This months article concerns Dairy Cows - a rare species in Caithness, but not extinct yet.
The increasing milk yield and associated energy requirement of the modern dairy cow over recent years has led to challenges in achieving good nutritional management. Incorrect feeding of diets that are high in rapidly fermentable carbohydrates and low in fibre can lead to prolonged and repeated periods of abnormally low rumen pH which is referred to as Subacute Ruminal Acidosis (SARA).
Rumen pH fluctuates during any 24 hour period and is determined by the dynamic balance between the intake of fermentable carbohydrates, the production of volatile fatty acids (VFAs), the buffering capacity of the rumen and the rate of VFA absorption. Normal rumen pH is considered to be 5.8 and above. When pH drops below this for more than a few hours a day, a cow is considered to have SARA.
The major clinical signs of SARA are reduced or cyclic feed intakes and decreased milk yield and efficiency of milk production. Diarrhoea or variable dung consistency is also noticed along with regurgitated cud balls. SARA has also been associated with laminitis and subsequent hoof overgrowth, sole ulcers and sole abscesses. The lameness can be a significant source of chronic pain and financial loss and generally does not appear until weeks or months after the episode of SARA. Repeated episodes of SARA will also lead to inflammation and damage in the rumen wall. This is painful and will cause reduced feed intakes. If the damage to the rumen wall is severe, bacteria will leak from the rumen into the circulation, leading to abscesses in the liver and the lungs and infections elsewhere in the body. Therefore farms with SARA will often have high rates of culling and unexplained deaths.
SARA is diagnosed at the herd level rather than on an individual cow basis. Milk recording data can be used to monitor the fat: protein ratios of high risk groups such as fresh calvers (TMR feeding) and peak yielders (supplementary concentrate feeding). Low individual butterfat levels and low fat:protein ratios suggest SARA. Regular body condition scoring is a good way of monitoring nutritional management, whilst scoring rumen fill monitors feed intakes. Scoring the consistency of the dung is useful and dung samples can be given a sieve score to assess the amount of undigested fibre present. In a good sample, less than one third of faeces will be left in the sieve and the remaining fibre will be short length. Loose samples and samples showing excessive amounts of undigested fibre suggest SARA. Analysing rumen fluid samples taken directly from the rumens of 12 or more at risk cows can also help to diagnose SARA, although this is an invasive, time consuming technique and timing of sample collection is important. Samples with pH <5.6 suggest acidosis whilst pH values from 5.6 to 5.8 are borderline. If >25% of animals tested have a low pH then the group is at risk of SARA.
SARA can be prevented by adhering to some basic rules. Correct transition feeding of pre-calvers will prepare the rumen for the lactating ration by increasing the surface area for VFA absorption and providing a healthy microbe population. Fermentable carbohydrate in the lactation ration should be balanced by long fibre of 5cm in length to promote saliva production and rumination (cows will tend to sort straw from the ration if it is poorly chopped) and dramatic changes in the ration and periods of feed deprivation should be avoided. Addition of dietary buffers such as sodium bicarbonate will help, and supplementing the diet with microbials such as yeasts help promote a healthy rumen flora.