An exploratory assessment of mineral supplementation on circulating minerals, herd performance, and reproductive success in American Plains Bison

1 Introduction

The American Plains Bison (Bison bison) are one of the two subspecies of the American bison, also known colloquially as buffalo. These large mammals, an iconic symbol of the Great Plains, once roamed the vast prairies of North America in the tens of millions. Custer State Park (CSP), South Dakota, was the last large park established in 1914 for the preservation of buffalo (Gordon, 1995). The management practices used on bison in CSP were established in 1961 and are still maintained today, with only minor modifications. Surplus animals were removed by an annual auction, trophy bulls were used for hunting, calf deliveries made through sealed bids, and a calculated number of bison were slaughtered to have available for the operated lodges on the park (Gordon, 1995). Throughout their time and overall distribution amongst the United States, bison have been thought of as obligate grazers (Boyd and Gates, 2006). However, there is a growing body of evidence that suggests bison diets include substantial quantities of forbs and shrubs (Jorns et al., 2020). The forbs and shrubs may be providing nutrients such as minerals for bison that are not as readily available in grasses. In commercial bison operations, these forbs and shrubs are not as readily available, which may lead to dietary gaps in available nutrients. In bison diets, the circulating mineral concentration is poorly understood and lacks research. Many factors such as mineral concentrations and secondary compounds affect grazer nutrition, but protein and energy concentrations of the plant’s bison eat determine their performance and reproduction (Craine, 2021). Reproductively active females have higher energy demands than males due to gestation and lactation, with energetic requirements peaking during early to midsummer (Main et al., 1996; Forsyth et al., 2005). Comparing to midsummer peak, bison have a “winter slump.” When days are short and temperatures fall, feed supply (or access to feed) decreases, voluntary intake decreases, digesta transit rates from the rumen and post ruminal sections of the gut decrease, and BW (or weight gain for young animals) decreases (Huntington et al., 2019).

As bison formerly roamed large expanses of the United States (U.S.), the requirements that the body needed and signaled to consume could be found while roaming. With the continuing repopulation of the American bison in the United States herds are in confined ranges compared to the vast plains of the U.S. With this shift in housing, the natural minerals available and wide variety of different native forages are more limited making receiving essential minerals challenging compared to their free roaming ancestors. The mineral elements most likely to be lacking under grazing conditions for ruminants are Ca, P, Na, Co, Cu, I, Se, and Zn. In some regions and under specific conditions, Mg, K, Fe, and Mn may be deficient and excesses of F, Mo and Se can be extremely detrimental (McDowell, 1996). Forages with low crude protein (CP) or digestible organic matter (DOM) often have a slower passage rate that can lead to decreased intake which lowers weight gain and reproduction (Soest, 2018). In most countries of the world, the principle means by which cattle producers attempt to meet mineral requirements of their grazing herds is through the use of free-choice dietary minerals (McDowell, 1996). This observational study sought to quantify the effects of supplemental minerals on circulating minerals and effects on pregnancy in bison cows in Western South Dakota.

2 Materials and methods

2.1 Locations and experimental design

This study used a cross-sectional design to compare trace mineral levels in female bison across two locations, Custer State Park (CSP), or the unsupplemented control herd (CON) and Kremer Buffalo (KB), which acted as the supplemented, treatment herd (SUPP). CSP is South Dakota’s State Park and wildlife reserve in the Black Hills. KB is a privately owned and managed operation that resides in the Black Hills of South Dakota and borders one side of the park’s fence. Though both locations are separate from one another, they are managed by the same individual and are employed with the same management strategies. Climate, forage species, and other environmental variables are comparable due to location and management similarities.

2.2 Management and experimental parameters

A subset of 73 (SUPP) and 84 (CON) females aging 1–17 years old were selected at random for inclusion in the study in year 1. In year 2, a subset of 60 SUPP and 130 CON were selected at random, proportionally distributing across age to equally represent the whole herd. Individually sampled animals were categorized by location and age. Yearlings were excluded from pregnancy status analyses.

All bison were managed on a semi-arid, mixed-grass prairie ecoregion, suitable for supporting free-ranging bison and other native species. The CON herd utilizes a continuous grazing management strategy, solely relying only on the native forage of the park. The herd had access to multiple pastures which were all mixed grass prairies, composed of smooth bromegrass, western wheatgrass, switchgrass, and unidentified mixed old growth. Animals had equal access to all pastures prior to collection procedures. The SUPP herd uses a similar grazing system with similar pasture species makeup, plus additional supplementation of free-choice Redmond Bison 90 (Redmond Agriculture, Redmond, Utah, U.S.A; Table 1) and a dose of Multimin^® (Axiota, Fort Collins, Colorado, U.S.A; Table 2) at time of processing which occurs once per year in the fall.

Table 1. Guaranteed analysis Redmond Bison 90 provided at supplemented ranch (SUPP) for years 1 and 2.

Ingredient Statement: Unrefined Salt, Molasses, Sodium Selenite, Zinc Oxide, Manganous Oxide, Copper Sulfate, Cobalt Carbonate, Ethylenediamine Dihydroiodide (EDDI).

Table 2. Guaranteed Analysis MULTIMIN 90 provided at supplemented ranch (SUPP).

Ingredient Statement: ¹as zinc oxide; ²as copper carbonate; ³as manganese carbonate; ⁴as sodium selenite; ⁵as preservative. .

Blood, fecal, and forage samples were collected along with performance data at the time of round-up on October 9-11^th, 2024 and October 5-9^th, 2025 (CON) and November 9^th, 2024 and October 25^th, 2025 (SUPP). Blood samples were collected via jugular venipuncture. Fecal was collected during the pregnancy status checks via fecal grab. Pregnancy status was determined by a licensed veterinarian via a portable veterinary ultrasound machine. Forage samples were collected by random spot clippings. All sample results of minerals present were recorded in mg/l or mg/kg DM.

2.3 Blood and fecal collections

Blood samples were collected while processing animals through a handling system using a neck extender attachment for jugular venipuncture. A single 12 cc syringe fitted with an 18x1” needle was used to collect blood, which was subsequently divided between two 6 mL anticoagulant tubes: a K₂EDTA tube (VACUETTE^®, Greiner Bio-One International GmbH, Kremsmünster, Austria; lavender top) and a trace-element serum tube (BD Vacutainer^®, Becton, Dickinson and Company, Franklin Lakes, NJ, USA; royal blue top). The lavender top tube was stored on ice, as whole blood, for transport back to the Bison Studies Laboratory at SDSU (Brookings, SD, U.S.A). The blue top tubes were centrifuged for plasma collection and stored on ice for transport. Fecal samples were collected at the time of pregnancy status check. A liter sized Ziploc bag was filled with feces from each cow and stored on ice for transportation back to the SDSU Bison Studies Laboratory for subsequent analysis.

2.4 Performance recording

Individual animal data was recorded at the time of sample collection to account for potential physiological and production-related factors on mineral status. Age, weight, reproductive status, and lactation status were documented for each bison in this study.

Age was recorded based on previous ranch records and branding management strategies. Body weight was measured using a livestock scale built into the chute at time of handling. Reproductive status was determined using an IBEX^® EVO^® portable veterinary ultrasound system (E.I. Medical Imaging, Loveland, CO, USA) operated by a licensed veterinarian and classified as either pregnant or open. Yearlings were excluded from reproductive status analysis due to their immature breeding age.

2.5 Laboratory analysis methodologies

2.5.1 Analysis of forage, intake, and water

Forage was collected using a range hoop with the area of 924 cm² (34.3 cm diameter). Clippings were taken 25.4 mm from the stem base in every grazing pasture with guidance from park/producer management. As the SUPP herd borders the CON herd, both sample locations are grazed on the same native species of forage. Brome grass, wheatgrass, and switchgrass were all present on each ranch and were separated into monoculture paddocks. Forage was sorted by paddock and dried in the oven at 75°C for 12 hours. Once dried, samples were weighed again and then ground in a Thomas-Wiley Laboratory Mill at 4mm and analyzed via Energy Dispersive X-Ray Fluorescence Spectrometry for mineral concentrations.

Supplemental mineral intake was estimated by utilizing records from the producer regarding the amount of mineral that was fed and the amount of time between feeding which was averaged across individual animal.

A general mineral analysis was conducted via Energy Dispersive X-Ray Fluorescence Spectrometry to identify mineral intake via water consumption.

2.5.2 Analysis of fecal

Fecal samples were composited while wet to ensure homogeneity and minimize variability within the sample. Once composited, samples were dried in the oven at 75°C for a period of 24 hours to remove residual moisture for analysis. Dried fecal samples were ground in a Retsch ZM 200 centrifugal mill equipped with a 5mm sieve to achieve a suitable particle size. The ground fecal was analyzed via Energy Dispersive X-Ray Fluorescence Spectrometry for mineral concentrations.

2.5.3 Analysis of blood

Blood samples were obtained via jugular venipuncture using sterile techniques to minimize the risk of contamination. Each sample was split between two anticoagulant tubes. The tubes designated for whole blood analysis were processed by extracting 1.5 mL of the blood from the tubes and analyzed via Energy Dispersive X-Ray Fluorescence Spectrometry.

2.5.4 Energy dispersive X-ray fluorescence spectrometry

An EDX-7200 (Shimadzu Scientific Instruments, Kyoto, Japan) was utilized in determining all minerals in this study. Individual standard curves for Zn, Mn, Co, Cu, Se, and Mo were created in the Bison Studies Laboratory using ICP grade analytical standards (Fisher Scientific). Ranges for standards were determined via previous literature of circulating minerals and serial diluted to produce the standard curve. A subset of samples were then analyzed in the EDX-7200 and results were compared of the same samples analyzed via ICP by a third-party laboratory for result validation.

2.6 Statistical analysis

All statistical analyses were conducted in R. Due to the study design, individual animals were not considered independent experimental units; rather, location (ranch) was considered the experimental unit. Univariate mineral data are presented descriptively, and no univariate inferential analyses (e.g., ANOVA) were conducted. Multivariate patterns were evaluated using PERMANOVA based on distance matrices.

For all mineral concentrations (blood, fecal, and forage), means, standard deviations (SD), minimum, and maximum values were calculated and summarized by location, year, age class, and pregnancy status where applicable. Yearling animals were excluded from pregnancy-related analyses.

To evaluate multivariate relationships among mineral profiles, principal coordinates analysis (PCoA) was conducted using Euclidean distance matrices generated from standardized mineral concentration data. Multivariate differences in mineral profiles were assessed using permutational multivariate analysis of variance (PERMANOVA) implemented via the adonis2 function in the vegan R package. Models included fixed effects of location, year, pregnancy status, and their interactions where appropriate. Significance was assessed using 999 permutations. Because of the limited number of experimental units (locations), PERMANOVA results are interpreted as exploratory indicators of multivariate structure rather than confirmatory statistical tests. Therefore, emphasis is placed on patterns of separation observed in PCoA ordinations in conjunction with descriptive summaries.

Prior to analysis, non-detectable values and missing entries (e.g., “ND” or “.”) were treated as missing (NA). Mineral variables were evaluated for completeness, and only variables with sufficient observations were included in multivariate analyses.

3 Results and discussion

3.1 Forage and fecal mineral concentrations

Minerals concentrations in the forage did not differ among the two treatment herd locations, likely reflecting the comparable pasture composition across both locations (Table 3). Although mineral concentrations did not differ, numerical variations were observed among individual sampled forage types, including brome grass, wheat, and switchgrass, with total forage averages summarized across the locations (Table 4). These differences are consistent with expected variation among forage species and may contribute to heterogeneity in mineral intake at the animal level (Yoshihara et al., 2013). Selenium was not detectable in any forage sample analyzed, suggesting limited dietary selenium contribution from forage sources at both locations. Selenium is often deficient in forage but can be found through soil to plant transfer or animal grazing which can maintain adequate blood levels (Arthington and Ranches, 2021).

Table 3. Forage mineral values (mean ± SD) of select minerals from both unsupplemented (CON) and supplemented (SUPP) locations.

Values represent mean ± standard deviations forage mineral concentrations (mg/kg DM) for American Plains bison by year.

ND, Not Detectable.

Table 4. Forage mineral values of select minerals from both unsupplemented and supplemented locations.

Values represent forage mineral concentrations (mg/kg DM) by forage species for Year 1. Selenium concentrations were below detection limits for all forage samples. .

ND, Not Detectable.

Fecal mineral concentrations exhibited numerical differences between CON and SUPP herds across both years (Table 5). Fecal Mn and Zn concentrations were numerically greater in the CON herd in both years, while fecal Mo concentrations were substantially greater in the SUPP herd during both Year 1 and Year 2. Fecal Cu concentrations showed greater variability across years, with higher values observed in SUPP animals during Year 2. Selenium was not detectable in fecal samples from either herd in either year (Table 5). Because fecal mineral concentrations represent unabsorbed dietary minerals, greater fecal concentrations in specific minerals may reflect either increased intake or reduced absorption efficiency depending on the mineral and its interactions within the diet. For example, antagonistic interactions among trace minerals, particularly between molybdenum and copper, are well established in ruminants and can influence mineral utilization (McDowell, 1996).

Table 5. Fecal mineral values (mean ± SD) of bison in Western South Dakota in both unsupplemented (CON) and supplemented (SUPP) groups.

Values represent mean ± standard deviation fecal mineral concentrations (mg/kg DM) for American Plains Bison by treatment group and year. Selenium concentrations were below detection limits for all samples.

ND, Not Detectable.

Multivariate analysis revealed a strong effect of location on fecal mineral profiles in both years (Table 6). In Year 1, location explained 86.6% of the variation in fecal mineral concentrations (R² = 0.866; P = 0.001), indicating a pronounced separation between SUPP and CON herds. Similarly, in Year 2, location explained 91.0% of the variation (R² = 0.910; P = 0.001). Fecal mineral concentrations reflect the fraction of consumed minerals that are not absorbed and are subsequently excreted, providing insight into both intake and utilization efficiency. The high proportion of variation explained by location in both years indicates that supplementation was a dominant factor influencing fecal mineral composition. Separation between groups was consistent across years, suggesting a strong and repeatable effect of supplementation on fecal mineral composition.

Table 6. Permutational multivariate analysis of variance (PERMANOVA) results for fecal mineral profiles in bison by management group (CON vs. SUPP) within Year 1 and Year 2.

*Degrees of freedom.

Fecal mineral concentrations (ppm) were analyzed using Bray–Curtis dissimilarity matrices. PERMANOVA was conducted separately within each year to evaluate the effect of management group (CON = control, no supplementation; SUPP = supplemented). R² represents the proportion of variation in multivariate mineral profiles explained by management. P-values were generated using 999 permutations. Analyses are presented as descriptive and exploratory due to limited replication at the herd level.

Collectively, these results highlight the importance of evaluating multiple biological matrices when assessing mineral status. While circulating mineral concentrations provide information on immediate physiological status, fecal mineral concentrations offer additional insight into mineral utilization and loss. The integration of these measures provides a more comprehensive understanding of mineral dynamics in grazing bison systems.

3.2 Circulating blood mineral concentrations by location and pregnancy status

Mean circulating blood mineral concentrations for bison cows differed numerically between the SUPP and CON herds (Table 7). Across locations and years, circulating concentrations of Mn, Co, Cu, and Zn were numerically greater in the SUPP herd compared with the CON herd. Selenium concentrations were numerically similar between locations, with slightly greater values observed in CON animals during Year 1 and in SUPP animals during Year 2. Circulating Mo concentrations were comparable between locations in both years (Table 7). These numerical differences in circulating mineral concentrations may reflect variation in minerals associated with supplementation strategies. The elevated concentrations in SUPP bison potentially reflect effective uptake from supplementation and improved bioavailability compared to forage alone. Higher circulating concentrations of Mn, Co, Cu, and Zn in supplemented animals are consistent with known roles of supplementation in improving mineral intake in grazing ruminants, particularly in systems where forage alone may not meet requirements (McDowell, 1996). However, the relatively modest differences between herds and the reduced separation observed in Year 2 highlight the complexity of interpreting circulating mineral concentrations. Blood mineral levels are tightly regulated through homeostatic mechanisms and may not directly reflect short-term dietary intake, particularly when animals are not experiencing overt deficiency or excess.

Table 7. Circulating blood values (mean ± SD) of select minerals for bison cows (age = 1-20 years old) at the unsupplemented ranch (CON) and supplemented ranch (SUPP).

Values represent mean ± standard deviation circulating whole blood mineral concentrations (mg/l) for American Plains Bison cows by treatment group and year. Samples were analyzed as whole blood without separation.

*CON, Unsupplemented; SUPP, Supplemented.

In Year 1, separation between SUPP and CON herds was observed in PCoA space (Figure 1). This visual separation aligned with PERMANOVA results, although location explained a moderate proportion of the variation in circulating mineral profiles with 19.9% of the variation explained in circulating mineral profiles (R² = 0.199; P = 0.001; Table 8). These results suggest a moderate influence of supplementation on circulating mineral status during the first sampling year. In contrast, separation between SUPP and CON herds in Year 2 was less pronounced (Figure 1), with greater overlap observed between groups. Although PERMANOVA results remained significant (P = 0.002), the proportion of variation explained by location was substantially reduced (R² = 0.047), indicating that only 4.7% of the variation in circulating mineral profiles was attributable to supplementation (Table 8). This suggests a weaker effect of supplementation during the second sampling year. Univariate visualization of individual mineral concentrations further supported these trends, with patterns of separation between SUPP and CON herds varying by mineral and year (Figure 2).

Table 8. Permutational multivariate analysis of variance (PERMANOVA) results for circulating blood mineral profiles in bison by management group (CON vs. SUPP) within Year 1 and Year 2.

*Degrees of freedom.

Circulating blood mineral concentrations (ppm) were analyzed using Bray–Curtis dissimilarity matrices. PERMANOVA was conducted separately within each year to evaluate the effect of management group (CON = control, no supplementation; SUPP = supplemented). R² represents the proportion of variation in multivariate mineral profiles explained by management. P-values were generated using 999 permutations. The proportion of variation explained by management was moderate in Year 1 and low in Year 2, indicating a limited influence of management on circulating mineral profiles.

Differences in the magnitude of separation between years may reflect variation in environmental conditions, forage mineral availability, or other management factors influencing mineral status. Collectively, these findings indicate that while supplementation influenced circulating mineral profiles, the extent of this effect varied between years.

Biological variation associated with the independent sampling of animals between years may have also contributed to differences in multivariate patterns. Because different individuals were evaluated each year, variation in age distribution, physiological status, and reproductive stage may have influenced mineral metabolism and circulating concentrations. Trace minerals such as copper, zinc, and selenium play critical roles in immune function, oxidative balance, and reproductive processes, and their circulating levels can vary with physiological demand (Soest, 2018; Arthington and Ranches, 2021). As a result, differences in herd composition or reproductive dynamics between years could partially explain the reduced separation observed in Year 2. Management-related factors may further contribute to these patterns. Although supplementation strategies remained consistent, differences in intake behavior, supplement access, or grazing distribution could influence the degree to which supplemented animals consumed and utilized trace minerals. Additionally, environmental stressors such as temperature extremes, drought conditions, or forage availability may alter both mineral intake and metabolic demand, further influencing circulating profiles.

Collectively, these findings suggest that while supplementation contributed to differences in circulating mineral status, its relative impact was influenced by year specific environmental and biological conditions. This highlights the importance of considering temporal variation when evaluating mineral status in grazing systems and supports the interpretation of these results as preliminary, descriptive indicators of mineral dynamics in American Plains bison.

3.3 Pregnancy rates by percentage by location

To evaluate the influence of reproductive status on circulating mineral profiles, analyses were restricted to animals older than 1 year and conducted by year to account for independent sampling. Reproductive performance in this study was evaluated using pregnancy status due to additional reproductive parameters, including age at puberty, age at first calving, calving interval, and calf survival, were not available for either herd. Although these measures would provide a more comprehensive assessment of reproductive efficiency, pregnancy status remains a useful indicator of reproductive success within the context of this exploratory comparison.

When summarized by pregnancy status across both herds, circulating mineral concentrations were generally similar between open and bred cows (Table 9). Bred cows exhibited numerically lower Mn and Cu concentrations compared to open cows, while numerical trends suggest that Zn, Se, and Co may be elevated in bred bison relative to open cows (Table 9). These patterns may reflect differences in mineral utilization associated with gestation, as Meyer (2020) explains pregnancy is known to increase mineral demands for fetal development and maternal tissue accretion. Selenium has strong direct evidence of improving cattle reproduction through embryonic survival, postpartum uterine health, and conception-related outcomes (Mehdi and Dufrasne, 2016). At the same time, zinc mainly supports reproduction through hormone regulation and antioxidant defense (Swenson et al., 2020). However, pregnancy status was assessed at a single time point, and animals were not followed across gestation.

Table 9. Values* (mean ± SD) of select minerals by pregnancy status of bison cows (age 2-20 years) in both herds in western South Dakota.

Values represent mean ± standard deviation on circulating whole blood mineral concentrations (mg/l) for American Plains Bison cows, excluding 1 year olds, by pregnancy status and year. Samples were analyzed as whole blood without separation.

Animals who were not bred at time of palpation = open.

Pregnancy rates, expressed as percentages, differed numerically between the two treatment herds across the two years of observation (Figure 3). In the CON herd, 89.2% of females were confirmed pregnant at the time of palpation, whereas the SUPP herd exhibited a lower observed pregnancy rate (70.7%) during year 1 (Figure 3). In year 2, pregnancy rates differed numerically, but with the CON herd experiencing 64.3% confirmed pregnancies and the SUPP herd having 86.2% (Figure 3). Because each treatment was represented by a single herd (n = 1 per treatment), differences between locations should be interpreted cautiously and cannot be attributed solely to supplementation. This reversal in herd level pregnancy percentages between years likely reflects differences in sampling intensity and population representation rather than true shifts in reproductive performance between locations. During Year 1, pregnancy status was recorded for all animals processed during the round-up, whereas in Year 2, pregnancy status was determined from a randomly selected subset of approximately 150 animals. Reduced sample size and random subsampling in Year 2 may have increased variability and altered the apparent distribution of reproductive outcomes within each herd.

Multivariate analysis indicated that pregnancy status did not significantly influence circulating mineral profiles. In the pooled analysis, year explained a significant proportion of variation (R² = 0.082; P = 0.001), while the interaction between location and pregnancy status was not significant (P = 0.125; Table 10). Within individual years, no significant effects of pregnancy status or location by pregnancy interactions were detected. In Year 1, the interaction between location and pregnancy status was not significant (P = 0.603), and similar results were observed in Year 2 (P = 0.378; Table 10). These findings suggest that multivariate mineral profiles were not consistently associated with reproductive status across years.

Table 10. Permutational multivariate analysis of variance (PERMANOVA) results for circulating blood mineral profiles in bison evaluating the effect of management group and pregnancy status within Year 1 and Year 2.

*Degrees of freedom.

Circulating blood mineral concentrations (ppm) were analyzed using Bray–Curtis dissimilarity matrices. PERMANOVA was conducted separately within each year to evaluate the effects of management group (CON = control, no supplementation; SUPP = supplemented) and pregnancy status (open vs. bred). R² represents the proportion of variation in multivariate mineral profiles explained by the model. P-values were generated using 999 permutations. df = degrees of freedom. No significant effects of management group or pregnancy status on circulating mineral profiles were detected in either year, with very low proportions of variation explained by the model. These results suggest that variation in circulating mineral profiles was largely driven by factors other than management or pregnancy status.

Although multivariate analyses did not detect significant differences, descriptive evaluation of individual mineral concentrations indicated numerical variation between open and pregnant animals within certain locations and years. However, these patterns were inconsistent and characterized by substantial variability, limiting interpretation of pregnancy-related effects.

The observed differences in pregnancy percentages between herds may reflect variation in herd management practices rather than nutritional factors alone. The most likely influence on this pregnancy rate difference was the presence of bulls in the CON herd. A bull-to-cow ratio of 1:8 at the SUPP herd and 1:2 at the CON herd were observed during the rut prior to this study’s collection. The Kansas Buffalo Association suggests that a standard bull-to-cow ratio for bison is approximately 1:15 (LLC, 2025). Though both ranches were below the recommended ratio, the lower bull-to-cow ratio in the CON herd likely reduced competition, allowing for significantly more cows to be bred. Additionally, reproductive success in grazing systems is influenced by a combination of nutritional, environmental, and management-related factors. While mineral status is an important component of reproductive performance, energy balance and overall nutritional value is often primary drivers of fertility in ruminants (Craine, 2021; Huntington et al., 2019). The similar circulating mineral profiles observed between open and bred cows suggest that mineral status alone was not a limiting factor for reproduction in these herds during the study period. Environmental, genetic, or herd-specific stressors may also play important roles in fertility status. During the rut, male and female bison spend approximately 1 month in large mating groups, with males tending to potential mates in an effort to maximize breeding potential (Larter, 1988). Because pregnancy status was assessed at one point in time and reproductive management factors were not controlled experimentally, no causal inferences can be made.

4 Conclusion

This study provides preliminary, descriptive insight into forage, fecal, and circulating mineral dynamics, as well as reproductive outcomes, in American plains bison managed under contrasting supplementation strategies within a shared regional environment. While forage mineral concentrations were similar between locations, fecal mineral profiles demonstrated a strong and consistent separation between herds, indicating that supplementation substantially influenced mineral intake and excretion patterns. In contrast, circulating mineral concentrations exhibited more moderate and variable differences between supplemented and unsupplemented bison, suggesting that physiological regulation and environmental factors play an important role in maintaining mineral homeostasis.

Reproductive outcomes were not consistently associated with mineral status, as circulating mineral concentrations were similar between open and bred cows across both years. Instead, observed differences in pregnancy rates between herds appeared to be more closely related to management factors, particularly bull-to-cow ratios and herd dynamics during the breeding season. These findings highlight the multifactorial nature of reproduction in grazing bison systems and suggest that mineral supplementation alone is unlikely to drive reproductive performance.

Collectively, these results emphasize the importance of evaluating mineral status using multiple biological matrices and across time when assessing nutritional strategies in bison. While supplementation influenced mineral dynamics, its effects were context-dependent and moderated by environmental and biological variability.

Given the limited availability of species-specific nutritional data for bison, this study contributes important baseline information to support the development of targeted management strategies. However, because results are descriptive in nature and experimental units were limited, findings should be interpreted as preliminary. Future controlled studies are needed to establish definitive mineral requirements for bison and to further evaluate the role of supplementation in optimizing health, productivity, and reproductive success.

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Footnotes

Edited by: Peter Erickson, University of New Hampshire, United States

Reviewed by: Brianna R. Beechler, Oregon State University, United States

Yallappa M. Somagond, National Research Centre on Mithun (ICAR), India