Effects of feeding <i>B. subtilis</i> and <i>B. licheniformis</i> on growth and carcass characteristics of feedlot lambs

This study investigated the effects of dietary Bacillus subtilis and Bacillus licheniformis supplements on the growth performance, carcass characteristics, dressing percentage, morbidity, and mortality of weaner lambs under commercial South African feedlot conditions. The blinded field trial included 649 Merino lambs, with an average age of four months and an average weight of 30 kg at the start of the trial. These lambs were randomly allocated to either a probiotic-supplemented diet or a control diet. Lambs were weighed, ear-tagged, and stratified by initial body weight before allocation to treatment groups. Lambs were initially allocated at 65 lambs per pen across 10 pens (five pens per treatment group, 325 lambs per treatment); however, one lamb died during transport prior to pen allocation, leaving a total of 649 lambs enrolled in the study. Probiotic supplementation did not significantly affect overall average daily gain, but differences were observed in interval growth rates between days 15 and 28. Lambs receiving probiotics had numerically higher feed intakes, but a descriptively higher feed conversion ratio compared to the controls. The probiotic treatment group also had a lower dressing percentage and higher incidences of low-grade rumen parakeratosis. No significant differences were observed between the groups for overall morbidity and mortality during the feeding period. These findings suggest that probiotic supplementation can improve growth during the first 28 days of the feeding period; however, the negative effects on rumen and lung health (bronchopneumonia and rumen parakeratosis) that developed in the probiotic-supplemented lambs during the feeding period should be noted. (Submitted 22 November 2025; Accepted 23 March 2026; Published 03 April 2026)