Is it all right to turn horses out to graze a wheat field just starting to head out? There has been NO drought stress.

It may be O.K. to turn out your horses on the wheat field that has no drought stress if three other conditions are not present.
First and second, if cloudy/rainy weather or very cool weather has not occurred, you still should be fine to graze out the wheat without fear of nitrate or prussic acid poisoning problems that can occur on stressed grass crops, fields, or pastures. Cloudy/rainy weather particularly on high fertility soils can cause new growth of grass …