What is opposite gender of a cow?

The opposite gender of a cow is a bull. Bulls are renowned for being more aggressive than cows, and are also more muscular. A male bull which has been castrated is referred to as a bullock, or steer. Here's a picture of a bull for reference:

Humans have learnt to use the fiery temperament of the bull for their own entertainment, and despite the controversy, bulls are still used for sport across the globe. In America bull riding is a popular sport, which as the name implies, involves the riding of bulls. This isn't doesn't involve a leisurely canter like you might do with your favourite pony, instead it is more typically clinging on one handed to the bucking beast for about 8 seconds, if you're lucky.

The Running of the Bulls still takes place in Spain to this day, and involves provoking a group of bulls to stampede down delegated streets whilst a group of brave, (or mad!) people try desperately to evade their horns, feet, and generally just try to get out of their way and stay alive. If this has sparked your curiosity, bare in mind that bulls can run at speeds of about 35 mph!

Aside from sport however, bulls are commonly used for beef and breeding, and many live long and happy lives enjoying their bachelorhood on the farm!