My girl last had her heat beginning of June. She is a working breed. Usually she has a heat cycle every 8 months however June was only her second heat. In the past few days I noticed her humping my other dogs (not normal for her) and she was becoming clingy. The other dogs ocassionally smelled her as well. There's no blood, no swelling, etc. I took a chance and took her to our studs house. He instantly would not leave her alone and for the next 36 hours they had multiple slip ties and yesterday apparently she stood for him and flagged her tail but today refuses to even stand for him. Has anyone had experience with silent heats? Are slip ties sufficient in getting a bitch pregnant?

You certainly can get a litter from a slip mating(they cant be called slip tie, as no tie took place), as long as the timings are correct.

At only her second heat she is really too young to bred from (unless she was unusually old for her first). 

A bitch of this size breed should be over two years of age for her first litter so normally after a third or later season.

Not only is that better for her, as she will have had a chance to mature both physically and mentally but her hormones should have settled into a more predictable pattern.

Litters can result fro slip matings but in my bred (another primitive natural type) it rarely does (only heard of one litter, a singleton, in over 20 years of breeding) and is usually a sign of mating too early.

She just turned 2. She didn't get her first heat till a little after a year old.

If you progesterone test this will give an indicator if she's in season or not.
I'm going to make the assumption as you've mated her she's had all.the health tests done relevant for her breed too?

It's going to be a case of wait and see and getting a scan done when she is around 4-5 weeks post mating to see if she's pregnant or not.

Yes I have. I have ovulation pads that showed she was ovulating but the pads only work when a bitch is in season or else they will give off false positives so it didn't really help me.

As she was only in season in June, so wouldn't normally be expected to be in season again till the New Year at the earliest, I'd be more suspicious that she has a slight infection whcih can make them attractive to males.

I'd get her blood tested to check her progesterone levels, but also check the levels of white blood cells for infection. She could also be having a slight phantom pregnancy that's making her humpy.

ovulation pads are useless you need a progesterone test.
get her to the vets and do that and check that she doesn't have any infection because some males get confused and belive the smell of an infection means the bitch is in season.
She maybe having a phantom.. Get the vet to rule out pyo too

> As she was only in season in June, so wouldn't normally be expected to be in season again till the New Year at the earliest, I'd be more suspicious that she has a slight infection whcih can make them attractive to males.

Me either!    And other than some slight infection, sometimes clogged anal glands can appeal to males!!   But I doubt any bitch would have stood for a male unless she was both in season, and in 'standing heat' at that.   And if she is having a false pregnancy, that too could mean her hormones are all over the place = 'humpy'

Slip ties can produce litters - one of my stud dogs never managed a full tie but much as I didn't let him mate many bitches, of those he did mate, he only missed to one and she came back to my other male on her following heat, and missed again.   He managed to tie with his bitches too.    She then went to another, outside stud dog, and again didn't conceive.

> As she was only in season in June,

oops I was reading it as/assuming they had mated the bitch in July and were waiting on an imminent birth???

>I was reading it as/assuming they had mated the bitch in July and were waiting on an imminent birth??

No, they're saying they think she's come in season again and they've taken her to the stud dog now, but he's only been mounting and not penetrating. I'd suspect a phantom pregnancy and possibly an infection as well. There's no indication of how experienced any of the dogs or their owners are.

Having re-read your original post & taken in the details a bit more, I agree with some of the other posters that she may well be having a phantom pregnancy(although a little later than it would normally show signs).
If June was only her 2nd season there would be a cycle pattern yet but would be odd to be in season again only 3 months after the last, although I have read some whom have had bitches in season of 4 month intervals.
It wouldn't be a bad idea to go to the vets & get her checked out just to make sure its nothing serious.