Background: The effect of heat stress on livestock production is a worldwide issue, where animal performance is influenced by exposure to high environmental temperatures, indicating the existence of possible genotype-by-environment interactions (G´E). The main objectives of this study were to (1) detect the time periods in which heifer fertility traits are more sensitive to the exposure to high environmental temperature and/or humidity, (2) investigate G´E due to heat stress in heifer fertility traits, and (3) identify genomic regions associated with heifer fertility and heat stress in Holstein cattle.
Results: Phenotypic records for three heifer fertility traits (i.e., age at first calving, interval from first to last service, and conception rate at the first service) were collected, from 2005 to 2018, for 56,998 Holstein heifers raised in 15 herds in the Beijing area (China). By integrating environmental data including hourly air temperature and relative humidity, the critical periods in which the heifers are more sensitive to heat stress were defined as <=30 days before the first service for age at first calving and interval from first to last service, or 10 days before and <= 60 days after the first service for conception rate. Using reaction norm models, significant G´E was detected for all three traits regarding both environmental gradients, proportion of days exceeding heat threshold and minimum temperature humidity index. Through single-step genome-wide association study, PLAG1, AMHR2, SP1, KRT8, KRT18, MLH1, and EOMES were suggested as candidate genes for heifer fertility while HCRTR1, AGRP, PC, and GUCY1B1 were for heat tolerance.
Conclusions: The critical periods in which reproductive perfromances of heifers are more sensitive to heat stress are trait-dependent. Thus, detailed analysis should be conducted to determine this particular period for other fertility traits. The considerable magnitude of G´E and sire re-ranking indicates the necessity to consider G´E in breeding schemes. This will enable selection of more heat-tolerant animals with high reproductive efficiency under harsh climatic conditions. The candidate genes identified to be linked with response to heat stress provide a better understanding of the underlying biological mechanisms of heat tolerance in dairy cattle.