Bacillus spp. supplementation promotes feed efficiency and alters metabolic responses of preweaning Angus × Holstein calves.

This experiment evaluated the effects of feeding a Bacillus-based direct-fed microbial (DFM) on the growth rates and metabolic responses of Angus × Holstein crossbred calves. After a 1-wk adaptation period, 80 calves were blocked by sex, initial BW (47 ± 5.1 kg, average ± SD), and age (15 ± 3.8 d, average ± SD) into the following treatments: (1) milk replacer (MR) with the addition of maltodextrin (CON; n = 40; 15 females and 25 males), or (2) MR with the addition of 1 g of a Bacillus-based DFM (BAC; n = 40; 16 females and 24 males) containing Bacillus paralicheniformis 809 and Bacillus subtilis 810 and 9 g of maltodextrin/head per day. Treatments were offered for 63 d and animals were raised in 8 pens of 10 calves each. The dose of BAC was applied with the first MR feeding through an automatic milk feeder, and calves were fed increasing amounts of MR until reaching 1.74 kg of MR at 13 d; this amount was maintained until 42 d, and it was gradually decreased until 63 d, when calves were completely weaned. Animals were followed one more week after weaning. Blood samples were collected from all calves at the beginning and at weaning for determination of different markers of growth, metabolism, and immunity. For all statistical analyses, calf was considered the experimental unit. The proportion of calves with fecal score >1 was greater in CON during wk 4 and 7 of the study, but greater for BAC than CON at wk 6. Calves from CON had a greater ADG on wk 8, whereas opposite results were observed on wk 9. For MR intake, CON calves had a greater intake on wk 1 and 2 than BAC, whereas total DMI was greater for CON on wk 2, but greater on wk 9 for BAC. Lastly, the same treatment × week interaction tended to be observed on feed efficiency (FE), as BAC calves had a greater FE on wk 1 and 2 than CON and tended to be greater on wk 3. At weaning, serum gamma-glutamyl transferase, total serum protein, and nonesterified fatty acid concentrations were greater in BAC, whereas serum glucose and haptoglobin concentrations tended to be greater and serum lactate concentration tended to be lower in BAC-fed calves. In summary, Bacillus spp. supplementation positively affected the FE of preweaning Angus × Holstein calves while also modulating their metabolic and immune responses. Nonetheless, additional studies are warranted to understand the metabolic pathways leading to such responses, if any.