I decided to make this post because I assumed it would help others with grading their horses. So here we are:

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Genetics

As most of you know, certain breeds carry certain genes. If you are looking for a specific gene like Mushroom or Manchado, it is important to always gene test your horses and foals. If you don't, your horse might not carry the genes you're looking for; you may never even know about it.

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Multiple horses

If you are trying to obtain multiple grades, it is easier to grade a few horses at the same time. I use this method alot when trying to breed a rare gene into another horse as it ensures I am not inbreeding. 

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Patience is key

This is pretty self-explanatory; grading is a hard and slow process. Being impatient won't make anything better or different.

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Rescue Center

Sometimes, you can find 75-87% crossbreeds in the in-game rescue. This can give you a head start with grading, and you can also find multiple other popular breeds there for cheap; I found lots of good ones using this method. It is also important to check the rescue regularly as more horses are surrendered each day, meaning you could find even better horses to help you with the grading process!

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Studs and Broods

Crossbreeds up for stud and brood can also help with grading. Sometimes there will be DNA tested horses that are 75% or more of a specific breed, similar to what I said about the rescue. These horses can also help you with the grading process.

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If I missed anything else, feel free to post it on this thread and I will add it! I hope this helps other players!