ABSTRACT

Two separate field experiments were conducted during 2015 and 2016 cropping seasons at the National Root Crop Research Institute (N.R.C.R.I.) Umudike to evaluate the effect of vegetation residue, crop spacing and weed control methods on the emergence pattern, density of mimosa (Mimosa invisa Mart.) and yields of two cassava varieties of contrasting morpho-types. The sites used for the experiments were predominately infested with M. invisa Mart. The experiments were laid out in split-split plot in randomized complete block design (RCBD) with three replications. For the experiment on the effect of residue and weed control methods on cassava yield, the main plot consisted of two residue management practices (burning and no burning). The sub-plot consisted of two cassava varieties of contrasting morphology (TME 419 - sparse branching variety and NR 8082 - profuse branching variety) and the sub-sub-plot consisted of four weed control methods (hoe weeding at 4, 8 and 12 weeks after planting (WAP), S-metolachlor (1160 g/ha) + atrazine (1480 g/ha) applied pre-emergence followed by hoe weeding at 12 and 16 WAP,  S-metolachlor (1160 g/ha) + atrazine (1480 g/ha) applied pre-emergence followed by trifloxysulfuron sodium (8 g/ha) applied post-emergence at 8 WAP  and Weedy check). In the experiment on effect of crop spacing and weed control methods on cassava yield, the main plot treatments were three crop spacing; 1 m × 0.6 m, 1 m × 0.8 m, and 1 m x 1 m. The sub-plot treatments were two cassava varieties of contrasting morphology (TME 419 and NR 8082) while the sub-sub-plot treatments were four weed control methods (hoe weeding at 4, 8 and 12 weeks after planting (WAP), S-metolachlor (1160 g/ha) + atrazine (1480 g/ha) applied pre-emergence followed by hoe weeding at 12 and 16 WAP,  S-metolachlor (1160 g/ha) + atrazine (1480 g/ha) applied pre-emergence followed by trifloxysulfuron sodium (8 g/ha) applied post-emergence at 8 WAP  and Weedy checks). Data collected were subjected to analysis of variance and means separated using least significant difference (LSD). The results obtained showed that hoe weeding at 4, 8 and 12 weeks after planting (WAP), S-metolachlor (1160 g/ha) + atrazine (1480 g/ha) applied pre-emergence followed by hoe weeding at 12 and 16 WAP, and S-metolachlor (1160 g/ha) + atrazine (1480 g/ha) applied pre-emergence followed by trifloxysulfuron sodium (8 g/ha) significantly (P ≤ 0.05) controlled M. invisa density in both cropping seasons at 12 to 16 WAP. At 10 months after planting (MAP) and 2 months after harvesting, significantly higher densities of M. invisa were observed in plots that were hoe weeded at 4, 8 and 12 (WAP) and S-metolachlor (1160 g/ha) + atrazine (1480 g/ha) applied pre-emergence followed by hoe weeding at 12 and 16 WAP whereas, NR 8082 with the highest canopy cover at 1 m x 0.6 m spacing reduced the mimosa seedling emergence and other weeds compared with TME 419. Reduction in plant spacing from 1m x 1m to 1 m x 0.6 m significantly (P ≤ 0.05) reduced weed density, and increased root yield in both cropping seasons whereas residue management methods did not control M. invisa density. Plots treated with S-metolachlor (1160 g/ha) + atrazine (1480 g/ha) applied pre-emergence followed by trifloxysulfuron sodium (8 g/ha) provided the highest return on investment of N209.19 and N616.68 /ha per naira invested in 2015 and 2016 respectively, and effectively controlled M. invisa in the cassava field