Crop Canopy Effect in Managing Glyphosate Resistant Kochia in Soybean.

Kochia (Kochia scoparia) is becoming increasingly difficult to manage in central South Dakota as biotypes resistant to glyphosate emerge.  Whereas other crops have tank mixes to help control glyphosate-resistant kochia (GR-kochia), soybeans have limited options. Rotation with crops where other herbicides can be applied is a promising solution. In addition, previous research suggests that kochia seeds lack dormancy mechanisms, making kochia infestation reduction possible if populations are managed aggressively. However, there is not much documentation quantifying kochia performance in different crop canopies. The objective of this study was to quantify kochia growth in different crop canopies to develop integrated management recommendations for GR-kochia. Corn, soybeans, field peas, wheat and fallow field plots were established in 2010, 2011 and 2012 in Brookings SD. Two kochia cohorts, each consisting of 100 seeds, were sown at crop planting and approximately 28 days after crop planting to quantify germination and survival.  In addition three evenly spaced single kochia plants, were established from seed in each plot to quantify kochia biomass and seed production. Kochia germination was inconsistent among the crop treatments. Soybeans emergence however was equal to or greater than the other treatments in each year which demonstrates a potential competitive advantage of kochia in soybeans relative to other crops. The magnitude of kochia shoot and seed reduction was similar among the different crop canopies. Relative to the fallow treatment, all crop canopies reduced (P<0.05) kochia biomass and seed production by more than 90%. Kochia biomass production was positively correlated with seed count per plant (R² = 0.98).  Rotation with other crops and timing of herbicide applications have great potential in managing GR-kochia in soybeans. Additional research is needed to quantify the combined effects of crop competition and aggressive management in depleting kochia infestations.