Can you eat beef when you are breast feeding?

Yes
It's safe to eat whilst you're breast feeding, though it can affect you it is unlikely to affect your baby.
Beef is the culinary name for meat from bovines, especially cattle. Beef can be harvested from cows, bulls, heifers or steers.
Beef muscle meat can be cut into steak, roasts or short ribs. Some cuts are processed (corned beef or beef jerky), and trimmings, usually mixed with meat from older, leaner cattle, are ground, minced or used in sausages.
Meat is animal flesh that is eaten as food. Humans are omnivorous, and have hunted and killed animals for meat since prehistoric times. But the advent of civilization allowed the domestication of animals such as sheep, pigs and cattle, and their use in meat production on an industrial scale.