TerraVet is a brand of long acting oxytetracycline for cattle. It is given as an injection every 3 days. Some people have decided to use it off label for chickens, since antibiotics are hard to get in feed stores, so any dosage may or may not be helpful especially given orally. Here is an older thread that might be useful: https://www.backyardchickens.com/threads/miracle-la-200-dosage-and-vetrx-for-crd-treatment.631458/
If you are treating MG, a respiratory disease, Tylosin powder for the water may be a better choice. Tylan 50 injectable from some feed stores can be given orally as well. Here is where to find Tylosin: https://jedds.com/products/tylosin-powder