I'm not sure what the rules on this community are about non-professionals posting and answering. I have arm-chair researched Sumer in the Jemdet Nasr period (around 3000 BCE) when Uruk was a large metropolis. Much like most things Sumer, the research is lacking as we're simply dealing with too old societies who lived in a part of the world that has changed so much due to climate change and soil salinization over the 7000 years civilization has existed there.
The cities of Jemdet Nasr period were structured into irrigated and unirrigated land. The latter was used as grazing ground for sizable quantities of goat and sheep, which required pastoral people to herd them. During the Ur III period, there were possibly around 350 000 sheep, which would require a considerable non-sedentary workforce to manage which were possibly imported from the surrounding hilly & mountaineous countryside.[1]
[1] Sumer and the Sumerians by Harriet Crawford, "Patterns of Settlement and agriculture", pages 40, 58, 59