My dog is female and she is leaking brownish fluid but no smell or pus. Is it the start of her cycle?

If you have concern about this, then please talk to your vet. Internal problems are not anything to mess with.

Dog ownership is a learning process and you should seek medical advise from the real experts.

It could be her heat cycle coming, or it could be something else. Dogs don't need to have litters in order to get uterine infections such as Pyometra. Of course, drainage is always better then nothing. But still, speak with your vet about your concerns.

Plus, how old is the dog? Age does make a difference for the types of advice you're getting.

Impossible for any of us to say for certain, but there are several other possibilities.

The discharge you describe is not what is to be expected from a normal 'season', regardless of whether at start or finish. It may not be entirely a blood discharge... uterine 'infections', called 'mucometra, or a type of uterine problem similar to pyometra, but not an infection, can produce a brownish discharge. A malignancy in either the urinary or reproductive tract may cause such a discharge. An abnormal season may result in an abnormal discharge even. Some dogs have "silent" seasons... no outer evidence is seen to her being in an actual season, except many male dogs are capable of telling and also taking advantage of the opportunity... can't rule out a miscarriage, either!

I would tend to favor the more common, least complicated reason, but would want to check out the more serious possibilities before giving any hint at an "all clear" for her. I suspect your vet would feel similarly. Sorry we can't give a more definitive answer... that's up to someone who can examine and test properly before 'ruling' on a potentially serious condition.

"Give your vet a visit for this" is the best, most accurate suggestion we can give.