What You Feed Your Cows Impacts Greenhouse Gas Emissions

How does feed production and diet formulation affect dairy greenhouse gas emissions?

The short answer: Dairy producers select forage crops based on the nutritional needs of their herd, crop suitability to the regional climate and soils, and their farm production system.

The carbon footprint of a dairy farm is an estimate of all its greenhouse gas (GHG) emissions from the herd, field, manure, and feed management subsystems minus any carbon storage in soils and vegetation. Climate-friendly forage crops are those which significantly increase net carbon accumulation in the field and do not appreciably increase GHG emissions from enteric fermentation when consumed by cattle.

Break it down: Forage crops are grasses or legumes fed to cattle in the form of pasture, hay, or silage (fermented and ensiled material). Forages include annuals (e.g., corn silage, sorghum-sudan grass) and perennials (e.g., alfalfa, meadow fescue), and may be grown alone or in multispecies mixes. Forage selection is one of several factors that impacts a farm’s carbon footprint.

• Major dairy farm GHG emissions include methane (CH₄) from enteric fermentation, manure storage, and field emissions; nitrous oxide (N₂O) from field emissions and manure storage; and carbon dioxide (CO₂) from farm energy and fertilizer use.
• CH₄ production from enteric fermentation is normally the largest source of dairy farm GHG emissions and reduced enteric methane production is associated with the use of more digestible feeds. When long term manure storage is used, this can also be a major source of GHG emissions.
• N₂O emissions from crop fields are another significant source of dairy GHGs. N₂O field emissions are largely driven by nutrient applications (synthetic fertilizer and manure) but are also a function of soil type and microclimate.
• Forages differ widely in their capacity to accumulate carbon in plant biomass and soil, and in the GHG emissions that are produced in the field and from dairy cattle digestion via enteric fermentation.
• An ideal climate-friendly forage crop would have the following characteristics:
  1. Low nutrient requirements;
  2. High forage quality;
  3. Significant resource allocation to below-ground root growth; and
  4. Would not deplete existing soil organic carbon stocks.

The big picture: Dairy producers, whether managing their animals on pasture or in confinement, select forages to balance their herd’s nutritional needs and milk production goals while ensuring they have sufficient forage to feed their cattle throughout the year.

• Dairy cattle in confinement are fed combinations of forages, grains, and feed supplements in a total or partial mixed ration.
• These diets are formulated to optimize milk production and consistency of diet delivery, while feed for grazing herds is managed through pasture species selection and management practices to optimize forage productivity, quality, and milk production.
• In both confinement and grazing systems, perennial crops are more likely to accumulate carbon in soil and plant biomass than annual crops due to their longer growing season and greater allocation of energy to root development.
• However, annual crops like corn silage are an increasing part of the dairy cattle diet due to their high forage productivity, the simplicity of one harvest event, nutritive properties, and digestibility to support milk production.

In short, achieving net-zero emissions in dairy systems will require improved management practices applied to the herd, manure, field and feed. Integration of perennial forages into the dairy farm system can increase carbon accumulation in soil and vegetation, and selection of perennials with high forage quality is important for maintaining or reducing the emissions produced during cattle digestion.

Photo courtesy of Getty/Unsplash.