Genetic Analysis of Heat Stress for Conception Rate across Parities in the Netherlands

Fertility is known to be negatively affected by heat stress. Our aim was to evaluate the impact of heat stress on estimated breeding values for conception rate (CR) for Holstein cattle in the Netherlands, by modeling sire breeding values for CR within each parity as a function of THI, using a broken stick random regression model. Our results showed a clear reduction in average CR at THI levels of 70 of up to 15% for parities 1 through 3, starting after a THI threshold of 60, while no clear reduction in CR was observed for parity 0. The broken stick model estimated 2 breeding values for each sire, based on information of daughters experiencing different THI levels at the time of insemination. The first breeding value modeled the intercept, i.e., the baseline breeding value at THI values below the identified threshold, while the second breeding value modeled the slope, which reflects the change in breeding value with changes in THI above the threshold. The results revealed a clear pattern of increased genetic variance and heritability at higher THI values for parities 1 through 3, while the random regression was not significant for parity 0. Across parities 1 through 3, estimated genetic correlations between CR at THI values of 60 and 65 ranged from 0.93 to 0.97, and between THI values of 60 and 70 ranged from 0.79 to 0.88. This suggested some meaningful re-ranking between THI 60 and 70, while selection at THI values below 60 should still improve CR at high THI. In line with previously reported genetic correlations, CR in maiden heifers is clearly different from CR in lactating cows. For lactating cows, a high genetic correlation (between 0.88 and 0.96) was found between the intercepts in the different parities. Genetic correlations between the slopes suggested that the genetic component of heat stress for CR is most similar between parities 2 and 3, and between parities 1 and 2, but quite different between parities 1 and 3.