Research study by Agriculture & Agri-Food Canada
By Danyel Bueno Dalto, J. Jacques Matte, & Jérôme Lapointe
The National Research Council [1] recommends 80 to 100 mg/kg of dietary zinc (Zn) and 5 to 6 mg/kg of copper (Cu) for 7 to 25 kg piglets, but the swine industry worldwide uses levels up to 3,000 mg/kg of Zn and 250 to 500 mg/kg of Cu during the first weeks post-weaning [2, 3, 4] as a strategy to reduce the prevalence of diarrhea and improve growth performance.
Although essential in pigs’ nutrition, the environmental impacts related to the use of such high levels of zinc oxide (ZnO) and copper sulphate (CuSO4), as well as the concerns related to the development of bacterial multi-resistance to antibiotics and metals, led European countries to restrict their use in swine diets starting in 2022.
In addition to these issues, our research team has recently reported negative effects of high dietary levels of ZnO and CuSO4 on post-weaning piglets’ health. For example, systemic Zn, Cu and Fe concentrations were unbalanced in different organs, whereas energy production and the antioxidant system were impaired. Such results were not observed with lower dietary ZnO levels (under similar CuSO4 levels), indicating that an optimal dietary Zn/Cu ratio may improve those health aspects.
However, there is still limited understanding of the interplay between the modulation of Zn and Cu in different organs, which hampers the development of nutritional strategies to optimize trace mineral homeostasis and increase robustness of post-weaning piglets. Therefore, this study aimed to better understand the biological consequences of different dietary Zn/Cu ratios on Zn and Cu regulation in post-weaned pigs.
One hundred sixty piglets were selected at weaning (21 days of age) and randomly assigned to one out of four experimental one-phase post-weaning diets (without antibiotics) supplemented with 100 (LZn) or 3,000 (HZn) mg/kg of Zn as ZnO in combination with 6 (LCu) or 130 (HCu) mg/kg of Cu as CuSO4. Piglets were slaughtered at days 21, 28, and 42 for the collection of serum and body tissues.
Growth performance
The combination of high levels of dietary ZnO and CuSO4 (HZnHCu) impaired piglets’ growth, whereas beneficial effects of high dietary CuSO4 levels were detected only when ZnO levels were low (LZnHCu). However, these results have to be carefully interpreted, taking into consideration the different intestinal and metabolic fates between ZnO and CuSO4. Considering the optimized experimental environmental conditions of this study, it is possible that the negative effects of high ZnO levels on metabolism (see below) have overweighed its eventual positive effects (mainly local at the intestinal lumen) observed under more challenging environments (high pathogenic pressure). For CuSO4 that has mainly systemic actions, the influence of the experimental environment was likely less disturbing.
Zinc
High dietary ZnO levels decreased the expression of genes related to intestinal Zn absorption which was not efficient in proportionally reducing jejunum, liver, kidney, and serum Zn concentrations (Figures 1A and 2) that, in fact, sharply increased (five- and 12-fold in serum and liver, respectively) during the three weeks of supplementation compared to pre-treatment values (d21). Serum Zn concentrations averaged 4.1 mg/L in HZn groups at d42, which was reported to impaired performance in pigs [5], supporting the lower growth performance in HZn groups discussed above.
In LZn groups, liver Zn concentrations at d28 and 42 represented 71 and 66 per cent, respectively, of pre-treatment values, whereas serum values remained constant throughout the experimental period. Such results suggest that 100 mg ZnO/kg of diet during the first weeks post-weaning may not completely fulfil the piglets’ nutritional needs and, according to Hansen et al. [6], may increase the risk of diarrhea.
Copper
Piglets supplemented with HZn had greater jejunum Cu concentrations than LZn (Figure 2A), which may be attributed to an impairment of Cu efflux from intestinal cells caused by the activation of a mechanism responsible for trapping minerals in different organs.
This mechanism is controlled by an enzyme (metallothionein; MT) that is responsive to dietary Zn levels but that has a much greater affinity for Cu. In LZn groups, however, absorbed dietary Cu may have been released from intestinal cells with low interference of MT and reached the liver at different levels depending on the total amount of dietary Cu absorbed in LCu and HCu diets. The high expression of MT in liver of HZn animals, promoting greater trapping of Cu in hepatic cells, was expected to increase liver Cu concentrations but, contradictory, hepatic values were not far from those detected in LCu groups (Figure 2B), which likely did not receive enough dietary Cu (see discussed below).
Therefore, the high hepatic Zn content in HZn groups stimulated a greater MT expression in this tissue irrespective of hepatic Cu concentrations, which were limited either by the low Cu intake (LCu) or the significant sequestration of Cu within enterocytes by MTs in HZn groups. The greatest hepatic and serum concentrations of Cu in LZnHCu groups further support this reasoning.
Interestingly, hepatic Cu concentrations were lower than pre-treatment values (d21) from d28 not only for HZn but also for LCu groups (Figure 2B) whereas serum values (Figure 1B) were lower for all treatments.
Besides further supporting that HZn diets impair Cu metabolism independently of dietary Cu levels, it also indicates that 6 mg of Cu/kg of diet (LCu) is not sufficient to fulfil the post-weaning piglets’ requirements for this mineral.
High dietary Zn supplementation increased MT expression and Cu concentrations in kidney, which was reported as an indication of impaired Cu reabsorption after filtration in the kidney [7, 8], further contributing to decrease serum Cu levels.
Similarly to a previous study of our laboratory, serum and hepatic Cu concentrations were not higher than pre-treatment values (d21) in neither dietary treatment groups at any post-weaning time (d28, and 42).
Piglets treated with 100 mg Zn/kg of diet (LZn) had insignificant impacts on Cu parameters (unlike HZn groups), whereas HCu diets (130 mg/kg) contained more than 20-fold the NRC [1] recommendation for post-weaning piglets.
Therefore, although dietary ZnO levels have a decisive impact on Cu metabolism, it cannot be ruled out that other key factors are modulation Cu absorption/bioavailability in the post-weaning piglet.
Conclusions
During the first weeks post-weaning, piglets’ Zn requirements appear not to be satisfied by 100 mg ZnO/kg of diet.
In contrast, high dietary levels of ZnO were not efficiently regulated in different organs, resulting in remarkably high serum Zn concentrations that may likely have impaired piglets’ growth performance. Such high body Zn levels disturbed Cu metabolism significantly limiting hepatic and serum Cu concentrations.
However, independently of these major Zn effects, 6 mg Cu/kg of diet apparently did not meet the Cu requirements of post-weaning piglets.
A low dietary Zn/Cu ratio is desirable to avoid compromising piglets’ health by high systemic Zn concentrations and to avoid a potential Cu deficiency under long-term high dietary ZnO supplementation during the post-weaning period. BP
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