Why did the horses calm down in Melancholia?

Before that scene, while he was observing the Melancholia, Claire came home with some sedatives. John followed her and saw her putting them to the drawer. In that time, he was sure that the planet was not going to crash them so he was calm and he said "Are you going to kill us all?" This means the sedatives are powerful enough to kill a person.
The horses were making too much noise, as Justine said, and after finding out that Melancholia will hit Earth, he panicked as well. He took the sedatives from the drawer and gave normal doses to the horses to calm them down before commiting suicide. That's what made them calm down.