Effects of Protein Supplementation and Microbial Inoculation on Fermentation Quality, Aerobic Stability, and In Vitro Digestibility of Maize Silage

The study evaluated the effects of replacing soybean meal (SBM) with marula oilcake (MOC) at equal inclusion (10% fresh weight) levels in whole-crop maize silage treated with or without lactic acid bacteria inoculants on fermentation characteristics, nutritive value, aerobic stability, and in vitro nutrient degradability. Maize was ensiled with SBM or MOC in a non-iso-nitrogenous 2 × 3 factorial design and either inoculated or uninoculated with Lalsil Fresh or Sil-All 4×4 for 90 days. Protein sources differed significantly (p < 0.05). The MOC showed high DM, EE, GE, and ADL, whereas SBM had high CP, ash, and IVOMD. Fibre fractions (aNDF and ADF) were similar (p > 0.05). The SBM control showed significantly high (p < 0.05) LA, NH3-N, CP, IVOMD, propionic acid, and early gas production, indicating efficient fermentation. The SBM + Lalsil maintained low pH, and early OM, CP, and GE degradability. The SBM + Sil-All achieved the highest (p < 0.05) OM, NDF, and ADF degradability and acetic acid production than other treatments. The MOC control showed low (p < 0.05) pH, high fibre and GE, reduced butyric acid, and low 48 h gas production, indicating slower fermentation but improved stability. The MOC + Lalsil had high (p < 0.05) DM, low CO2 and yeasts and moulds, and the highest (p < 0.05) CP degradability, propionic acid, and peak gas production at 12 h. The MOC + Sil-All showed high (p < 0.05) GE and WSC with peak GE degradation at 12 h, but low NDF degradability and reduced gas production. Overall, SBM improved degradability and fermentation efficiency, particularly with Sil-All, whereas MOC enhanced energy density and aerobic stability, with Lalsil optimising protein utilisation. Matching inoculant type to protein source is essential to optimise silage quality and rumen fermentation. Further research should assess different inoculant inclusion rates and include a maize-only control, and evaluate protein source inclusion under iso-nitrogenous conditions to allow more accurate comparisons.