Question

What will happen if more cattle were brought into the ecosystem

Answer 1

The last bit of your question, YES these domestic animals have a huge ecological importance!  Especially if you released them into the wild.

When I first read the question, I wasn't so sure about this.  I figured they would probably just wander around for a while, possibly a few months or years, and then die out little by little as they competed for food and fell to predators.  

However, in the meantime they would likely eat everything, and then attract large numbers of predators, who would then have many babies.