Allelochemicals Biologica... - Name

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ANA LUISA ANAYA
nutsedge. Without herbicides, hairy vetch did not suppress weeds 8 wk after cover
crop desiccation. Half rates of atrazine and metolachlor (1.1 + 1.1 kg al ha (-1))
reduced total weed density more effectively in no cover crop than in hairy vetch. Half
rates of atrazine and metolachlor controlled redroot pigweed, Palmer amaranth, and
goosegrass regardless of cover crop. Full rates of atrazine and metolachlor [2.2 + 2.2
kg al ha (-1)] were needed to control large crabgrass in hairy vetch. Control of yellow
nutsedge in hairy vetch was marginal even with full herbicide rates- Yellow nutsedge
population increased and control with herbicides declined the second year, particularly
with half rates of atrazine and metolachlor. All cover crops except hairy vetch alone
reduced emergence, height, and yield of sweet corn. Sweet corn yields from half rates
of atrazine and metolachlor equalled the full rates regardless of cover crops.
It is presently not known what effect wheat root residues have in regulating
dicotyledonous (dicot) weed emergence in no-till management systems. Past research
has focused almost entirely on the role of shoot residues, while the role of root residues
in weed control has been essentially ignored. A field study was designed by Blum et
al. (2002) to determine the respective effects of wheat shoot and root residues in
regulating the emergence of three dicotyledonous weed species (morning-glory, pigweed
and prickly sida). Glyphosate-desiccated wheat plots and fallow plots were surface
seeded with morning-glory, pigweed and prickly sida during the spring of 1996 and
1997. Weed seedling emergence was determined for two months during each
experimental period in plots with or without wheat shoot and/or root residues. The
resulting data suggested that: a) the closer desiccation of the wheat cover crop occurred
to the initial emergence of pigweed seedlings, the lower the emergence of that weed,
b) the effects of wheat shoot and/or root residues on dicot weed seedling emergence
vary considerably for the different weed species ranging from stimulation to inhibition
and c) the role of root residues appear to be much more important to regulating weed
emergence than that of surface shoot residues, Differences in soil moisture and
temperature associated with the presence or absence of wheat residues could not be
used to explain the observed treatment effects.
The growth of four summer season crops, namely Cyamopsis tetragonoloba,
Sorghum vulgare, Pennisetum americanum and Zea mays, in fields with or without
residues of the preceding sunflower crop was poor. Crop density, weight of seed or
grain and total yield were significantly lower in sunflower fields than in the control
fields (i.e. those without previous sunflower crops). Growth in terms of plant height
and biomass was drastically reduced after 60 days. The effect was more pronounced
in the fields where sunflower residues were allowed to decompose than in those where
residues were completely removed. The soil collected from sunflower fields (both
with and without residues) was found to be rich in phenolics, which in a laboratory
bioassay were found to be phytotoxic. The reduced growth and yield of crops can be
attributed to the release of phytotoxic phenolics from decomposing sunflower residues
(Batish, et al., 2002).
John and Narwal (2003) assessed that Leucaena leucocephala is the most
productive and versatile multipurpose legume tree in tropical agriculture and has
several uses, thus called ‘miracle tree’. It is a popular choice for intercropping with