I have a female and two males (same breed). One male is neutered and the other is not.
The last heat, there was only one male - the neutered one. They tied several times and no puppies-great!
I had him neutered because he grew up to have some faults outside the breed standard. I got another male with great characteristics and very well representative of the standard. He is now almost a year old. The problem is now the bitch is in season again, she only wants to mate with the neutered male. When the new guy tries, she VERY aggressively bites at him and he leaves her alone. I've removed the neutered one from the situation for a few hours at a time, and she still won't let the new guy near her. We're only on about the second day of her allowing mating. Does anyone know if she will eventually allow the new guy, or how to get her to?

Your male is too young to have had his Hips assessed for Dysplasia so should not be used yet for breeding until he is old enough, and also should have Clinical eye testing.

I assume he has already been DNA.tested for hereditary disease conditions within the breed?

https://www.petgeneticslab.co.uk/products/foresight-health-australian-shepherd

Of course the bitch requires the same.

If either is a carrier for the same recessive condition then they should not be mated together. One partner must test clear for recessive conditions

Have you had the younger male Pennhipped for HD? if not you should wait until he is at least 12 months before he is hip & elbow scored.

I take both your entire dogs are fully DNA tested and at least one is normal if the other is a carrier for any of the genetic mutations or normal by inheritance.

Plus clinical eye & hearing testing

scewing:  > I had him neutered because he grew up to have some faults outside the breed standard

Could I just ask if you would have made that decision to neuter if no breed standard faults had been found?

NOT Neutered guy

> He is now almost a year old

He is very young to be put to a bitch IMO and I learned the hard way that NOT DNA Testing, obtaining hip & elbow scores (of which he would have to be a minimum of 12 months at time of X Rays) and eye testing will do you NO favours in the longterm and certainly not give you credability in the Show Arena.

NB An aggressive female usually means - not ready and can put an inexperienced Stud off for life!
Maybe rethink your timings and obligations in producing Healthy Litters?

Are you in the US? If so then he won't have been hip scored which needs doing and under the OFA scheme its min of 2yrs for scoring, so he is too young to be used if you are being responsible.

Have you progesterone tested your bitch? If she is telling this boy off then either she is not ready or gone over. Get this done and if you don't have help find someone who can hold your girl while you guide him in to what he needs to be doing. Progesterone testing is needed to see where she is and when if not already ovulated where you are. Not every bitch ovulates the same time each season, some could be ready day 4 and others as late as day 28.

He could have been"Pennhipped"from 16 weeks of age, PennHip is of course a US based assessment of hips

That's the earliest but they recommend scoring again at 6 months and then a year old. Why would you not follow the scheme recommended by the AKC and breed clubs and use OFA?

You only get a snap shot at 16 weeks. Anyone scoring then and never again is not one I would go anywhere near.

FYI what the breed club recommend in terms of health testing and saying hip scoring under the OFA

http://mascusa.org/breed/health

> under the OFA scheme its min of 2yrs for scoring, so he is too young to be used if you are being responsible.

OFA allow Prelims from 12 months and then re x-ray at 2 for final result.

Many use preliminary grading to use a male before 2

But the whole thing is no breeding until properly scored and a CHIC is given. Prelims aren't there to breed earlier, if gives you an idea

I agree,

> We're only on about the second day of her allowing mating

Health testing aside as it's been said above How do you know she is allowing maiting if she won't let the stud near her? You havnt allowed the neutered one to mate her have you? Sounds like she isn't ready yet you progesterone tested her to see what's happening?

> The last heat, there was only one male - the neutered one. They tied several times and no puppies-great!

This may be part of your problem. Why did you allow them to mate last season? Your female maited last season with this male she lives with she may prefer the make she is more familer with to this new young pup.

Really? You have an in depth knowledge of PennHip assessment scheme & why using the results(of 16 week  testing) as a guide has improved the state of hips in subsequent generations?

The assessments done at 16 weeks have been proven to be accurate during the development of the scheme. PennHip assessment measures the hops differently to OFA etc & of course involves 3 Xrays not the single one the other schemes

I read their own website and its what they recommend. All under their FAQ's

The health testing doesn’t really matter when you consider we should not breed from a dog who isn’t even a year old since you have no idea of the ‘final product’ of dog you have. Particularly with males temperament is not established until later.

Tectona I agree on this too