Changing views on the role of the uterus in post-partum reproductive function in beef cows

The paradigm for post-partum management of well-being and reproductive function in beef cows is changing. For many decades, research focused on the hypothalamic-pituitary-ovarian axis in attempts to manage beef cows to resume estrous cycles more quickly, because uterine involution was not considered a factor that was limiting to reproductive function in post-partum beef cows (Kiracofe, 1980). Questions remained, however, about the prevalence of subclinical endometritis in beef cows, and about the impact of sub-clinical reproductive tract health on subsequent reproductive performance and well-being (McNeel et al., 2017a).

In a pair of papers published in this issue of the Journal of Animal Science, Ault and colleagues (Ault et al., 2019a,b) investigated the uterine and vaginal microbiome in post-partum beef cows using next generation sequencing and demonstrated that a less diverse uterine microbiome in the post-partum interval close to breeding was associated with a decreased risk of conception to timed artificial insemination. A similar result was reported for the vaginal microbiome of post-partum dairy cows (Bicalho et al., 2017). Beef cows, therefore, may not have as many cases of clinical metritis, but some of the same components of post-partum reproductive health may be influencing post-partum re-breeding efficiency in beef cows as in dairy cows. The results of studies examining the influence of endometrial cytology on reproductive performance in post-partum beef cows have been mixed (Santos et al., 2009; Ricci et al., 2017); however, the papers by Ault and colleagues indicate that the application of next generation sequencing technologies may allow more accurate determination of the relationships between post-partum reproductive tract health and re-breeding efficiency.

In addition to influencing the fertility of the cow, the microbiome that the calf is exposed to during birth can also influence health and performance of that calf (Lima et al., 2019). No attempt was made to relate the genera associated with fertility of the dam to the genera associated with health and performance of the calf. It will be necessary to determine how these two responses interact to develop best management practices for the well-being of the cow and the calf.

Questions remain about the repeatability within a fertile or infertile cow from year to year and factors that contribute to diversity of the reproductive tract microbiome. If there is a significant repeatability, does this mean that some beef cows are more susceptible to post-partum reproductive health problems as would be indicated by results in dairy cows (McNeel et al., 2017b), and does that indicate a host by microbiome interaction as has been suggested for the rumen and its microbiome? Is a less diverse reproductive tract microbiome in beef cows associated with causal factors such as late term abortions, calf birth weight, dystocia, retained fetal membranes, or twinning? Some of these factors were associated with decreased microbiome diversity in the study in dairy cows (Bicalho et al., 2017), and influenced lifetime productivity in beef cows (Echternkamp et al., 2007; McNeel et al., 2017a). Is there a relationship between the diversity of the uterine microbiome and the rate of uterine involution? While earlier studies observed no relationships between bacterial populations and rate of involution (reviewed in Kiracofe, 1980), perhaps this needs to be revisited with modern genomic technologies. As a result of the current papers, it can be concluded that post-partum reproductive health plays a role in re-breeding efficiency in beef cows as well as dairy cows and may influence calf well-being and performance. Therefore, the recent requests for applications by AFRI for proposals to investigate the microbiome of the reproductive tract are timely and applicable with many questions to be answered.