Shawn P. Conley, State Soybean and Wheat Extension Specialist

John Gaska, Outreach Specialist

University of Wisconsin, Madison

 

In January, we published an article entitled Read the Tag and Check the Germ in 2008 that discussed the 2008 soybean seed quality issues. This article is another reminder to growers to check the percent germination of all of your soybean seed lots prior to planting and adjust your soybean seeding rates accordingly. In a normal year, we assume 90% of the live soybean seed we plant will emerge. Therefore to estimate our stand population we conduct the following calculation:

 

Seeding rate x % germ x % expected emergence = estimated stand

 

In this example, our grower drills 180,000 seeds per acre of 94% germ seed, and assumes 90% emergence. The estimated soybean stand will be = 152,280 plants/acre. If the grower planted 80% germ seed the estimated soybean stand would be = 129,600 plants/acre. Under most environmental conditions 129,000 plants/acre would produce 100% yield potential, however if we do not achieve our assumed 90% emergence rate due to poor early season growing conditions, we rapidly approach stands where yield loss may occur. 

 

We all understand the importance of planting date on soybean yield. In 2008, however I would be extremely cautious if considering planting soybean into cold wet soils. The questions regarding seed quality and vigor coupled with the lack of replant seed make me a little leery this year to really push our soybean plantings.

 

Lastly, it takes soybean longer than corn to emerge. Corn on average takes ~125 GDU’s base 50° F where soybean takes ~145 GDU’s base 50° F (Robinson 2008). Across planting dates, we saw a range of 7 to 30 days for soybean to emerge. A soybean is considered emerged when the cotyledons get above the soil surface. Planting date also greatly affected the percentage of seeds that emerged (Figure 1). Note the environmental impact of planting date and year on emergence across 2006 and 2007.

 

Figure 1. Planting Date Effect on Percent Soybean Emergence at W. Lafayette, IN 2006 and 2007.

 

 

 

Reference

Andrew Robinson, 2008. Early planting influence on yield, protein, oil and seed vigor on indeterminate soybean. M.S. Thesis.