PERTANIKA JOURNAL OF TROPICAL AGRICULTURAL SCIENCE

The major forage in tropical livestock systems is napier grass (Pennisetum purpureum), and the first input applied to improve its yield is nitrogen. How big a response that is remains to be seen. Field trials are discordant, and many compare multiple nitrogen rates to one untreated control, giving undue weight to the evidence. This meta-analysis synthesises the tropical record that accounts for that dependence. English-language articles were retrieved from Scopus, ScienceDirect and Google Scholar following PRISMA. Effect sizes are reported as Hedges’ g and pooled using a random-effects model with cluster-robust variance estimation, with each study (rather than each comparison) serving as the unit of analysis. Twelve studies fulfilled the criteria. Nitrogen increased organic matter, crude protein, plant height, fresh biomass yield and tiller number (P < 0.05), crude protein being the most consistent response. There were no changes in dry matter, crude fibre, NDF or ADF. The ether extract, stem diameter, leaf area and leaf number were only studied twice each and could therefore not be reliably assessed. There is no single optimal rate that can be defended because few dose subgroups included 3 independent studies. The practical message is not as broad as the primary literature suggests. Moderate nitrogen supply, before the stand lignifies, increases protein and tillering, but higher rates add little.