Effect of Replacing Lucerne Meal with Seriphium plumosum Meal in the Diet on Growth Performance, Methane Emission, In Vitro Rumen Fermentation Characteristics, and Rumen Microbial Community Composition of Nguni Steers

This study assessed the effects of replacing lucerne meal with Seriphium (S.) plumosum meal on growth performance, methane emissions, rumen fermentation characteristics, and rumen bacterial community composition of Nguni steers. Twenty-eight 22-month-old Nguni steers (average body weight 300 ± 10 kg) were randomly assigned to diets containing 0%, 10%, 20%, or 30% of S. plumosum meal in a completely randomized design. The diets were formulated to be isocaloric to meet the nutrient requirements of growing Nguni steers. Feed intake, nutrient digestibility, and live weight gain were measured over a 90-day feeding period. Methane emissions were recorded using a laser methane detector, while rumen fermentation characteristics were evaluated in vitro. Rumen microbial communities were characterized using 16S rRNA gene sequencing. Increasing S. plumosum inclusion had no effect (p > 0.05) on dry matter intake, nutrient digestibility, live weight gain, or methane emissions, indicating that replacement of lucerne meal did not compromise production performance. However, dietary treatments influenced (p < 0.05) gas production throughout the incubation period (3–48 h). The 10% diet (T2) produced the highest gas volumes, with values declining at 20% (T3) and 30% (T4). Cumulative gas production after 48 h followed a similar pattern. Significant linear, quadratic, and cubic responses (p < 0.01) indicated a non-linear response of gas production to increasing dietary inclusion of S. plumosum. In vitro organic matter digestibility, ammonia–nitrogen concentration, volatile fatty acid profiles, and the acetate-to-propionate ratio, demonstrated changes (p < 0.05) in rumen fermentation dynamics. Heatmap analysis of the rumen microbiota revealed clear shifts in bacterial community composition with increasing S. plumosum inclusion in the diet. Therefore, S. plumosum meal has potential as a locally available alternative forage resource for partial replacement of lucerne meal in Nguni steer diets.