Sustainability & efficiency

Lifetime Net Merit Index ($NM) and the correlated response on methane emissions

---

Abstract

Genetic selection is a very powerful tool for achieving lasting gains in environmental sustainability. Contrary to improvements in nutrition and management, changes achieved through selection are incremental, cumulative, and permanent, which makes genetic improvement a very cost-effective strategy. However, the impact that genetic selection has had on reducing the environmental footprint of dairy farming is neither well-documented nor well-communicated to stakeholders, consumers, and society. In addition, the additional benefits achieved recently through the inclusion of traits such as feed efficiency remain unknown and should be quantified. Therefore, there is a critical need to better understand and describe what the dairy industry has achieved by using selective breeding to reduce the environmental impact of dairy farming. The objective of this study was to quantify the changes in methane emissions over the past 5 decades due to the use of US economic selection indices. We evaluated 12 different selection indices from the 1971 Predicted Difference Dollars (PD$) to the 2025 Lifetime Net Merit Index (NM$). Note that PD$ included only milk and fat yield while the current 2025 NM$ includes 17 traits and composites. We calculated the response to selection for 24 traits and type composites, plus three methane emission traits, namely methane production (CH4, g/d), residual methane intensity (RMI, g/d), and residual methane yield (RMY, g/d). Trait RMI was calculated as methane production adjusted for milk energy and metabolic body weight, while RMY was calculated as methane production adjusted for dry matter intake. The correlated responses were calculated as a function of the genetic and phenotypic correlations among the traits and each of the indices. For milk, fat, protein, residual feed intake, and the three methane traits, correlations were computed using a dataset of 1,980 mid-lactation Holstein cows, whereas for the other traits, correlations were calculated using gPTAs obtained from the Council on Dairy Cattle Breeding. We observed a consistent decline in the correlated response in CH4, from 6.28 to 1.26 PTA change/year in the period 1971-2025. Similarly, we observed a consistent decline in the correlated response in residual methane traits, from 1.36 to -0.48 PTA change/year for RMI and 1.30 to 0.98 PTA change/year for RMY in the period 1971-2025. Overall, our results indicate that changes to the NM$ over the past five decades, especially the inclusion of body size and residual feed intake, have led to a marked decrease in the correlated response in methane emission traits. The inclusion of methane traits into US selection indices in the near future will undoubtedly further reduce the environmental impact of dairy farming.

Keywords: 2026

How to Cite:

Cole, J., Martinez-Boggio, G., Peà±agaricano, F., Tricarico, J. & Weigel, K., (2026) “Lifetime Net Merit Index ($NM) and the correlated response on methane emissions”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2285468. doi: https://doi.org/10.31274/wcgalp.23717

Rights: 1