Dietary Zinc-Amino Acid Complex Does Not Affect Markers of Mammary Epithelial Integrity or Heat Stability of Milk in Mid-Lactating Cows

Supplying dietary zinc in excess of traditional requirements has clear impacts on the gut epithelium, but little research has explored whether similar impacts on the mammary epithelium may occur. Our objective was to determine the effects of supplemental Zn sources, in excess of minimal requirements, on markers of mammary epithelial integrity in blood and in milk as well as the heat stability of milk in mid-lactation cows. Twelve multiparous Holstein cows (132 ± 21 days in milk and 51 ± 3 kg/day milk) were blocked according to milk yield and enrolled in a replicated 3 × 3 Latin square experiment. Experimental periods were 21 days, with 17 days allowed for diet adaptation and 4 days for sampling. Treatment sequences were randomly assigned to animals and treatments were as follows: (1) 0.97 g Zn/day provided as ZnSO₄ (34.5 mg supplemental Zn/kg diet DM; 30-ZS), (2) 1.64 g Zn/day provided as ZnSO₄ (56.5 mg supplemental Zn/kg diet DM; 60-ZS), and (3) 0.55 g Zn/day provided as ZnSO₄ plus 1.13 g Zn/day provided as a zinc-methionine complex (58.2 mg supplemental Zn/kg diet DM; 60-ZM). Treatments were administered once daily as an oral bolus containing all supplemental trace minerals. Rumen-bypass methionine was also included in the 30-ZS and 60-ZS boluses to provide metabolizable methionine equivalent to that provided in 60-ZM rations. Milk samples were assessed for electrolytes, somatic cell transcript abundance of genes related to zinc metabolism, and heat coagulation time. Whole blood samples were analyzed for Na and K concentrations, and plasma samples were analyzed for lactose concentration. Cows fed 60-ZS or 60-ZM had greater zinc intake compared to 30-ZS. Dry matter intake and milk fat content tended to be greater in 60-ZS and 60-ZM cows compared to 30-ZS. Somatic cell linear score was similar among treatments. Treatments neither affected markers of mammary epithelial integrity in blood nor in milk of cows, including plasma concentration of lactose, milk concentrations of Na⁺ and K⁺, and SLC30A2 and CLU transcript abundance. Treatments had no effect on milk N fractions or heat coagulation time. This study provided no evidence that supplemental Zn above the established requirements can improve blood-milk epithelial barrier or heat stability of milk in healthy mid-lactation dairy cows.