Evaluation of urea–molasses-treated rice straw as a partial replacement for Brachiaria mutica in diets of growing Boer crossbred goats

This study evaluated the effects of diets formulated with different levels of urea–molasses-treated rice straw (UMS) as a partial substitute for para grass in growing Boer crossbred goats under Mekong Delta feeding conditions. A 5 × 5 Latin square design was applied with five dietary treatments, five periods, and five goats (Boer × Bach Thao). Experimental diets were formulated with UMS replacing para grass at 0, 25, 50, 75, and 100% of dry matter (UMS0–UMS100). Increasing UMS inclusion linearly reduced nutrient intake; goats fed UMS100 had the lowest dry matter, organic matter, neutral detergent fibre, acid detergent fibre, and hemicellulose intakes (p < 0.05). Overall rumen fermentation variables did not differ among diets (p > 0.05); however, higher UMS levels decreased ruminal pH and increased NH3–N concentrations before feeding and at 3 h post-feeding (Linear, p < 0.05). Apparent digestibility of DM, OM, crude protein, and hemicellulose increased with UMS level (p < 0.05), whereas total digestible nutrient intake was unchanged (p > 0.05). Growth observations suggested lower ADG and poorer feed conversion values at high UMS inclusion levels (p < 0.05) and did not differ between UMS0 and UMS25. These findings suggest that diets formulated with moderate UMS replacement levels may provide a practical supplementary roughage strategy during periods of forage shortage under Mekong Delta conditions.