The last thing producers want to hear at pregnancy check time, is the call of “Open!” from their veterinarian. It is often assumed that open females failed to conceive; however, fertilization rates in beef cattle typically range between 90-100%. Nonetheless, only around 70% of fertilizations result in conception. Studies indicate that in domestic livestock embryonic loss during early pregnancy may account for around 30% reduction in pregnancy rates. According to several estimates, this can represent a loss of over $1 billion to the livestock industry. Animals that experience early embryonic loss may conceive late in the breeding season or not at all, which can further increase production and financial losses. Although numerous genetic and/or physiological factors such as chromosomal defects, genetic interactions, and other abnormalities play a role in embryo survival, producers can manage stress and environmental factors to minimize losses early in gestation.
Critical periods of embryonic development
When thinking about how management strategies may influence conception rates, it is important to keep in mind critical periods of embryonic development. Maternal recognition of pregnancy in cattle occurs 15-17 days after fertilization, when the embryo produces a protein called bovine interferon-τ that signals its presence. This is the first step in ensuring that the uterine environment will support embryonic development. The embryo attaches to the uterus around day 19 after conception. Between days 22 and 25, the placenta begins to develop and the embryo becomes fully attached, a process that is complete by day 42. At this point, the embryonic period ends and the conceptus is referred to as a fetus, with most of the major tissues, systems and organs already formed. Approximately 80% of early embryonic losses occur before day 17, with 10-15% of losses between day 17 and 42, with only around 5% after day 42.
Handling and transportation
When animals become stressed due to handling or shipping, they release hormones that can change the uterine environment and influence embryo survivability. If cattle need to be moved to pasture following artificial insemination (AI), research indicates that transportation should be conducted within the first 1 to 4 days after AI. This is an ideal time since the embryo is still in the oviduct and should not be affected by changes in the uterus. If cattle are not transported immediately after AI, it is best to wait until after day 42 (6 weeks post-AI) when the placenta is attached and the pregnancy is well established. After this point, the embryo is less susceptible to environmental challenges, although it is still possible for losses to occur. Transportation stress is influenced by weather, distance to be traveled, and conditions inside the truck such as number of cattle in each section. Handle cattle as gently as possible and avoid overcrowding trucks or trailers in order to minimize stress.
Heat stress has also been documented to play a role in embryonic death due to elevated uterine temperatures and potential impacts on the quality of oocytes available for fertilization. Major factors driving this response are temperature and humidity. The Livestock Weather Hazard Guide, available from The Samuel Roberts Noble Foundation, Inc. can provide guidelines for using a temperature-humidity index to determine the hazard potential. Although it is difficult to manage extreme temperature shifts, providing shade or fans during breeding can help reduce heat stress. Also, sorting and handling cattle should be avoided during the hottest parts of the day.
The bottom line
With the majority of pregnancy losses occurring in the embryonic period, it is critical to mitigate losses in livestock by managing stress and avoiding transportation when the embryo is most susceptible to mortality.