Zearalenone is a mycotoxin that naturally occurs in 78% of globally sampled TMR diets according to the dsm-firmenich mycotoxin survey. Its effects can impair reproduction and cause spontaneous abortions, resulting in enormous economic losses to dairy producers. Here's how to recognize[IA2] the effects of zearalenone in dairy cattle.
When addressing mycotoxins on dairy farms it is common to focus on the direct losses in production and health status. With zearalenone, the worries are different: since it has a very similar structure to estrogen, zearalenone can bind to the receptors located in distinct areas of the body and cause delays in reproduction, spontaneous abortions, ovarian cysts and other reproductive challenges.
Estrogen prepares the reproductive organs of the cow for breeding and influences behavior that encourages mating. During the estrous cycle, estrogen concentration remains low and stable until the estrus (aka heat), when it increases abruptly. A normal estrus lasts one day and once it is passed, if no oocyte is fertilized, the concentration of estrogen resides again. (Figure 1)
Figure 1. Hormonal fluctuations along the estrus cycle. (adapted from Senger (2003).
When zearalenone is ingested and reaches the rumen, microbes can transform the molecule but this change does not result in an actual detoxification. The two main resulting molecules of this metabolic process are α-zearalenol (α-ZEL) and β-zearalenol (β-ZEL) which can be 60 or 0.2 times as estrogenic (respectively), with the biggest portion becomes α-ZEL (figure 2, Gruber-Dorninger et al., 2021). This means that zearalenone’s estrogenicity is heavily increased, resulting on even bigger hormonal unbalances in animals.
Figure 2. In vivo degradation of zearalenone in rumen cannulated dairy cows (Gruber-Dorninger et al., 2021)
During an experiment by Mahmoud et al. (2014) two different levels of mycotoxin contamination in the diet were evaluated: 200ppb ZEN, ppb AfB1 and 400 ppb ZEN, ppb AfB1. The two treatments were provided to Holstein dairy cows from calving and blood samples were taken at day 45 postpartum. This trial helps elucidate quite clearly what the effects of zearalenone on the diet of a dairy cow.
Figure 3. Changes in concentration of main reproductive hormones when two different zearalenone treatments were fed to Holstein dairy cows.
The first clear effect of a change on zearalenone contamination in the diet is that all the concentrations of hormones in blood (such as estrogen, progesterone, luteinizing hormone, T3, T4 and prolactin) were affected significantly by the treatment. The cow has a normal level of estrogen that would naturally increase only right before estrus. If zearalenone is on circulation binding with receptors of estrogen, this alters the natural concentration of other hormones. This hormonal imbalance can result in unsuccessful reproduction, lack of response to hormonal treatments for artificial insemination, as well as false heats.
The increase of zearalenone in the diet resulted in a reduction of progesterone, also known as the “pregnancy hormone” because it ensures the physiological conditions are right to keep the embryo. In Figure 1 it can be observed how estrogen and progesterone regulate each other, when estrogen is high, progesterone is lower, and the opposite. Thus, having any increase of zearalenone, which will behave like estrogen, can result in the decrease of progesterone levels, thereby losing the conditions to keep the embryo and increasing the likelihood of a spontaneous abortion.
Additionally, an increase in zearalenone in the diet resulted in a lower blood LH concentration (Figure 3). LH hormone is responsible for the successful release of the oocyte once the follicle is mature. Zearalenone-induced LH reduction can impede therefore the release of the oocyte from the follicle, making it become a cyst. A way that reproductive experts use to identify a cyst with an ultrasound is by measuring its diameter. Follicles measuring a diameter above 20mm, are categorized as a cyst.
Interestingly, both treatments resulted in an average follicle diameter of more than 20mm but what was surprising was how linear the response was, with double ZEN concentration resulting in double follicle diameter.
Figure 4. Graphic comparison between a normal ovary with successful ovulation with a polycystic ovary without possibility to ovulate.
Any sort of delay in the reproduction of dairy cows is a great economic concern. According to different authors (De Vries, 2006; Eicker & Fetrow, 2003; Hovingh, 2009), the cost of a cow spontaneously aborting can be 550-1200 USD, and every extra open day costs 5.20-6 USD (Cabrera, 2014). Additionally, breeding problems are one of the top reasons for cow replacement. A cow that takes longer to become pregnant has a 75% higher possibility of being culled (Cabrera, 2014).
When addressing poor reproductive efficiency in a dairy farm, it is essential to ensure that mycotoxins are not the limiting factor to reach reproductive potential. Once this burden has been removed, other strategies to promote successful reproduction can be implemented.
Considering the economics of cow reproduction, zearalenone can represent relevant losses if not addressed properly. A deactivation strategy for zearalenone is necessary in order to ensure an adequate hormonal balance and successful reproduction.
11 January 2024
Ignacio holds a bachelor's in Agriculture Sciences and a master's in Sustainable Animal Nutrition and Feeding. He´s the global Marketing Manager for Ruminants at dsm-firmenich, and has supported farmers, nutritionists, and health experts to overcome the negative impact of mycotoxins in different parts of the World.
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