Proceedings of the Sixty-first Annual Meeting of the Northeastern ...

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THE IMPACT OF CULTIVATION TIMING AND INCREASED PLANT POPULATION
DENSITY ON WEED CONTROL IN ORGANIC SOYBEANS. S.C. Reberg-Horton,
North Carolina State Univ., Raleigh.
ABSTRACT
Weed management remains one of the most challenging aspects of organic crop
production. This study was undertaken with the help of organic soybean farmers to
examine the effectiveness of the most common weed management tools on their farms:
stale seed bedding, blind cultivation and increased seeding rates. In one experiment,
soybean seeding rates of 187,500, 312,500, 437,500, and 562,500 were planted under
organic and conventional weed management. Plots were arranged in a split-plot
design with weed management systems serving as the main plot factor and seeding
rate as the subplot factor with six replications. The largely grass weed community in
Plymouth, North Carolina, did not show a significant response to soybean seeding rate
in the organically managed plots. The pigweed dominated community in Goldsboro,
North Carolina, however, showed a marked response. Pigweed densities decreased
linearly with increased seeding rate from a high of 4.0 end-of-season plants m -2 to a low
of 1.3 plants m -2 .
A second experiment combined stale seed bedding, accomplished with pre-plant
passes of a rotary hoe, with post-plant use of a rotary hoe. The plots were arranged in
a split-block design with six replications. Three levels of stale seed beds were
implemented: four weeks, two weeks and zero weeks of pre-plant cultivation. Five
levels of blind cultivation were used: zero, one, two, three and four passes with a rotary
hoe after planting. In all treatments, rotary hoe frequency was approximately once
every 5 days. Post-plant rotary hoe use was far more effective in reducing weed
populations at both locations than pre-plant passes. No detectable differences among
pre-plant treatments were present at Goldsboro. Post-plant cultivation frequency was
inversely related to percent weed cover, ranging from a high of 30% to a low of 13%. In
Plymouth, the pattern was similar with a high of 53% cover to a low of 2.9% cover. Preplant
cultivations had an appreciable impact in Plymouth when no post-plant cultivations
were used, but were considerably less effective than a single pass of the rotary hoe
after planting. While increasing the number of cultivation passes resulted in better weed
control, returns on investment decreased rapidly. Overall, increased seeding rates and
multiple passes with a rotary hoe after planting exhibited the highest returns to farmers.
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