As many as half – if not more – of UK dairy cows are thought to be suffering from Sub-Acute Ruminal Acidosis (SARA), which is eroding farm profitability.

To help prevent acidosis, producers should spread balanced forage and concentrate feeds over the day, provide starchy feeds little and often and limit parlour concentrate feeds to 4-4.5kga milking, says Adam Clay, DairyCo extension officer.

They should also ensure separately fed forage is of a good quality and available ad-lib, and encourage maximum saliva production with salt or salt licks and adequate water.

Optimal rumen health is critical for feed usage and dry matter intakes, and therefore has a direct impact on milk yields and constituents, says Mr Clay. The ideal rumen pH is 6-6.5 and, although this will fluctuate throughout the day, just two hours below a pH of 5.8 can result in SARA.

“SARA depresses production and appetite and can create a vicious circle of falling pH in the rumen,” says Mr Clay. “When acids pass through the rumen wall they cause metabolic acidosis, which in severe cases can lead to shock and death.”

Rumen health can be easily managed through careful feeding, but to get the feed type and management right it is essential to have a basic understanding of how the rumen works.

“One of the four compartments of a cow’s stomach, the rumen is essentially a huge fermentation vat containing about 130 litres of chewed up feed, bacteria and saliva,” says Mr Clay.

“On top of this soup lies a ‘rumen mat’ of fibrous course material which acts as a filter. This rumen mat stabilises the rumen and filters smaller particles, which have been regurgitated and re-chewed, through to the rumen liquor below.”

Fermentation of small feed particles beneath the rumen mat is aided by bacteria, which operate best at about pH6. When pH falls below this level, the bacteria become less efficient, slowing digestion. This, in turn, reduces feed intake and cud chewing, which makes the problem worse by lowering essential saliva production.

In a healthy rumen, feed is broken down into volatile fatty acids (VFAs) – a key energy source for the cow. These VFAs are then absorbed through the rumen wall into the bloodstream via small finger-like papillae.

“The type of feed has a direct impact on both the effectiveness of the rumen mat, and the size of the rumen papillae,” says Mr Clay. “Low-energy diets can reduce the size of the papillae, therefore reducing the absorption site for nutrients. This means cows won’t be using feed efficiently. Conversely, increasing energy in the diet can stimulate papillae growth.”

High-forage diets are also important, as they form the basis of the rumen mat. They also encourage more cudding, which stimulates saliva production, an important element in rumen buffering. “Excessive amounts of concentrates, on the other hand, decrease rumen pH, reduce feed intake and microbial production and depress butterfats.”

The ideal ratio of forage to concentrate feed is 60:40 on a dry-matter basis and rations should be well mixed to avoid selective feeding. “Cows burrowing through the longer fibre to get to the palatable concentrated feeds are more likely to get SARA, so it is important to balance the need for a decent forage chop length with the need for a unified feed.”

Cows commonly spend between eight and 10 hours a day chewing the cud, and at rest, producers should see more than 60% of the herd ruminating, says Mr Clay. “This is an easy way to get an accurate idea of how your herd’s rumen health is shaping up.”

Although the rumen operates most efficiently at a pH of between 6 and 6.5, most high yielding diets, with large amounts of starchy feed, depress this to below 6 for much of the day, and quite often to 5.5 for part of it. This is when SARA can occur, which not only decreases digestive efficiency, but is linked to displaced abomasums and laminitis.

“It is vital to appreciate the dynamic state of the rumen environment and the extent to which changing feeds or feeding systems can alter rumen conditions – for better or worse,” he warns.