The use of nitrogen fertilization in tropical grasslands is a strategy that may reduce the pressure of livestock production on biome areas with humid forests. The objective of this study was to assess the use of different doses of nitrogen in Guinea grass (Megathyrsus maximus (Jacq.) con. Mombasa) cultivated in the humid tropics. Doses of 0, 10, 20, 30, 40, and 50 kg of N ha⁻¹ application⁻¹ were applied during two consecutive years in experimental plots. The experimental area is located in the northeast of Pará, Brazil, with a predominantly tropical climate according to the Köppen classification. The morphogenic, production, and qualitative characteristics of the forage were evaluated. The morphogenic variables, rate of leaf appearance and rate of leaf elongation, in addition to daily accumulation of forage and crude protein content, increased with increasing doses of nitrogen. The morphological structure of the pasture was not modified. The number of cuts increased, whereas the number of days of recovery decreased, in the rainy season with increasing doses of nitrogen. The use of nitrogen fertilization does not alter the structure of the pasture; however, it improves the morphogenic, production, and chemical characteristics of Guinea grass.

The objective of this study was to evaluate the effects of monensin supplementation on animals receiving warm-season grass with limited supplementation. In Exp. 1, treatments were a factorial combination of 2 stocking rates (1.2 and 1.7 animal unit [AU] [500 kg BW]/ha) and supplementation with monensin (200 mg/d) or control (no monensin) distributed in a complete randomized design with 3 replicates. Thirty Angus × Brahman crossbred heifers (Bos taurus × Bos indicus) with initial BW of 343 ± 8 kg were randomly allocated into 12 bahiagrass (Paspalum notatum) pastures and supplemented with 0.4 kg DM of concentrate (14% CP and 78% TDN) daily for 86 d. Herbage mass (HM) and nutritive value evaluations were conducted every 14 d, and heifers were weighed every 28 d. There was no effect (P ≥ 0.97) of monensin on HM, herbage allowance (HA), and ADG; however, animals receiving monensin had greater (P = 0.03) plasma urea nitrogen (PUN) concentrations. The stocking rate treatments had similar HM in June (P = 0.20) and July (P = 0.18), but the higher stocking rate decreased (P < 0.01) HM and HA during August and September. Average daily gain was greater (P < 0.01) for the pastures with the lower stocking rate in August but not different in July and September (P ≥ 0.15). Gain per hectare tended to be greater on pastures with the higher stocking rate (P ≤ 0.06). In Exp. 2, treatments were 3 levels of monensin (125, 250, and 375 mg/animal per day) and control (no monensin) tested in a 4 × 4 Latin square with a 10-d adaptation period followed by 5 d of rumen fluid collection and total DMI evaluation. Blood samples were collected on d 4 and 5 of the collection period. Ground stargrass (Cynodon nlemfuensis) hay (11.0% CP and 52% in vitro digestible organic matter) was offered daily. The steers received the same supplementation regimen as in Exp. 1. Total DMI was not different among treatments (P = 0.64). There was a linear increase (P ≤ 0.01) in propionate and a tendency for decreased acetate (P ≤ 0.09) concentrations in the rumen with increasing levels of monensin; however, there was no effect (P ≥ 0.19) of monensin levels on ruminal pH and ruminal concentrations of butyrate and ammonia. In addition, there was no effect (P ≥ 0.73) of monensin levels on plasma concentrations of glucose, insulin, IGF-1, and PUN. In summary, monensin supplementation effects were not detected at either stocking rate and may not be effective in increasing performance of beef cattle grazing low-quality warm-season grasses with limited supplementation.

SUMMARY: The objective of this study was to evaluate morphogenetic and structural characteristics of Guinea grass pastures subjected to two grazing frequencies (90 and 95% light interception) and two post-grazing heights (30 and 50 cm), during the rainy season and dry year. The leaf appearance rate varied only in the summer, the grazing pastures managed at 90/50 (light interception/post-grazing height) presented higher values in the leaf appearance rate (0.440 leaf/tiller.day), compared with those managed at 90/30 (0.275 leaf/tiller.day) and 95/50 (0.228 leaf/tiller.day), respectively. In the winter/beginning of the spring, pastures managed at 90/30 (0.03 cm/tiller.day) presented lower stem elongation rate in comparison with those managed at 90/50 (0.19 cm/tiller.day) and 95/30 (0.16 cm/tiller.day). Management strategies do not compromise the tissue flow in tillers and, consequently, can be utilized for the management of Guinea grass.