Effect of heat stress on methane emissions of Dutch Holstein cows

Abstract

Over the past decade, climate change has raised the importance of addressing heat stress in dairy cattle. The Temperature-Humidity Index (THI) is a key tool for assessing the animals’ response to varying weather conditions, serving as an indicator of heat stress. Studies suggest that higher THI is linked to reduced milk production and compromised health. Despite this, little is known about the effect of temperature and humidity on methane emissions of dairy cattle. Our study aims to investigate the potential impact of temperature and humidity on methane emissions in the Dutch Holstein population. We analyzed 132,960 weekly methane concentration (CH₄c) records from 7,669 cows across 72 commercial farms in the Netherlands spanning from 2019 to 2023. Each methane record was paired with weekly THI data computed from meteorological records provided by the Nederlands Meteorologisch Instituut (KNMI). Weekly THI values were calculated using the National Research Council formula, resulting in indexes ranging between 28 and 72. At the population level, a repeatability animal model included fixed effects such as herd-year-season interaction, week of lactation, and parity-age of cow at calving interaction (parities = 1, 2, 3, ≥4). Random effects included animal additive genetic effects and permanent environment effects. Methane concentrations showed a significant increase starting at a THI value of 46. At the individual level, a reaction norm model focusing on THI values higher than 46 (THI₄₆₊) was implemented. An interaction between animal additive genetic effect and THI46+ level using Legendre polynomials of first order was fitted, resulting in different aggregate Estimated Breeding Values (EBVs) at different THI46+ values per animal. Results demonstrated a significant THI effect (P < 0.001) on CH₄c at a population level. Estimated aggregated heritabilities at different THI₄₆₊ level for  CH₄c ranged between 0.11 (at THI 55 and 56) and 0.50 at THI level 70 (SE=0.01). The permanent environment ratio ranged between 0.19 at THI level 70 to 0.35 at THI level 56. Based on the EBV for CH₄c at the THI value lower than 46 (that is, in a thermo-neutral environment), cows were ranked into top (high emitting animals, n=50) and bottom (low emitting animals, n=50) groups. The results revealed that aggregate EBVs for low-emitting cows tended to increase as THI levels rose, whereas high-emitting cows showed decreasing EBVs at higher THI₄₆₊ levels. This could potentially impact the selection of CH₄ emissions reduction strategies in a future affected by climate change (global warming) and/or in countries with different temperatures and humidity levels.