The current study evaluates the effects of particle size (physical factor) and starch availability rate (chemical factor) on the response of young dairy calves fed high-starch, forage-free starter diets. A 2 × 2 factorial design with two grain sources (corn grain [CG] and barley grain [BG]) and two delivery forms (whole grain and steam-flaked grain) was used to assess growth performance, rumen fermentation, and microbial protein synthesis (MPS). The MPS was estimated based on the urinary excretion of purine derivatives (PD). Forty-eight female Holstein calves (3 days old, with an average initial body weight of 40.5 ± 1.93 kg SD) were randomly assigned to one of the following treatments: whole corn grain (WCG), steam-flaked corn grain (SFC), whole barley grain (WBG), or steam-flaked barley grain (SFB) (n = 12 per treatment). The milk feeding schedule was identical among treatments until weaning (day 53), after which calves received their respective experimental diets until day 73. Grain type did not affect starter intake, average daily gain (ADG), or feed efficiency in dairy calves. Calves fed the BG diet had greater ruminal butyrate concentrations (P = 0.009) and lower ruminal acetate concentrations (P = 0.029) compared to those fed the CG diet. Regarding processing method, flaking the grain, compared to feeding whole grain, improved starter intake during the pre-weaning period (P = 0.041). Calves fed the SFB diet had greater ADG compared to other treatments (P = 0.039) during the pre-weaning period and, accordingly, had the greatest feed efficiency (P = 0.048) and withers height (P = 0.049) during the same period. The concentration of total volatile fatty acids (VFA) was greater when calves were fed flaked grains compared to whole grains (P < 0.05). The greatest urinary excretions of allantoin and PD were observed in calves fed the SFB diet compared to the other diets (P < 0.05). Feeding SFB resulted in the greatest urinary urea nitrogen concentration, indicating the lowest nitrogen efficiency (P = 0.037). Feeding whole grains reduced blood glucose and beta-hydroxybutyrate concentrations but increased blood urea nitrogen concentration compared to steam-flaked grains (P < 0.05). Steam-flaked barley yielded greater ADG than whole barley. In conclusion, under the conditions of the current study, although the particle size distribution was relatively similar between steam-flaked grains and whole grains, the greater fermentation rate in steam-flaked grains can improve animal growth performance, which is mostly related to optimized rumen fermentation and MPS. Our findings indicated that the rate of starch availability can be more crucial than grain particle size in young calves. The effect of different processing methods on ruminal microbial species activity needs further evaluation in future studies.
