Written with contributions by Angie Lemminger, SDSU Graduate Research Assistant.
Introduction
Body condition scoring (BCS) has long been used by herders, pastoralists, and herd managers for enhancing stockmanship and monitoring animal health and welfare. BCS is an effective management tool as it allows managers to safely assess the health of their mature animals quickly and non-invasively. This technique provides time-efficient valuable insights to monitor animal health. It is important to monitor rapid changes, especially those outside of expected seasonal changes, as this may indicate health-related issues, including poor nutrition, gastro-intestinal parasite infestation, or disease infection.
The term “body condition” was first defined by Murray in 1919 as the ratio of body fat to nonfat components in a live animal. Later, body condition scoring was formally developed for sheep in 1961, it is a non-invasive technique to estimate the fat reserves on key regions of an animal. For bison, the BCS values have been refined to focus on five key regions: spine, rump, hips, thigh, and long ribs. While there may be other body regions that stakeholders also consider, these five regions both reveal animal health and are easily and accurately compared among observers. Each body region is scored separately and then averaged to result in the individual’s overall BCS. In the case of having difficulty in discerning a score for a region or individual (e.g., either 2 or 3), the midpoint can be noted (e.g., then 2.5). Those individual overall measures are then averaged to have a whole-herd BCS for that month.
Like many hoofed wildlife species, bison (Bison bison) naturally fluctuate their body mass in response to the seasons by approximately 15%. During winter, bison reduce their total metabolic rates by 46‒59% (Rutley and Hudson, 2000), therefore they lose body condition over the course of winter which is described as “winter-slump” (Huntington et al., 2019). However, once spring arrives and temperatures increase, their metabolic rates increase in-kind and their body conditions improve over spring, especially for cows anticipating calving and nursing.
Familiarizing yourself with key regions of bison anatomy
Scoring should focus on these areas of the bison. Key anatomical regions include spine, rump, hips, thigh, and long ribs (Figure 1). These regions have been shown to be informative while reducing variability when comparing scores among multiple observers (Zielke et al., 2018).
5×5 BCS Scale for Bison
A five point scale is used to score five body regions on bison for their body condition score. For each of the five focal regions above, they are scored on a 5-point scale starting from 1: severe emaciation, to 5: severe obesity. The relative change of 1 BCS value is roughly equivalent to 90-110 lbs (~10%) of body mass for mature female bison, or 150-200 lbs (~10%) of body mass for mature male bison. The five body conditions are represented in Figure 2. We provide a region-by-region detailed breakdown at the end of this document.
How to tally the scores for individuals
It is recommended to conduct BCS monthly. Each body region is scored individually, and the scores are averaged to determine the animal’s overall BCS. After scoring each of the five body regions to a 1-5 value, then sum those 5 values and divide by 5 (the number of body regions). Over time and with practice, you may be able to assign a single BCS value for the whole animal. Below is an example of assessing BCS for all five body regions (Table 1).
| A. Spine | B. Rump | C. Hips | D. Thigh | E. Long Ribs | Sum (A+B+C+D+E) | Average (Sum ÷ 5) |
|---|---|---|---|---|---|---|
| 2.5 | 2.5 | 2 | 2.5 | 3 | = 12.5 | 2.5 |
How to tally the scores for the whole herd
It is recommended to survey 10% of your herd per month, or at least 30 individuals, whichever is lesser. This provides a robust survey of your herd so that you are sure to find overperforming and underperforming individuals in the herd. It is natural and normal for aging individuals — greater than 15-years-old for cows and 10-years-old for bulls — to have declining BCS as they approach and enter aged class. It is recommended that you pair these BCS scores with other regular surveillance work, such as monthly fecal collection for parasitological assessments, especially for poor BCS individuals.
After each individual is scored, you may then average all of the BCS scores for your herd. This number, while only representing a portion of your herd, is representative of the whole herd at that time (Table 2).
| Date | Individual # (Ear Tag) | Sex | Age (years) | Spine | Rump | Hips | Thigh | Long Ribs | Sum | Individual Average |
|---|---|---|---|---|---|---|---|---|---|---|
| April | 1 | F | 15 | 2.5 | 2.5 | 2.0 | 2.5 | 3.0 | = 12.5 | 2.5 |
| April | 2 | F | 12 | 3.0 | 2.5 | 2.0 | 2.5 | 3.0 | = 13.0 | 2.6 |
| April | 3 | F | 10 | 3.0 | 3.0 | 2.0 | 2.5 | 3.0 | = 13.5 | 2.7 |
| April | 10 | F | 5 | 3.0 | 3.0 | 2.0 | 2.5 | 3.0 | = 14.0 | 2.8 |
| Sub-sampled Herd Average: | 2.65 | |||||||||
How to use monthly scores for your herd
Recording your herd average BCS values provides a record of performance for your animals at different times of the year. Over time, you’ll be able to track how they perform in drought, normal, and wet years. Furthermore, you’ll also be able to then use your own herd as a reference to help identify overperforming and underperforming individuals for that season. Across the whole year, production-oriented bison herds should strive for an average BCS of 3.5 for the herd, while conservation-oriented herds should strive for an average BCS of 3.0 for the herd.
Understanding the expected changes during key periods is crucial for interpreting these scores correctly. Two of the most significant periods are the "winter slump" and the breeding season. During the winter slump, typically from December to April, it is natural for mature cows to lose 10-15% of their pre-winter body mass. However, a loss exceeding 20% can signal potential health or nutritional problems and may negatively impact future reproductive success. It is critical to manage this period carefully, as cows with either very low (BCS ≤2) or very high (BCS ≥4) scores are less likely to conceive. Similarly, the breeding season places significant energy demands on bulls. It is expected and natural for mature bulls to lose 10-15% of their pre-rut body mass due to increased activity and reduced foraging. For this reason, bulls should be evaluated as close to the breeding season as possible when they are at their peak condition. As with cows in winter, a body mass loss of 20% or more during the rut could indicate an underlying illness or injury that warrants closer observation.
As briefly indicated earlier, scores naturally fluctuate throughout the seasons of the year. During the growing season, as for all mammals, metabolic rates are higher for bison but are reduced during the winter season (Figure 3). Keeping track of how your bison herd performs from year to year is a useful measure to then anticipate how your herd may naturally respond to impending droughts or wet years.
| Date | Pasture | # in Herd | Age/Class | Status | # of Animals Scored | Average Score | Expected Seasonal Score | Notes |
|---|---|---|---|---|---|---|---|---|
| January | 1 | 100 | Bred cows | Bred | 10 | 2.65 | 3.1 | Should provide more protein |
| February | 1 | 100 | Bred cows | Bred | 11 | 2.60 | 2.75 | Decline has slowed |
| March | 2 | 100 | Bred cows | Bred | 10 | 2.60 | 2.5 | - |
| April | 2 | 100 | Cows | Calving | 15 | 2.55 | 2.25 | - |
| May | 3 | 100 | Cows | Lactating | 12 | 2.8 | 3.0 | - |
| June | 4 | 100 | Cows | Lactating | 8 | 3.4 | 3.5 | - |
| July | 5 | 100 | Cows | Lactating | 10 | 3.4 | 3.5 | - |
| August | 6 | 100 | Cows | Breeding | 10 | 3.5 | 3.6 | - |
| September | 7 | 100 | Bred cows | Bred | 10 | 3.55 | 3.6 | We’re on track |
| October | 8 | 100 | Bred cows | Bred | 10 | 3.55 | 3.5 | Round up |
| November | 9 | 100 | Bred cows | Bred | 10 | 3.3 | 3.4 | - |
| December | 9 | 100 | Bred cows | Bred | 10 | 3.2 | 3.2 | - |
Why use BCS?
BCS also has significant implications for reproductive success in cow bison—scores of 4 or higher may indicate excessive fat at calving, while scores of 2 or lower are associated with reduced fertility. These evaluations should inform management decisions, including the need for targeted health interventions at both the individual and herd level.
In particular, using the BCS may help you to guide how much to adjust the delivery of winter bale/swath grazing required for your herd based on total weight, because, the relative change of 1 BCS value is roughly equivalent to 90-110 lbs (~10%) of body mass for mature female bison, or 150-200 lbs (~10%) of body mass for mature male bison. Winter-slump frequently results in body mass declines of 10-15% each winter for both sexes. Accounting for winter-associated BCS decline may help to reduce hay waste from unconsumed hay.
Region-by-Region Detailed BCS
The spine can be viewed from either from behind or from the side. The individual bones along the spine should ideally be slightly visible, but not prominent; muscle sloping away from the spine should be moderately filled. A bison with spine scoring a 1 would appear to have very sharp, visible bones and steep-angled muscle. A bison with spine scoring a 5 would have bones that are not visible and buried in fat and muscle has little slope and flattened profile.
BCS: Spine = 1
BCS: Spine = 4
The rump is best viewed from either side of the body. The bones should ideally be slightly visible, but not sharp. The rump muscles should be flat and angular. A bison with a rump scoring a 1 should have muscles that are caved-in. A bison with rump scoring a 5 would be covered in fat and no bones visible; rump is full and bulging-out.
BCS: Rump = 1
BCS: Rump = 5
The hips are best viewed from behind. The hip bones should ideally be slightly visible, but not sharp. The muscle sloping away from the hips should be flat and angular. A bison with hip scoring a 1 would have very sharp, prominent edges that are sharp and muscle is caved-in. A bison with hip scoring a 5 would have bones that are not visible and buried in fat; rump is full and bulging-out.
BCS: Hips = 1
BCS: Hips = 4
The ribs are best viewed from the side of the body. The ribs should ideally be slightly visible in summer (not visible with winter coat) with 3-4 ribs showing but not sharp or distinct; edges should be round and fleshy. A bison with ribs scoring a 1 would be prominent with many ribs visible in both summer and slightly less visible in winter. A bison with ribs scoring a 5 would not be visible in summer or winter; and buried in fat.
BCS: Ribs = 1
BCS: Ribs = 5
The thighs are best viewed from the side of the body. The thigh depression (sulcus line between the quadriceps and hamstring) should ideally be slightly visible, but not prominent; muscle should be moderately filled. A bison with thighs scoring a 1 would appear to have a very deep sunken depression with visible bones of the hip and rump. A bison with thighs scoring a 5 would have no visible thigh muscle depression, buried in fat.
BCS: Thigh = 1
BCS: Thigh = 5
Conclusion
It is important to monitor body condition scores for poor condition and rapid changes as this may indicate poor nutrition, gastro-intestinal parasite infestation, or disease infection. BCS also has significant implications for reproductive success in cow bison—scores of 4 or higher may indicate excessive fat at calving, while scores of 2 or lower are associated with reduced fertility. These evaluations should inform management decisions, including the need for targeted health interventions at both the individual and herd level. Establishing a frequent and regular parasite monitoring program for your herd is important to become proactive, instead of reacting to health problems. Contact your veterinarian or local extension office for labs that can conduct this testing.
Assessing the body condition scores (BCS) of a herd is an effective decision support tool, as it enables managers to optimize feeding quality, implement targeted supplementation strategies, and proactively address potential health concerns. This proactive approach enhances overall health and improves management efficiency. Observers should strive to evaluate a representative sample of bison to ensure accurate assessments. When possible, physically handling bison can strengthen the correlation between visual estimations and actual body condition, refining the accuracy of future assessments.
Acknowledgements
This work and AL were supported in part by Rocky Mountain Bison Association and the Center of Excellence for Bison Studies and JMM was supported in part by the USDA–NIFA Hatch project award no. 1026173, the USDA–NIFA Multistate project award no. 7004803, the USDA–Natural Resource Conservation Service Conservation Innovation Grant project award no. NR243A750011G006. We also thank the National Bison Association and National Buffalo Foundation for their collaboration on this project.
References
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