Jeff Lowe's animals at the 'Tiger King' zoo have not been getting proper veterinary care for years ... according to the feds who've suspended his license to run the zoo.

As we reported ... the U.S. Department of Agriculture yanked Jeff's exhibitor's license for Joe Exotic's old GW Exotic Animal Park for 21 days, and now we know why the USDA finally made the call to punish him.

According to new legal docs, obtained by TMZ ... the USDA cites Lowe for numerous failures in providing vet care for his animals dating back to November 2017.

The USDA points out that the number of animals at Lowe's zoo has exploded from 29 in 2017 up to 212 in 2020, and alleges he's simply incapable of taking care of all the lions, tigers, lemurs, monkeys and other animals he's accumulated.

Jeff's also accused of submitting false documents related to veterinary care and records. Specifically, the USDA claims he submitted reports from its attending vet in 2019 and 2020 ... even though that vet claims she resigned from the zoo in June 2018.

According to the docs ... Jeff and his wife, Lauren, have also violated regulations in exhibiting animals without sufficient distance and barriers between them and the public.

And of course, the USDA cites countless examples of allegedly mistreated animals from its recent investigation -- as we first told you -- like the 16-week-old lion cub that was too weak to stand, geriatric wolves in pain and a 17-year-old dead tigon that was not properly cared for or disposed of.

Finally, the USDA claims Lowe does not possess the required paperwork -- acquisition records and disposition records -- for a bunch of the zoo animals.

Lowe tells us the allegations of improper veterinary care have never been an issue until the most recent USDA inspection, which he previously dismissed as the government being "overzealous."