Vaginal Microbiota Composition and Its Relationship with Fertility in Repeat Breeder Dairy Cows.

Milk production in dairy herds is determined by both intrinsic and extrinsic factors, with reproductive efficiency serving as a primary determinant. Infectious, nutritional, and management-related challenges can reduce this efficiency. Following parturition, cows are more susceptible to clinical disorders due to a temporary loss of integrity in the cervix, vagina, and vulva, which allows environmental bacteria to ascend and alter the vaginal microbiota. These microbial changes may disrupt endocrine responses related to conception and contribute to repeat breeder cow syndrome (RBCS), which is defined as failure to conceive after three or more inseminations. This study investigated associations among cultivable vaginal bacteria, circulating progesterone and glucose concentrations, and reproductive performance in 30 fourth-parity Holstein cows with a body condition score of 3.5. Cows were classified by reproductive history as repeat breeders (RBCS; n = 14) or controls (CTL; n = 16). Vaginal mucosal samples were collected at insemination and cultured on blood agar and MacConkey media under aerobic and microaerobic conditions. Bacterial identification was conducted using Gram staining and standard biochemical assays. Blood samples were collected at insemination, on day 5 post-insemination, and every two days thereafter to measure progesterone and glucose concentrations. Fertility outcomes were analyzed using PROC GLIMMIX, and hormonal data were analyzed using mixed models with repeated measures. The bacterial genera identified included Bacillus, Escherichia coli, Staphylococcus, Klebsiella, Proteus, Streptococcus, and Actinomyces. Progesterone and glucose concentrations did not differ significantly between groups (p > 0.05). However, the fertility rate (p < 0.05; CTL:87.50% vs. RBCS:57.14%) and number of attempts to conceive (p < 0.001; CTL:2.5 vs. RBCS:6.7) differed statistically between treatments. A higher prevalence of S. hyicus was detected in RBCS cows, and E. coli, S. hyicus, and Proteus spp. were more frequently detected in non-pregnant cows. These findings suggest that the identified cultivable vaginal bacteria are associated with reproductive status in dairy cows.