Meta-analysis for heat tolerance traits in Holstein in France, the Netherlands and Spain
In the context of climate change, livestock production systems face the challenge of ensuring that, although more and more exposed to heat-stress conditions, animals will be able to remain healthy while maintaining production in satisfactory quality and levels, responding to consumers demand. As part of the European project RUMIGEN, France (INRAE, Idele), Spain (INIA, IRIAF), and the Netherlands (WUR) studied the response of different dairy traits to heat-stress conditions, for Holstein cattle breed.
Performances of production (milk, fat and protein yields) and udder health (somatic cell score) traits were investigated under different meteorological conditions. These meteorological conditions were represented by the temperature humidity index (THI) averaged over three days around the day of record. A THI=50 was considered as neutral condition (i.e. no heat/cold-stress) and used as reference for level comparisons. Specific rates of changes in production/health traits at a given THI were measured as the slopes (first derivative) of the estimated reaction-norm curves for each trait. Genetic evaluations were performed by each country using test-day records of cows in their first lactation between 2010 and 2020, to estimate both levels and slopes for each trait. Breeding values (EBVs) and reliabilities were obtained for sires with at least 20 daughters.
A meta-analysis was performed to estimate the genetic correlations between the three countries. For each country, deregressed proofs and effective record contributions were computed using a single trait model from the EBVs, reliabilities, and variance components estimated at a national level for specific THI values. For both levels and slopes, the genetic correlations between the three countries were then estimated using the MACE approach (Schaeffer, 1994).
For the levels at THI=50, the estimated genetic correlations between countries ranged from 0.85 to 0.98. The low heritability of the slopes and the small proportion of performances recorded under heat-stress conditions impacted the quality of the deregression process. Thus, inferences on the slopes’ genetic correlations between countries were weak. Conversely, the levels of performances under heat stress conditions are more heritable and highly correlated with level under neutral THI, enabling stronger inferences on these traits, regardless of less data. The consistency of these traits will be studied in the next step of our study.
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