So, my general advice is this, especially since you are just starting:

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Colts

-Cull any that are not up two on week 3 (week 3 is your first sign of how strong a foal will be)

-Cull any that are not up week 6 (week 6 is basically your week 7, but for SD)

Of course, I recommend starting off with mares for SD since it is easier to breed to high SDLB studs compared to mares.

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Fillies

-Cull any not up one week 3

-Cull any not up week 6

Once you establish a solid herd, I adjusted my expectations for my fillies to be the same for the colts.

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Edit: So basically, your week 3 is your week 4, week 6 is your week 7 in relation to how people view AD. I like to think of it that way at least, could be wrong, xD. Anywho, I am always happy to help if you need more advice. Oh, I also stop tracking at week 12, but I think up to week 9 is enough, personally.