Negligible long-term non-CO2 GHG emissions are created by feeding byproducts to dairy cows.
By Dr. Mary Beth de Ondarza Almost 30% of the world’s agricultural products actually end up as “byproducts”. Dairy cows recycle nutrients from feed byproducts into nutritious milk and dairy products. Feeding byproducts to animals greatly reduces the need for alternative disposal, such as burning or landfill dumping. Although, byproduct feeds have been fed to dairy cows for decades (Ajila et al., 2012), previous surveys do not adequately put a figure on their use in today’s US dairy diets. As with everything the cow eats, a portion of byproduct feed is converted into methane and nitrous oxide. Enteric methane emissions are highly dependent upon rumen digestibility of feeds, while manure methane and nitrous oxide emissions vary in regard to excretion of undigested nutrients and manure management. These non-CO2 greenhouse gases (GHG) contribute to global warming. But, since byproduct feeds replace forages and grains that would otherwise be included in the dairy cow’s diet, minimal additional non-CO2 GHG emissions are generated. Documenting the extent of byproduct feed inclusion in US dairy diets and discerning the impacts of byproduct feeding on GHG emissions will help further establish the environmental sustainability of the US dairy industry. For this reason, Dairy Management Inc. sponsored a research project (de Ondarza and Tricarico, 2021) to achieve the following objectives: 1. Understand what types of byproducts and how much of each type are fed per US milking cow (A “milking cow” was defined as a lactating cow in addition to associated dry cows and replacement heifers). 2. Quantify the enteric and manure-related GHG emissions resulting from feeding byproducts to milking cows and compare these emissions to those of other byproduct disposal methods. 3. Quantify the nutritional contributions byproducts make to US lactating cow diets (dry cows and replacement heifers excluded).
Byproducts feeds survey Questionnaires were sent to dairy nutrition consultants and feed industry representatives in the US regarding their usage of 63 different byproduct feeds as well as the number of lactating cows and milk production of the dairies they served. Survey results described 33.5% of US lactating cows and 35.7% of US milk production in 2019, making this the most comprehensive survey of US byproduct usage in dairy cows.
Consumption of each byproduct and byproduct type on a regional and US basis were determined using questionnaire answers weighted according to 2019 USDA regional milk cow numbers and milk production. Each day US dairy cattle consume 12 kg AF (8.2 kg DM) byproduct per milking cow (lactating cow in addition to associated dry cows and replacement heifers) or 319 g AF (219 g DM) byproduct per kg milk produced (Table 1). Cows in the West consume the most byproduct followed by the South, Northeast, and the Midwest, reflecting regional variations in byproduct cost, availability of byproduct and whole grains, as well as ability to manage feed byproducts on the farm. In 2019, US milk cows ate 32 to 41 million Mt (As-Fed) of feed byproduct based on extrapolation from either USDA milk production or USDA milk cow numbers (Table 1).
The 63 feed byproducts included in the survey were categorized into 18 feed byproduct types (Table 2). Daily consumption of feed byproduct types per milking cow in the US in 2019 is presented in Table 3. Corn distillers’ grains are fed at the highest rate of all byproducts at a US average of 1,404 g DM/milking cow/d with cows in the West consuming the most (2,289 g DM/milking cow/d). Canola meal was ranked second at a US average of 1,359 g DM/milking cow/d with cows in the West consuming the most (2,374 g DM/milking cow/d). Soybean meal was ranked third in US byproduct usage at 1,229 g DM/milking cow/d. Feed byproducts provide valuable nutrients, with the US average byproduct blend containing 30% CP, 30% NDF, 11% sugar, and 7% fat on a DM basis.
Non-CO2 GHG emissions from byproduct feeds The 100-year Global Warming Potentials (GWP100) of 34 and 298 were used to express methane and nitrous oxide emissions, respectively, in terms of “CO2 equivalent emissions” (CO2-eq) (Myhre et al., 2013). The equation of Niu et al. (2018) based on a dataset containing 1,084 observations across 45 studies conducted in the US between 1962 and 2016 was used for estimating enteric methane (g/d) from feed byproducts. On average, US milking cows produced 7654 CO2-eq, g/d of enteric methane from 8.2 kg DM byproduct daily in 2019 (Table 4). Enteric methane production from byproduct is greatest for the South and West (8080 and 8933 CO2-eq, g/d) given their greater byproduct feeding rates (9.6 and 10.9 kg DM/d). Average US milking cow enteric methane yield is 932 g CO2-eq per kg byproduct DM. Mean US enteric methane intensity is 207 g CO2-eq from byproduct/kg of milk.
Methane emissions from manure were calculated using an Environmental Protection Agency equation based on manure volatile solids (EPA, 2011). Direct nitrous oxide (g/d) from manure and indirect nitrous oxide (g/d) from manure were calculated using equations based on fecal nitrogen (EPA, 2019). According to EPA (2019), US dairy manure is managed as follows: 40% anaerobic lagoons, 24% liquid/slurry systems, 18% solid storage, 12% daily spreading, 4% pasture, range, or paddocks, and 1% deep pit storage. These waste management system usage estimates are based on state or regional usage data from various sources (e.g. Census of Agriculture, EPA’s Office of Water, USDA, and opinions from experts) and not from farm-level estimates. Although this represents a limitation, it is currently the only available source of data on the percent manure distribution among waste management systems by operation on a state level. The collection and summarization of primary farm-level data is needed, for example through an updated and more comprehensive survey, to provide a better representation of how manure is managed on US dairy farms.
On average, US milking cows produced 5,580 CO2-eq, g/d from manure methane and manure nitrous oxide arising from 8.2 kg DM byproduct daily in 2019 (Table 4). Manure emission intensities averaged 151 CO2-eq/milk (g/kg) from byproduct. Regional production of manure methane followed byproduct consumption, but it was greatly affected by regional differences in waste management systems, with a predominance of liquid manure handling and storage (e.g. anaerobic lagoon and liquid/slurry) in the West compared to the other regions. The low predicted manure methane values for the Northeast are largely due to the predominance of the daily spread manure management system suggested by EPA (2019) for that region.
Dietary nutrients, GHG emissions from byproduct feeds in the lactating diet Using the dairy nutritionist survey data only, byproduct feeds consumed by lactating cows (dry cows and replacement heifers excluded) were calculated for the US and its four regions (Table 5). Lactating diets were formulated with byproduct feed blends for each region and the US and using typical regional forages and whole grains.
Daily feed byproduct consumed ranged from 8.65 kg AF (6.41 kg DM) for the Northeast to 12.46 kg AF (8.54 kg DM) in the West with a US average of 11.04 kg AF (7.62 kg DM). This represents a US average feed byproduct inclusion of 32% of the lactating diet with regional values ranging from 27% in the Northeast to 36% in the West. On average, feed byproduct supplies 54% of the crude protein needed by US lactating cows, ranging from 44% in the Midwest to 66% in the South. Feed byproduct is used to supply 61% of the sugar in lactating cow diets, ranging from 52% in the Midwest to 72% in the South. Lactating cows also receive an average of 46% of their dietary fat from byproduct sources, ranging from 33% in the Northeast to 56% in the West. On average, 36% of non-CO2 GHG emissions generated by US lactating dairy cows result from the fermentation of byproduct feeds. Due mainly to high byproduct usage, the West region has the highest percentage (38%) of lactating cow non-CO2 GHG emissions from byproduct while the Northeast has both the lowest percentage of lactating cow non-CO2 GHG emissions from byproduct (34%) and the lowest byproduct feeding rate. GHG emissions from byproduct feeds disposed by alternate methods Greenhouse gases are generated when byproduct disposal occurs by alternative means other than feeding to animals. Landfill disposal involves a combination of anaerobic and aerobic fermentation resulting in 50% of the carbon converted into methane with the remaining carbon converted into biogenic CO2 (EPA, 2010). Methane from landfill disposal was calculated based on kg of organic carbon (IPCC, 2019). Composting emissions were estimated using emissions factors from EPA (2010). Emissions from direct combustion of byproducts were calculated using stationary combustion emission factors for agricultural byproducts (EPA, 2014). Byproduct feeds contribute to enteric and manure non-CO2 GHG emissions like all other feeds in the cow’s diet. Since byproduct feeds replace forages and grains in the diet, their impact on non-CO2 GHG emissions is obtained after subtracting the emissions from the forages and grains they replace (Table 6). Feeding byproducts to dairy cows results in considerably less non-CO2 GHG emissions than landfill disposal (49-fold) and composting (4.7-fold) while supplying valuable nutrients for milk production. The US EPA (2020) considers recycling food surplus and byproducts into animal feed as a preferred choice over composting, burning, and landfill (Papargyropoulou et al., 2014) (Figure). Conclusions This study describes the use of byproduct feeds by US dairy cows based on the most thorough survey conducted on this topic so far. Dairy cattle utilize considerable amounts of different types of byproduct feeds, following current recommendations for the sustainable management of unavoidable food waste. Dairy cattle recycle important nutrients from agricultural byproduct streams into milk and dairy foods producing considerably less non-CO2 GHG emissions than landfill disposal and composting. Negligible long-term non-CO2 GHG emissions are created by feeding byproducts to dairy cows since they substitute forages and grains in the diet. References Available upon request.
Dr. Mary Beth de Ondarza is with Paradox Nutrition, LLC, West Chazy, NY.
