eCow - Subacute Ruminal Acidosis (SARA)
With the growing demand for milk and reducing costs on farms, methods for increasing productivity have included a switch from high fibre diets to diets high in grains (carbohydrates). While this method of increasing productivity does provide results, there are substantial downsides associated with the health of the cow which are often not visible to the farmer. Large quantities of grain in a cow’s diet often leads to a more acidic rumen environment, termed acidosis. A common form of acidosis often undiagnosed in herds is known as subacute ruminal acidosis or SARA. Long term consequences of SARA can include decreased levels of milk fat, decreased milk production and unnecessary culling of cattle.
SARA is diagnosed as transient periods of depressed ruminal pH (below about 5.5), usually following a large meal. Transitioning a cow’s diet from a high-forage, high fibre diet to a high-energy, easily digestible, high grain diet leads to a rapid influx of volatile fatty acids (VFA) into the rumen. These VFA are an expected by-product of bacterial fermentation and absorbed by the rumen wall to provide nutrients for the cow, but they can easily build up if the rumen is not adapted to remove them as quickly as they are being produced. High concentrations of VFA in the rumen increase the overall acidity and therefore lower the pH of the rumen.
Cows suffering from SARA do not always show clinical signs and as such are extremely difficult to diagnose without directly testing the pH of the rumen.
Primary signs of SARA in cows can include any of the following:
- Reduced appetite
- Reduced cud-chewing
- Undigested grain in faeces
- Foamy faeces containing gas bubbles
- Reduced milk fat content
- Reduced milk yield
As well as these, secondary signs can appear much later and are difficult to diagnose ante-mortem. Unlike the epithelial cells lining the abomasum, ruminal epithelial cells are not protected by mucus and as such are vulnerable to acidic damage. Prolonged periods of low pH in the rumen can lead to rumenitis and eventually erosion and ulceration of the ruminal epithelium, allowing bacteria to colonise the papillae and, through portal circulation, can reach and infect the liver. If bacteria clear the liver they may go onto colonise the lungs, heart valves, kidneys or joints resulting in pneumonia, endocarditis, pyelonephritis and arthritis respectively. One of the most common and impactful effects caused by prolonged acidosis is laminitis, resulting in lameness. Lameness has been described as the most important animal welfare issue concerning cows and, along with reproductive failure and low milk yield, is a major cause of culling. Several symptoms of SARA can lead to culling but can also have much farther reaching consequences involving human health. In beef cattle, enterohemorrhagic E.coli such as 0157:H7 can reside in the intestine and through shedding caused by SARA can be passed to humans through uncooked meat.
There are two likely scenarios where SARA commonly arises. The first is during the transition to a high energy milking diet, often post calving, and the second is when inaccuracies in the calculation of dry-matter-intake (DMI) lead to unsuitable forage to grain ratios in diets. After calving cows require a much greater supply of food to allow milk production to proceed unhindered, usually provided by the farmers in the form of an increased grain/forage ratio in their diet. A sudden transition such as this is unhealthy for the cow and not expected by the rumen wall, which takes time to adjust to a different chemical cocktail. It’s important to note that the introduction of higher quantities of grain can cause SARA regardless of the position in the calving cycle, only that calving is a common time to increase the grain in a diet.