Plant breeding for organic and sustainable, low-input agriculture

Selection for weed tolerance in soybean
Johann Vollmann and Helmut Wagentrist
Department of Applied Plant Sciences and Plant Biotechnology, BOKU - University of Natural
Resources and Applied Life Sciences, Vienna, Austria
Soybean (Glycine max [L.] Merr.) has recently proved to be an interesting grain legume crop for
organic farming in Central European countries (Bavec & Bavec, 2006). As a consequence, early
maturity, acceptable agronomic performance and quality features such as high seed protein
content, large seed weight or yellow hilum are important features for food grade soybeans. As
soybean is slowly developing during early stages of crop establishment, competition of the
soybean crop with seed-propagated weeds is important, and weed control is a major challenge
for farmers. Genetic variation in weed suppression or weed tolerance of soybean is largely
unknown. As soybean does not show significant allelopathic effects in general, genetic
differences in competitiveness against weeds might be due to mechanisms such as rapid
development during juvenile stages (Jannink et al., 2000), branching habit or weed shading
through increased leaf area or leaflet shape. Therefore, an experiment was initiated in order to
identify differences in competitiveness of soybean cultivars against weeds, by which selection
criteria for weed tolerance might subsequently be established.
During the 2005 and 2006 growing seasons, twelve soybean genotypes were either grown in
weed-free plots or in plots with heavy weed infestation in a split-plot experimental design at a
location near Vienna/Austria. Weed infestation was simulated by broad-sowing of winter oilseed
rape seeds after the emergence of soybean. Grain yield as well as other agronomic and seed
quality characters were recorded for pure stands and simulated weed stands, and grain yield
reduction by weed infestation was determined for individual genotypes.
Weed infestation had statistically significant effects on various characters of soybean. Across all
genotypes, weed infestation caused a delay in time to maturity by about 2 days, a reduction in
plant height by 12 cm and a decrease in grain yield by 450 kg/ha (i.e. 20 % in average).
Thousand-seed weight was increased from 157 to 165 g, and seed protein content was also
significantly increased from 342 to 350 g/kg by weed infestation. Yield reduction of individual
cultivars was between 0 and 48 % and between 0 and 20 % for the 2005 and 2006 growing
seasons, respectively. Moreover, yield reduction of individual genotypes was clearly
reproducible over the two years indicating a genetic difference in this character. The degree of
yield reduction by weed infestation for individual cultivars was not related with leaf area, but
significantly correlated with time to maturity. Therefore, the lesser yield reduction in early
maturing cultivars as compared to late cultivars should be considered as weed tolerance rather
than weed suppression under the experimental conditions applied.
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