The used dosage in the current study was selected according to the survey of Kehoe et al. ; they used 0.5 to 0.6 mg/d of NU. They also concluded that the yeast cell contents compared to pure NU could result in better performance ; because of that, the yeast NU was supplemented in the current study.
According to Table 2, there were no differences in initial BW, wither or hip height among experimental treatments. Although the time effect was significant and DMI increased with the advancement in the study, total DMI (milk DM + starter DM) during pre-weaning was similar among treatments. Nevertheless, a tendency for the day by treatment interaction (P = 0.10) showed that NU supplemented calves more rapidly increased starter intake compared with the control group. Accordingly, pre-weaning ADG and BW at d 30 and 55 as well as feed efficiency were not different for experimental groups. Treatments had no effect on post-weaning ADG and feed efficiency, but starter intake was linearly increased as NU was supplemented (P < 0.04); because of that, the final BW linearly improved in NU supplemented calves (P < 0.02). Similar to ADG measures, hip and wither heights were not affected by treatment during the pre- and post-weaning periods.
In contrast to our hypothesis, DMI and growth performance were not affected by NU supplementation during pre-weaning period. Although Kehoe et al.  did not observe any difference in feed intake and efficiency between treatments over a 6-wk period, they reported that the control group tended to consume less starter during wk 6 compared with the yeast-derived NU supplemented calves. Similar to the current study, rats fed a regular diet without extra supplementation compared with rats fed dietary nucleotides showed no declined growth rates . In malnourished children, nucleotide intake was shown to enhance growth in weight, length, and head circumference in infants born small  and to increase biomarkers that could influence catch-up growth . Hill et al.  also found no effect of NU in milk replacer on pre-weaning DMI. In contrast with the current study, however, Hill et al.  reported the decreased pre-weaning ADG and feed efficiency probably because NU was supplemented at a very high dosage and as an N source in milk replacer. Unexpectedly, post-weaning starter intake linearly increased in NU supplemented groups without any change in ADG or feed efficiency. Kehoe et al.  and Hill et al.  indicated that post-weaning feed intake was not affected by nucleotide treatment. Considering the more tendency of NU fed calves to consume more starter when they arrived to weaning, it appears that the beneficial effect of NU on starter intake has postponed to the post-weaning period. Wood et al.  suggested that weaning may disrupt the permeability of the GIT that diminishes with age. On the other hand, it has been proposed that NU can improve intestinal epithelium repair, gut development [16, 17] and intestinal environment . Therefore, it appears that the intestine environment has been improved and, in turn, has led to better intake during post-weaning period. Recently, Adab et al.  also showed that Zn-glycine (which has been shown to improve small intestinal integrity) results in increased DMI around weaning and post-weaning period. Because of the greater post-weaning DMI, the heavier final BW in NU fed calves was not surprising. Król  also showed higher final BW in claves fed yeast nucleotides in milk replacer, which is in line with the current results.
The day with fecal score ≥ 3 tended to linearly (P = 0.10) decreased with NU supplement throughout the study; this was mostly because of reduced days with diarrhea during the first month of life (P < 0.05). The diarrhea is one of the most causes of mortality and morbidity in pre-weaning period (which can cause the growth performance to be disrupted; ) and it has been found that NU nucleotides improve intestinal maturation  and aid recovery from diarrhea [22, 23].
Fewer days with the abnormal scores in NU fed calves could confirm their better health condition. In line with the current survey, days with abnormal fecal score was linearly decreased when NU was supplemented in Hill et al.  study during the pre-weaning period and throughout the study; the reductions in days with abnormal fecal scores were attributed to poor digestion of NU, which might have increase fecal output of solids. Similar to our results, Król  observed that the fecal score was worse in claves fed yeast nucleotides in milk replacer during the first month of life. Fecal scores were not influenced by treatment in other study  probably because of low dosage and the source of NU; however, in that study treatment by week interaction revealed that control group (calves without any additive) had higher fecal scores during wk 2, 3 and 4 compared with NU-treated calves.
In line with the current results, days with medical treatments (Table 3) did not differ when NU was fed at 0, 10, or 20% of milk replacer DM . It was expected that days with medical treatments would decrease because days with loose feces had reduced. It has been shown that dietary NU can affect immune function and may have beneficial effects on gastrointestinal tract growth and maturation , probably affecting medical treatment days. Jiao et al.  showed that sows receiving nucleotides had increased fecal Lactobacillus counts and decreased Escherichia coli counts at weaning day; however, they found no difference in fecal score and diarrhea in piglets and concluded that the nucleotides could influence intestinal health and have positive effects on excreta microflora in sows at weaning day without no impact on medical treatment days.
Although the number of used drug and treatment bouts were not different among groups (Table 3), in control group 4 heads out of 10 claves received serum therapy during pre-weaning period while only 2 out of 10 calves in each NU treated group were subjected to serum therapy (Table 3). This is very important based on the cost of current therapy and the long-term effect of the therapy early in life on future productive performance of calves. Heinrichs and Heinrichs  concluded days of illness and days treated before 4 mo had significant effects on first-lactation production of Holsteins.
Hip width linearly decreased when NU was increased in milk replacer ; less feed intake was the main cause of declined hip width in study of Hill et al. . Furthermore, hip and withers heights were not affected by treatments in other study . It appears that the NU effects on skeletal growth measurements is minor and the principal place on which NU has some effects is the small intestine, as it was observed in fewer days with abnormal fecal scores.