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UNL explains breeding stressors

by Wyoming Livestock Roundup

            As breeding season quickly approaches, cows and heifers are faced with a variety of stressors from the metabolic pressure of providing for a calf to unpredictable changes in the environment.

            According to Nebraska Extension Beef Educator Sydney O’Daniel and Nebraska Extension Beef Cattle Reproductive Physiologist Dr. Rick Funston in a University of Nebraska-Lincoln (UNL) publication dated May 1, stress during early pregnancy is well documented to cause embryonic death and loss of pregnancy.

Early pregnancy

            “If we are going to talk about early embryonic loss, I think it is important to go over what happens in the first 42 days of pregnancy so we can understand how stress will impact the embryo,” O’Daniel explains during an episode of UNL’s Beefwatch podcast where she discusses her and Funston’s article.

            “Fertilization occurs on day zero, and from day one to day four, the embryo stays in the oviduct,” she continues. “On day five, the embryo travels down the uterine horns to the uterus, where it hangs out for awhile and goes through some developmental phases until around day 15, which is when maternal recognition of pregnancy occurs.”          

            O’Daniel further explains from day 15 to day 42 the embryo will begin attaching to the uterus, and development of the placenta will occur.

            “On day 42, the embryo becomes a little more protected because it is attached to the uterus and less susceptible to changes in the uterine environment resulting from stress,” she says.

Transportation stress

            With this in mind, O’Daniel and Funston note there are some key areas to review management practices in order to minimize stress and early pregnancy loss.

            The first is transportation.

            “Transporting cows to summer pasture oftentimes coincides with the breeding season, especially if cows or heifers are synchronized and artificially inseminated and need to be near working facilities,” O’Daniel and Funston say. “It is important to plan transportation or other stressors strategically to prevent early pregnancy loss.”

            “This window of time is crucial for blastocyst formation, maternal recognition of pregnancy and adhesion to the uterine wall,” they explain. “When cattle are loaded into a trailer and transported to a new place, they may become stressed and release a cascade of hormones that can alter the uterine environment, making it less ideal for supporting pregnancy.”

            O’Daniel and Funston reiterate prior to day five, the embryo is still in the oviduct and protected from the uterine environment, while after day 42, the embryo has implanted in the uterus and is less susceptible to a changing environment.

            “While transporting on days five through 42 pose the greatest risk, waiting to haul cows and heifers until a week or two after day 42 may help prevent late embryonic loss as well,” they say. “Ultimately, stress during those critical time points may disturb important early embryonic.”

            Therefore, O’Daniel and Funston recommend if cattle need to be hauled, producers should do so during days one through four or after day 60.

Heat stress

            Another stressor producers should be aware of during the breeding season, according to O’Daniel and Funston, is heat stress.

            “Research conducted at Oklahoma State University found cows exposed to heat stress eight to 16 days after breeding had decreased progesterone concentrations, increased prostaglandin concentrations and reduced embryonic weights,” they explain.

            During the podcast, O’Daniel points out how important it is to have correct hormone levels in order to maintain a pregnancy.

            “Progesterone is the hormone that supports pregnancy, so if progesterone levels drop and prostaglandin levels increase, the success of the pregnancy is compromised,” she says.  

Pregnancy diagnosis

            O’Daniel and Funston say implementing a pregnancy diagnosis method is key for making sound managerial decisions regarding open females. However, they also point out some pregnancy diagnosis methods carry more risk than others in terms of pregnancy loss.

            “Currently, there are three options available for pregnancy diagnosis – transrectal palpation, transrectal ultrasound and blood testing,” O’Daniel and Funston explain.

            They note blood tests are the least invasive method, but results are not immediate which may not be practical for some producers.

            “Transrectal palpation and transrectal ultrasound allow for immediate results, but pose a greater risk for early pregnancy loss and can vary based on the stage of pregnancy and the skill of the technician,” O’Daniel and Funston say.

            “Additional research from CSU found transrectal palpation can cause nearly a two-fold increase in fetal losses compared to transrectal ultrasound, and heifers evaluated by inexperienced technicians had nearly a two-fold increase in fetal losses compared to those evaluated by experienced technicians,” O’Daniel explains.

            She notes the same study also found heifers evaluated prior to 50 days post-breeding had a three-fold increase in pregnancy loss.

            “Transrectal palpation and transrectal ultrasound can be utilized as early as 35 days and 25 days, respectively, but that might not be wise,” O’Daniel states. “Waiting to diagnose pregnancy until after day 42 or even day 60 can increase pregnancy rates and reduce embryonic losses.”

            Hannah Bugas is the assistant editor for the Wyoming Livestock Roundup. Send comments on this article to

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