A medically important antimicrobial drug may not be administered in feed or water to cattle, swine, or poultry unless ordered by a licensed veterinarian through: A medically important antimicrobial drug prescription; or A veterinary feed directive. On or after January 1, 2018, and subject to subsection (c) of this section, a medically important antimicrobial drug may be administered to cattle, swine, or poultry if, in the professional judgment of a licensed veterinarian, the medically important antimicrobial drug is necessary: To treat a disease or infection; To control the spread of a disease or infection; or For a surgery or medical procedure. On or after January 1, 2018, a medically important antimicrobial drug may be administered to cattle, swine, or poultry if, in the professional judgment of a licensed veterinarian, the medically important antimicrobial drug is necessary for prophylaxis to address an elevated risk of contraction of a particular disease or infection. Notwithstanding subsection (e) of this section, administration of a medically important antimicrobial drug for the purpose of prophylaxis may not exceed 21 days unless federal label directions require a longer period of use. Unless administration of a medically important antimicrobial drug is consistent with subsection (b)(1) of this section, a medically important antimicrobial drug may not be administered in a regular pattern to cattle, swine, or poultry. A medically important antimicrobial drug may not be administered to cattle, swine, or poultry solely for the purpose of: Promoting weight gain; or Improving feed efficiency. Except as provided in paragraphs (2) and (3) of this subsection, a medically important antimicrobial drug may not be administered to cattle, swine, or poultry for a period longer than 21 days. A medically important antimicrobial drug may be administered to cattle, swine, or poultry for a period longer than 21 days if the federal label directions for the drug require a longer period of use. A licensed veterinarian may extend administration of a medically important antimicrobial drug for not more than 21 days if, after conducting an on–site visit, the veterinarian determines that the extension is necessary to treat or control the spread of disease or infection. A licensed veterinarian may grant additional extensions of not more than 21 days, provided that the veterinarian conducts an on–site visit before each extension. On or before January 1, 2021, the Department shall adopt regulations prohibiting the routine administration of a medically important antimicrobial drug to dairy cattle entering a dry cycle except when necessary based on an assessment of the presence of an intramammary infection.