Role of dairy in the carbon footprint of US beef.

In PNAS, Eshel et al. (1) present a novel methodology for calculating a suite of ecological impacts from livestock and poultry production in the United States. The authors acknowledge two complementary and valuable approaches in ecological accounting: bottom up and top down. Top-down analyses, however, are only valuable insofar as they adequately represent the structure of the study system. By neglecting the role of the dairy system in beef production, Eshel et al. dramatically overestimate the greenhouse gas (GHG) emissions attributable to beef. The authors derive the GHG intensity of a consumed megacalorie of beef (kg CO2e·Mcal ) from published beef cattle life cycle assessment (LCA) studies. The authors then multiply this estimate by megacalories of beef consumed in the United States to calculate national GHGs from beef. These calculations are problematic because a nontrivial portion of beef calories are sourced from dairy production systems, where the life cycle emissions of producing dairy beef, a coproduct, are much lower. Dairy’s contribution to beef includes culled cows and bulls, as well as dairy bull and heifer calves not held as replacements that enter the beef system as veal or feeder cattle. Of these dairy cattle, the US Department of Agriculture only reports slaughter of calves and dairy cows. Thus, it is difficult to estimate dairy’s contribution to the beef supply, and improved data collection at the national level would facilitate better modeling. However, public and peer-reviewed data are currently available to approximate this relationship. Using a US livestock feed requirements model by Peters et al. (2), we estimate that 17% of all beef calories produced originated in the dairy system (Table 1). The major factor reducing the footprint of dairy beef is the allocation of burdens between dairies’ two products: milk and beef. Thoma et al. attributed 87% of dairies’ farmgate GHGs to milk, with the remainder allocated to beef (3). For calves that leave the dairy to be fed in the beef system, this is only part of their life cycle impact. However, maintaining breeding cows typically accounts for a large proportion of total farm-gate emissions in intensive beef systems. Unsurprisingly, the least GHG intensive and most land-use efficient beef production systems in a recent meta-analysis were dairy based, with emissions intensities that were half as large as the beef GHG values used by Eshel et al. (1, 4). Using the national beef consumption figures in Eshel et al., our estimates of 17% of beef calories as dairy sourced, and a published LCA value of 15 kg CO2e/edible kg dairy beef (4), we find Eshel et al. may have overestimated the climate impact of beef by 18,520 × 10 kg CO2e/y. To put this perspective, this is 3.4 times the national GHG burden attributed to eggs by Eshel et al. (1). The magnitude of this preliminary result points to the need for more robust modeling of the interconnection between beef and dairy systems in top-down ecological accounting.