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Originally Posted by Rumils's acquaintance

...the really awful part is that any such offspring would only be chopped up to be used as seed or fertilizer for other Orcs being grown in the pits of filth where they are farmed (with various other ingredients added during the gestation in their womb of filth in order to specialize the crop). This is where the addition of human remains would be important to the creation of your "Man-Orc" or Uruk-Hai, but even this would take many generations. Maybe 20 years for five to ten generations, depending upon the quality of Orc wanted, the longer the time, the better the quality of Orc. So, Uruks would probably be four to five years per generation. That is still a very short time to breed Orcs, as a HUGE Army could be raised in relatively short times.

Also, many of the Orcs in a batch might not make it, or they might be cannibalized as food by those "Hatching" early.

Originally Posted by Wikipedia, article on Warhammer 40k "Orks"

Their cross-breeding with fungi allows them to reproduce asexually, which explains the lack of Ork females. This reproductive process basically involves the dispersion of spores from a decomposing Ork corpse, such that killing an Ork literally creates more Orks, explaining why Orks are such a difficult enemy to combat and how, along with their extreme physical hardiness, they can very easily infiltrate and build up large populations virtually anywhere. The only known solution to this problem is to incinerate the entire area to destroy all Orks and Ork spores. Because Orks are so genetically similar to plants and fungi, their diets are extremely flexible - Orks can be used not only as meat, but also as yeast and phytochemical-containing fibrous plantlike sides. Thus, Ork Pie is a staple of Ork diets, particularly on the battlefield, where little other food is available, and dead Orks are in abundance.