Forage and straw inclusion in calf grain starter has beneficial effects on rumen development, average daily gain (ADG), and dry matter intake of dairy calves. However, the optimum age when roughage should be included in the starter diet has not yet been defined. To determine optimum time of straw inclusion, 40 Holstein calves (36.7 ± 5.0 kg) were assigned at 3 d of age to 1 of 4 treatments in a completely randomized block design lasting until weaning at 60 d. Treatments (% of dry matter) were: control starter without wheat straw (WS) inclusion until d 60 (CON); 90% calf starter concentrate (CSC) + 10% chopped WS offered from d 3 to d 60 (WS3); 90% CSC + 10% chopped WS offered from d 21 to d 60 (WS21); 90% CSC + 10% chopped WS offered from d 41 to d 60 (WS41). All WS was mixed with the starter, and all calves received 8 to 2 L of whole milk daily in a step up-step down manner according to farm management protocol. Total feed consumption and body weight (BW) gain were monitored until weaning at d 60. Effect of diet was analyzed within specific time periods using the JMP version of SAS with the effect of WS evaluated as an orthongnal contrast (WS versus no WS) at each period. Solid food consumption at d 60 (827, 947,1064 and 1151 ± 24 g/d for CON, WS3, WS21, and WS41, respectively, was different among treatments (P < 0.05 with inclusion of WS positive (P < 0.001). The 60 d ADG (0.74, 0.78, 1.00 and 0.94 ± 0.03 kg/d for the respective treatments with CON and WS3 being lower than WS21 and WS41 (P < 0.05) and with a positive WS effect (P < 0.001). The 60 d BW followed a similar pattern (69.8, 69.9, 79.8, and 73.2 ± 1.9 kg/d for treatments CON, WS3, WS21 and WS41, respectively (P < 0.05) and inclusion of WS still positive (P < 0.05). Rumen β-hydroxy butyric acid was not affected by treatments. The days that calves experienced a negative health event were also not affected by treatment. In conclusion, WS inclusion at d 21 of age is recommended based on weaning BW, DMI and A
