Effect of nitrogen levels and weed management methods on weed abundance and yield of upland rice (Oryza sativa L.)

Weed infestation and inherent low soil fertility are among the major factors attributed to the low yield of rice in Nigeria. Field trials were therefore conducted to evaluate the effect of nitrogen application levels and weed control methods on growth and yield of upland rice (var. NERICA 2) at the Teaching and Research Farm of the Federal University of Agriculture, Abeokuta (07° 15'N, 03° 25'E) during 2015 and 2016 cropping seasons. Three nitrogen (N) levels (0, 60 and 90 kg/ha) were evaluated and they constituted the main plot treatments, while three weed control treatments, viz: pre-emergence application of Orizo Plus® (propanil plus 2, 4-D) at 2.0 kg a.i ha-1, Orizo Plus® at 2.0 kg a.i ha-1 followed by supplementary hoe weeding (SHW) at 6 weeks after sowing (WAS) and three hoe-weeding regimes at 3, 6 and 9 WAS, and a weedy check constituted the sub-plot treatments. All the treatments in different combinations were laid out in a randomized complete block design with a split-plot arrangement with three replicates. Results indicated a significant (p≤0.05) increase in weed density and dry matter with an increase in N application level from 0 to 90 kg ha-1. Similarly, crop vigour and plant height increased significantly (p≤0.05) with increasing N application levels up to 90 kg ha-1. However, 60 and 90 kg N ha-1 were at par in increasing the number of tillers, leaf area index and yield attributes of rice. All the weed control methods resulted in a significant (p≤0.05) reduction in weed density and dry matter with subsequent increase in rice growth and yield than the weedy check. Pre-emergence application of Orizo Plus® followed by SHW at 6 WAS and three hoe-weeding regimes resulted in significantly (p≤0.05) lower weed density and dry matter, and a higher number of tillers, panicle weight and grain yield than a sole application of Orizo Plus®. With Orizo Plus® followed by one SHW or three hoe-weeding regimes, increasing N application levels resulted in a significant (p≤0.05) increase in grain yield of rice. However, with Orizo Plus® applied alone, increasing N application levels did not increase rice grain yield. These results suggest that Orizo Plus® at 2.0 kg a.iha-1 followed by one SHW at 6 WAS integrated with N application at 90 kg ha-1 is adequate to effectively control weeds and increase rice yield in the rainforest-savannah transition zone of Nigeria.