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Traditional starch-rich concentrates for milk-fed calves induces an acidotic ruminal environment even with small daily intakes. A new feeding regimen including use of low-starch concentrate was formulated and tested for impact on ruminal environment and -development in 2 to 10 wk old calves. Eight Holstein calves (48 ± 1 kg BW at birth) implanted with ruminal cannulaes (20 mm ID) at d 7 ± 1 of age were allocated to one of two feeding regimens. Calves on control treatment (CON) had free access to a barley-based concentrate (319 g starch/kg) from wk 1. Calves on alternative treatment (ALT) had no access to concentrate until wk 4 when they got free access to a low-starch concentrate (68 g starch/kg). All calves had free access to artificiallydried grass hay, and water and were fed skim-milk-based milk-replacer (4.6 L/d in wk 1 to 2, 6.4 L/d in wk 2 to 7, and 3.2 L/d in wk 7 to 8). Every wk, 8 ruminal samples/calf were collected from 0730 to 2400. There were no treatment effects on ADG (802 ± 35 g/d) or total solid feed DMI (concentrate + hay; 78 ± 4 kg). On ALT compared with CON, average ruminal pH was higher (6.39 vs. 6.04 ± 0.09, P < 0.05) and minimum ruminal pH was higher (6.14 vs. 5.57 ± 0.09, P < 0.01). A relatively large treatment x wk interaction was found for all VFA data. Especially from wk 2 to 4 the butyrate profile and the ruminal butyrate concentration was smaller (P < 0.001) for ALT (1 to 3 ± 1 mmol/L) compared with CON (6 to 9 ± 1 mmol/L). However, when concentrate was introduced to ALT calves ruminal butyrate increased to a higher (P < 0.001) level (12 to 19 ± 1 mmol/L) compared with CON (8 to 13 ± 1 mmol/L). No differences between treatments in ruminal mass, length, shape and aggregation of ruminal papillae were detected at wk 10. Our results show that it is possible to formulate rumen-friendly low-starch concentrates without compromising growth or ruminal development in milk-fed calves. [ABSTRACT FROM AUTHOR]