The Defined Daily Dose Veterinary / year refers to the number of days per year which the average horse has received a dose of antibiotics. If a practice only treated one horse, and it had five days of oral potentiated sulphonamides, the result is 5. If the practice treated that horse and vaccinated four others (so five horses in total) the DDDVet/yr would be 1 (one horse has five days antibiotics and four have none so the average horse got one day that year). Further if you vaccinated another four horses (8 in total) and treated one with ten days of ceftiofur, the DDD would be 1.875 (5 potentiated sulphonamides + 10 ceftiofur / 8 horses).

DDDVet per year is therefore independent of the mg of antibiotics used, which would be higher in the potentiated sulphonamides and four vaccines case (say 30000mg x 5 days / 500kg x 5 horses = 60mg/kg) while the ceftiofur and potentiated sulphonamides example might be lower at 32.8 mg/kg (30000 potentiated sulphonamides x 5 days + 1400mg ceftiofur x 10 days / 500kg x 10 horses = 32.8 mg/kg).

In essence using mg/kg, the overall figure would be made better despite using more reserved antibiotics. Hence the preference for DDDVet/yr.