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The objective of this study was to investigate the prevalence of subclinical endometritis 4h after AI and its effect on first service conception rate (FSCR) in dairy cows. A total of 201 Holstein-Friesian cows with no signs of clinical endometritis were examined 4h after first AI for signs of subclinical endometritis. Endometrial samples were collected from the uterus using the cytobrush technique. The proportion of polymorphonuclear cells (PMN) in the cytological sample was used to characterize an inflammation of the endometrium. Cows were categorized into three groups according to the proportion of PMN in the sample. Cows with 0% PMN (n=115) were assigned to group Zero, cows with >0-15% PMN (n=59) to group Medium, and cows with >15% PMN (n=27) to group High. Pregnancy diagnosis was performed between days 38-44 after AI by palpation of the uterus and its contents per rectum. The FSCR was significantly higher in group Medium than in groups Zero and High (57.6% vs. 39.1% and 29.6%). Statistical analysis revealed an interaction between parity and PMN group. Primiparous cows were at higher risk of being classified into group Medium than multiparous cows (OR=2.27, P=0.01). Primiparous cows in group Zero had lower odds of pregnancy after first AI than primiparous cows in group Medium (OR=0.3, P=0.02). A comparison with cows that were not examined for subclinical endometritis showed that the collection of endometrial samples itself had no effect on FSCR.