Influence of maize herbicides on weed seed bank diversity in a humid forest agro-ecology of south-eastern Nigeria
A Screen house study was carried out to determine the weed seed bank diversity following preemergence weed control in maize (Zea mays L.) farm. The Soil used for the study was collected from Faculty of Agriculture Research and Teaching farm (04o 90 889`N and 006o 92 240` E a.s.L 2,7 m ) of the University of Port Harcourt, Rivers State, Nigeria (04o 54 538eN and 006o 55 329e; a.s.l. 17m) . The weed seed bank study was conducted at the Green house, Centre of Ecological study University of Port Harcourt. Soil for this study was collected at three depths-0-5cm, 5-10cm and 10-15cm respectively. Nine core samples were collected in a diagonal transect from each maize plot in a manner that depicts a \Z. and bulk for each soil depth. The soil for the study was collected 40 weeks after the pre-emergence maize herbicides treatment from previously treated maize weed control trial. The herbicide treatments were as follows: Primextra Gold at 1.5kg ai ha-1 ; Primextra Gold at 3.0kg ai ha-1 ; Pendimethalin + Atrazine at 1.0 + 1.0kg ai ha-1 ; Pendimethalin + Atrazine at 2.0 + 2.0 kg ai ha-1 ; Gardoprim A (Atrazine + Terbutylazine) at 1.0kg ai ha-1 ; Gardoprim A (Atrazine + Terbutylazine) at 2.0kg ai ha-1. Hand Weeded x 2 (3+6 WAP), Unweeded and Weed Free treatments were the controls. The size of the pots used was 0.029m2. Result shows that hand weeded treatments: weedfree (0.996), weed x 2 (1.069) had lower diversity indices compared to the herbicides with a mean index of 1.34 and the unweeded with an index of 1.187. It was also observed that the pre-emergence herbicides used may have controlled grasses more than broad-leaved weeds. The result of this study showed that the herbicide (Pendimethalin + Atrazine) at 2.0 +2.0kg ai ha-1 was more effective in reducing weed seeds (seed bank) than the other five herbicide treated plots, with a mean diversity index value of 1.083, while the highest weed diversity was recorded in plot treated with Gardoprim A herbicide (Atrazine +Terbutylazine) at 2.0 +2.0kg ai ha-1, with a mean diversity index of 1.260. . The mean average weed diversity was highest (1.214) in 0-5cm soil depth followed by 1.149 in 5-10cm soil depth and then 1.068 in 10-15cm depth. This shows that as soil depth increases the number of weed species decreases. The result also implied that Gardoprim A may not provide some residual control of late emerging weed seeds or seedlings that germinate several months after the initial treatment. This calls for a careful selection of appropriate pre-emergence weed control program in diverse arable cropping systems that will provide lasting weed control for late emerging weeds in the system.
Keywords: Index of diversity, seedling emergence, soil depth and pre-emergence