Got a source for "last month of life"?

Penn State University Extension says "...approximately 95% of the cattle in the United States continue to be finished, or fattened, on grain for the last 160 to 180 days of life (~25 to 30% of their life), on average."[1]

Oklahoma State University Extension also cites a study that compares "growth and carcass attributes of calves finished for 98 to 105 days in a grass system or a legume system"[2]

That puts us between 3 to 6 times longer than you stated, and gives us the context for how much of the average cattle's life that is. (USDA Prime, Choice, and Standard are all 30 to 42 months. Select is under 30 months.[3])

1: https://extension.psu.edu/grass-fed-beef-production 2: https://extension.okstate.edu/fact-sheets/finishing-beef-cat... 3: https://www.ams.usda.gov/grades-standards/slaughter-cattle-g...