Based on the given feature description, the key feature that can help determine the grain yield of the soybean cultivar is the "Seeds per meter/linear" feature. Generally, soybean cultivars with higher seeds per meter/linear tend to have higher grain yields.

Here is the analysis and dictionary for the given task:

```json
{
	"low": ["FTR 3190 IPRO", "FTR 4288 IPRO", "MONSOY M8606I2X", "M 8644 IPRO", "ADAPTA LTT 8402 IPRO", "FORTALEZA IPRO", "MONSOY 8330I2X", "SUZY IPRO", "FTR 4280 IPRO", "LYNDA IPRO", "PAULA IPRO", "ELISA IPRO"],
	"high": ["NK 8770 IPRO", "NK 8100 IPRO", "NK 7777 IPRO", "GNS7900 IPRO - AMPLA", "NEO 760 CE", "74K75RSF CE", "96R29 IPRO"]
}
```

Please note that the list of cultivar names provided in the dictionary is based on the assumption that the "Seeds per meter/linear" feature is a good indicator of grain yield. These lists may not represent an exhaustive analysis and may vary depending on other factors that were not included in the given information.