Short communication: Variation in feed efficiency hampers use of carbon dioxide as a tracer gas in measuring methane emissions in on-farm conditions

Abstract

Breeding cows for low CH₄ emissions requires that the trait is variable and that it can be recorded with low cost from an adequate number of individuals and with high precision, but not necessarily with high accuracy if the trait is measured with high repeatability. The CH₄:CO₂ ratio in expired breath is a trait often used as a tracer with the production of CO₂ predicted from body weight (BW), energy-corrected milk yield, and days of pregnancy. This approach assumes that efficiency of energy utilization for maintenance and production is constant. Data (307 cow-period observations) from 2 locations using the same setup for measuring CH₄ and CO₂ in respiration chambers were compiled, and observed production of CH₄ and CO₂ was compared with the equivalent predicted production using 2 different approaches. Carbon dioxide production was predicted using a previously reported model based on metabolic BW and energy-corrected milk production and a currently developed model based on energy requirements and the relationship between observed CO₂ and heat production (models 1 and 2, respectively). Animals used were categorized (low, medium, and high efficiency) according to (1) residual feed intake and (2) residual milk production. Model 1 underestimated CH₄ production by 15%, whereas model 2 overestimated CH₄ by 1.4% for the whole database. Model 1 underestimated CO₂ production by 2.8 and 0.9 kg/d for low- and high-efficiency cows, respectively, whereas model 2 underestimated CO₂ production by 0.9 kg/d for low-efficient animals but overestimated it by 1.2 kg/d for high-efficiency cows. Efficient cows produce less heat, and consequently CO₂, per unit of metabolic body weight and energy-corrected milk than inefficient cows, challenging the use of CO₂ as a tracer gas. Because of biased estimates of CO₂ production, the models overestimated CH₄ production of high-efficiency cows by, on average, 17% relative to low-efficiency cows, respectively. Selecting low CH₄-emitting cows using a CO₂ tracer method can therefore favor inefficient cows over efficient cows.

Keywords: CO(2) tracer method; breeding; dairy cow; methane.