Chapter 3: Bison Nutrition

How much crude protein does the food of the bison contain on the Northern Great Plains, in the aspen parkland and tall grassland of southern Canada (Alberta, Saskatchewan) during the different seasons of the year? How much crude protein does the northern part of the Great Plains contain during each month, in fall, winter and spring? How much energy and protein does the bison need, to grow and to maintain its body weight? How much does this large animal eat in winter and how much in summer? How much aboveground dry matter (g DM/m²) must grow at least, so that the bison will be able, to live there (marginal bison habitat)? What have scientists found now out about this?

Lorne Klein is a Canadian export on the bison in Saskatchewan, Canada. In Saskatchewan and the other prairie provinces of Canada, the ranchers are raising now the bison, just like cattle. That is why they have learned now during the last years much about the bison, where it is able to live and where not, and what it must eat and how much. Lorne Klein writes in his research report, Bison Pastures and Grazing Management (1998):

"Contrary to common belief, the Western Canadian herds trailed north in winter to the parklands. The parkland offered shelter from winter storms and a greater certainty of snow for a water source. The native fescue range, which had been grazed slightly in summer by a small resident population, offered a large volume of stockpiled forage for winter grazing. With normal autumn moisture, a flush of green growth, late in the season, ensured reasonable quality.

"In summer, the bison grazed the short grass prairie of SW Saskatchewan and SE Alberta. The cool season grasses were flowering and the warm season grasses were actively growing. Again, the timing of grazing this region ensured maximum forage quantity with good forage quality. During the summer, the bison were dispersed into smaller herds and constantly on the move. In winter, they tended to gather in larger groups and only moved, when forage supplies were depleted." Klein, L. (1998:1)

Bison Physiological Adaptation

"Bison have the ability, to digest low quality forages more completely than beef cattle. Studies have shown, bison can extract from 5% to 13% more nutrients from various low quality feeds. This advantage occurs, when protein levels are 8% or lower. Explanations for this increased efficiency are a higher level of nitrogen recycling, differences in rumen micro flora, and longer feed retention time in the rumen (79 hours for bison vs 69 hours for cattle). At protein levels above 10%, cattle digest feed equal to or better than bison.

"The bison’s metabolic rate decreases from summer to winter. In summer, the maintenance energy requirement of a 1000 lb (454 kg) cow is estimated at 22.4 Mcal/day. In winter, the requirement is about 12.5 Mcal/day (44.2% less in winter). This reduction lowers the feed quality and quantity, required in winter. Dry matter intake of cows is estimated to be 2.2 – 2.8% of body weight during summer. In winter, appetite drops to 1.4-2.0% of body weight." Klein, L. (1998:2)

Bison are very adapted to cold weather

"The lower critical temperature (temperature, at which an animal increases feed intake or expands extra energy from body reserves to stay warm) of six month old bison calves is colder than –30°C. The lower critical temperature of adult bison has not been measured, but is expected, to be significantly lower. As a comparison, lower critical temperature for beef cows in mid winter is about -20°C. Cold resistance in bison is due to their excellent hair coat and to reduced physical activity. Under extreme cold weather, it has been demonstrated, physical activity reduces thermal insulation.

"Mature bison females in good condition are able, to lose 10-15% of body weight from January to June. ... A lower body condition in spring will allow cows, to gain weight in June and July. It is important, to have them on a rising plane of nutrition starting in mid June, to flush for breeding in August and September. Young females, in their second and third winter, can also lose 10-15% body weight from January to June, provided, they enter the winter in good condition." Klein, L. (1998:2).

Estimated nutrient requirements of female bison (dry matter basis)

After: L. Klein (1998:3) Table 1. * % TDN = % of dry matter, bison is able to digest.

Bison Grazing Behavior

"Studies have shown, bison do not simply forage at random. The animals select locations in a paddock and upper parts of the vegetation, that are higher in feed value, than field average. To compensate for reduced grazing time per day, especially during the rut, bison will select the actively growing plants, with higher protein and energy. At times, bison are hard to figure out. In early spring, they have been observed, to ignore new lush growth in favour of the previous season’s growth." (1998:3)

Comment: The same we see among the Dall sheep in the Yukon, at the beginning of spring. Also up there some of these mountain sheep may still eat the dry brown grass from last year, even though fresh green grass, rich in protein and low in fiber, is beginning to sprout up. I have seen this myself. Why do they do that? – That is quite simple. Because their microflora in their digestive tract is not able yet, to digest this fresh green grass. Their microflora is still adapted to the dry winter feed, low in protein and high in fiber.

"Bison can adjust their diet to as high as 20-40% shrubs (often willows), when grasses are limited. Forcing them, to browse at these levels, is not recommended, because production may be compromised. Producers have noted, they browse on willows and poplar leaves, even when there is plenty of grass available. They have also been observed, to select for weedy forbs, such as dandelion, but the times and intervals are unpredictable." Klein, L. (1998:4).

Mature bison bull in Yellowstone National Park, Northwestern United States. The bison in Yellowstone National Park is a mixture of Prairie and Wood (Mountain) bison. Photo by M.D. Beal. From: Margaret Mary Meagher, The Bison of Yellowstone National Park (1973:39) Fig. 14.

Winter Grazing

"Bison are naturally adapted to winter grazing. Before European settlement, they followed predictable movement patterns. Contrary to popular belief, the majority of the bison in Western Canada converged to the Brown Soil Zone (SW Saskatchewan and SE Alberta) for their summer range. For fall and winter range, they migrated to the Black and Grey-Wooded Soil Zones (Manitoba, East and North Saskatchewan, North and West Alberta). Bison migrated to the parklands in winter for

1. shelter from winter storms
2. higher volume and quality of forage, and
3. availability of snow as a water source.

Undoubtedly, losses, due to starvation and reduced calf crop, would have been extreme in some winters. In many winters, losses would likely have been higher, than what producers would tolerate today. ... There are reports of bison, grazing in two feet of snow. Some animals require training, to function with winter grazing." Klein, L. (1998: 

Bison hair-coat: how warm?

How warm is the hair coat of the bison in winter? How does its under-wool compare with that of the muskox and other animals?

Cheryl Ermel is a West Canadian expert on bison fiber. She sent me on 13 December 2000, through Lorne Klein, Canadian bison expert in Weyburn, Saskatchewan, the following report about the bison fiber. They are raising now the bison in southern Canada just like cattle: "Each spring, Bison shed their winter undercoat, which is a fine, soft "down". Bison down has properties common to other exotic fibres: cashmere from goats, down from camels or yaks, qiviut, the undercoat of the musk ox.

"There are several sizes of fibre in a full bison coat, ranging from the long stiff guard hairs (1), to a stiff dark brown hair (2), a medium brown hair (3), a fine white hair (4), and lastly the insulating down fibres (5). Down has a staple length of 1 - 2 inches (2.54-5.08 cm), and has a crimp to it (= is curly) by micron size, down feels similar to cashmere, and is only slightly larger than qiviut fibres. Bison down is extremely warm, soft, and durable, contains no lanolin." Ermel, Cheryl (written comm. 13 Dec. 2000)

This shows me: The under-wool ("down" in Canada) of the bison is in winter just as long and as warm as that of the muskox. So the bison should be able to endure just as much cold in winter, as the muskox. Theoretically. But there is an important difference. The muskox must stay out on the open arctic tundra in winter, where there are no trees. It is able, to endure the deepest cold and strongest blizzards, right on top of the bare ridges of the hills. Up there the cold is especially severe,

Why is the muskox able to stay in this deep cold? Because above its layer of warm under-wool, there is a cover of long guard hair. This long guard-hair covers nearly its whole body, even the belly and its short legs, nearly down to its hooves. The hair-coat of the muskox in winter we may compare to a person wearing a warm sweater or down-sleeping bag covering his whole body. And on top of his sweater or down sleeping bag, he is wearing a warm, wind-proof parka, going nearly down to his feet. In this way, the muskox is optimally adapted to its life on the arctic tundra. It does not move around much. It does not have to walk far between its winter and its summer range. If it would have to travel as far as the reindeer and bison, it would soon overheat. Because its winter hair-coat covers nearly its whole body. So it would not be able to get rid of the heat fast enough.

The bison is also covered in winter with a warm layer of under-wool. This under-wool is just as long and seems to be just as warm, as that of the muskox. But only the front-half of the bison is covered with long upper hair. The hind part of its body is not covered with the long guard hair, like that of the musk ox. When looking at a bison from a distance, it seems, as if the hind half of its body were shorn. The bison resembles a person, who is wearing a warm sweater, covering most of his body. On top of this warm sweater, he is wearing a short, wind-proof parka. But this wind-proof parka covers only the upper part of its body: his head, arms, and the upper part of its rump. How does the bison then survive the cold winter on the Great Northern Plains of the northern United States and southern Canada, and on the large meadows of the mixed boreal forest further north? What does it do in winter, when it got very cold, when there was a blizzard?

The people, who have seen this themselves, tell us in their reports, preserved till now: During a mild winter, when there was little snow, the herds of plains bison used to stay out on the prairie, on the short grassland. When the winter was very cold, the bison in the western part of the Great Northern Plains then moved further west. Into the foot-hills of the Rocky Mountains. The plains bison in the central part of the Great Northern Plains used to move then north, into the aspen-parkland and mixed boreal forest of Saskatchewan – not south!. And the plains bison in the eastern part of the Great Northern Plains used to walk then eastwards, into the shrub-land and forest of eastern Manitoba. Also when there was a blizzard, these herds of bison than hurried into the woods.

West of the original range of the northern plains bison, on the watershed of the Rocky Mountains (like where is now the Yellowstone Park), the wood bison was living. North of the former range of the plains bison, in the central part of the Northern Great Plains, the wood bison was living. It did not move out into the Great Northern Plains. Now herds of plains bison, mixed with wood bison and herds the pure wood bison are living on the meadows and mixed boreal forest of Canada. In winter, when it is very cold, and when there is a blizzard, these northern bison herds also leave the large meadows and move into the deep forest. In the deep forest, it is warmer, there is no wind, and the snow is softer, than out on the open meadow. These bison will only come out into the open again, when it is not so cold and windy anymore.

Why does the bison not stay out on the open plains all winter, like the muskox? Why does the bison not just lie down on the open plain, when there is a blizzard, like the muskox on the arctic tundra? Why does the bison run into the deep forest and stay there, when it is very cold and very stormy? – Because the bison would then freeze to death. Because the bison is not as well protected against deep cold and blizzards (against the wind-chill-effect), as the muskox. The bison is not able to endure as much cold, as the muskox on the arctic tundra, because the hind half of its body is not covered with long upper hair. The hind part of the bison appears, as if it were clipped, as if it were naked.

Why is not the whole body of the bison also covered with long upper hair, hanging down nearly to its hooves, like that of the muskox? This has to do with their type of living. The muskox moves around very little. And its winter and summer ranges usually lie only a few miles apart. Many herds of bison had to travel hundreds of miles each spring from its winter range to its summer range. And in autumn they had to walk again hundreds of miles from their summer range to their winter range. Also the plains bison, mixed with the wood bison in the Slave River lowland, further north, has to walk now quite far between its summer and winter ranges.

What would happen, if the whole body of the bison were covered up with a long and warm hair-coat, like the muskox? The bison would not be able then, to travel very far. It would not be able then, to get rid of its heat fast enough. It would then soon overheat. And it would starve to death, because it would not be able, to walk then to those places, where it would find enough to eat. Thus, also the bison has been perfectly designed for its specific type of living. This also shows us that the bison would not be able to live on the arctic tundra, like the muskox. During the time of the woolly mammoth, large herds of steppe bison were living also in the Far North, together with the steppe horse, the muskox and the reindeer. But they were living then up there in a temperate climate, without an arctic climate, without ice and snow.

Marginal aboveground dry matter for bison

Lorne Klein, Canadian bison expert in Saskatchewan, central Southern Canada, says in his booklet Bison pastures and grazing management, July 1998, on page 3: "To meet forage quantity requirements, a pasture sward of not less than four inches (10.16 cm) is recommended. Another measure is to graze only, when there is more than 300 lbs/acre (33.626 g/m²) of dry matter." - Does this mean that the average bison, weighing 450 kg, is able, to live the year round on 33.6 g DM/m² aboveground dry matter, that this bison is able to maintain there its body weight and to grow? If not, how much g DM/m² yr does the 450 kg, 800 kg and 1000 kg bison need at least, so that it will be able to live there the year round, to maintain its body weight and to grow?

How dense (g/cm³) may the drifted, wind-packed snow in winter be, where the bison is still able to feed? At which snow density will it not feed there anymore, according to new research?

Lorne Klein explained to me on March 19, 2000: "The 33.626 grams/meter square is a suggested minimum during the summer months, when forage is actively growing. Most producers (= bison ranchers) like to have a higher quantity for grazing, otherwise production (= growth of the bison) may be sacrificed. There was no research addressing your question specifically, but 33.626 grams/meter square is most likely not enough for winter conditions with snow cover. Producers grazing standing forage (= where their bison graze) in winter, like to have at least a twelve inch (30.48 cm) height (= of the plant-cover).

"I can't answer your snow density question. I'm not aware of any research. I believe Bison tend to use their face to move snow rather than their hooves, so I do not believe they can survive in real hard snow." - Comment: The bison is still able to find enough food, where the plant-cover in winter is at least 12 inches (30.48 cm) tall. That is here an aboveground ground dry mass of 40.351 gDM/m² year.

Lowest aboveground plant matter (dry weight) per year

for winter grazing, above starvation

This table shows us, how much aboveground dry matter must grow at least in summer, so that the large grazer and browser will be able to live there during the winter. This has only to do here with the lowest amount of aboveground dry matter, growing per year, not the amount of protein, which it contains, where the animal should still graze. Below this amount, it will starve to death, due to lack of energy. This table is based on the lowest winter grazing for bison of 40.351 g DM/m² per year (L. Klein, written comm. Dec. 19, 2000 and Lorne Klein (1998:3).

This table also agrees quite well with the annual amount of aboveground dry matter, where the bison, of average body weight (450 kg) (J. Hudson and S. Frank, 1987:71-75), and large body weight (1250 kg) (Belovsky, G. E. 1986:36), is still able to live, to maintain its body weight, and where it will starve to death. It also agrees quite well with the annual amount of fodder, where the muskox is able to live. The 100-kg reindeer or caribou needs more aboveground dry matter per year, than this table shows us, about twice as much, because its metabolic rate is much higher, than that of the muskox.

And it agrees quite well, where the African elephant is still able to live, at 255 g DM/m² growing from 300 mm of rain per year (Laws, R. M. 1970:3); and where the elephant will starve to death with a full stomach, at 200 g DM/m² per year (Phillipson, J. 1975:). That is, where they will be able, to maintain their body weight, and where they will starve to death, due to lack of energy. This clearly proves to us that the elephant or mammoth is not able to live in an arctic or subarctic climate, just like the muskox and the reindeer of today. It would starve up there to death, due to lack of energy.

After R. J. Christopherson et al. (1978:52), Table 1. Feed intake, initial weight, average daily gain for a 96 day winter feeding period.

Metabolic rate (kcal/kg^.75/day) for bison calves during winter

Adapted from R. J. Christopherson et al. (1978:52) Table 2.

Metabolic rates of bison

This shows us, how much metabolizable energy (ME) the bison in southern Canada needs during the different seasons of the year. From "Seasonal energy expenditures and thermoregulatory responses of Bison and Cattle" by R. J. Christopherson, R. J. Hudson, and M. K. Christopherson, Dept. of Animal Science, Univ. of Alberta in: Can. J. Anim. Sci. 59: 611-617 (1979)

Metabolizable Energy

How much digestible crude protein and metabolizable energy does it need, to maintain its body weight and to grow?

H.W. Reynolds, R. D. Glaholt, and A.W.L. Hawley state in their article, "Bison" in J. A. Chapman and G. A. Feldhammer, eds., 1982, Wild Mammals of North America: Biology, Management and Economics, John Hopkins University Press:

"In winter at –30°C, the metabolic rate of bison was 718 kJ (171 kcal) per kg metabolic body weight per day. At 10°C, the metabolic rate of bison was 934 kJ (233 kcal) per kg metabolic body weight per day. Peden et al (1974): Digestibilities were greater in bison, than in cattle for fall and winter forages, in which the crude fiber (CF) content was high and the CP content was less than 7 percent. For spring and summer forages, in which the CP content exceeded 7 percent and the CF content was low, differences in digestibilities between bison and cattle were not evident." (1982:978)

"The nutritional requirements for bison can be considered similar to those of cattle. However, the greater assimilation of crude protein and energy by bison can be taken into account, when feeding poor-quality rations. The digestibility of crude protein and gross energy in poor-quality feeds might be considered to be about 5 percentage units greater, than for cattle, and feed requirements could be adjusted accordingly." Reynolds, H.W. et al. (1982:1001)

P. Jerome Martin, Northwest Feed Research, Edmonton, Alberta, Canada, states in "Digestive and grazing strategies of animals in the arctic steppe": "Ruminants, such as bison, can consume less feed per unit of body weight, than horses or elephants and, hence, require forage, which is somewhat higher in protein and lower in fiber." (1982:259).

"Mean retention time of digesta were 65-69 hours (h) for cattle versus 79 and 78 h for the bison and yak. There is no evident relationship between retention time of digesta and digestibility of dry matter. The authors concluded that: ‘Differences in the digestive efficiency of bison and yak, relative to the domestic cattle, when consuming poor quality feeds, may be the result of an improved nitrogen economy in the gastro-intestinal tract, resulting from longer retention times of digesta in the sites of microbial fermentation.

"Bison digested more efficiently than cattle, when high-fiber, low-protein (< 7%) forages were consumed in the fall and winter (Peden, et al., 1974). There is no difference in digestibilities between species, when higher quality forage was consumed in spring and summer. In this trial, bison were less selective than cattle, with respect to the quality of forage consumed.

"Hawley (1978) reported, that slough sedge (Carex atherodes Spreng.) was the most important forage for bison in the Slave River lowlands. The mean apparent digestibility coefficients (%) of slough sedge, when fed to bison and cattle, respectively, were: dry matter, 51 and 46, energy, 51 and 45; protein, 38 and 28; and acid detergent fiber (ADF), 47 and 42. The mean daily dry matter intakes for bison and cattle in a summer trial were each 1.6% of body weight). However, in a winter trial, the dry matter intake of cattle was 2.0% of body weight, which was significantly higher, than that of bison (1.6% of body weight)." Martin, P. J. (1982:262).

"The elephant, a monogastric, has a very large cecum (= a stomach with only one compartment); thus, as in the horse, fermentation products may be available for absorption posterior to the stomach, thus minimizing the elephant’s requirement for high-quality dietary protein. Like the horse, the elephant can effectively utilize large quantities of high-fiber, low-quality forage." Martin, P. J. (1982:262).

Low protein

"Protein concentrations in plants decrease with decreasing maturity of the plants. Animals require specific amounts of protein, to meet requirements for maintenance, reproduction and lactation. An inadequate protein intake in the ruminant may cause decreased feed intake, weight loss and failure to conceive. Animals can store some protein in the blood, liver, and muscles for subsequent use, but rely heavily on daily consumption of this very important nutrient." Martin, P. J. (1982:262).

"Protein requirements, as a percentage of the diet, vary directly with rate of growth and level of production. Young, rapidly growing animals need more protein as a percentage of their diet, than do adult animals. Protein, in excess of growth requirements, is used by the animal as a source of energy."(1982:264)

"Hawley (1978) found, that bison ate less food during the winter than cattle, but utilized it more efficiently. He suggested, that reduced feed intake may cause a slower rate of passage and, hence, increased digestibility and that this ‘adaptive strategy’ of bison allowed them, to survive the winter on a limited amount of low-quality feed." Martin, P. J. (1982:264).

Bison in Montana

What have scientists found out about the bison in the Yellowstone Park?

Margaret Mary Meagher, Research Biologist, National Park Service, says in her book, The Bison of Yellowstone National Park (1973): The bison of the Yellowstone National Park is mixture of the Plains buffalo (Bison bison) and The Mountain (Wood) buffalo (Bison b. athabascae), the wood bison. Of these herds about one third to one fourth are wood buffalo, the other two third to three quarter Plains buffalo.

Hair coat. The head, forelegs, hump, and shoulders are covered with longer hair (about 6 inches (15.24 cm) on shoulders and hump), but the hair on the flanks and hindquarters is much shorter, about 1 inch (2.54 cm), so that some bison look, as though they had been clipped. M.M. Meagher (1973:38)

Feeding in winter. Observations during the mild and average winters of the study period indicated, that snow depth did not limit forage availability. Bison commonly cleared fairly deep snow (by swinging the head in a sideways motion) with apparent ease. McHugh (1958) observed bison feeding in snow up to 4 feet (121.92 cm) deep. At higher population levels, snow depth may limit access to forage. Meagher (1973:73) - Captive bison gained mass, while supplementally fed a 15% crude protein diet during January-March, compared with bison, fed a 6% crude protein diet, that lost 9.9 ± 1.0% of their mass. DelGiudice, G. D. (1994:30).

Bull bison in Yellowstone National Park foraging in winter in snow about 2.5 feet (76 cm) deep. From: Margaret Mary Meagher, The Bison of Yellowstone National Park (1973:74) Fig. 26.