FERTILITY IN HOLSTEIN HEIFERS: GENETIC ASPECTS AND OPTIMIZATION OF REPLACEMENT STRATEGIES IN THE DAIRY HERD

Heifer management plays a key role in the dairy herd as it can account for more than 20% of total production costs. Fertility is an essential aspect to rear heifers efficiently and maximize lifetime performances, and it can be enhanced through effective management strategies and genetic selection. The recording of pregnancy diagnoses has allowed new traits to be collected early in life such as the interval from first to last insemination. The objectives of the present thesis were to: i) identify genetic and non-genetic factors affecting Italian Holstein heifer fertility, and propose an aggregate index for heifer fertility; ii) investigate genetic and genomic aspects of age at first calving; and iii) describe a practical tool to help farmers optimize replacement management. Chapter 1 investigated environmental and genetic aspects of conception rate, non-return rate at 56 d, interval from first to last insemination, and age at first service in heifers to develop a genetic evaluation. Chapter 2 performed genetic and genomic evaluation of age at first calving. Chapter 3 combined genetic and phenotypic aspects into a tool for the farmers to define the replacement needs and reduce replacement costs. Results of the present work indicated that heifer fertility should be considered as an additional trait in the breeding objectives of Italian Holstein. Genetic selection for age at first calving can reduce the actual phenotypic mean of this trait without negatively affect other reproductive performances. Sexed semen allows farmers to produce more replacements than the actual needs. However, given current market conditions, this is not the most convenient choice for farmers. Thus, providing a tool that helps dairy farmers to optimize replacement strategies based on their herd performances is appropriate. Overall, an optimal replacement management, both from environmental and genetic point of view, is expected to improve animal welfare and contribute to reduce replacement costs and environmental impact.