manual labor vs picking

manual labor and picking both are nouns.

Word	Noun	Adjective	Verb	Adverb
manual labor	Yes	No	No	No
picking	Yes	No	No	No

As nouns, picking is a hyponym of manual labor; that is, picking is a word with a more specific, narrower meaning than manual labor:

Other hyponyms of manual labor include application, coating, covering, bodywork, handling, planking, wiring.

manual labor (noun)	picking (noun)
labor done with the hands	the act of picking (crops or fruit or hops etc.)
	the quantity of a crop that is harvested