The aim of the study was to assess the average daily gain (ADG), the growth rate of linear measurements and cortical index changes of the third metacarpus (ICOR) of foals from 173 ± 8 to 363 ± 12 day of life. Sixteen Mangalarga Marchador weaned colts were divided by age and allotted randomly to one of four dietary treatments (n = 4): A – basal diet (Cynodon pasture + 1.2 kg · 100 kg^–1 body weight (BW) of concentrate + water and mineral mix ad libitum); B – basal diet + 5 g · day^–1 of yeast culture (109 cfu · g^–1 Saccharomyces cerevisiae Sc47); C – basal diet + 2 g · day^–1 of bacteria (108 cfu · g^–1 Bifidobacterium bifidum, Enterococcus faecium, Lactobacillus acidophilus, Lactobacillus plantarum) and yeast culture (105 cfu · g^–1 S. cerevisiae); and D – basal diet + 544 FTU of phytase per kg of concentrate per day. Data was analysed as repeated measures and fixed effects of age, treatment and their interactions accordingly to SAS® procedures. The additives did not affect the ADG or growth rate of linear measurements (P > 0.05); however the effect of age was observed. Furthermore, the ICOR was greater in 273 day-old foals fed diet B, which may suggest an increase in mineral uptake and bone development. So it can be concluded that a 100-day diet supplementation with S. cerevisiae Sc47 may increase bone development of weaned foals, however more studies are necessary to assure the beneficial effect of yeast addition on equine growth rate.