Climate change and greenhouse gas emissions receive a lot of attention in the media and with consumer-facing businesses. A lot of attention has focused on sources of greenhouse gases, especially related to fuel sources we use and food sources we eat. When it comes to food sources, ruminant animals, particularly cattle, receive a lot of attention for the large amount of methane produced. Methane is greenhouse gas with a global warming potential approximately 28 times greater than carbon dioxide, making it a relatively potent greenhouse gas, but not the most potent. 

Methane is produced during anaerobic (i.e., without oxygen) fermentation of organic material such as the digestion of feed in the rumen or degradation of manure in liquid storage facilities. Methane produced in the rumen of cattle and sheep has long been an area of study as it is a loss of energy from the feed that the animal does not get to use for growth, which is why it has been the target for development of interventions. Our understanding of this process is quite in depth and development of methods to reduce methane production in the rumen of cattle is an ongoing area of research. Many feed additives have been tested for their ability to reduce methane production and many are commercially available. 

Compared to other greenhouse gases such as carbon dioxide and nitrous oxide, methane is relatively short-lived in the atmosphere – approximately 12 years compared with 100+ years. From the standpoint of combating climate change, reduction in methane emissions could have the most immediate impact because of the short life of methane in the atmosphere. Thus, methane has become a focus for reducing overall greenhouse gas emissions. 

Methane is part of the biogenic carbon cycle where plants containing carbohydrates are consumed and digested by cattle, which produce and eructate, or belch, methane into the atmosphere. Methane in the atmosphere is converted to carbon dioxide over a 12-year period, and the carbon dioxide is assimilated by plants into carbohydrates. With a steady herd size, cows grazing pasture do not increase the amount of methane in the atmosphere, but when we increase the number of cows, then the amount of methane in the atmosphere increases. Vice versa, when we decrease the number of cows the amount of methane in the atmosphere decreases.  

The original method to compute global warming impact of greenhouse gas emissions did not account for the shorter life span of methane in the atmosphere and did not consider the trend in methane emissions with increasing or decrease herd size. A new method of calculation now accounts for the shorter life of methane in the atmosphere and the trend in herd size, which results in lower methane emissions overall (Figure 1). 

As mentioned above, methane is produced from manure. Storage of dry manure allows for aerobic (i.e., with oxygen) degradation of organic material such that little methane is produced. In contrast, storage of liquid manure creates an anaerobic degradation of organic material resulting in substantially more methane production. It does not matter whether the manure came from a ruminant animal or not as to the methane produced from manure – it primarily determined by the manure storage type. In current livestock production systems, beef cattle and poultry manure are stored primarily as dry manure, whereas dairy cattle and swine manure are stored primarily as liquid manure.  

Storage of manure in the dry form produces very little methane regardless of calculation method as can be seen with beef cattle and poultry in Figure 2. However, storage of manure in the liquid form produces a significant amount of methane as can be seen with dairy cattle and swine. The population of beef cattle decreased, and the population of dairy cattle was relatively constant from 2010 to 2020 resulting in the new method estimating lower enteric methane emissions than the old method (Figure 1). However, the amount of manure stored in liquid form has been increasing in dairy cattle operations resulting in increased manure methane emissions using the new compared with old method (Figures 1 and 2). 

Thus, of the total methane emissions from 2010 to 2020, enteric methane accounted for the most emissions at 75% using the old method, whereas manure methane accounted for the most emissions at 65% using the new method. With the new method likely the most accurate, addressing manure methane emissions may have the greatest immediate impact on total greenhouse gas emissions because a readily available and feasible technology in manure digesters already exists. The leading roadblock to implementation of this technology is cost, but public policy and consumer facing business could make substantial headway with financial incentives for dairies and swine operations to install manure digesters. 

Figure 1. Estimated cumulative emissions for enteric and manure methane production of the U.S. livestock industry from 2010 to 2020 using the old and new methods of calculation. MMT = million metric tons. Adapted from Beck et al. (2023;