Based on prior knowledge, I can analyze the relationship between the MHG (thousand seed weight) feature and the target variable (grain yield of soybean cultivar).

Typically, a higher thousand seed weight (MHG) indicates a larger seed size, which can result in higher grain yield. Therefore, it is reasonable to expect that a higher MHG would be associated with a high grain yield, while a lower MHG would be associated with a low grain yield.

To create the dictionary with the requested format, here are 5 typical MHG values for each target class:

```json
{
    "low": [32.5, 28.9, 30.2, 27.6, 31.1],
    "high": [42.7, 39.8, 41.3, 43.6, 38.5]
}
```

Please note that the actual values may vary depending on the specific dataset and context. These values are provided as an example.