In mid-October I attended the 86th Cornell Nutrition Conference in East Syracuse, NY, an event renowned for its wealth of information and engaging discussions. The week-long conference featured an array of dynamic speakers, thought-provoking sessions, and opportunities to connect with seasoned professionals eager to share their insights with those new to the field.
One standout session was by Dr. Stephanie Hansen, a professor of animal science at Iowa State University and an accomplished mystery novelist. Her talk, titled “Optimal vs. Adequate: Trace Minerals in Transition Cows,” explored the intricate balance between sufficient intake and optimal physiological response to trace minerals in the diets of transition cows. The session delved into key trace minerals, including zinc, copper, and selenium. I was particularly intrigued by the discussion on selenium (Se), as Dr. Hansen emphasized its pivotal role in reproductive health, immune strength, and overall success in dairy cattle.
The transition period, encompassing the three weeks before and after calving, is a pivotal stage in a dairy cow's lifecycle. During this time cows experience substantial physiological transformations as they transition from pregnancy to lactation. This window is also associated with significant changes in the metabolic and nutritional needs of a cow. Supporting a transition cow at this time can be integral to setting them up for a productive lactation.
Appropriate trace mineral supplementation is of particular importance, as there is often a narrow margin between toxicity and deficiency. Each mineral plays a different role in the diet of a cow and can be used to fill some gaps that a herd may be experiencing. In transition cows, trace mineral supplementation (or the lack thereof), can have an impact on the health of both the dam and the offspring. Selenium is an example of a trace mineral that can have a strong influence on reproduction and immunity in a herd.
Excessive reactive oxygen species (ROS) production during the periparturient period can induce oxidative stress, a condition linked to various health issues in dairy cattle transitioning from late gestation to lactation, including retained placenta, fatty liver, ketosis, mastitis, and metritis (Xiao et al., 2021). Selenium, a crucial component of antioxidants, can mitigate oxidative stress and potentially prevent these diseases.
Research by Jaaf et al. (2020) demonstrated that supplementing dairy cows with low amounts of Se-biofortified alfalfa hay increased blood and liver selenium concentrations. Notably, Se readily crosses the placental barrier, even at the expense of maternal stores (Abd El-Ghany et al., 2007). This can lead to maternal Se deficiency as fetal demand increases, reducing antioxidant accumulation and activity Supplementation during the third trimester may be necessary to maintain adequate maternal Se levels. Consequences of maternal selenium deficiency include reduced milk production due to Se's essential role in milk component synthesis. Additionally, a weakened immune system is a significant effect, increasing the susceptibility of cows to various infections such as mastitis, metritis, and other diseases. Se deficiency can impair ovarian function and embryo development, leading to reduced fertility rates and increased risk of abortions. Furthermore, it can weaken the muscles involved in labor, increasing the risk of dystocia. (Abd El-Ghany et al., 2007).
A study by Ceko et al. (2015) using X-ray fluorescence (XRF) imaging revealed that Se is concentrated in granulosa cells of large bovine follicles. Van Emon et al. (2020) proposed that this localization and Se's antioxidant properties protect oocytes from oxidative stress and DNA damage during folliculogenesis. In vitro research by Basini and Tamanini (2000) showed that Se directly stimulates granulosa cell proliferation and estradiol synthesis. Estradiol, a key hormone in the reproductive cycle, plays a crucial role in follicular development and ovulation. By stimulating the release of gonadotropin-releasing hormone from the hypothalamus, increased,estradiol production, stimulated by Se, triggers the preovulatory luteinizing hormone surge, which is essential for ovulation to occur (Van Emon et al., 2020). This research suggests that selenium (Se) plays a crucial role in enhancing reproductive success in cows by protecting oocytes from damage and stimulating follicular growth. In summary, by protecting oocytes, stimulating follicular growth, and regulating the hormonal cascade leading to ovulation, selenium contributes significantly to improved reproductive performance in cows. This translates to increased fertility rates, reduced risk of abortions, healthier calves, and enhanced overall reproductive efficiency, ultimately leading to increased profitability for dairy farmers.
In the United States, selenium supplementation in livestock feed is regulated by the Food and Drug Administration. The FDA establishes maximum allowable levels of selenium in animal feed to prevent toxicity. The maximum allowable level of selenium in dairy cattle feed is 0.3 ppm (parts per million) (Ullrey, 1992). However, these are maximum limits, and the optimal level of selenium supplementation will vary depending on factors such as the selenium content of the forage, the animal's physiological status (e.g., pregnancy, lactation), and the risk of selenium deficiency in the region.
In conclusion, Dr. Hansen's presentation highlighted the critical role of trace minerals, particularly selenium, in supporting the health and productivity of transition cows. Selenium plays a multifaceted role in immune function, reproductive health, and antioxidant defense. While appropriate supplementation is essential, it is crucial to adhere to regulatory guidelines and avoid excessive selenium intake, which can lead to toxicity. Continued research is needed to further refine our understanding of selenium's role in dairy cow health and to develop optimal supplementation strategies for different production systems.
— Hannah Jones