Influence of Cropping System, Land use Intensity and Weed Management Practice on Seedbank and Floristic Composition of Weeds

ABSTRACT 

There is a growing concern about the sustainability of agricultural production systems in subSaharan Africa. Thus, interest in the modification of cropping systems that contribute to the regulation of weed populations and reduce the negative impact of weeds on crop production is increasing. Understanding of weed seedbank dynamics will improve efficiency of weed management practices. This study was designed with the following objectives: (i) examine the relative time and pattern of weed seedling emergence as influenced by weed control methods, cropping systems and climatic factors; (ii) examine the impact of previous landuse intensities on pre-cultivation seedbank and weed infestation during the current cropping season, under different cropping systems and weed management practices and (iii) compare the volume of precultivation weed seedbank with floristic composition under the different landuse intensities as influenced by cropping and weed management systems. In pursuant of these objectives, field trials were carried out at the University of Ilorin Teaching and Research Farm (TRF) and Institute of Agricultural Research and Training (IAR & T), Obafemi Awolowo University, Ile-Ife, Ballah sub-station during the 2009 and 2010 cropping seasons. The experiment at the TRF was designed as a randomized complete block with a splitplot arrangement and three replications. The main plots consisted of four cropping systems (maize/cowpea intercrop, sole maize, sole cowpea and no cropping) while the sub plots consisted of three weed control methods (chemical weed control, hand weeding and no weed control). Weed seedling emergence and dry matter production were monitored continuously in the same fixed quadrats (0.5 m2 ) at 3, 6, 8, 10, 12 and 15 weeks after planting (WAP) and discretely in a different set of fixed quadrats (0.5 m2 ) at 5, 9 and 12 WAP. Crop yields were determined at harvest. The experiment which was conducted at the IAR & T Ballah sub-station was established on three plots of land with known cropping history. The study was laid out as a randomized complete block design with factorial arrangement. The treatments at each landuse site comprised of two current cropping systems (sole cropping and no cropping) and three weed management practices (chemical weed control, hand weeding and no weed control). Data on weed density and dry weed biomass were assessed continuously in the same fixed quadrats (0.5 m2 The major findings of the study were: ) at 3, 6, 9, and 12 WAP. The density of weed seedbank from the two locations was estimated from pre-sowing and post-harvest soil samples using the direct seedling germination method. Rainfall and temperature data were collected in both years. Data collected were subjected to analysis of variance, regression and correlation analyses. (i) weed seedlings emerged throughout the periods of assessment with two peaks at 3 and 10 WAP in 2009 and at 3 and 8 WAP in 2010 under all cropping and weed management systems; (ii) there were positive correlations between field weed emergence and rainfall (r = 0.99, p ≤ 0.001); and field weed emergence and air temperature (r = 0.98, p ≤ 0.001); (iii) weed infestation was higher in sole cropped fields (150 seedlings/m2 ) compared to 136 seedlings/m2 (iv) aggregate crop yields were significantly (p ≤ 0.05) higher in the intercropped than in the sole plots, even though, component crops yields were higher under the sole cropping than in intercrop; in the intercropped fields; (v) previous landuse significantly (p ≤ 0.05) positively influenced both the weed seedbank density and floristic composition of the different fields with annual weeds predominating in continuously cropped fields, while perennial weeds were dominant in fallow fields; (vi) there was a strong positive relationship (r = 0.98, p ≤ 0.003) between weed seed number in the soil (seedbank) and the number of emerged seedlings on the field; and (vii) on the average 15.8 - 30.6 % of weed seeds in the seedbank emerged. The above findings showed that an accurate estimate of the soil weed seedbank in combination with knowledge of weed emergence pattern can serve as valuable tools for predicting future weed infestations in arable crops. It is concluded that the diversification of cropping systems can provide an efficient means of managing weeds, leading to reduction in dependence on herbicide use.

Well Researched and Ready to use Ph.D Thesis, page numbers: 223, Department: Agronomy

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