We use DNA from hair roots to test for single-gene traits such as inherited disorders and coat colours. DNA in hair samples can last for five years or more if you treat it like potatoes: keep them dry and in the dark.
The main killers for DNA in horse hair samples are water and UV light. UV usually comes from sunlight, though I don’t imagine a tanning bed would be very good for it either. UV basically smashes DNA into little pieces, which is exactly what it does to DNA in your skin too if you forget to use sun protection. Smashed up DNA is not good for DNA testing.
Wet samples are also a problem, most likely because water allows excessive bacteria to grow on the sample. Some bacteria can eat DNA. We have learnt that if we get a sample that is visibly wet, it’s not even worth trying to get DNA.
Just one or two days of exposure to sunlight or water will make your sample unusable for DNA testing.
So if you take a sample for testing, take it from a dry horse, put it in a paper envelope, and if you’re not sending it the next day keep it in the dark (and definitely not on the dashboard of your car!). Don’t freeze or refrigerate, it won’t extend the life of the sample and it may cause water to condense.