Impact of Fertilizer Type and Harvesting Age on Nutrient Composition, In vitro Gas Production, and Fermentation Characteristics of Napier Grass (Pennisetum purpureum)

Abstract

The study evaluated the effects of fertilizer type (control, NPK, cattle dung, and poultry litter) and harvesting age (4, 6, 8, and 10 weeks) on the proximate composition, cell wall fiber fractions, in vitro gas production parameters, and fermentation kinetics of Napier grass. A 4 × 4 factorial arrangements were used under a completely randomized design, with four replicates per treatment. Proximate and fiber analyses were conducted following standard procedures, while in vitro gas production was measured using the Menke and Steingass technique. Fermentation kinetics were estimated using the Gompertz 3P model. Results showed that fertilizer type significantly influenced crude protein (CP), ether extract (EE), ash content, and acid detergent fiber (ADF) (P < 0.05). NPK fertilization produced the highest gas volume (22.00 ml), organic matter degradability (OMD, 40.96%), metabolizable energy (ME, 6.03 MJ/Kg DM), and short-chain fatty acid (SCFA, 0.46 mmol/200mg). The control treatment had the highest dry matter degradability (54.00%) and fermentation rate (0.120 ml/hr). Harvesting age significantly affected all measured parameters, with CP declining from 13.95% at week 4 to 10.90% at week 10, while fiber fractions increased with advancing maturity. Gas volume, OMD, ME, and SCFA were highest at week 4 (20.25 ml, 40.00%, 5.84 MJ/Kg DM, and 0.42 mmol/200mg, respectively). Significant interaction effects were observed between fertilizer type and harvesting age for most parameters. It is concluded that NPK fertilization combined with harvesting at 4–6 weeks optimizes the nutritive value and rumen fermentation efficiency of Napier grass.