EXPRESSION OF ESTRUS IN DAIRY COWS: PHENOTYPING, GENETIC VARIABILITY, AND ITS ASSOCIATION WITH REPRODUCTIVE PERFORMANCE

Abstract

The objective of this thesis was to evaluate novel estrus expression traits in lactating dairy cows. Bi-hourly activity and rumination data from 10,335 lactating cows wearing wireless sensors were collected from April 2021 to December 2022, resulting in a dataset with 46,048,112 records. An algorithm that quantifies changes in activity and rumination was developed to detect and describe estrus events. Four estrus expression traits were defined: estrus detection (yes/no), estrus duration (number of consecutive bi-hourly positive estrus events), activity strength (difference in activity during and prior to estrus) and rumination strength (difference in rumination during and prior to estrus). The association between estrus traits and pregnancy success was evaluated using alternative logistic regression models. The addition of a detected estrus event, estrus duration or strength, contributed significantly to explaining pregnancy success, relative to insemination in the absence of a detected estrus. We assessed the heritability of estrus expression traits using a sliding 21-day window between 21 to 100 DIM. We also evaluated the heritability of number of estruses between 11 to 70 DIM, during the voluntary waiting period. Heritability estimates for presence/absence of estrus were around˙ 0.20±0.02 in the period 21 to 50 DIM, and 0.09±0.03 in the period 50 to 100 DIM. Similar results were obtained for estrus duration and strength. The trait number of estruses between 11 to 70 DIM showed a heritably estimate of 0.23±0.02. Overall, estrus expression traits can be measured in a digital, standardized, and scalable framework, and these traits are significantly associated with pregnancy success. Notably, these estrus expression traits are heritable, which implies that breeding for improved estrus expression has the potential to boost estrus detection, insemination, likelihood of pregnancy, and pregnancy rates without relying on hormonal treatments.