm weather are a reduction in milk fat and an increase in lameness. In an experiment conducted in Florida with almost 23.000 observations, milk fat dropped from 3.85 to 3.31% when temperatures increased from 48.2 to 96.8oF (Beede et al., 1985).
The increase in the incidence of hoof lesions associated with laminitis occurred several weeks after the cows started to suffer from heat stress. In 10 Wisconsin dairies, September was the worse month of the year for hoof lesions with the incidence exceeding 16%.
Opposed to the other months, lameness caused by lesions were more frequent than those lameness issues originated from hoof infections. The authors suggested that heat stress, which starts to affect cows in July in Wisconsin, could have been the cause of this increased incidence later into the fall season.