No, it is not necessary to thaw a small roast before cooking it in a slow cooker for five or six hours. Slow cookers are designed to cook food, including frozen meat, safely over an extended period of time at low temperatures.
However, there are some important considerations when cooking frozen meat in a slow cooker:
Cooking time: When cooking frozen meat in a slow cooker, it will take longer to reach the desired level of doneness compared to thawed meat. You mentioned cooking for five or six hours, but depending on the size and thickness of the roast, it might require additional cooking time.
Safety: While slow cookers are designed to cook frozen meat safely, it's crucial to ensure that the internal temperature of the meat reaches a safe level to kill any potential bacteria. Use a meat thermometer to check the internal temperature, and the roast should reach at least 145°F (63°C) for beef and 165°F (74°C) for poultry.
Liquid and Seasoning: Since frozen meat releases moisture as it thaws, you may want to adjust the amount of liquid in your recipe or reduce it slightly to avoid having too much liquid in the slow cooker. Additionally, seasoning may need to be adjusted as frozen meat may require more to penetrate the surface.
Browning: Browning the meat before placing it in the slow cooker can enhance the flavor, but this step is not feasible with frozen meat. However, you can still sear the thawed portions of the meat before cooking if you desire.
Keep in mind that while it's safe to cook frozen meat in a slow cooker, for best results and more even cooking, it's recommended to thaw the roast before placing it in the slow cooker. If you're short on time and need to cook from frozen, be sure to plan for the extra cooking time and ensure the meat reaches a safe internal temperature.