Do Bison Produce as Much Methane as Cows?

1 Bison methane flux observations in the context of other grazing systems

To put our observations into a bigger picture and to help us make grazing systems that produce the least amount of greenhouse gases in the future, we need to look into methane emissions from other grazing systems. From this point of view, our simple seasonal scaling exercise may have under- or over-estimated methane emissions from bison grazing systems for a number of reasons that need to be taken into account when analyzing the results. Methane emissions from cattle have been seen to be around 10% higher in the summer than in the winter (Todd et al. , 2014; Prajapati and Santos, 2018, 2019), which means that the methane flux measurements we make in the winter may be lower than what full-year measurements would show. Similar to what Prajapati and Santos (2019) found when they measured the same thing with beef cattle in a feedlot in the winter: about 75 g CH4 per animal per day. This suggests that bison and cattle grazing systems may have similar methane efflux. Our study pasture shares features with both feedlot and grazing systems with important implications for methane efflux. The bison were free to graze (Fig.  2) but were also supplied supplemental hay at regular intervals (Table S2). What this means is that it is likely that the animals were well-fed, which is not always the case in a wildland bison grazing system during the winter. It was seen that cattle in Africa produced more methane per unit of feed when they ate less food than usual during the dry season, when food is scarce (Goopy et al. , 2020). When food is scarce in the winter, bison in natural grazing systems may also produce more methane per unit of feed, but less total methane efflux if they eat less feed. This is because there is a strong link between feed intake and methane production (Johnson and Johnson, 1995).

We didn’t see any big changes in methane efflux throughout the day. However, it’s important to note that we could only observe during the day because we didn’t have enough information to know where the animals were at night. Other studies have observed higher methane efflux from cattle during feeding times (Gao et al. , 2011), but bison often graze at night, so it’s not clear if their methane flux changes during the day and at night, which would have effects on how flux changes over time. Methane efflux was not significantly higher during days when supplemental hay was provided (p=0. 075), which suggests that the animals’ ability to graze all day, even when they weren’t given extra food, canceled out any diurnal methane efflux cycle that might have been present if they had been fed at certain times.

Methane efflux is also affected by nutritional needs. For example, dairying buffalo cows are thought to release more methane than other buffalo (Cóndor et al. , 2008). The study herd had a lot of pregnant females (Table S1), which means they have higher metabolic needs. This means that methane flux values may be higher than in a herd with fewer pregnant animals. Out of all the measurements we took, there is no proof that bison have more or less methane efflux than normal cattle. It is important to measure methane flux all year long to understand how bison methane efflux changes with the seasons and come up with reasonable annual totals.

Paul C. Stoy CORRESPONDING AUTHOR

American bison (Bison bison L. ) have recovered from the brink of extinction over the past century. Bison reintroduction creates multiple environmental benefits, but impacts on greenhouse gas emissions are poorly understood. Bison are thought to have released about 2 Tg of enteric methane annually, which is a small amount of the 9 to 15 Tg annually that they are thought to have released before industrialization. However, few measurements have been made because bison move around a lot while grazing and it can be dangerous to measure non-domesticated animals. We use eddy covariance to measure the flow of methane and carbon dioxide from a bison herd on a fenced-in pasture during the day in the winter. Methane emissions from the study area were negligible in the absence of bison (mean ± standard deviation = −0. 0009 ± 0. 008 µmol m−2 s−1) and were significantly greater than zero, 0. 048 ± 0. 082 µmol m−2 s−1, with a positively skewed distribution, when bison were present. We used two separate flux footprint models and estimates of where bison were taken from automated cameras to come up with a mean per-animal methane efflux of 58 5 µmol s−1 per bison, similar to eddy covariance measurements of methane efflux from a cattle feedlot during winter. When we add up all the observations made over time using conservative uncertainty estimates, we get a confidence interval of 2091 (25 percent confidence) with a 95% confidence level between 2054 and 20109%C3%A2%C2%80%C2%89g%C3%82%20CH4%C3%82%20per%C3%82%20bison%C3%A2%C2%80%C2%89d%C3%A2%C2%88%C2%921. The results showed that estimating the location of bisons caused the most uncertainty (46% of the total uncertainty), followed by the flux footprint model (33% of the total uncertainty) and the eddy covariance measurements (21% of the total uncertainty). This suggests that making more accurate estimates of animal locations is a good place to start lowering uncertainty. Annual measurements are ultimately necessary to determine the full greenhouse gas burden of bison grazing systems. Our findings show that greenhouse gas emissions from various ruminant grazing systems need to be compared. They also show that eddy covariance could be used to measure methane efflux from animals that are not kept as pets. Share.

The American bison (Bison bison L. ) was hunted to near extinction during European expansion across North America (Flores, 1991; Isenberg, 2000; Smits, 1994). In the United States in the late 1800s, there were less than 100 reproducing individuals on private ranches. This was down from a population of 30 million to 60 million animals in the 1800s (Hedrick, 2009). The about 500,000 bison that live today are there because of the work of private landowners, government agencies, and sovereign American Indian tribes (Gates et al. , 2010; Sanderson et al. , 2008; Zontek, 2007), all of whom have spurred a growing interest in bison reintroduction. The number of bison is likely to grow even more, which makes it more important for researchers and land managers to figure out how their growth will affect the environment.

The ecological role of bison has become better understood as populations have recovered (Allred et al. , 2001; Hansen, 1984; Knapp et al. , 1999). Bison like to eat grasses more than other plants (Plumb and Dodd, 1993; Steuter and Hidinger, 1999). This makes forb diversity better (Collins and Steinauer, 1998; Hartnett et al. , 1996; Towne et al. , 2005). During the winter, they like to graze in certain meadows, and during the growing season, they look for the most energy-dense plants everywhere (Fortin et al. , 2003; Geremia et al. , 2019), often in areas which have recently burned (Allred et al. , 2011; Coppedge and Shaw, 1998; Vinton et al. , 1993). All of these observations suggest that bison choose high-quality forage over large amounts. This may affect their methane emissions, which all ruminants do because methane emissions are linked to feed quality (Hammond et al. , 2016), including cellulose and hemicellulose intake (Moe and Tyrrell, 1979). It’s still not clear how much methane bison produce when they eat mostly cellulose-rich grass, since they like fresh leaves, and if taking care of bison can increase or decrease the greenhouse gas impact of farming with ruminants.

Since 2016, the amount of methane in the air has been rising quickly for reasons that are still not clear (Nisbet et al. , 2019), and there is an urgent need to improve our understanding of its surface–atmosphere flux. It is estimated that between 2030 and 2040 percent of human-made methane emissions are caused by fermentation in livestock (Kirschke et al. and Gerber et al. (2013) say that cattle alone produce about 5 Pg of carbon dioxide equivalent per year. , 2013; FAO, 2017). Methane emission estimates from livestock have tended to increase as more information becomes available (Beauchemin et al. , 2008; Thornton and Herrero, 2010; Wolf et al. , 2017), further emphasizing their critical role in global greenhouse gas budgets (Reisinger and Clark, 2018). Earth system management needs to cut down on greenhouse gas emissions that aren’t needed. Cutting down on enteric methane sources is seen as a good way to do this (Boadi and Wittenber, 2002; DeRamus et al. , 2003; Herrero, et al. , 2016; Hristov et al. , 2013; Johnson and Johnson, 1995; Moss et al. , 2000).

Bison in North America are thought to have been responsible for some 2. 2 Tg yr−1 (Kelliher and Clark, 2010; Smith et al. , 2016) of the 9–15 Tg yr−1 of pre-industrial enteric methane emissions (Thompson et al. , 1993; Chappellaz et al. , 1993; Subak, 1994). During the pre-settlement period in the United States, emissions from wild ruminants made up almost 90% of current emissions from domesticated ruminants, based on an estimated historical bison population size of a million animals (Hristov, 2012), showing how important bison were to methane fluxes in the past. The current and future contributions of non-domesticated ungulates to methane fluxes are uncertain (Crutzen et al. , 1985). Prior studies either used inventory methods or scaling equations that weren’t based on methane efflux measurements from bison. The only direct observations of bison methane flux that we know of were of 30-gram methane per kilogram of dry food intake from a female bison that was confined and fed alfalfa pellets (Galbraith et al. , 1998), more dry matter than elk (Cervus elaphus) and white-tailed deer (Odocoileus virginianus), and about the same as dairy cows fed high maize silage (Hammond et al. , 2016). Cattle methane emissions tend to be greater when fed alfalfa than grass (Chaves et al. and others (2006), so the published values might not give a good picture of how much methane bison release in the wild, where it hasn’t been measured yet.

We use the eddy covariance method (Dengel et al.) to measure the flow of methane from a herd of bison on winter pasture. , 2011; Felber et al. , 2015; Prajapati and Santos, 2018; Sun et al. , 2015). We use flux footprint analyses and automated cameras to find the locations of bison to estimate methane flux on an individual animal level. We then talk about our findings in the context of eddy covariance methane flux measurements from other ruminants.

The study site is a 5. 5 ha fenced pasture on the Flying D Ranch near Gallatin Gateway, Montana, USA (45. 557∘, −111. 229∘), on a floodplain immediately west of the Gallatin River (Fig.  1). Daily high temperatures average 1. 6 ∘C, and daily low temperatures average −11. 5 ∘C at Bozeman Yellowstone International Airport (BZN), located 24 km north-northeast of the site, during the November–February measurement period. BZN records an average of 18. 2 mm of precipitation per month during November–February, almost entirely as snowfall. A herd of 39 bison entered the pasture on 17 November 2017 and left on 3 February 2018. Before the bison went into the pasture on November 16, 2017, the landowners measured their average weight, which was 329 kg with a standard deviation of 28 kg. The bison ranged in age from 0 to 12 years old. 5 to 7. 5 years old (Table S1). Bison ate a mix of perennial grasses that were grown in the area and perennial grass hay that was grown in nearby fields (Table S2) and delivered every three days on average (Table S3). This way of managing the bison is similar to how pasture and feedlot systems work in some ways.

A 3 m tower was installed near the center of the study pasture during November 2017 (Fig.  1) and surrounded by electric fencing to avoid bison damage. Four game cameras (TimelapseCam, Wingscapes, EBSCO Industries, Inc. , Birmingham, Alabama, USA) were mounted to the tower and pointed in cardinal directions. Two additional game cameras were mounted near the pasture edge facing the tower. Every 5 minutes, cameras caught s, and Fig. 1 shows an individual captured by the south-facing camera on the northern edge of the study pasture.  2. Bison locations at the 0. For the 5â€h time period of the eddy covariance measurements, the bison locations were manually assigned to squares in a 20â€m grid that was placed on top of the pasture area (Fig.  1). The 20-meter grid size is the smallest one that we thought would help us figure out where the bison were based on features of the field that could be seen with a camera. However, we still consider these observations to be just a guess and are open to correction. As explained in the section on spatial uncertainty below, we test how sensitive estimates of bison methane efflux are to estimates of where the bison live.

A Hukseflux NR01 net radiometer (Delft, the Netherlands) mounted on a wall was used to measure incident and outgoing shortwave and longwave radiation, as well as the net radiation. 5 m a. g. l. (meters above ground level). An SR50 sonic distance sensor (Campbell Scientific Inc. , Logan, UT, USA) was installed at 1. 3 m to gauge snow depth, and air temperature and relative humidity were measured at 2. 25 m using an HMP45C probe (Vaisala, Vantaa, Finland). Average 0–30 cm soil moisture and temperature were collected using CS650 probes (Campbell Scientific). Meteorological variables were measured once per minute, and 0. 5 h averages were stored using a CR3000 datalogger (Campbell Scientific).

Three-dimensional wind velocity was measured using a CSAT3 sonic anemometer (Campbell Scientific) at 2. 0 m a. g. l. (meters above ground level). Carbon dioxide mixing ratios were measured at 10 Hz using an LI-7200 closed-path infrared gas analyzer (LI-COR Biosciences, Inc. ) with an inlet placed at the same height as the center of the sonic anemometer. Methane mixing ratios were measured at 10 Hz using a LI-7700 open-path infrared gas analyzer (LI-COR Biosciences, Inc. , Lincoln, Nebraska, USA) with the center of the instrument likewise located at 2. 0 m and a 22 cm horizontal distance from the sonic anemometer; open- and closed-path infrared gas analyzers for eddy covariance work about the same in the field (Detto et al. , 2011; Deventer et al. , 2019). We use the atmospheric convention in which flux from the biosphere to the atmosphere is positive. Measurements were taken during the winter days from 7:00 a.m. to 17:00 p.m. local time so as not to drain the batteries and to make sure there was enough light to use game cameras to figure out where the bison were. Flux measurements began on 14 November 2017 and ended on 14 February 2018.

Bison are dangerous and will charge humans. Their presence made it harder to get data and keep game cameras in good shape. Some high-frequency flux measurements were lost, and cameras were turned off during very cold periods, which caused measurements to be lost. Flux measurements and pictures were taken at the same time from January 7th to February 13th, 2018, except for January 10th when instruments were covered in snow. For the weeks of November 14th through November 29th, 2017 and December 31st through January 6th, 2018, flux data were collected without any game camera footage.

Methane and carbon dioxide fluxes were calculated using an EddyPro (LI-COR Biosciences, Lincoln, Nebraska, USA). Standard double rotation, block averaging and covariance maximization with default processing options were applied. Getting rid of spikes was done according to Vickers and Mahrt (1997), and spikes were thought to be more than 3 5 standard deviations from the mean mixing ratio for carbon dioxide and more than 8 standard deviations from the mean mixing ratio for methane, since the bison herd is likely to cause methane spikes every once in a while. Following Dumortier et al.’s advice, the default dropout, absolute limit, and discontinuity tests were run with the default settings.  (2019), and the Moncrieff et al.  (1997) and Moncrieff et al.  (2004) low- and high-pass filters were applied. The Webb–Pearman–Leuning correction (Webb et al. , 1980) was applied to calculate methane efflux using the open-path LI-7700 sensor. It was thought that the storage flux in the 2â€m airspace below the infrared gas analyzers would be small, so it wasn’t included in the flux calculation. Flux measurements where the quality control flag was higher than one after Mauder and Foken (2011) (also see Foken et al. 2004, 2004) were thrown out, and all the fixes that were made when bison were present led to a decrease in methane flux of 2014%C3%A2%C2%80%C2%89%. Methane flux measurements that were higher than 1 µmol/m2 s−1 and carbon dioxide flux measurements that were higher than 20 µmol/m2 s∂1 were thrown out after the probability distribution of the observations was looked at. We checked how sensitive flux measurements are to the friction velocity (u*) to see if measurements taken when there wasn’t enough turbulence should be left out of the analysis, even though the flux measurements were only done during the day.

Fact Check: Methane Emitted By Bison Does NOT Compare With Levels Expelled By Cows

Do cattle produce methane?

And this is true. Cattle do produce methane as do bison and other ungulates. This gas contributes to global warming and much is being done to identify a way to suppress this naturally occurring gas to mitigate those impacts. But cattle actually mimic what bison and elk did naturally and are needed to help maintain healthy grasslands.

Did bison release more methane than cows?

The implied claim here is wrong. While cows aren’t the biggest source of greenhouse gases, they do still release methane into the atmosphere. However, limited research suggests bison in the 1800s may have released a similar, though possibly lower, amount of methane than cattle today.

How much methane does a wild bison eat a year?

The researchers found bison fed a diet of sun-cured alfalfa pellets emitted about 30 kilograms of methane each year, far less than the roughly 100 kilograms per year emitted by cattle. A 2021 paper says wild bison could have produced even less methane, noting that their diet on the prairie would involve eating forage that would lead to less gas.

Why do bison eat a lot of methane?

The word is commonly used to refer to bison, their North American cousins. The focus on the animals’ rear ends also misses their most potent source of methane, Mitloehner said. The anatomy and digestive habits of cattle and bison cause almost all of the methane to escape from their mouths, a process known as enteric emission.