On-field assessment of critical period of weed interference in okra [Abelmoschus esculentus (L.) moench] field in Ibadan, a rainforest-savanna transition eco-zone of Nigeria.

The critical period of weed interference is defined as the period in the life cycle of a crop during which it must be kept free of weeds to prevent unacceptable yield losses. The critical period of weed interference in okra [Abelmoschus esculentus (L.) Moench] was determined in the field trials during two cropping seasons (2006 and 2007) in the crop garden of the Department of Crop Protection and Environmental Biology, University of Ibadan, Ibadan located in the rainforest-savanna transition ecological zone of Nigeria. The study, consisting of two sets of treatments was carried out in a randomized complete block design with three replicates. In the first set of treatments, the crop was kept free of weeds for the first 2, 4, 6 and 8 weeks after planting (WAP) and subsequently weeds were allowed to grow (weed-free then weedy, WF-WD) until harvest, to determine the growth stage at which emerging weeds would no longer reduce the crop yields. In the second set of treatments, weeds were permitted to grow with the crop for an increasing length of time as listed above and subsequently the plots remained weed-free (weedy then weed-free, WD-WF) to determine the period at which weeds emerging with the crop began to suppress the growth of the crop and reduce its yield. Seasonlong weed-free (WD-0-WF) and weedy (WF-0-WD) treatments served as checks to compare weed competition and yield losses with other treatments. Season-long weed interference with okra resulted in 42.3% and 33.3% reduction in plant height, 54.9% and 45.5% reduction in stem diameter and 71.2% and 79.5% reduction in okra shoot dry weight in 2006 and 2007, respectively. Weeds that interfered with okra from 2 WAP resulted in 79.8% and 72.5% fresh fruit yield reduction in 2006 and 2007 respectively, and weed interference with okra from 8 WAP resulted in 19.8% and 19.6% yield losses in 2006 and 2007, respectively. Thirty one (31) weed species were enumerated in the study area with dicotyledons being the most prevalent (80.7%). The most important of the weed species among the dicotyledons was Synedrella As. J. Food Ag-Ind. 2009, Special Issue, S288-S296 289 nodiflora (RIV=12.5%) and among the monocotyledons was Panicum repens (RIV=9.5%). The Shannon-Wiener Index and Equitability index for the community was 2.804 and 0.817, respectively. In 2006, weeds that interfered with okra for the first 2 WAP resulted in 17% yield reduction while the season-long interference resulted in a 90.2% yield reduction. Also in 2007, competition between weeds and okra for the first 2 WAP and season-long interference resulted in 16.4% and 92.2% fresh fruit yield reduction, respectively. Results in the two cropping seasons showed that, to prevent >10% fruit yield losses in okra, the field must be kept free of weeds throughout the entire growing cycle.