From research I see it's best to give this 7-10 days after mating.

Any reason it can't be given sooner?

It can: it has to be given in two doses: the first either during the season or 7-10 days after mating, and the second dose 1 - 2 weeks before whelping.

See datasheet.

Sorry - I meant sooner as in sooner than 7 days after mating. 4 or 5 for example ? Does it interfere with the sperm ?

I've given it 24 hours after mating. Can be given as soon as in season too.

I've given the 1st injection to my girls at various times, some before mating, especially if the girls are going off to an outsider to be mated, as I couldn't always guarantee I could get them to the vets within the 7 days after mating. Those that we've mated here have been done during the fertile days, and up to 7 days after mating, either way most have resulted in pregnancy, and most have been decent size litters.