F.P. Campos, D.R.O. Nicácio, P. Sarmento, M.C.P. Cruz, T.M. Santos, A.F.G. Fari, M.E. Ferreira, M.R.G. Conceição, C.G. Lima
Improvements in the nutritional quality of tropical grasses depend on the proper use of grazing management and nitrogen fertilization as well as climatic conditions. The aim of this study was to evaluate the effect of nitrogen (0, 125, 250, 375, 500 or 625 kg/ha, applied as urea), grazing cycles (GCs) and growing degree days (GDD) on the chemical composition and in vitro digestibility of dry matter (IVDMD), organic matter (IVOMD) and neutral detergent fibre (NDFD) of hand-plucked samples of Xaraes palisade grass (Urochloa brizantha cv. Xaraes). A total of 24 plots of 112 m2 each were cultivated with Xaraes palisade grass was evaluated and carried with repeated measures (GCs) and one treatment factor (N rate) in completely randomized block design, with four replications. Grass samples were collected after 26 days of regrowth from plots that were managed under rotational stocking (26 days of rest and 2 days of occupation) in six GCs (1–6). To achieve the target height of post-grazing residues (24–27 cm), dry Holstein cows were used with a body weight (BW) of 527 kg (SD 36 kg), considering a consumption of 2.5% of BW and 40% estimated mass loss of pre-grazing forage. All of the nutrient concentrations were affected by N fertilization and GCs. The crude protein (CP) content increased linearly with the N rate in each grazing cycle (P < 0.05). NDF decreased linearly with the N rate in each grazing cycle (P < 0.05). N fertilization increased the IVDMD, IVOMD and NDFD with a linear or quadratic response depending on the GC (P < 0.05). The average maximum point that was obtained in GC4 in the regression equations for IVDMD, IVOMD and NDFD as a function of N fertilization was 533 kg N/ha/year. The GDD had a quadratic effect on the chemical composition and digestibility of Xaraes palisade grass with distinct N doses. N fertilization stimulated the formation of cell walls with better-quality fibre and resulted in a higher forage digestibility according to the climatic conditions and GDD. The greatest improvements in forage quality were obtained, with N rates greater than 382 kg/ha/year.
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