Unfortunately, I had a typo when I posted this quote. So, here it is again corrected:
"He [Ennis] was still working there in September when Alma Jr., as he called his daughter, was born and their bedroom was full of the smell of old blood and milk and baby shit, and the sounds were of squalling and sucking and Alma's sleepy groans, all reassuring of fecundity and life's continuance to one who worked with livestock."
This makes it more clear that it was Ennis who works with livestock, and that he considered his wife and children to be like livestock.