Here is a little blurb from the Merck Manual:
Fenbendazole (SafeGuard , Panacur) is approved in the USA for use in growing turkeys at the rate of 14.5 g/ton of feed (16 ppm), fed continuously as the sole ration for 6 days for the removal of Ascaridia dissimilis and Heterakis gallinarum. No withdrawal time is required. One study indicates a possible negative effect on sperm quality by the drug. It has been suggested that an alternative drug be used for treatment of breeding toms or that the sperm number and frequency of artificial inseminations be increased. Fenbendazole is not approved for use in other poultry in the USA but is effective against Ascaris when administered once at 10–50 mg/kg; if needed the treatment can be repeated after 10 days. At 10–50 mg/kg, fenbendazole when administered daily over 5 days is effective against Capillaria. Fenbendazole is also efffective against other nematodes when administered at 10–50 mg/kg/day for 3–5 days or as a single dosage of 20–100 mg/kg, or added to the drinking water at 125 mg/L for 5 days or to the feed at 100 mg/kg. Fenbendazole should not be administered during molt, because it may interfere with feather regrowth.
Albendazole (Valbazen) administered as a single oral suspension (5 mg/kg bird weight) was reported effective against A galli, H gallinarum, and Capillaria obsignata. The drug also has been reported effective against cestodes if administered at 20 mg/kg. There are no published withdrawal times
