Diets that promote the production of SCFA stimulate the ruminal development. Feeding hay may decrease energy intake of calves while whole corn grain (WCG) may be an alternative fiber source, with no TDN intake dilution, improving C4 and C3 concentration and performance. Thirty-nine Holstein calves were used in a randomized block design, considering sex, birth date and weight at 21d of age (48.7 ± 0.82 kg), when the supplementation started, to compare: 1) Control: starter concentrate (24% CP, 14% NDF, 46% NFC); 2) Hay: starter concentrate, supplemented with free choice chopped Tifton hay; and 3) Corn: starter concentrate supplemented with free choice WCG. Animals were fed 6 L/d until 21d and then 4L/d of whole milk until gradual weaning at 56d of age. The rumen fluid was collected 2 h after feeding at 6th and 8th week. Data were analyzed using the MIXED procedure of SAS and the means were compared using Tukey test (P < 0.05). Hay supplementation increased total DMI (Control: 365.5^b; Hay: 619.4^a; Corn: 428.3^b g/d), without affecting the NFC intake. There was a diet and age interaction, in which hay supplementation increased NDF intake after the 5th week. When compared with WCG, free access to hay promoted an increase in concentrate DMI (Control: 366.3^ab; Hay: 577.7^a; Corn: 320.6^b g/d), NDF (Control: 51.2^ab; Hay: 79.1^a; Corn: 44.3^b g/d) and NFC intake (Control: 170.8^ab; Hay: 265.4^a; Corn: 148.1^b g/d). Therefore, calves supplemented with hay presented higher ADG (Control: 483.3^b; Hay: 647.8^a; Corn: 486.2^b g) and final BW (Control: 61.1^b; Hay, 68.9^a; Corn: 62.9^b kg). Hay supplementation increased C2 (Control: 49.0^ab; Hay: 52.0^a; Corn: 46.4^b mM/100mM), compared with the WCG. When compared with hay, free access to whole corn grain promoted an increase in C3 (Control: 36.7^ab; Hay: 33.0^b; Corn: 37.1^a mM/100mM), and decreased C2:C3 ratio (Control: 1.36^ab; Hay: 1.60^b; Corn: 1.28^a). However, C4, total SCFA and ruminal pH were unaffected by solid diets, suggesting that WCG did not cause acidosis. In summary, the inclusion of WCG improves the energy efficiency in milk-fed calves without compromising rumen health. However, the hay inclusion can stimulate an early solid diet intake, improving performance.