Abstract

Unlike the effect of prilled urea, which is well known, the effect of granular urea fertilization on growth of many staple crops is still being assessed across regions in Nigeria and its effect on consequent weed growth and dynamics is not well known. Therefore, this study was conducted, in the year 2018 after a maize -granular urea fertilization trial on the Ultisols of the Niger Delta, Nigeria, to determine the effect of the granular urea fertilization on weed growth and diversity. Weed data was collected, two weeks after maize harvest, from plots that received the following seven treatments: No granular urea (Control), nitrogen (N) from granular urea (GU) at 75 Kg N/ha (GU75) and 100 Kg N/ha (GU100), N from prilled urea (PU) at 75 Kg N/ha (PU75) and 100 Kg N/ha (PU100), N from a combination of GU and poultry manure at 75 and 25 kg N/ha (75GU + 25FM) and N from a combination of PU and poultry manure at 75 and 25 kg N/ha (75PU+25FM). Using RIV% of > 10%, four dominant weed species found were: Cyprus rotundus (a perennial sedges; RIV 22.2%), Elusine indica (an annual grass; RIV 16.8%) and two annual broadleaf species; Oldenlandia corymbose (RIV 13.08%) and Aspilia Africana (RIV 10.03%). The highest RIV (112.99%), weed density (697.33/m2) and weed dry weight (697.33g/m2) were found in the Control while the lowest RIV (87.16%), weed density (383.33/m2) and weed dry weight (20.73g/m2) were on plots that received high urea (100PU and 100GU). Weed density and RIV% were found to have strong, negative relationship (R2=0.77, 0.71 respectively) with mean number of maize leaves at 70 days after sowing. Thus, the use of granular urea (particularly at 100 kg N/ha) during the maize growing period suppressed subsequent weed growth and diversity (particularly annual broadleaf types) with prolonged shading effect of the maize canopy contributing strongly to this. The use of granular urea on farmlands in the Ultisols of Rivers State could be encouraged rather than the less environmentally friendly prilled urea.