Intake, digestibility and nitrogen utilization of three tropical tree legumes: I. As sole feeds compared to Asystasia intrusa and Brachiaria brizantha

The tropical tree legumes Paraserianthes falcataria, Gliricidia sepium, and Calliandra calothyrsus were fed to ram lambs to evaluate their potential as feeds. Dry matter intake, digestibility of dry matter, neutral detergent fiber and nitrogen, and digestible energy content were determined through a digestion study. The herbaceous dicot Asystasia intrusa was included as an underutilized source of nitrogen and Brachiaria brizantha was included as a standard tropical (C4) grass. Of the tree legumes, C. calothyrsus had the highest level of soluble phenolics (SPHE), averaging 38% of dry matter, and soluble proanthocyanidins (SPRO), averaging 13.7 absorbance units per gram (AU g−1) of dry matter. P. falcataria was intermediate, averaging 15% SPHE and 4.8 AU g−1 SPRO, with G. sepium the lowest, with 5% SPHE and 0.4 AU g−1 SPRO. Dry matter intake (percent of body weight) was lowest for C. calothyrsus-fed lambs, averaging 2.0%, compared with 3.2% for P. falcataria and 2.5% for G. sepium. Intakes were similar for A. intrusa and B. brizantha, averaging 2.6%. C. calothyrsus also had the lowest dry matter digestibility, averaging 55%, compared with 61% for P. falcataria and 63% for G. sepium, which were similar. Highest dry matter digestibility was obtained for A. intrusa, averaging 72%, and B. brizantha, averaging 65%. Forages had similar rank for neutral detergent fiber digestibility. Proanthocyanidins in the tree legumes may have bound with cell wall carbohydrates, resulting in a reduction in dry matter and NDF digestibilities. Digestible energy (kcal g−1) was highest for G. sepium, averaging 3.0, and ranged from 2.6 to 2.7 kcal g−1 for C. calothyrsus, P. falcataria, A. intrusa and B. brizantha. Fecal N was higher from the lambs fed tree legumes (average, 0.419 g kg−1 BW/day) compared with A. intrusa or B. brizantha (0.261 and 0.159 g kg−1 BW/day, respectively). This was attributed to higher fecal NDF-N, averaging 0.329 g kg−1 BW/day, from the tree legumes versus 0.162 g kg−1 BW/day for A. intrusa and 0.048 g kg−1 BW/day for B. brizantha. Consequently, apparent and true N digestibilities were lower for the tree legumes, averaging 61 and 69%, respectively, versus 73 and 84% for A. intrusa, and 76 and 93% for B. brizantha. Within the tree legumes, C. calothyrsus had lowest apparent and true N digestibility, averaging 51 and 57%, while P. falcataria and G. sepium had apparent and true N digestibilities averaging 67 and 76%. Proanthocyanidins and phenolic compounds in the three legumes, especially C. calothyrsus, were associated with reduced forage quality.