Nutritional contributions and non-CO2 greenhouse gas emissions from human-inedible byproduct feeds consumed by dairy cows in the United States

Abstract

Dairy cattle recycle nutrients found in human-inedible byproduct (BP) feeds to make nutritious milk, eliminating the need for disposal by alternative methods. Dairy nutrition consultants and feed industry representatives in the US were surveyed to quantify milking cow BP consumption. Survey data described 33.5% of US lactating cows and 35.7% of US milk production in 2019 in the most comprehensive BP feeds dataset collected so far. On a daily basis, US dairy cattle consumed 12 kg AF (8.2 kg DM) BP per milking cow (including dry cows and replacement heifers) and 0.32 kg AF (0.22 kg DM) BP per kg milk produced. This equates to 32 to 41 million Mt (AF) (22–28 million Mt DM) total BP consumed by US milking cows in 2019. Regional and US BP usage by type were calculated as well as the nutritional contributions BP feeds make to dairy cow diets. On average, 32% (DM basis) of the US lactating cow diet (excluding dry cows and replacement heifers) is BP feed, supplying 54, 61, 46, and 27% of the protein, sugar, fat, and fiber necessary for milk production. Dairy cows generate greenhouse gas (GHG) emissions as a consequence of enteric and manure fermentation. Published equations were used to describe and compare non-CO2 GHG emissions from feeding BP to milking cows or disposing by other means. On average, US milking cows produced 225 g/d of enteric methane (eCH4) from BP in 2019, with 27.4 g eCH4/kg of BP DM, and an eCH4 intensity of 207 g carbon dioxide equivalent emissions (CO2-eq) from BP/kg of milk. Manure from BP fed to US milking cows yielded 138 g/d manure methane (mCH4) and 3.02 g/d manure nitrous oxide (mN2O), with 16.76 g mCH4/kg of BP DM and 0.368 g mN2O/kg of BP DM, and a mCH4 intensity of 151 g CO2-eq from BP/kg of milk. Feeding BP to US milking cows to partially replace non-BP feeds such as forages and whole grains, generates 70 CO2-eq/BP (g/kg DM) of non-CO2 GHG emissions while landfill disposal, composting, and combustion emit 3448, 328, and 31 CO2-eq/BP (g/kg DM), respectively. Feeding BP to dairy cows represents an effective vehicle to recycle valuable nutrients that are either indigestible by humans or undesirable for direct human consumption into milk and dairy foods. Since BP replace forages and grains that would otherwise be included in the dairy cow's diet, minimal or even less long-term non-CO2 GHG emissions are generated by feeding BP to dairy cows.