The Procaine is safe on Chickens. At least according to the below listing and the few I have had to treat Bumble foot with have not died yet.
From the List that I posted a link to.
Procaine Penicillin G / Procaine Benzylpenicillin (intermediate-acting; includes numbing agent called Procaine that slows absorption)
Injectible liquid
Pro-Pen-G, Durvet Pen Aqueous, others
Dura-Pen (Procaine combined with Benzathine)
Many feed stores (*Injectible is kept in store's refrigerator with vaccines), vets $9-$26
Penicillins, Benzylpenicillins Bumblefoot, Wounds, Fowl Cholera, Necrotic Enteritis, Ulcerative Enteritis, some strains of Ornithobacterium Rhinotracheale (ORT), Erisipelas (used along with Benzathine Pen) Comparison to other Penicillins: Medium-low potency effect; active for medium amount of time in bird's sytem (Procaine slows absorption); Procaine is poorly soluble so best given by injection.
*Avoid giving acidic foods or supplements if dosing orally.
*In feed, general dose is 100-300 mg/day (This recommendation needs to be double-checked).
? For Cholera: (?3 ccs by mouth?) Chronic problems may require long- term or periodic treatments.
? For Necrotic Enteiritis: 1,500,000 u/gal water for 5 days
For wounds & Bumblefoot: 1/4 to 1/2 cc daily for 5-7 days subcutaneous or in breast muscle.
*See special notes on "Treating with Penicillin"