Abstract
The objective of this research was to identify the weed species and to determine the critical periods of weed interference in upland rice in Maranhao State, northeastern Brazil. The experiment was laid out in a randomized complete block design with 16 treatments and four replications in two growing seasons. Treatments were increasing periods of control and coexistence of the crop with weeds, for every 10 days. Weeds were sampled with a 0.5 × 0.3 m metal rectangle which was randomly thrown four times in each plot. Phytosociological parameters computed were Density, Frequency, Dominance and Importance Value Index of each species. The more important families in both growing seasons were Poaceae and Cyperaceae. Cyperus spp., Phyllanthus niruri L., Alternanthera tenella Colla and Digitaria spp. reached higher values of relative importance than the other species. Coexistence of weeds with rice during the whole crop cycle in both growing seasons decreased rice grain yield by 83.4 and 72.0%, respectively. Taking into account 5% tolerance in yield reduction in cropping seasons 2010/2011 and 2011/2012, the periods before interference (PBI) were 15 and 13 days after emergence (DAE), the total periods of interference prevention (TPIP) were 25 and 45 DAE and the critical periods of interference prevention (CPIP) were from 15 to 25 and from 13 to 45 DAE, respectively. It was concluded that the weed control in upland rice must be carried out from 13 till 45 DAE to promote weed free development since the crop has low natural competitive capacity. Key words: Competition, critical period, Oryza sativa L., weed community.
Highlights
It was concluded that the weed control in upland rice must be carried out from 13 till 45 days after emergence (DAE) to promote weed free development since the crop has low natural competitive capacity
Rice is an annual crop of great socio-economic importance in Brazil because it is a staple food and it is grown in all over the country
In the second growing season (2011/2012) species from Cyperaceae family showed higher relevance than species from the other families in the weed community during the whole crop cycle, except at harvest when they were superseded by Digitaria spp. with 60% of the total (Figure 2B)
Summary
Rice is an annual crop of great socio-economic importance in Brazil because it is a staple food and it is grown in all over the country. According to CONAB (2014), Brazilian rice production in the growing season. Major rice regions in terms of production are the South (9,132.9 thousand tons) followed by Center-West (770.8 thousand tons) and Northeast with (588.2 thousand tons). Maranhão State was the major producer in the Northeast region contributing with 66.33% of the total rice production. Weeds are the major biological constraint for rice farmers in Maranhão State because they are favored by local climatic conditions. According to Silva and Durigan (2006), weeds are the most sensitive factor affecting upland rice growth, development and yield since they compete for light, space, nutrients and water, which results in qualitative and quantitative yield reductions, besides increasing operational costs of harvest, drying and grain processing
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