The effects are evident after 6 hours of transport.
As most horse owners are aware, transporting a horse for any length of time is a stress inducing event. Numerous studies have been conducted that document the negative effects of transportation stress including, dehydration and respiratory illnesses. However, there has not been a lot of research into effective methods for reducing this stress and its associated consequences. Researchers at Texas Tech University, began to tackle this problem by performing two studies to analyze the effects of transportation stress on the horse's body.
For the first study, 20 quarter horses were hauled for 26 hours. The researchers found increased levels of cortisol, a stress hormone, in the horses after transport. In the second study, 30 horses were hauled for 24 hours. As with the first study, the horses had increased levels of cortisol. However, the increased cortisol levels were found to occur after just six hours of transport and remained elevated for the duration of the transport period. The horse's respiration levels were also elevated during transport at the 6 and 12 hour marks but had returned to normal at the 18 and 24 hour marks.
These studies confirm that horses do experience stress as a result of transport, and that those effects are evident after 6 hours of transport.