Formulating and describing diets for ruminants is not simple. Energy concentration can most easily be described in terms of Mcal (mega-calories) per pound. Wintering bison diets on poor quality hay with 2 to 4 pounds of grain may be in the 45 to 47 Mcal/lb range. Warm up or transition diets with 8 to 10 pounds of grain and medium quality hay may be in the 55 to 58 Mcal/lb range. Diets with free choice grain intake and minimal forage will be in the 62-64 Mcal/lb range as bison will consume some forage if offered. Ultimate control comes with totally mixed rations fed in fenceline bunks.
Protein
Protein needs to be treated entirely different in bison diets than bovines. Bison recycle nitrogen efficiently, an evolutionary response to very low protein diets from mature grasses during several months of the year. This recycling may cause high blood urea nitrogen levels from modestly high protein levels in the diet. The specific results of high nitrogen levels is unknown but we do know that it is metabolically expensive in energy terms to deaminate amino acids. Determining maximum and or optimum protein levels is high on the research priority list. In some areas, many feeds contain protein levels higher than many bison producers consider optimum making it difficult to formulate diets. Eleven or 12% protein is considered the maximum from anecdotal experience.
Minerals
Mineral requirements have not been determined for bison. Anecdotal experience again suggests a calcium to phosphorous ratio of 1:1, unlike the 2:1 ratio for bovines. Selenium requirement in bison is thought to be greater than bovines at .02 parts per million (ppm). Mineral nutrition in bison has been based on trial and error experience. Considering the value of bison, it may be practical to offer a complete mixed mineral that includes several micro-minerals including cobalt, copper, iron, iodine, zinc, molybdenum, and others, especially if animals look unthrifty or some undiagnosed problems exist. Minerals are available as inorganic (i.e. sulfates and oxides) or chelated (i.e. proteinates and methionates). The bioavailability of the chelated minerals is thought to be higher but they are more expensive. Local soil conditions may dictate specific mineral formulations to counter deficiencies or toxicities from plant material or water. Interpreting toxicity and deficiency symptoms from bovines may be useful.
Feed intake
Bison seem to naturally self-limit intake with less dry matter consumed per unit body weight than bovines. Bison also consume feed in several small meals throughout the day vs. fewer large meals observed in bovines. This habit maintains a more uniform ruminal environment and may contribute to more complete nutrient extraction by bison vs. bovines.
Feed intake for bison changes with the season presumably due to some evolutionary mechanism that in effect says “eat more in the fall to store up fat for winter, don’t try to find feed in the winter because it will take more energy to forage for feed than will be available from the feed you find, eat more in the spring to gain back what was lost during the winter, and eat modest amounts in the summer as the grass is nutritious and other things such as calving and breeding are important.” It has been documented that bison physical activity, respiration, heart rate, and metabolism slow down in proportion to cold exposure until extreme conditions are encountered. This seasonal reduction in intake may contribute to enhanced digestion when feed is abundant, commonly called compensatory gain. In other species, intestinal villa were observed to increase in length when high fiber, low nutrient density feeds were fed, thus increasing the total surface area for nutrient absorption and improving feed efficiency. It seems logical to manage bison rations to take advantage of their natural eating habits. Winter feeding strategies may include only modest energy supplementation if poor quality hay is fed or offer a better quality hay, such as native grass harvested during the vegetative stage during the winter. However, there is some evidence that diets with modest grain levels (5-10 lb/hd/day) fed during the winter produce satisfactory gains, especially in milder climates. Cold temperatures and photoperiod may both be responsible for triggering any seasonal patterns but we cannot separate them or practically control them.
The need for some forage and/or fiber in the diet for proper rumen function is antagonistic to maximum gain, which requires high concentrate levels. One approach which may be useful is to use feeds that are high in digestible fiber, commonly considered the hemi-cellulose fraction. This fraction can be determined with a lab analysis of the feed or diet. Laboratory analysis of feedstuffs produces a value for ADF or acid detergent fiber, which is basically indigestible cellulose and lignin, and a value for NDF for neutral detergent fiber. The hemi-cellulose fraction is the difference between the NDF value and the ADF value. Feeds that have a relatively high digestible fiber content need to be included in research trials with high starch grains to confirm this hypothesis.
Anecdotal information and some research trials suggest that high grain diets with corn or barley at 60 to 75% of intake produce optimum gains. Higher grain diets have been fed but the nutritional stress of minimum or no forage rations can be problematic. The value of a warm-up period is important, with 50 days or more providing greatest gains once high grain diets are fed.