Cause:
Excessive consumption of carbohydrate rich feed. The sudden ingestion of large quantities of highly fermentable carbohydrate rich feed, such as ground or rolled barley, wheat or oats, can cause the rumen to produce excessive lactic acid. The lactic acid kills many of the rumen micro flora and enters the circulatory system to cause metabolic acidosis, a toxic condition causing physical symptoms.
Grain overload can occur when any cereal grains fed. Bison are considered somewhat resistant to grain overload, and the incidence is much lower in bison than in cattle. Cereal grains can be fed to bison with a much greater degree of safety than they can be fed to cattle.
Clinical signs:
Grain overload has been seen in bison (19). In the early stages bison may be uncomfortable, mildly ‘colicky” and anorexic. Diarrhea always accompanies grain overload. As the condition progresses bison will become depressed, start to stagger, and eventually become recumbent.
In cattle it usually takes 24 to 48 hour after grain consumption for this condition to develop. The mortality rate can be high, and in some cases dead bison are the first indication of a problem. Founder and abortion can follow grain overload.
Postmortem findings:
The rumen contents will be very thin. If whole, or rolled grain is being fed, there should be a large quantities of grain in the rumen and abomasum.
Diagnosis:
The diagnosis can often be made from the history and clinical signs. The history doesn’t always include sudden access to grain. Grain overload can occur in calves that are being fed whole oats on a free choice basis (19).
The pH of rumen fluid that is obtained from sick animals via a stomach fluid can be determined. Cattle being fed a roughage diet have a rumen pH between 6 and 7, those on a high grain diet have a rumen pH of 5 to 6. A rumen pH of less 6 in cattle being fed a roughage diet and a rumen pH less than 5 in cattle being fed a grain diet are considered significant (19). These values have not been established for bison.
Treatment:
In mild cases, withhold water for 24 hrs. Remove the grain that is causing the problem, and provide long grass hay free choice. Antacids, such as baking soda and magnesium oxide may be administered orally to correct the acidosis.
In severe cases rumenotomy and rumen lavage have been used in cattle. The efficacy of these treatments in bison is unknown. The stress associated with these procedures may be too severe to warrant their use. Severe cases will be unresponsive to treatment, and slaughter should be considered.
Control:
Many bison ranchers provide bison calves and yearlings with grain, especially whole oats, in a free choice manner. The assumption is commonly held by many bison ranchers that bison will only eat what they need. Although this method of feeding bison is commonly practiced, there have been instances when bison have died of grain overload under this management (19). If the incidence of grain overload increases in bison being fed free choice grain, feeding programs that limit quantity of grain being fed to bison need to be implemented.