Contributing Author: Elisa Marcondes de Souza, UT Extension Beef Intern with Dr. Saulo Zoca

Pregnancy loss represents one of greatest obstacles to reproductive efficiency in beef cattle production, directly impacting both productivity and the sustainability of production systems. While maternal, embryonic, and environmental factors have been widely studied, the bull’s contribution, particularly regarding management and semen quality, has received less attention.

Most reproductive management strategies focus primarily on ensuring conception, with relatively little investment in pregnancy maintenance, especially during the early stages of gestation. Previous studies indicate that most losses occur before day 32 of pregnancy. Male-related factor such as poor semen quality, abnormalities in sperm formation, and failures in embryo-to-uterus signaling play a direct role in these events. Such failures can result in inefficient fertilization, impaired embryonic activation, and consequently early disruptions in embryonic development. In this context, this article aims to discuss how the bull influences pregnancy loss and how he contributes to effective reproductive management practices.

Semen Quality

Sperm production, or spermatogenesis, takes approximately 61 days in cattle. During this period, any factor affecting the bull’s health or management, such as heat stress, illness, or inadequate nutrition, can compromise semen quality. This means that events occurring weeks before mating may negatively affect the bull’s fertility for an extended period. Classical studies evaluating the impact of testicular heat stress reported that semen quality (motility and morphology) was normal up to 10 days post insult but decreased considerably afterwards and did not return to normal quality until approximately 45 days post insult.

Parameters such as sperm motility and morphology, evaluated during a Breeding Soundness Exam (BSE), are fundamental for successful fertilization; however, abnormalities in these traits not only reduce the chances of conception but also compromise embryonic development during the early stages. It is important to remember that fertilization is only the beginning of a long and complex journey. Abnormal sperm can still fertilize an egg, but this does not guarantee success in subsequent stages, especially in embryonic genome activation, when the embryo begins to function independently.

Sperm DNA integrity is one of the factors that can influence this process. Alterations in the genetic material are strongly associated with early pregnancy losses, especially by day 16 of gestation, when maternal recognition of pregnancy should occur. One of the main consequences of DNA fragmentation is the insufficient production of interferon-tau (IFN-τ), a cytokine secreted by the early developing placenta (trophectoderm) that signals to the maternal system that gestation has begun. This signaling is what maintains the corpus luteum (CL) alive and active. Without it, the  CL regresses and pregnancy is terminated, even before it can be detected by conventional methods, causing many losses to go unnoticed in the field. Recent studies have also identified sperm proteins directly associated with fertility, showing that semen quality influences embryo viability more broadly than previously understood.

In some recent studies the impact of bull over nutrition was investigated. In an observational study, semen quality of yearling bulls developed in similar conditions were compared during their first BSE. It was observed that bulls with greater fat deposition (subcutaneous backfat) had decreased semen quality, especially the percentage of morphologically normal sperm, and were more likely to fail their BSE. Further, in a controlled experiment, mature bulls managed to be at a moderate body condition or over conditioned, had little difference in semen quality, yet embryo production and development were negatively affected when semen from over conditioned animals was used. These findings emphasize the importance of proper bull management prior to the breeding season and the need for thorough semen evaluation during a BSE to avoid using subfertile bulls and compromising herd reproductive efficiency.

Paternal Genetics and Embryonic Abnormalities

The genetic material inherited from the sire and the dam can carry lethal recessive mutations or structural defects that render embryonic development inviable within the first days post-fertilization. These problems only manifest when both parents are carriers and the recessive genes interact. Identifying these early lethal genes is challenging; but although silent, these anomalies significantly reduce pregnancy rates and herd reproductive efficiency. It is important to highlight that such mutations are more commonly observed in straightbred (purebred) cattle than in crossbred populations. Importantly, a single sire carrying these embryonic lethal genes can generate a large population of female carriers, potentially compromising herd fertility for generations.

Final Considerations

Data analysis clearly show that the bull’s influence on pregnancy maintenance, especially during early embryonic development, is undeniable. Although pregnancy losses have historically been attributed mainly to females, recent advances highlight the need to rigorously evaluate paternal contributions as well. Factors such as low sperm motility, morphological abnormalities, and DNA fragmentation are closely linked to fertilization and embryonic activation failures. Furthermore, genetic mutations inherited from the sire can compromise the embryo before implantation even occurs. Other contributing factors remain under investigation.

Considering all these aspects, the bull should not be viewed merely as a fertilizing agent but as a central pillar of reproductive efficiency. Evaluating his contribution in depth, by integrating genetic, functional, and environmental data, will be essential to reduce embryonic losses and optimize herd productivity. Investing in sire selection and monitoring programs (BSE) is not just good practice; it is a necessity to ensure sustainability and profitability in modern beef cattle production.

References available by request to the authors.