Meta-analysis of the effects of organic and inorganic zinc supplementation on performance traits and diarrhea incidence in suckling calves

This meta-analysis aimed to investigate and compare the effects of organic and inorganic zinc supplementation on performance traits and diarrhea incidence in suckling calves. A comprehensive literature search identified 15 eligible studies (<strong>published between</strong> 2019 to 2024), providing data for 44 comparisons on body weight (BW), 70 on average daily gain (ADG), 66 on total dry matter intake (TDMI), 34 on starter intake, 53 on feed conversion ratio (FCR), and 34 on diarrhea incidence. A multilevel random-effects model was employed to estimate the effect size of zinc supplementation, with the effect size expressed as the standardized mean difference (SMD). Publication bias was assessed using funnel plots and Egger's test. Data from the 15 selected studies were analyzed using Stata software (version 18), and heterogeneity among studies was evaluated using the Q- test and the I-squared (I²) statistic. The results demonstrated that both organic (SMD = 0.64, <italic>p </italic>&lt; 0.05) and inorganic (SMD = 0.72, <italic>p </italic>&lt; 0.05) zinc significantly improved ADG in suckling calves. The highest BW was observed in calves supplemented with organic zinc (SMD = 0.58, <italic>p </italic>&lt; 0.05). Organic zinc also significantly increased starter intake compared to inorganic zinc (SMD = 0.40, <italic>p</italic> &lt; 0.05). Both forms of zinc increased TDMI in suckling calves (<italic>p </italic>&lt; 0.05). A significant reduction in diarrhea incidence (measured as percentage of diarrheic calves in each group) was observed with both zinc sources (<italic>p</italic> &lt; 0.05). Multivariate meta-regression analysis showed that experimental duration and zinc supplementation method were significant sources of heterogeneity for starter intake, ADG, and FCR (<italic>p</italic> &lt; 0.05). In conclusion, this meta-analysis suggests that organic zinc supplementation has a more profound effect on improving performance traits in pre-weaning suckling calves compared to inorganic zinc sources. Additionally, zinc supplementation (regardless of form) effectively reduces diarrhea incidence in suckling calves. These findings underscore the importance of ensuring adequate zinc levels in the diets of calves to support gastrointestinal health and overall performance.