Expression of estrus in dairy cows: Phenotyping, genetic variability, and association with productive and...

Dairy cow estrus is marked by acute changes in behavior, such as increased activity and decreased rumination. These changes can be measured on the farm using wearable sensors and other monitoring systems. The main goals of this study were to quantify the expression of estrus in lactating dairy cows, evaluate its genetic variability, and assess its relationship with milk production and reproductive performance. Data consisted of 46,048,112 bi-hourly activity and rumination records collected using automated monitoring devices in 10,335 lactating dairy cows during 2 years on a large commercial farm. We developed an open-source, freely available algorithm that quantifies changes in activity and rumination to infer the duration and intensity of estrus. First, we evaluated the association between estrous traits and pregnancy success using alternative logistic regression models, adjusting by lactation, year-season, and sex-sorted or conventional semen. The statistical analysis showed that the addition of a detected estrus contributes significantly to explaining pregnancy success, relative to insemination in the absence of a detected estrus. Similar results were found for estrous duration and intensity. Second, we assessed the heritability of estrous traits using a sliding 21-d window between 21 and 100 DIM. We also evaluated the heritability of number of estrus between 11 and 70 DIM (voluntary waiting period). The statistical models included herd-season and lactation as fixed effects and cow as a random effect. Heritability estimates for presence/absence of estrus were around 0.20 ± 0.02 in the period 21-50 DIM and 0.09 ± 0.03 in the period 50-100 DIM. Similar results were observed for the duration and intensity of estrus. The number of estrus during the voluntary waiting period (11-70 DIM) showed a heritability estimate of 0.23 ± 0.02. Finally, we evaluated the relationship between estrous traits and productive performance, including average milk production in the period 11-70 DIM and also 305-d mature equivalent milk production. All estrous expression traits showed negative genetic correlations with productive traits, ranging from -0.14 ± 0.06 to -0.37 ± 0.05. Overall, the duration and intensity of estrus can be measured in a digital, standardized, and scalable framework, and these traits are significantly associated with pregnancy success. Notably, estrous behavior traits are heritable, which implies that breeding for improved expression of estrus has the potential to improve on-farm detection and increase insemination and pregnancy rates. Additionally, estrous traits are genetically unfavorably correlated with milk production.