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 to have high nitrogen content?in extreme cases causing high nitrates or nitrate poisoning possibilities. Prussic acid problems are more prevalent in forage sorghums and sudangrass than in small grain plant growth due to the higher hydrocyanic acid (HCN) content in these grass plants. Very cool (near frost) weather or extreme drought can also create problems with either prussic acid or high nitrates, particularly more so when these stressed plants get a shot of rain that then produces quick growth that carries these high levels of HCN or nitrates.
If the wheat is not stressed by any of the two previous factors, consider a third factor?the stage of the wheat field. Higher feed value as well as better quantity is obtained when small grains are at the boot (preheading) stage or even at the vegetative stage (higher crude protein but lower quantity at vegetative stage).
When wheat heads out, if the variety is not awnless, these awns on the wheat can cause problems with mouth sores on horses. In extreme cases of awned wheat being eaten, the horse can actually quit eating or slow down on eating and consequently not maintain his weight in the field even with lush plant growth seen everywhere.
Consider these three additional concerns before using a wheat field for open grazing, and, even if grazing, watch the horse’s health to determine if weight maintenance or gain is being achieved (depending on each horse) and if additional ration supplements may be needed. Below is a benchmark of feed value from small grains to keep in mind. It will also show when higher feed value is obtained with these crops.
Small grain, hay or silage
Vegetative 12-17 31-35 53-60 60-64 96-114
Heading 9-12 35-45 55-65 53-60 77-104
Dough 9-11 37-50 60-70 48-57 66-93