| Summary: | This study aimed to determine the protein and carbohydrate fractions as well as the in situ rumen degradability of <i>Brachiaria decumbens</i> silage (BDS) supplemented with soybean hulls. Five soybean hull inclusion levels were used: 0, 10, 20, 30, and 40% of the fresh matter of <i>B. decumbens</i> grass, distributed into a completely randomized design with five replications. The inclusion of soybean hulls caused a linear decrease (<i>p</i> < 0.001) in carbohydrate fractions A + B1 and a linear increase (<i>p</i> < 0.001) in carbohydrate fraction C. The percentage of non-protein nitrogen fraction increased linearly (<i>p</i> < 0.001), but the nitrogen fractions B<sub>1</sub> + B<sub>2</sub> and B<sub>3</sub> presented a negative quadratic effect (<i>p</i> < 0.01) with soybean hull level and fraction C presented a linear decrease (<i>p</i> < 0.001). The dry matter (DM) degradability of soluble fraction (A) and the undigestible DM decreased linearly (<i>p</i> < 0.01) with the soybean hull level. The potentially degradable water-insoluble portion (DM fraction B) and degradability rate (c) of the DM fraction B increased linearly (<i>p</i> < 0.001) with soybean hull level. The crude protein (CP) fraction A presented a linear increase (<i>p</i> < 0.001) with soybean hull inclusion; however, soybean hull levels caused a linear decrease (<i>p</i> < 0.001) in the CP level of fraction B. The degradable insoluble fraction of NDF (D) of the silage increased linearly (<i>p</i> < 0.001) and the indigestible NDF fraction of the silage was linearly decreased with the soybean hull level (<i>p</i> < 0.001). The inclusion of intermediate levels (20–30%) of soybean hulls provided better protein and carbohydrate fractions and better quality of BDS.
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