Based on my analysis of the given feature and task, here is the dictionary with the possible values of the feature "Cultivar" for the target class "Low" and "High":

```json
{
	"low": ["NEO 760 CE", "MANU IPRO", "77HO111I2X - GUAPORÉ", "NK 7777 IPRO", "FTR 4280 IPRO", "LYNDA IPRO", "FTR 3868 IPRO"],
	"high": ["GNS7900 IPRO - AMPLA", "97Y97 IPRO", "BRASMAX OLIMPO IPRO", "NK 8100 IPRO", "82HO111 IPRO - HO COXIM IPRO", "83IX84RSF I2X", "ADAPTA LTT 8402 IPRO", "98R30 CE", "FORTALEZA IPRO", "MONSOY 8330I2X", "SUZY IPRO", "TMG 22X83I2X", "EXPANDE LTT 8301 IPRO", "FORTALECE L090183 RR", "LAT 1330BT", "ATAQUE I2X", "SYN2282IPRO", "82I78RSF IPRO", "M 8644 IPRO", "MONSOY M8606I2X", "NK 8770 IPRO", "FTR 4288 IPRO", "FTR 3190 IPRO"]
}
```

Please note that this analysis is based on the assumption that the given feature "Cultivar" has a strong relationship with the target variable "grain yield". However, without additional information or a proper analysis, the accuracy of this relationship cannot be guaranteed.