Is methane emission genetically the same trait in young bulls and lactating dairy cows?

It is of interest to examine whether methane (CH₄) emission is genetically the same trait in young bulls and lactating dairy cows. The aim was therefore to estimate the genetic correlation between CH₄ emissions for Norwegian Red young bulls and lactating cows. Measures of CH₄ from GreenFeed (GF) were available from Geno's test station for young bulls and from GF units installed across 14 commercial dairy herds. Data from the years 2020 to 2023 were included. After data editing, the dataset consisted of 771,989 GF visits from 1,370 cows, and from the test station we had 112,071 GF visits from 244 young bulls. Each young bull had, on average, 40 d with CH₄ data, measured at 11 to 12 mo of age. Each visit to the GF provided an estimate of the animal's daily CH₄ emission originating from rumen fermentation. The traits analyzed were grams of CH₄ per animal per day, calculated as the daily average. The mean (SD) for cows and young bulls were 406 (108) and 222 (46) g of daily CH₄, respectively. A bivariate linear animal repeatability model was used to estimate (co)variance components. The estimated heritability (SE) of CH₄ emissions was 0.39 (0.04) for cows and 0.49 (0.15) for young bulls, and the estimated genetic correlation (SE) between the 2 traits was 0.63 (0.22). The large SE reflects that the genetic correlation was estimated based on information from relatively few animals, and results should therefore be interpreted cautiously. However, the results suggest that phenotyping future young artificial insemination bulls is valuable for the genetic evaluation of methane emissions in Norwegian Red, even if CH₄ emission is not exactly the same trait genetically in young bulls and lactating cows.