Effects of supplementing a <i>Bacillus</i> -based direct-fed microbial on feed intake, apparent total-tract nutrient digestibility, growth rates, and metabolic responses of <i>Bos indicus</i> beef heifers during the dry and early vegetative seasons

Abstract This experiment evaluated intake, nutrient digestibility, growth rates, and metabolic responses of Bos indicus beef heifers fed Bacillus spp. when offered a low-quality tropical hay [≤5% crude protein (CP)] and then transitioned to lush pastures during the early vegetative stage. Nellore heifers (n = 48; 20 ± 1 mo) were ranked by initial body weight (BW; 312 ± 26.4 kg) and randomly assigned to Tifton 85 hay-based diet and a protein supplement offered at 0.30% BW with or without (CON; n = 24) the inclusion of a Bacillus-based direct-fed microbial (DFM; BAC; n = 24). Animals were housed individually throughout the experiment, resulting in 48 drylot pens. The experimental period lasted 140 days, while individual full BW and blood samples were collected every 28 days. In the last three days of each 28-day period, fecal grab samples were collected from all heifers to assess the apparent total-tract digestibility. After the dry season, heifers were transitioned into lush pastures for 15 days and offered the same treatments (total experimental period = 156 days). All data were analyzed with SAS (version 9.4). Dry matter (DM) availability was measured at the beginning and end of the grazing period, whereas BW and blood samples were collected over the 15-day period. During the dry season, no treatment effects were observed (P ≥ 0.20) for feed intake or ADG. Dry matter and CP digestibility were greater for BAC (P ≤ 0.05). Heifers fed BAC had greater blood urea-N (BUN) and plasma insulin-like growth factor-I (IGF-I; P ≤ 0.02), while mean serum glucose tended (P = 0.09) to be greater. Treatment × day effects (P &amp;lt; 0.01) were observed on BW change during the 15-day grazing season. The BAC-fed heifers tended to lose (P ≤ 0.09) more BW from day 0 to 1, but gained (P &amp;lt; 0.01) more BW from days 0 to 15 of the period. During the 15-day grazing period, heifers fed BAC had a greater overall BW change and average daily gain (ADG) when compared with CON cohorts (P &amp;lt; 0.01). No differences were observed for DM availability at the beginning of the grazing period (P = 0.61), whereas paddocks assigned to BAC heifers had less DM available after 15 days (P = 0.02). In summary, feeding Bacillus spp. increased DM and CP digestibility, serum hormones and metabolites associated with energy and protein metabolism in beef heifers fed a low-quality tropical hay, without effects on growth rates. When the heifers were transitioned to lush pastures for 15 days, Bacillus spp. supplementation improved the growth rate of the heifers, positively impacting the entire growth of the herd.