I really hate to add scientific data to this but the truth of the matter is that a womans estrogen level peaks in the middle of the 28cycle starting with the day she begins menstrating. This is the "fertile period". The level then begins to drop and the testosterone level elevates causing her to flush out her womb, not being pregnant, and start the entire thing over again. The female hormone level is at it's lowest point while in need of tampons.

Just thought I would bring that up.

One other thought is that when all mammals are coming into "heat" or a fertile period of time, they bleed. Not humans, in humans it is the oposite. Maybe it is the blood from a womb that attracts the bucks, and not the urine. If we are thinking estrogen urine, then urine from days 12-16 would be the best.