As prey animals, sheep evolved with the ability to harvest their food quickly with very little chewing, then retire to a safe place to further process their meal. Sheep are unable to directly digest the cellulose in forages and must rely on billions of microorganisms in the rumen (bacteria, protozoa and fungi) for fermentation and digestion. The byproducts of these microbes provide the sheep with needed nutrients (protein, energy, B vitamins and vitamin K). The health of the entire animal is reliant on the health of the rumen microbes.
The rumen is often referred to as a large fermentation vat and is the first and largest of the four compartments of a sheep’s stomach. It is located on the animal’s left side and takes up a large portion of the abdominal cavity with a capacity of up to three gallons in a mature sheep.
Lambs are born with a non-functioning rumen. Bacteria begin to populate the rumen shortly after birth, but the microbial population isn’t adequately developed for efficient digestion for several weeks. Creep feeding a highly digestible grain mix with soft, high-quality hay encourages early rumen development and should be offered, especially to bottle lambs, starting at 7-10 days of age. The rumens of lambs fed only milk and forage develop more slowly, usually not becoming fully functional until 50-60 days old.
The rumen microbes begin the process of digestion as soon as food enters the rumen. Partially digested food (cud) is regurgitated. Tough fibers are chewed into finer particles, mixed with saliva (a natural buffer or antacid), then swallowed again, with more fermentation by the microbes. Strong, muscular contractions in the rumen continually mix the contents, and aid in regurgitating cud and belching to release accumulated gases. This process goes on for several hours daily, usually while the sheep is at rest.
The various rumen microbes work together to digest the food particles, but have very distinct functions. Some digest starch and sugars (grain, concentrates, lush forage) while others digest cellulose (forages, hay). A balance in the types of microbes is maintained based on the sheep’s diet. It takes time for the microbial population to adjust to feed changes. Minor fluctuations are not usually a problem, but anything that seriously alters or destroys the balance of microbes in the rumen, such as rapid diet changes, illness or antibiotic use, can cause an abrupt drop in rumen pH, leading to acidosis. Since saliva acts as a powerful antacid, any prolonged reduction in cud chewing also increases the risk of acidosis.
Acute acidosis is a serious condition and may be fatal. Even mild forms can cause permanent damage to the gut lining, affecting both feed intake and nutrient absorption, and resulting in poor weight gain and/or chronic diarrhea. Decreased production of B vitamins in the rumen may result in deficiencies, especially Vitamin B1 (Thiamine), leading to polio. Acidosis may cause liver abscesses and/or laminitis (founder). A related condition, bloat, occurs when the rapid fermentation of high starch/sugar diets causes a build-up of gas in the rumen. This trapped gas puts pressure on the diaphragm, making breathing difficult and putting the animal at risk for sudden death
To prevent damage to the rumen, the best advice is to start low and go slow to allow the microbial population to adjust and rebalance. Introduce grain or lush pastures slowly, then increase incrementally over 1-2 weeks. Feed long stemmed grass hay before grain or prior to grazing lush pastures to slow intake and encourage cud chewing and the natural antacid effect of saliva. Feeding whole kernel corn, rather than cracked or ground, slows digestion and helps avoid sudden drops in pH. Supplementation with B Complex and/or Thiamine may be necessary when feeding highly fermentable diets.
By: Kathy Bielek and Roxanne Newton, EAPK Communications Committee