Effects of partial substitution of barley with maize and sugar beet pulp on growth performance, rumen fermentation and microbial diversity shift of beef calves during transition from a milk and pasture regimen to a high-concentrate diet

Effects of partial substitution of barley with maize and beet pulp (BP) on animal performance, rumen fermentation and rumen microbial populations were evaluated in thirty-nine newly weaned calves transitioned from a milk and pasture regime to a high-concentrate ration. Diets were: a barley-based growing diet (C); and two diets in which barley was partially replaced with maize and 10% BP (BP10) or 22% BP (BP22) on as fed basis. Wheat straw was offered ad libitum in all cases. Three adaptation protocols were followed: in Protocol 1 animals were abruptly shifted to the C diet, whereas calves in Protocols 2 and 3 were gradually adapted and received diets BP10 and BP22, respectively, for 10d; thereafter a 50:50 mixture of their diet and diet C until day 14, and finally, from day 15, diet C. All animals were slaughtered at a target live weight (LW) of ca. 500 kg. The experiment had then three phases (Ph): Ph1 (0-10d), Ph2 (11-14d) and Ph3 (15d-slaughter). Concentrate intake and proportion of straw in the diet were affected (P<0.01) by the interaction dietary protocol by phase, showing animals in Protocol 3 the lowest concentrate intake and the highest straw proportion in Ph2. Daily gains, LW at the end of each phase, dressing percentage (59 +/- 0.4%, on average) and total concentrate intake along the experiment (995 +/- 21.3 kg DM, on average) were similar (P>0.05) among adaptation protocols. Daily rumen pH, and concentrations of lactate and volatile fatty acids were not affected by protocol. At the end of the trial, protozoa persisted in the rumen of all calves with an average concentration of 5.89 log cells/mL. Diversity dropped throughout the study; most protozoa belonged to genera Entodinium (97.5%) and Isotricha (1.7%), with minor presence of Dasytricha (0.05%), Epidinium (0.43%) and Subfamily Diplodiniinae (0.23%). Adaptation protocol had no effect on rumen bacterial population or diversity; however, both were affected by day of sampling indicating a shift in bacterial community in response to the concentrate inclusion. Although diversity dropped with concentrate inclusion, there was an increase after d10. In conclusion, the application of different protocols to adapt beef cattle to high-concentrate diets had no effect on animal performance and rumen fermentation which were not negatively affected by the abrupt transition to the high-concentrate diet. This suggests that calves might cope with those feeding conditions probably due to the microbial adaptions to a new rumen environment.