For global estimates (~73%)

Previous studies have estimated that 73% of all antimicrobials sold globally are used in animals raised for food

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7766021/pdf/antibiotics-09-00918.pdf

For US in particular (~80%)

Of all antibiotics sold in the United States, approximately 80% are sold for use in animal agriculture; about 70% of these are “medically important” (i.e., from classes important to human medicine).2 Antibiotics are administered to animals in feed to marginally improve growth rates and to prevent infections, a practice projected to increase dramatically worldwide over the next 15 years.3 There is growing evidence that antibiotic resistance in humans is promoted by the widespread use of nontherapeutic antibiotics in animals. Resistant bacteria are transmitted to humans through direct contact with animals, by exposure to animal manure, through consumption of undercooked meat, and through contact with uncooked meat or surfaces meat has touched.4

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4638249/