Could Your Culling Strategy Reduce Methane Emissions?

Understanding the source

Government officials and industry organizations have identified methane emissions from cattle operations as a priority in our country’s effort to reduce GHG in the atmosphere. Early efforts have included construction of methane digesters and capturing methane from manure storage facilities. These efforts are significant but only address a portion of a dairy farm’s overall GHG emissions. Depending on the animal housing environment and manure management practices, manure methane emissions can contribute anywhere from 5-50% of a dairy's GHG emissions.

The remaining emissions come from manure nitrous oxide, on-farm fuel combustion, upstream emissions associated with producing operating inputs (i.e., feed, fuels, etc.), and last but not least, directly from the cow in what is referred to as enteric methane emissions. Enteric methane is produced by the fermentation process in a cow’s rumen and digestive tract. This is the next frontier in reducing GHG emissions from dairy farms and an area that’s beginning to receive a lot of attention.

However, reducing enteric methane emissions is tricky. A cow’s ability to extract nutrients from feed inedible to humans is what makes her special. Efforts to lower enteric methane emissions that reduce rumen function, decrease productivity, or hamper the ability of the cow to upcycle feed ingredients result in challenging tradeoffs between economic and environmental objectives. Alternatively, high performance genetics can provide dual economic and environmental benefits.  

Elite genetics provide two-fold benefits

One verifiable strategy to reduce GHG emissions, and something that can be implemented today, is to increase cow longevity and reduce the number of replacement heifers. Farms around the world are already employing this genetic strategy as they select sires that excel for health, fertility and component traits. Healthier, longer-living cows create an opportunity for herds to reduce their culling rate and raise fewer replacement heifers. 

A life-cycle assessment was used to evaluate the impact of culling rate on GHG emissions from a commercial-scale 2,000-head Holstein dairy (Figure 1). We calculated the emissions for a range of culling rates and chose to compare two herds with 37% and 30% culling rates. A culling rate of 37% is the industry average.

Figure 1. The life-cycle assessment uses a control volume that includes mature dairy cows and replacement heifer development as shown in the control volume below (red). For this study, emissions are not allocated among output products, which include milk, manure, cull cows and day-old beef x dairy calves, but simply assigned entirely to the milk.

For this assessment, we keep all other model parameters constant, including diets, death loss rates, cull cow dressing percentage, emissions factors and more. Thus, the only difference among cases is the number of animals in each age group. 

As shown in Table 1, a reduced cull rate lowers the total number of cows and heifers at the dairy and shifts the demographics to include more cows that are third lactation and greater. Because the older cows are more productive, the total energy-corrected milk (ECM) yield of the dairy is about 1% greater for the reduced cull rate herd, and the enteric methane emissions are reduced due to having fewer animals in the system (Table 2). By combining these impacts, the Enteric Methane GHG Intensity is reduced by 3.1% and Total GHG Intensity is reduced by 4.2% for the herd with a reduced cull rate.  

Table 1: Head counts for two herds with 37% and 30% culling rate, respectively

CO_2e: Molecules of each GHG cause different amounts of warming when they are in our atmosphere. To account for this, we use units of carbon dioxide equivalent (CO_2e) to normalize the Global Warming Potential (GWP) of each GHG, such as CO₂ (1), CH₄ (27), and N₂O (273). Said differently, emitting 1 ton of CH₄ is equivalent to emitting 27 tons of CO₂.

Table 2: Methane emissions, total GHG emissions, methane intensity and total GHG intensity for two herds with 37% and 30% culling rates, respectively. Emissions are not allocated among output products.

Figure 2: Total GHG Intensity and Enteric Methane Intensity for cull rates ranging from 20%-50%.

Due to a lower culling rate and longer productive life, Herd 2 raises fewer replacements and has more mature cows. This means fewer non-lactating animals eating feed and producing methane. These differences may look small but throughout one year this 2,000-cow herd would reduce enteric methane emissions by almost 238 tons of methane when measured on a CO₂ equivalent basis.

Herd 2 also has a higher percentage of the milking herd producing at mature levels of production. This leads to higher production coupled with reduced enteric methane emissions – a win/win situation that results in a 4.2% reduction in methane emission intensity for the herd.

There are other economic advantages to raising fewer replacements that are frequently discussed, including lower feed costs and the ability to capture more value with crossbred beef calves. But the benefits in reduced enteric methane emissions may not be as obvious or frequently discussed. 

Don't let up on your strategy

There are tradeoffs when you consider managing a herd of older cows. Aged cows tend to have higher prevalence of health issues like mastitis, lameness and lower pregnancy rates. It is paramount that herds adopt a genetic strategy that emphasizes traits like mastitis resistance, lameness resistance and fertility. Using indexes like Herd Health Profit Dollars™ (HHP$™) and Dairy Wellness Profit Index^® (DWP$^®) will help generate cows that are less prone to these older cow issues and continue to be profitable producers later in life. 

Strategies for reducing enteric methane emissions are popular topics on dairy farms today. Reduced culling provides a meaningful and verifiable way to reduce methane emissions while improving productivity. It’s a strategy that doesn’t require additional investment and deserves consideration. Recognition of this strategy as a means to reduce emissions can extend the dairy industry’s successful, decades-long track record of reducing the environmental footprint of food production.