Early‐Life Respiratory Emissions of CO₂, CH₄, and N₂O in Pre‐Weaned Dairy‐Bred Calves

ABSTRACT Greenhouse gas (GHG) emissions from livestock are a major contributor to climate change, with cattle known to be the principal contributor through enteric fermentation, manure management, and metabolic processes. This study investigates the emission rates of CO₂, CH₄, and N₂O in pre‐weaned calves aged 12 to 86 days, focusing on the transitional phase from a purely milk‐based diet to the introduction of solid feed. Using a novel respiratory mask, repeated measurements of GHG emissions were collected from 65 calves to offer first insights into early‐life respiratory GHG emissions from calves. All three gases increased with age, with CH₄ showing a much stronger age dependence than for CO 2 or N 2 O, consistent with early developmental changes as calves begin to consume solid feed. Eructation events were observed only in calves older than 25 days, and were characterised by significant increases in CH 4 , but not CO 2 or N 2 O. Overall, CO 2 emissions dominated the GHG profile of the cattle, and although breath concentrations remained relatively stable, emission rates increased significantly with age, suggesting that changes in tidal volume and lung capacity were the primary drivers. Breed‐specific differences were limited and sensitive to model structure; after adjusting for age using a generalised linear model, British Blue‐cross calves exhibited higher CH₄ emissions and lower N₂O emissions compared to other breeds, though these differences were not statistically significant across all comparisons. These findings provide the first empirical characterisation of early‐life respiratory greenhouse gas emissions in calves, offering baseline data to support improved modelling of livestock emissions and informing future mitigation strategies that target the pre‐weaning period.