Monensin toxicity in two breeds of laying hens.

A case of monensin toxicity in laying hens is reported. The drug was incorporated, by error, at a concentration of 800 mg/kg in the rations which were fed for 4 days to light (Leghorn X Rhode Island Red, LR) and heavy (White Rock, WR) breeds of laying hens. Egg production (initial levels of about 80% and 57%, in the LR and WR hens, respectively) decreased markedly 24 h after feeding the monensin-supplemented rations, and 8 days later it was less than 10% in the LR hens and had stopped completely in the WR birds. Egg production in both strains regained normal levels 30 days from the beginning of the episode. Mortality started 48 h after monensin administration, reaching a peak on the 9th and 6th days in the LR and WR hens, respectively. Mortality was much higher in the WR hens (38.3%) than in the LR hens (3.7% in shed A and 8.3% in shed B). It seems, therefore, that the WR hens were more susceptible to monensin toxicity than LR hens.