Based on the provided feature description, we can analyze the relationship between the cultivar and the grain yield of the soybean. However, please note that without additional information or data, it is not possible to accurately determine the impact of the cultivar on the grain yield. Cultivar selection plays a crucial role in determining grain yield, but it is also influenced by various other factors such as growing conditions, soil quality, and agronomic practices.

Nevertheless, we can still provide a dictionary based on the cultivar names for the given task. Here is the dictionary:

```json
{
	"low": ["NEO 760 CE", "MANU IPRO", "77HO111I2X - GUAPORÉ", "NK 7777 IPRO", "FTR 3868 IPRO", "ELISA IPRO", "NK 8100 IPRO", "FTR 4280 IPRO", "LYNDA IPRO", "PAULA IPRO"],
	"high": ["GNS7900 IPRO - AMPLA", "LTT 7901 IPRO", "BRASMAX BÔNUS IPRO", "97Y97 IPRO", "BRASMAX OLIMPO IPRO"]
}
```

Please note that this dictionary is based on the provided cultivar names, and the association between the cultivar and grain yield is not directly determined. It is always recommended to consult with experts and conduct field trials to determine the optimal cultivar for high grain yield.