Field and laboratory experiments were performed to evaluate the allelopathic
potential of sorghum residues alone or in combination with reduced rate of trifluralin
herbicide on weeds in mung bean field. The field experiment was conducted during
2014 season at the Research Farm of Biology Department, College of Science,
Baghdad University by using randomized complete Block design (RCBD) to test the
effect of sorghum residues at rates of 5 and 10 t ha-1 alone or in combination with
reduced doses of trifluralin (30 and 60% of recommended dose) on weeds and mung
bean crop. Weedy check and label rate of trifluralin (2.4 Lha-1) were also included
for comparison. Each treatment was replicated four times. Total phenolics in field
soil amended with sorghum residues at 10 t ha-1 was determined in at 0, 14, 28 and
42 days of sorghum residues decomposition. Also bioassay experiment was
conducted to test the allelopathic effect of sorghum residues at 10 t ha-1
incorporated into the field soil on seedling emergence and growth of Portulaca
oleracea weed during the above mentioned decomposition periods. Results showed
that incorporation of sorghum residues at 5 t ha-1 reduced weed density by16.9 and
18.8% of control after 30 and 60 days from sowing, respectively. The reduction was
increased when sorghum residues were incorporated at 10 t ha-1 and reached to 30
and 30.5% of the control after 30and 60 days from sowing, respectively. However,
the suppression of weed population and dry weight biomass was further improved
when the plots were treated with 30 and 60% of label rate of trifluralin and sorghum
residues at 5and 10 t ha-1. The highest suppression of weed population and dry
biomass was achieved by application of reduced rate of trifluralin (60%) to plots
amended with sorghums residues at 10 t ha-1. Integration of reduced herbicide and
sorghum residues at 5and 10 t ha-1 resulted in more weed suppression than sole
application of the respected sorghum residues. The results also revealed that weed
suppression was directly translated into yield of mung bean. Application of
trifluralin herbicide at 60% of label rate in plots amended with sorghum residues at
10 t ha-1 recorded the highest biological and grain yield, number of pods per plants
and weight of 100 seeds. Chemical analysis indicated that total phenolics started to
increase at14 and 28 days of decomposition and decline thereafter until vanished 6
weeks of decomposition. Biological activity test of field soil revealed that
suppression of Portulaca oleracea weed was highly correlated with total phenolics
(R2 = 0.95for seedling emergence and 0.87 for dry biomass) of soil suggesting that
high weed suppression was mainly due to high activity of phenolics.