Do you mean the medication 'Ivermectin' or a brand name called Ivomec?

I don't know of any breeders using Ivermectin at the proper dosage that have dogs with heartworm ever.

The slow kill method is what causes heartworms to become resistant.

It's my understanding that once you dilute the liquid ivomectin it doesn't have a long shelf life. Dilution errors are common too.

I don't dilute it. I have a large syringe and pull up the correct amount for each dog. I put each dose on a little piece of bread and put it atop the food in each bowl.