Utilisation de la note d'embonpoint et de la composition en acides gras du lait en tant qu'indicateurs des performances de reproduction chez la vache laitière

Improving cow fertility by means of genetic selection has become increasingly important over the last years in order to overcome the decline in dairy cow fertility that has taken place over the past decades. However, fertility traits are difficult to measure and have low heritabilities. Consequently, indicator traits are of interest for breeding value estimation for fertility especially if these traits are easier to measure, have higher heritabilities and are well correlated with fertility. Therefore, the objective of this thesis was to investigate the opportunity of using either fatty acid contents (FA) in milk predicted by mid-infrared spectrometry or body condition score (BCS; i.e., a subjective measure of the amount of metabolizable energy stored in a live animal) as indicator traits of female fertility. Research conducted on BCS and fertility records from Canadian Ayrshire and Holstein cows indicated that BCS was heritable and showed a low to moderate favorable genetic correlation with fertility suggesting that higher BCS would be related to better fertility. Also, based on results obtained on Walloon data, selection for higher nadir BCS was suggested as useful to change BCS curve over the lactation and improve fertility. Furthermore, using records from Walloon Holstein cows, FA were demonstrated to be moderately heritable. Genetic correlations among FA and fertility were low to moderate and changed over the lactation. Overall, the pattern of genetic correlations of fertility with BCS and FA substantiated the known relationship between energy balance status and fertility. Body fat mobilization in early lactation induces BCS loss. Also, the release of long-chain FA in milk from the body fat mobilization inhibits de novo FA synthesis in the mammary gland, leading to a decrease of short- and medium- chain FA. To conclude, this research has shown that traits based on BCS and milk FA profile fulfill criteria to be considered as indicator traits to improve indirectly fertility of dairy cows