Taken together, the obtained data demonstrate that the differences in serum toxic and essential trace element and mineral levels between the cows with different milk productivity were more obvious in the pasture period when compared to the feedlot period. Specifically, after transition from feedlot to pasture high-productive cows were characterized by lower levels of toxic metals, higher circulating mineral concentrations, as well as a less profound decrease in essential trace element levels when compared to low-productive cows. Furthermore, serum toxic metal and metalloid levels were characterized by an inverse association with certain minerals and essential trace elements, being indicative of the potential antagonistic relationships. Finally, the differences in trace element and mineral levels were found to be associated with daily milk yield, but not milk protein or lipid content.
The observed differences in serum metal and trace element and mineral levels in cows between the feedlot and pasture periods may be mediated by feeding regimens. Specifically, indoor housing in winter involves concentrate feed supplementation (Magan et al. 2021) that is known to be associated with higher intake of essential and toxic trace elements (Rey-Crespo et al. 2013). O’Brien et al. (1999) demonstrated that milk Cu and I were found to be higher during winter periods, whereas no clear trend in the changes of Mn, Mo, Zn, Co, Cr, and Fe levels was observed (O'Brien et al. 1999). Correspondingly, administration of a concentrate-based ration was associated with higher blood I level (Lejeune et al. 2010). In addition, it has been demonstrated that the intake of Co is also associated with the administration of concentrate feeds (Orjales et al. 2018). In contrast to trace elements that are characterized by higher intake during indoor period, consumption of minerals (Ca, Mg, P, Na, K) was shown to be positively associated with time spent on pastures (Morales-Almaráz et al. 2021). Grazing outdoors on perennial ryegrass pasture was shown to be associated with higher milk Ca levels as compared to indoor housing on a total mixed ration (Gulati et al. 2018). The observed increase in Ca levels in the pasture period corresponds to higher Ca level in pasture grass in comparison to hay, silage, and chow administered during the feedlot period (Sizova et al. 2024).
The existing data on the variations in heavy metal intake and accumulation in dairy cows in different periods and housing systems are insufficient. However, Pastorelli et al. (2023) also reported higher milk Cd levels in winter as compared to summer period (Pastorelli et al., 2023). In winter higher Pb and Cd levels were observed in raw milk samples in N.R. Macedonia (Limani et al. 2022). Furthermore, the level of Cd, Cr, Pb, and Ni was found to be higher in milk samples from conventional and especially commercial farms as compared to the organic ones (Zwierzchowski et al. 2018). Hypothetically, higher intake of heavy metals may be associated with its higher content in feed concentrates. Specifically, earlier studies demonstrate that animal feeds frequently contain detectable heavy metal levels that sometimes even exceed the upper tolerable level (Dai et al. 2016). Of all components of Wisconsin dairy herd feed ration, mineral mix was shown to contain the highest levels of As, Cd, and Pb (Li et al. 2005). Moreover, a study by Li et al. (2019) originating from China reported that the levels of Cr and Pb in dairy feeds may exceed the recommended limits by a factor of more than 6 and 17, respectively (Li et al. 2019). Correspondingly, Pb and Cd levels in cattle tissues significantly correlated with feed heavy metal content (Hashemi et al. 2018). In addition, Al, As, Hg levels in feed also correlated with cow milk concentrations (Zhou et al. 2017).
However, the particular mechanism underlying relatively lower accumulation of trace elements in high-productive cows in comparison to the low-productive ones despite grazing on the same pastures is unclear. Hypothetically, the observed differences may be mediated by different genetic characteristics of cows with different milk production. Specifically, Upadhyay et al. (2015) demonstrated that gene ontology categories associated with metal ion transport (GO: 0030001) and metal ion transmembrane transporter activity (GO: 0046873) were found to be enriched in cows characterized by different milk production (Upadhyay et al. 2015). Furthermore, high-productive dairy cows were characterized by upregulation of hepatic GSTM4 gene (McCarthy et al., 2010), that is also known to be involved in detoxification (Gasmi et al. 2022). The assumption of the role of different genetic background in accumulation of metals and other trace elements is also supported by the observation by Denholm et al. (2019) who revealed a significant association between dairy cow genotype and blood and milk trace element concentrations (Denholm et al. 2019).
The results of group comparisons demonstrated that high-productive cows are characterized by higher levels of minerals and essential trace elements (except for Co and I), while having lower serum levels of toxic metals and metalloids. Furthermore, regression analysis also supported these findings demonstrating a positive relationship between Cr, Co, Mg, I, and Na levels with milk yield, whereas circulating toxic As and Pb were found to be inversely associated with milk production.
Toxic metal exposure was shown to be a risk factor for adverse health effects and lower productivity in dairy cows (Raikwar et al. 2008). Specifically, it has been also demonstrated that total accumulation of toxic metals in hair of dairy cows is inversely associated with milk production (Miroshnikov et al. 2021). Cd exposure is able to affect reproduction and milk production (Lane et al. 2015). An earlier study by Miller et al. demonstrated that dietary Cd exposure significantly decreases milk production in dairy cows (Miller et al. 1967). Serum Pb levels were shown to be inversely associated with body condition scores in dairy cows, although no relationship with milk yield was revealed (Denholm et al. 2022). In turn, reduction of Pb and Cd body burden was associated with an increase in daily milk yield in cows from an industrial area (Portiannyk, Mamenko, 2021).
In contrast to toxic metals, mineral intake is known to be essential for dairy cow health. Specifically, adequate circulating Ca was shown to be critical for postpartum health and reproductive performance in dairy cows (Jeong et al. 2018). Supplementation with oral Ca boluses was shown to increase daily milk yield (Oetzel, Miller, 2012). Similar effect was observed in multiparous cows of greater production potential, but not those with below average production potential (Martinez et al. 2016). Correspondingly, clinical hypocalcemia in multiparous cows was associated with lower daily milk production (Venjakob et al. 2018). At the same time, the results of certain studies did not reveal any significant association between serum Ca levels and milk productivity (Østergaard, Larsen, 2000).
Mg is also an essential mineral for dairy cows (Schonewille, 2013). An increase in milk yield is associated with higher Mg requirements with an increase in Mg intake and Mg absorption (Martens, Stumpff, 2019), therefore, high-productive cows are considered at higher risk of Mg deficiency (Pinotti et al., 2021). Oral Mg supplementation was shown to increase milk production and milk fat content in hypomagnesemic cows (Wilson, 1980). Correspondingly, supplementation with Mg butyrate significantly increased daily milk yield and milk protein and fat content (Fébel et al. 2023).
Reduction of dietary P content was also shown to result in decreased milk yield, while having no significant effect on milk fat and protein content in lactating dairy cows (Wu et al. 2000). These findings corroborate earlier data on the association between P deficiency and low milk production (Call et al. 1987). It is also proposed that dietary P deficiency may impair rumen microbiota resulting in reduced milk protein content (Elizondo Salazar et al. 2013).
Previous studies also demonstrated the role of Na and K in regulation of milk production. Specifically, milk yield was shown to be associated with K retention in dairy cows (Silanikove et al. 1997). It is proposed that K requirements in high-productive cows may be increased in early lactation period (Dennis et al. 1976). Specifically, an increase in milk production was associated with early hypokalemia (Plöntzke et al. 2022). Correspondingly, increased dietary potassium in early lactation may improve milk yield and milk fat content (Harrison et al. 2011). Na supplementation was shown to increase daily milk yield in cows grazing a tropical grass-legume pasture (Davison et al. 1980). In addition to increased milk yield, Na supplementation was shown to improve Ca and Mg metabolism in cows, also possessing beneficial effect on mammary gland health (PHILLIPS et al. 2000).
Essential trace elements are also involved in maintenance of dairy cattle health and thus milk production. Specifically, adequate B nutrition was shown to be beneficial for dairy cows and other animals (Abdelnour et al. 2018). Despite the lack of B supplementation on milk yield, it significantly improved milk composition and mammary gland health (Praveen et al. 2021), in parallel with a beneficial effect on metabolic regulation in dairy cows (Basoglu et al. 2017). In addition, B was shown to improve Ca and Mg bioavailability in cows (Baspinar et al. 2017).
The results of a recent meta-analysis demonstrated that Cr supplementation significantly improves milk production but not composition (Malik et al. 2023), also significantly influencing metabolic parameters by reducing non-esterified fatty acid levels and increasing glucagon concentrations (Malik et al. 2024). Given the role of Cr as an insulin-mimetic, it has been shown that Cr supplementation significantly modulates insulin-signaling pathway, although this effect was different in antepartal than post-partal period (Pantelić et al. 2018), being also dependent on the energy balance in these periods (Kegley, Spears, 1999).
Co deficiency is known to be associated with a wide spectrum of adverse effects including impaired growth and reproduction (Silva et al. 2021). Milk secretion is associated with higher B12 and Co requirements (Kincaid et al. 2003). In turn, Co supplementation was shown to improve daily milk yield in cows (González-Montaña et al. 2020). However, high dietary Co supplementation may affect both milk production and fatty acid composition (Karlengen et al. 2013). Correspondingly, we have observed a trend to increased serum Co levels in high-productive cows (Sizova et al. 2024).
Lactation is associated with decreased plasma Fe and Hb levels, being indicative of higher Fe requirements (Randhawa et al. 2009). Correspondingly, simultaneous Fe and Cu deficiency was shown to be associated with reduced milk yield (Abramowicz et al. 2021). Fe supplementation in cows consuming a diet adequate in Fe, did not improve daily milk production but significantly reduced somatic cell count in milk (Weiss et al. 2010). At the same time, Denholm et al. (2022) demonstrated that serum Fe levels were inversely associated with milk yield, while being positively correlated with body condition scores (Denholm et al. 2022).
Although earlier studies demonstrate that insufficient I intake may contribute to reduced milk production, later investigations failed to reveal an association between dietary I intake and milk yield (Niero et al. 2023). Specifically, no effect of I supplementation on milk production was observed in cows grazed on pastures with adequate I content (Grace, Waghorn, 2005).
In addition to lower levels of toxic metals and higher concentration of essential trace elements and minerals in high-productive cows compared to low-productive ones, a significant inverse correlation was observed between certain toxic and essential elements. These findings generally corroborate the earlier data on the adverse effect of toxic metal exposure on mineral metabolism in cows. Specifically, Cd accumulation in the organism following long-term dietary exposure was shown to affect hepatic and renal levels of essential metals (Smith et al. 1991). A significant inverse correlation of milk Pb and Cd with Ca and Mg levels, respectively, was observed in Simmental cows from an organic farm (Pilarczyk et al. 2013). In cattle from industrial areas, blood Pb levels was inversely associated with circulating concentrations of Cu, Co, and Fe (Patra et al. 2006).
In turn, improved mineral intake may possess protective effects against toxic metal overaccumulation. Specifically, administration of mineral blocks containing high levels of P and Ca were shown to reduce bioavailability of Pb in vitro (Pareja-Carrera et al. 2020). Correspondingly, mineral block supplementation was shown to reduce Pb accumulation in sheep by increasing its fecal excretion (Pareja-Carrera et al. 2021). Noteworthy, dietary Ca was shown to reduce liver Pb content in sheep (Pearl et al. 1983).