Soybean Competition and Weed Control Options at Low Populations.

Recent studies at the University of Kentucky have indicated that final plant populations near 247,000 plants per hectare are sufficient for maximum yields on full season soybean [Glycine max (L.) Merr.].  Previous recommendations for the state of Kentucky have always been significantly higher, in part to aid in competition with weeds.  This study was undertaken to determine the impact of lower soybean seed rates and populations on weed control and yield.  Asgrow 3906 soybean was planted into a no-till field at Lexington and a conventionally tilled field at Princeton.  Soybean seeds were planted in a randomized split plot design with the main plot of seed rate; 185,000, 309,000, and 432,500 plants per hectare.  The split plots were weed management with  a glyphosate treatment at three weeks after planting (3 WAP), five weeks after planting (5 WAP), seven weeks after planting (7 WAP), three and seven weeks after planting (3+7 WAP), and a weed free treatment, along with a weedy control.  At both locations, grain yield at the two higher seeding rates did not differ, while the lowest seeding rate yielded 13% lower at Lexington and 10% lower at Princeton.  At each location the 5 WAP, 7 WAP, 3+7 WAP, and weed free treatments resulted in similar yields, while the 3 WAP treatment yielded significantly lower at both locations and the weedy control yielded the lowest.  Under some circumstances, maximum yields can be achieved at populations less than 185,000 plants per hectare, but according to this data, seed rates of at least 309,000 plants per hectare, along with a timely application of glyphosate is a more dependable option.