The current NRC guidelines do not include recommendations for ?bre provision to dairy calves. Trials investigating the use of forage in starter have yielded inconsistent results. Because of these contradicting data, four trials were designed to examine how the dietary factors interactions may affect calf responses to forage supplementation. In addition, a fifth trial was conducted to investigate the effects of amount and type of solid feed provision on rumen development in veal calves. In the first experiment, thirty calves were assigned to different treatment consisting of control and experimental groups (contain 8 or 16 % alfalfa meal on DM basis) to determine the effects of different level of alfalfa meal on performance and rumen development in dairy calves. Results showed that starter intake, average daily gain and feed efficiency during the pre and post-weaning periods were not different among treatments. Alfalfa addition decreased corneum and epithelial layer thickness, but increased papillae length. In overall, results of this study indicated that alfalfa meal feeding had no negative effect on performance of dairy calves, also improved ruminal pH and rumen development in dairy calves. In the second experiment, fifty newborn dairy calves were used in a 2 × 2 factorial arrangement with the factors supplementation level (8% or 16% on DM basis) and particle size ( 2.92 mm or 5.04 mm as geometrical means) of alfalfa hay. In addition, a control group without alfalfa hay was used. Growth performance of alfalfa-fed calves did not differ from control calves, but alfalfa supplementation decreased corneum thickness of the rumen wall. Final body weight decreased when the level of long PS alfalfa hay increased from 8 to 16%, but increased when the level of medium PS alfalfa increased from 8 to 16%. Results, in general, indicate that adequate, but not excessive, physical stimulation is required for appropriate rumen development and growth of dairy calves. In the third experiment with a 2×3 factorial arrangement, sixty dairy calves were fed different grain sources (corn vs. barley) with different forage sources (alfalfa hay, corn silage or no-forage). Results showed that barley grain led to improved starter intake and ADG during the entire period, without negative effects on ruminal pH and dairy calves’ health. Corn silage compared with alfalfa hay and control calves increased BHBA at d 48, starter intake, ADG and final weight. However, ruminal pH and rumination time increased and non-nutritional behaviors decreased in alfalfa supplementation groups, but performance was not different with control calves. Contrary to our hypothesis, there was no interaction between forage and grain sources, however, the performance of dairy calves affected by the source of grain and forage. In the fourth experiment, the effects of, and interactions between, grains processing and forage provision were investigated on performance of dairy calves. The experiment was a 2×2 factorial arrangement with two methods of grains processing (finely ground vs. steam flake) each combined with or without corn silage provision. Starter intake, ADG and gain-to-feed ratio were not different between finely ground and steam flaked groups, although non-nutritional behavior decreased and TVFAs increased with steam flake feeding. Supplementation steam flake grains with corn silage improved starter intake and growth performance of dairy calves. Regardless of the grain processing method, the starter intake, ADG, and rumination time was greater in corn silage supplemented calves. In the fifth experiment, thirty-two male veal calves at 2 wk of age were randomly assigned to 4 treatments in a 2 × 2 factorial arrangement with SF level (SF; high vs. low) and forage to concentrate ratio (F:C; 50:50 vs. 20:80) as factors. At the end of the experiment, all calves were euthanized and rumen tissue sampled for rumen epithelial development measurements. The greater SF level increased corneum thickness, epithelium thickness, papillae length and muscle thickness, however, higher F:C ratio enhanced physical properties of the rumen as muscle and rumen wall thicknesses. The mRNA expression of genes involved in VFA metabolism (bPCCB and HMGCS2), traort of VFA (MCT-1, MCT-4) and protons (bSLC9A1, bSLC9A2), and urea traorters (UT-B and AQP7) increased with greater SF intake. However, a down-regulation in genes involved in elongation of fatty acids (bELOVL6, bELOVL7) and GPR43 was observed in high SF calves. In general, our studies demonstrate that rumen development and growth in dairy calves is a complex process, which may influence by nutritional factors interactions. Key words : forage, dairy calves, processing, grain, starter