Genetic and nutritional effects on reproduction in lactating dairy cows
Authors:   , Doyle Rachel C. 
Published: December 2023

Good reproductive performance is a prerequisite for profitable pasture-based seasonal calving systems of milk production. The objective of this dissertation was to elucidate the physiological mechanisms contributing to superior reproductive performance observed in high genetic merit lactating dairy cows compared to cows of an average genetic merit managed in a standardised environment. The studies specifically focused on characterising uterine health, ovarian activity, circulating concentrations of reproductive hormones, transcriptomes of the endometrium and corpus luteum, and circulating concentrations of pregnancy proteins during early gestation. A series of experiments were conducted using lactating dairy cows with a high genetic merit for fertility and milk production traits (Elite) and cow's representative of the national average genetic merit for milk production and fertility traits (NA). Study one examined the postpartum resolution of uterine infection, circulating concentrations of estradiol and luteal phase progesterone in Elite, NA, and Jersey cows. Results from the study support the idea that Elite cows have (1) faster resolution of uterine infection in the postpartum period and (2) greater circulating progesterone during the luteal phase compared with NA and Jersey cows. Study two investigated differences in the transcriptome of the endometrium and corpus luteum on day 7 and 14 of the estrous cycle, particularly in genes associated with CL and endometrial function. Combination of transcriptome analysis of the endometrium (day 14) and corpus luteum (day 7 and 14) identified upregulation of genes involved in luteal cell apoptosis, uterine receptivity and immune response in Elite compared with NA cows, which may contribute to the previously reported difference in reproductive phenotypes and fertility performance. Study three investigated the influence of cow genetic background and feed treatment on plasma pregnancy associated glycoprotein (PAG) and pregnancy specific protein B (PSPB) during early gestation. The results indicated (1) greater plasma PAG concentrations in cows on the high concentrate feeding treatment, (2) modest effects of sire and dam fertility and milk sub-indices were observed on the fold changes in PAG and PSPB in early gestation and (3) no effect of genetic group was observed on circulating PAG and PSPB concentrations. The results indicate that in early gestation the variation observed between cows in PAG and PSPB concentrations cannot be explained by cow genetic background but are influenced by maternal nutrition. Study four investigated the association between milk PAG S-N in early gestation with variables related to the dam, sire and gestating fetus. Dam genetic merit for milk production did not have a large effect on the PAG S-N. Cows with the greatest fertility sub-index had greater milk PAG-S-N compared with cows with intermediate and poorest fertility suggesting possibly a more favourable environmental for the establishment, maintenance of pregnancy and fetal development through faster placental development. Collectively, the results highlight the importance of the uterine environment, luteal phase progesterone, pregnancy proteins and differentially expressed genes in the corpus luteum and endometrium for the observed phenotypic fertility differences between genotypes. The study has identified key physiological mechanisms controlled by genetic merit for fertility traits. These results highlight the ability of the Economic Breeding Index to deliver cows with superior genetic gain and phenotypic fertility performance without antagonising milk production. The Next Generation Herd will play a key future role in the investigation of the effect and influence of a more holistic approach to genetic selection on phenotypic performance.