Open. It is a word no veterinarian enjoys saying during the herd check. Even more frustrating is calling open on a cow that just six weeks previously was called pregnant. While pregnancy loss is known to occur, it is hard to accept. When the pregnancies originated from in vitro-produced (IVP) embryos, it may appear that the pregnancy losses are more frequent but even harder to accept.
This increase in losses isn’t simply a perception. A study published in the Journal of Dairy Science in 2025 reported that pregnancy loss from day 32 to 62 of gestation was 18% in frozen IVP embryos compared to just 4% during the same time frame for A.I. pregnancies.
Utilization of embryo transfer, and more specifically in vitro-produced embryos, has increased dramatically over the past decade. In 2022, the number of in vitro-produced embryos was five times greater than in vivo-derived embryos. Much of the increase has been seen in dairy cows. In North America alone, over 125,000 dairy cows were used for oocyte pickup (OPU), compared to 78,000 beef cows.
Dairy producers have incorporated embryo transfer technologies as a way to increase the genetic progress of the herd, increase pregnancies in difficult-to-breed cows and, more recently, as a way of producing beef calves to capture higher market prices. As the production of embryos increases, working toward ways to maintain the pregnancies is crucial.
A difficult start
The miracle of life is a miracle due to all of the hurdles that must be overcome in the developmental process during a pregnancy. It is estimated that proper timing of insemination results in 80% to 90% conception. Almost immediately, however, pregnancy loss begins. There are estimates that only 50% of all ovulated oocytes in dairy cows will progress as a viable embryo past one week. Moreover, it is estimated that only 27% of IVP embryos will result in a live calf on the ground.
There are numerous causes of this poor embryonic development, ranging from genetic factors of the embryo – including lack of embryonic signaling to the dam – to environmental factors such as pathogens, heat stress and nutrition. Since these embryo transfer (ET) pregnancies are typically considered of higher value, seeking ways of improving conception rate and/or decreasing pregnancy wastage should be prioritized.
Nutrition is more than just energy
Most dairy producers understand that reproductive success is often closely related to body condition score. Lower condition is a consequence of negative energy balance and inadequate amounts of energy for cows to cycle and become pregnant. In addition to energy being needed, trace minerals play an incredible role in the reproductive cycle. Trace minerals – including copper, zinc, manganese and selenium – all impact the entire process. This starts from ovarian follicle growth and ovulation in the cow and spermatogenesis in the bull, through displays of sexual receptivity, conception and parturition. Deficiencies in these trace minerals can lead to anestrous, fertilization failure, embryonic death and fetal malformations. Trace mineral concentrations have been shown to be greater in the conceptus than in surrounding reproductive tissues, indicating the need for trace minerals in the developing embryo.
The importance of antioxidants
Trace minerals are essential components of enzymes such as superoxide dismutase (SOD) and glutathione peroxidase (GPx). These enzymes are antioxidant proteins that help protect the developing embryo from damage. Most people are familiar with hydrogen peroxide and the effect it can have on cells, killing both pathogenic (i.e., bacteria) and non-pathogenic cells. As embryos develop, relatively large quantities of several oxidative free-radical substances (known as reactive oxygen species) similar to hydrogen peroxide are produced. These antioxidants are needed from the time of ovulation through embryonic development. Without adequate levels of trace minerals in the dam, not enough antioxidants are available to neutralize the reactive oxygen species, potentially killing the embryo and losing the pregnancy.
Trace minerals are needed right from the start
Beyond the role that trace minerals have in regulation of hormone production and cyclicity, selenium and zinc play vital roles during the initial events necessary for conception. Selenium concentrates in large ovarian follicles in cattle, supporting ova maturation and shielding them from damage by oxidative free radicals. Furthermore, zinc helps prevent terminal fertilization error. Each of the male and female gametes (sperm and ova, respectively) contain one-half of the full complement of chromosomes. Should more than one sperm cell fertilize the ovum, excess chromosomes would be contained within the zygote (fertilized egg), causing a duplicity of chromosomes and leading to a terminal error. When a single sperm cell fertilizes the egg, zinc is released, causing what is referred to as a zinc spark. Zinc then alters the zona pellucida, or shell of the egg, preventing other sperm from being able to penetrate the egg. In addition, this zinc spark is a signal of viability to contribute to start the process of embryo development.
In vitro work has also demonstrated the need for trace minerals. Supplementation of copper, selenium, manganese and zinc during in vitro maturation and embryo culture has shown a reduction in the cumulus cell damage and apoptosis. Selenium supplementation improved the hatching rates of embryos compared to non-supplemented oocytes. Zinc, known for its role in cellular division, helps the embryo continue to multiply the number of cells as development continues.
Utilizing injectable trace minerals to decrease pregnancy loss
The known role of trace minerals in fetal development and pregnancy maintenance could be a key in improving the outcomes for IVP embryos. A recent study conducted at Ohio State University examined this very scenario. A total of 556 Holstein heifers were enrolled in the study, with half receiving an injection of trace minerals at the initiation of synchronization. Heifers receiving the injection of trace minerals had less than half the pregnancy loss (9.52 versus 22.2) as non-treated controls.
Previous studies transferring embryos produced through superovulation have shown that conception rates were higher in injectable trace mineral- supplemented heifers. Cattle treated with trace minerals 17 days before embryo transfer had 1.72-fold higher odds of being pregnant 48 days after ET compared to the control group. It is believed that the improved conception rate was due to the trace minerals' role in cellular enzymatic function, including both hormones and immune cells, thereby improving embryonic survival.
A promising and highly recommended strategy to improve the pregnancy rate of the transferred IVP embryos is to provide an injection of selenium, copper, zinc and manganese to the recipients at the onset of the synchronization protocol (17 days before the ET).
Conclusion
In vitro-produced embryos provide dairy producers with a way of capitalizing on genetics and help difficult breeders become pregnant. Those benefits are lost if a live calf isn’t born nine months later. While researchers continue to look for ways to improve the outcomes of live births, improving the trace mineral status of the cow may be a promising way to reduce hearing that dreaded word: open.
