Prussic acid will volatize to the atmosphere during the drying process.  Properly cured hay will have no toxicity levels from prussic acid.  Nitrates on the other hand never leave.

Silage. Sorghum silage is generally safe for feeding. Although it could contain toxic levels of prussic acid while in storage, much of the poison escapes as a gas during fermentation and when being moved for feeding. However, as a precaution, do not feed new silage for at least 3 weeks after harvesting and storing.

Hay. The prussic acid content of sorghum hay decreases as much as 75 percent while curing and is rarely hazardous when fed to livestock.

http://www.agry.purdue.edu/Ext/forages/publications/ay196.htm