put out to grass/pasture, to be

put someone out to pasture

Fig. to retire someone. (Based on put a horse out to pasture.) Please don't put me out to pasture. I have lots of good years left. This vice president has reached retirement age. It's time to put him out to pasture.
See also: out, pasture, put
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

put out to grass

Also, put out to pasture. Cause to retire, as in With mandatory retirement they put you out to grass at age 65, or She's not all that busy now that she's been put out to pasture. These idioms refer to farm animals sent to graze when they are no longer useful for other work.
See also: grass, out, put
The American Heritage® Dictionary of Idioms by Christine Ammer.

put out to pasture

1. To herd (grazing animals) into pasturable land.
2. Informal To retire or compel to retire from work or a full workload.
See also: out, pasture, put
American Heritage® Dictionary of the English Language, Fifth Edition.

put out to grass/pasture, to be

To be retired from active duty; to rusticate. This term, which refers to animals that are turned out to a meadow or range, particularly a horse that is too old to work, was transferred to human beings as early as the sixteenth century. John Heywood used it in his 1546 proverb collection: “He turnde hir out at doores to grasse on the playne.”
See also: grass, out, put
The Dictionary of Clichés by Christine Ammer
See also:
  • put (one's) back to (something)
  • put back to
  • put (something) on the block
  • put a cap on
  • put a cap on (something)
  • put (one's) back into (something)
  • put (something) in with (someone or something)
  • put (someone or something) on ice
  • put (someone) on ice
  • put on ice