The shrinking U.S. cattle herd was a key factor in Cargill’s decision to close its beef slaughter plant in Milwaukee last week, company officials said. The plant, which Cargill purchased in 2001, had the capacity to process 1,300 to 1,400 animals daily and employed 600 workers.

Cargill said it made the decision to shutter the plant as of Aug. 1 after an 18-month ...