Pot roasts also come from the fore- and hindquarters of the carcass. These muscles are more heavily exercised and contain more connective tissue, making them less tender. Moist-heat cooking takes more time, but the results are worth waiting for. The beef becomes fork-tender and develops a savory depth of flavor unique to slow-cooked beef.
Pot roasts from the chuck have more fat, and thus more flavor, than those from the round, but many beef chuck and round cuts can be used interchangeably in pot roast recipes, requiring only slight adjustments in cooking times. Take advantage of this fact when the cut specified in a recipe is not available, when certain cuts are on special or to accommodate family preferences.
Source: www.beefitswhatsfordinner.com
