It has been suggested that Zn can improve growth and health status when it supplemented during pre-weaning; however, the optimal time of supplementation has not well elucidated in rearing Holstein calves. Therefore, the aim of this study was to evaluate an organic source of Zn on the growth performance and health status of Holstein calves. Three treatments of 24 calves each were fed whole milk supplemented with no Zn (CON), ~100 mg/d of organic Zn from d 3 to 80 (Zn3), or ~100 mg/d of organic Zn from d 50 to 80 (Zn50). The source of Zn was Zn-glycine. Although total dry matter intake (DMI) was not affected by treatments during the first month of life, Zn supplement led to greater DMI during the second month (P < 0.02) and throughout the study compared to CON group; since Zn50 calves showed greater DMI from day 51 to day 80 of trial, the overall DMI was greatest for Zn50, followed by Zn3 and CON. From d 30 to weaning, Zn3 calves had greater (P = 0.04) average daily gain (ADG) than CON and Zn50, and tended (P = 0.06) to had a greater overall ADG. There were no effects of treatments on gain efficiency. Final wither height was greater in Zn3 compared to compared to others. Although overall β‑hydroxy butyric acid and alkaline phosphatase did not change among treatments, circulating Zn and glucose were significantly increased by Zn supplementation from d 3 (Zn3). The days with loose fecal scores were greatly decreased in Zn3 calves compared to others throughout the study. In general, the results showed that under the conditions of the current experiment, supplementing with organic Zn tended to have beneficial effects on DMI, ADG during the preweaning period but not the postweaning period; the current results also indicated preweaning fecal score was significantly improved when the calves were fed Zn from d 3.
