Proportion and digestibility of tree legumes in diets of grazing cattle in a tropical silvopastoral system

Abstract

In Silvopastoral Systems (SPS) the dynamics and complexity of the combination between grasses and legume trees can significantly impact forage intake and digestibility, affecting livestock performance. This research was composed of two parallel studies. The first one consisted of a field trial assessing three grazing systems: signalgrass (Urochloa decumbens Stapf.) monoculture, and two SPS, one composed of signalgrass + gliricidia [Gliricidia sepium (Jacq.) Kunth.], and another by signalgrass + mimosa (Mimosa caesalpiniifolia Benth), in a randomized complete block design with three replications. Yearling crossbred 5/8 (Holstein/Zebu) steers (175 ± 29 kg initial body weight) were used as experimental animals. Response variables included δ13C in diet and the proportion of C3 species in the feces. The second study was an in situ dry matter digestibility (ISDMD) assay using fistulated male buffalos, and it was evaluated the increasing levels of each legume from the SPS (0, 25%, 50%, 75%, and 100%) in replacement to signalgrass, under different incubation period (24, 48, and 96 h). The range of the carbon isotope (δ13C) found in the cattle's feces (-14.2 to -16.4 ‰), in both signalgrass monoculture pastures and SPS, evidenced the preference of the cattle under SPS for consuming grasses (C4) instead of tree legumes (C3), as both were available. The carbon isotope (δ13C) in plants with C4 metabolism ranges between -9 and -16‰, and in C3 from -22 to -34‰. Increases in the intake of legumes during the dry period might have occurred due to lesser herbage mass and nutritive value of the grass. The ISDMD reached its greater values (>80%) in the inclusion levels of 75 and 100% of gliricidia, at 48- and 96-h incubation (P<0.05). Reductions in the ISDMD coefficients were observed increasing the levels of mimosa in the diet (P<0.01). Adding 100% of mimosa led to decreases in the ISDMD of 54.4% (24 h), 56.8% (48 h), and 49.1% (96 h). The species of legume tree can significantly affect the preference, proportion of C3/C4 species, and the digestibility of the diet of grazing animals in SPS. The grazing cattle in SPS showed a preference for consuming grass instead of legume trees, however, gliricidia was preferred compared to mimosa, especially in periods of lesser grass availability.