USA: A Century in the Evolution of Methane Emissions in Dairy Production
- Alvaro Garcia (Dellait)
- Feb 24
- 2 min read
In 1924, the U.S. dairy herd consisted of approximately 21 million cows, each producing about 1,890 kg of milk annually and consuming approximately 9.7 kg of dry matter (DM) per day. Advances in genetics, nutrition, and management have resulted in significant improvements in intake, productivity, and efficiency.
Currently, the U.S. dairy herd numbers 9.36 million cows—a 55% reduction in herd size since 1924. Each cow now produces, on average, 24,000 pounds (10,977 kg) of milk annually and consumes approximately 60 pounds (27.2 kg) of dry matter per day. This shift has resulted in a threefold increase in feed efficiency, with modern cows producing significantly more milk per unit of feed consumed.
Despite increases in individual productivity and intake, total methane emissions have declined since 1924, due to reduced herd size and improved feed efficiency. This highlights the environmental progress made in dairy farming, as the industry produces more milk with fewer cows and lower overall methane emissions.
Using the Ellis Model, a common tool in agricultural and environmental studies, daily methane emissions for dairy cows in 1924 and 2024 were estimated based on their dry matter intake (DMI). Methane is measured in megajoules (MJ) to reflect its energy content, as each kilogram (kg) of methane contains approximately 55.5 MJ.
The estimated methane emissions of 11.32 MJ per day provides a reasonable approximation for modern dairy cows, although it is slightly lower than current estimates of 100 to 130 kg per year. This difference likely stems from the fact that the Ellis Model relies solely on DMI, without considering diet composition (e.g., forage versus concentrate ratios), which can influence methane production.
High-producing cows on high-energy diets may emit more methane than the model predicts. However, because the same model was used for 1924 and 2024, the comparison remains valid, allowing a reliable assessment of changes in emissions over the last century.
Despite the increase in individual methane emissions from modern cows due to increased feed intake, total emissions have decreased substantially since 1924, primarily due to the reduction in the cow population. Consequently, total emissions can be estimated at approximately 106 million MJ per day, compared to 126 million MJ per day for cows in 1924. This represents a 16% reduction in total methane emissions.
Methane intensity measures the amount of methane emitted per unit of milk produced, reflecting the efficiency of emissions relative to productivity. In 1924, methane intensity was approximately 1.16 MJ per kg of milk, with each cow producing 1,890 kg of milk annually. By 2024, methane intensity had fallen to 0.38 MJ per kg of milk, with cows now producing 10,977 kg of milk annually. This reduction in methane intensity highlights improvements in feed efficiency and productivity, resulting in lower emissions per unit of milk.
Although total emissions fell by 16%, methane intensity (emissions per unit of milk) improved by approximately 67.2%, indicating much greater productivity efficiency relative to emissions over the century. The analysis suggests that today's dairy cows are much more efficient, producing substantially more milk with lower total methane emissions than a century ago.
