The effects of organic zinc supplementation on production performance and health of high producing dairy cows

Abstract

Sustainable dairy production equally demands improvements in milk production as well as the well-being of dairy cows. In that context, zinc (Zn), an essential trace mineral, is hypothesized to improve both of those aspects as it serves as an activator of a large number of enzymes and protein factors required for nutrient metabolism, immune defense, and antioxidant systems. In recent decades, supplementation of organic Zn over inorganic Zn in the diet for lactating dairy cows has emerged as a result of the greater bioavailability of organic Zn compared to its inorganic counterparts. Among organic Zn, Zn-amino acid complexes (Zn-AA) are widely used in the livestock industry. There have been several studies comparing Zn-AA vs. inorganic Zn (ZnO or ZnSO4) related to dry matter intake (DMI) and milk production of dairy cows published in the literature. As a part of the literature review in this thesis, we performed a meta-analysis using data from nine such studies and the metafor package of the R statistical software. The mean difference (MD, MD = mean response for organic Zn – mean response for inorganic Zn) was used as the effect size of interest. Random effect models were employed to summarize the overall effect size and estimate the heterogeneity of the effect size between the studies. Mixed-effect models were employed then to explore the factors responsible for the heterogeneity. The random-effect model analysis revealed 0.42 kg/d, 0.57 kg/d, 18 g/d, 0.04%, and 23 g/d increases (P < 0.10) in DMI, milk yield, milk protein yield, milk protein %, and milk fat yield for complete replacement of inorganic Zn in the diet with Zn-AA. The added Zn concentration, stage of lactation, and duration of feeding organic Zn vs. inorganic Zn explained 24 to 72% of the heterogeneity of those effect sizes. The effect sizes of somatic cell count in milk (SCC) was significantly heterogeneous (P < 0.01) but the amount of inorganic Zn (mg/kg DM) being replaced with Zn-AA explained 97% of the heterogeneity of the effect size and had a positive relationship with it. Based on this relationship and the literature demonstrating positive relationships of SCC with oxidative stress of dairy cows, we hypothesized that Zn-AA supplementation would improve the immune response including immunoglobulin production and antioxidant capacity of high-producing dairy cows. To test this hypothesis, we determined the effects of increasing added Zn concentration in the diet from 76 to 96 mg/kg DM by using a Zn-methionine complex (Zn-Met) on SCC and immunoglobulin, antioxidant enzymes, and oxidative stress marker concentrations in the blood beside the effects on DMI and milk production of high producing dairy cows. In this experiment, 12 Holstein dairy cows (67 ± 2.5 days in milk) were randomly assigned to 1) a total mixed ration (CTL) containing 76 mg/kg of dry matter (kg DM) of Zn-Met (ZINPRO®, Zinpro Corporation, Eden Prairie, MN, n = 6) and 2) CTL top-dressed with extra 20 mg/kg DM of a new Zn-Met (+Zn-Met; Amipro Zn®, Debon Agri-tech Group, Shanghai, China, n = 6) for 70 d. Dry matter intake and milk yield were recorded daily and milk component concentrations and SCC were determined biweekly. Concentrations of Zn in blood and milk, and concentrations of reduced (GSH) and oxidized (GSSG) glutathione, malondialdehyde (MDA), catalase (CAT), superoxide dismutase (SOD), and immunoglobulins A (IgA), G (IgG), and M (IgM) in the blood were analyzed on d 0, 35, and 70. Treatment effects on all response variables except SCC were analyzed using the MIXED procedure of SAS with fixed effects of treatment, time, parity, treatment × time interaction, covariate effects of baseline measurements, and random effect of the cow. The SCC were analyzed using the GLIMMIX procedure of SAS with the Poisson distribution. Dry matter intake decreased by 1.2 kg/d for +Zn-Met compared to CTL throughout the study. The CTL had a greater milk yield (2.0 kg/d) than +Zn-Met only during the first 35 d, whereas the milk yield of +Zn-Met was 1.25 kg/d greater than CTL during the last 35 d. Milk protein and fat percentages, and fat yield were not affected by the Zn supplement. There was an interaction between treatment and time on milk protein yield as +Zn-Met had lower and greater milk protein yield than CTL during the first and last 35 d of the study, respectively. Zinc concentration in milk tended to be greater for +Zn-Met relative to CTL (4.48 vs. 4.06 ppm) and the Zn supplement tended to decrease SCC throughout the study. Serum Zn was similar between treatments on d 35 but tended to be higher for +Zn-Met on d 70 (1.06 vs. 0.81 ppm). The +Zn-Met was associated with lower plasma GSH: GSSG on d 35 and lower serum SOD on d 70 relative to CTL. The +Zn-Met did not affect the concentrations of IgA, IgG, IgM, MDA, and CAT. In summary, increasing added Zn content in the diet (76 to 96 mg/kg DM) using Zn-Met improved milk yield and SCC of dairy cows even though DMI decreased and some blood markers indicated increased whole-body oxidative stress. Future experiments designed to address the circadian rhythm of oxidative stress and day-to-day variability of oxidative stress markers in the blood would help understand the true effects of Zn supplementation on antioxidant capacity and oxidative stress of dairy cows. Measuring antioxidant enzyme activity beside the concentrations would also help capture those effects correctly. The Zn-Met product, Aminopro Zn® used in the present study seems to negatively affect the palatability. Further investigations with greater sample sizes is warranted to draw robust conclusions about the effects of Aminopro Zn® on DMI and the production performance of lactating dairy cows.