Heartworm prevention is sold as a 30-day thing for a combination of reasons. Mostly, a lot of people have an easier time remembering to do something at the same time every month than remembering to do something every six weeks, so dogs are more likely to get the stuff on a routine basis. In addition, if somebody does forget and is a week or two late (and it happens, quite a lot), they’re still on continuous coverage. Beyond that, and you’re starting to get into the range of needing to retest, although a lot of vets will tell you it’s okay to miss up to a month.
Hooch, I’m not sure that the concentration of ivermectin sold at feed stores does vary from brand to brand, but I do know that most liquid medications are available in varying concentrations. I saw a tech at a mixed practice damn near kill a dog one time in just that sort of situation. We had two different concentrations of the same drug, one for the small animals and one for the horses, and she grabbed the wrong bottle, pulled up the volume written on the chart, and gave the poor critter 10x the dose he was supposed to get. The dog pulled through, thank God, but that was one hell of a bad scene. And it’s an easy enough mistake to make if you’re not paying close attention; I’ve come damn close to doing the exact same thing a dozen times.
I also know that the injectable ivermectin we used to keep around had its dosages listed in volume per hundred pounds. Since most dogs don’t weigh hundreds of pounds, using this stuff in dogs requires some conversions. (Also, the dosage per pound tends to vary between species for some drugs, which is another reason you need to speak with your vet.) All it takes is one error in calculation to massively overdose an animal, and I’ll tell you from personal experience that EVERYBODY makes those sorts of mathematical errors once in a while.
I’m not saying you shouldn’t do it, I’m saying you need to be very careful about it. When you’re not very careful about drug, concentration, dosage, and route, really bad things can happen.