A slow cooker is an optimal choice for cooking a rump roast because of its consistent low temperature. With standard cooking temperature varying between 175 and 200 degrees Fahrenheit, it takes at least eight to 10 hours to cook a 3-pound rump roast on the lowest heat setting, and five to six hours on high. By slow cooking the meat, it becomes fall-apart tender, a state that is not achieved through quick-cooking methods.

The easiest way to cook a rump roast is to simply place it in your slow cooker with seasonings and aromatics, along with a liquid to assist in braising the meat. Start by rubbing a liberal amount of salt and pepper into the surface of the meat. Line the bottom of your slow cooker with vegetables such as carrots, potatoes, celery and onions, then place the roast on top of them. Add one cup of water, beef stock or chicken stock to the slow cooker, place the cover securely in place and set the cooker for eight to ten hours on low.

To lock in even more flavor and really make your rump roast shine, brown it first before putting it in the slow cooker. Rub your desired seasonings into the meat. Then, heat a couple of tablespoons of oil in a pan on medium-high, place the roast in the pan and turn it every couple of minutes until each side is browned. While the meat is cooking, line the slow cooker with the potatoes, carrots and celery. Remove the roast and place it on the vegetables in the slow cooker. Add another tablespoon of oil to your pan and saute chopped onions until they become translucent. Add a cup of water, beef stock or chicken stock to the pan, swirl it around to dislodge any browned pieces on the pan and pour the onions and liquid into the slow cooker. Set the timer for eight to ten hours on low.

If your slow cooker has a high setting, you can cook it even faster. Season your roast to taste and pour in a cup of cooking liquid, but simply decrease the cooking time to five to six hours on high. In this amount of time a rump roast will become tender, though not as tender as a long-cooked roast.