Acidosis

Sub-clinical acidosis is extremely rare in a pasture -based diets. Low rumen pH measurements can be observed in pasture based systems however this is a different condition to rumen acidosis observed on high starch or sugar diets. This is because a decrease in rumen pH in pasture based cows is usually caused by a rapid increase in fermentation acids e.g. acetic acid (such as in vinegar) associated with digestion. These are weak acids that are rapidly absorbed from the rumen and do not adversely affect digestion or microbial growth. In comparison, a drop in the rumen pH in a TMR- fed (high grain) cow is associated with the increase in lactic acid (a much stronger acid) which can cause acidosis. 

Acidosis is most common between day 7 and 14 of fodder beet transition, because the cow is ready for a high intake but the rumen is not, farmer patience in the transitioning process is dwindling, and the cows start eating any beets that they left behind earlier in the transitioning process.

The microbes operate in a fine balance around optimal pH with the rumen operating best at a pH between 6 and 7.

This pH balance is maintained by buffering agents in the saliva, which are produced by chewing the cud, and by absorption in the body of weak acids, (mainly volatile fatty acids), essential for milk fat production.

 When an animal ingests feed that is rich in fermentable carbohydrates or sugars that they are not accustomed to this can upset the balance.

• Readily fermentable carbohydrates cause problems because the lower fibre content of the diet results in less cud chewing so less saliva is produced. Saliva contains buffers that help stabilise the rumen pH.
• The high sugar content promotes the growth of lactic acid forming bacteria. Lactic acid is a stronger acid than volatile fatty acids that are produced during normal digestion processes so the pH drops.

At low rumen pH the microbes that digest fibre are less efficient and changes to rumen function result. In extreme situations the rumen stops working altogether. 

Acidosis is also known to damage the rumen papillae (small finger like projections in the rumen through which nutrients are absorbed) further reducing the ability of the animal to absorb nutrients.

In more severe cases the disease may progress to include metabolic acidosis, depression, dehydration, bloating and milk fever like symptoms.  Severe acidosis may result in cows going down, coma and death within 8-10 hours.

The implications of sub-clinical acidosis on non-lactating cow performance are less well researched but one area it will impact will be body condition score gain. 

Cows with clinical acidosis will go off their feed and with sub-clinical acidosis digestion of nutrients will be reduced so fewer nutrients will be available for body condition score gain.

Detecting sub-clinical acidosis in non-lactating cows is challenging as the best diagnostics appear to be VFA, lactic acid and ammonia concentrations, and rumen pH, none of which are easy to measure.

Oral drench affected cows with a slurry of magnesium oxide (2 handfuls; approx. 500 g mixed with water) 1-3 times per day until they improve and make sure they have alternative feed available. Cows with mild acidosis will be slower to walk to a new break but still act normal.