100 sheep 200 days: 100×200= original grass+new grass 200 days.

150 sheep eat 100 days: 150× 100= original grass+100 days new grass.

Therefore, the grass added every day is: (100× 200-150×100) ÷ (200-100) = 50 (copies).

Original grass:100× 200-50× 200 =10000 (copies) 10000÷(250-50)=50 (days).

A: 250 sheep can eat for 50 days. In order to prevent grassland desertification, this grassland can graze up to 50 sheep.