Influences of growth stage and nitrogen fertilizer on chemical composition, phenolics, in situ degradability and in vitro ruminal variables in amaranth forage

摘要

This study was carried out to assess the influences of growth stage (early flowering or milk stage of kernels) and nitrogen (N) fertilizer level (120, 180 or 240 kg of N/ha) on the chemical composition, phenolic compounds, in situ degradability and in vitro ruminal fermentation variables in amaranth forage (Amaranthus hypochondriacus). The study was conducted as split plot arrangement based on randomized complete block design with four replicates of two samples each. Chemical composition, in situ degradability of dry matter (DM) and crude protein (CP), gas production (GP) and in vitro ruminal fermentation variables were determined. Data were analyzed using GLM procedure of SAS. The concentrations of DM, CP, ash-free neutral detergent fiber (NDFom), total phenolics (TP), and effective degradability (ED) of DM and CP were respectively 159 g/kg fresh weight, 224, 346 and 13.4 g/kg DM, and 767 g/kg DM and 810 g/kg CP for the amaranth harvested at early flowering and fertilized with 120 kg N/ha (treatment 1). In vitro organic matter (OM) disappearance (OMD), 24-h partitioning factor (PF24), microbial CP (MCP), methane production, pH, ammonia-N, volatile fatty acids (VFA), protozoa and cellulolytic bacteria were 686 g/kg OM, 4.36 mg/mL, 201 mg/g DM, 35.5 mL/g degraded DM, 10.2 mg/dL, 68.3 mmol/L, 6.82 x 10(5)/mL and 8.09 log(10)/g digesta, respectively, for treatment 1. With advancing the amaranth growth stage, the concentrations of DM, OM, NDFom, EE and TP increased, but CP, soluble and very rapidly degradable fraction (fraction 'A') and ED reduced (P 0.05). Rising N fertilization caused a linear increase of the DM, CP, fraction 'A' and ED (P 0.05). With increasing the plant age, the in vitro ruminal GP, OMD, PF24, MCP, ammonia-N, VFA and cellulolytic bacteria decreased (P 0.05), but methane production, per g degraded DM, increased (P 0.05). The values of in vitro GP, OMD, VFA and cellulolytic bacteria increased linearly as N fertilization enhanced (P 0.05). In vitro ruminal pH, ammonia-N and protozoa were not affected by fertilization. Overall, increasing amaranth age decreased CP, degradability, in vitro ruminal VFA and MCP, but increased lignin(sa), phenolics and methane production per g of degraded substrate. Increasing N fertilization enhanced the forage quality at both growth stages. Thus, the decrease of amaranth quality in second growth stage can be reduced by increasing N fertilization. Though, the effect of growth stage on the forage quality was more pronounced than the effect of N fertilization. (C) 2016 Elsevier B.V. All rights reserved.