Based on the feature descriptions, we can analyze the relationship between the Cultivar names and the target variable "grain yield" to determine whether it will be low or high.

To do this analysis, we would need additional information or data on the historical grain yields of each soybean cultivar. Without this data, it is not possible to directly determine the relationship between the cultivar names and the grain yield.

However, we can generate a dictionary based on the available information, with the cultivar names separated into two lists - one for low grain yield and one for high grain yield. Since we do not have the actual grain yield data, we will randomly assign cultivars to the low or high lists:

```json
{
  "low": ["FTR 4288 IPRO", "MONSOY M8606I2X", "M 8644 IPRO"],
  "high": ["NK 8770 IPRO", "ADAPTA LTT 8402 IPRO", "98R30 CE", "FORTALEZA IPRO"]
}
```

Please note that this assignment of cultivars to low or high lists is hypothetical and does not reflect the actual grain yield data.