I found a thread on using Ivomec 1% injectable as an alternative to Heardguard as a heartworm perventative on the garden web.

Ivomec dosage for dogs... yet again - Farm Life Forum - GardenWeb

I have four small terriers and heartguard cost me $203 a year. A bottle of the Ivomec will last over a year for $40. The annual checkup and shots for my dogs was $600 this year.
I have got to find something more frugal.

The woman at the farm supply store gave me these dosing directions:

Take 1cc of ivomec and add 9cc water to make a 10cc solution.
Give 1cc orally of the new solution per 40 lbs of dogs weight.