The most significant limiting factor in the grasslands is

Rainfall

Explanation:

Grassland is an area where the highest proportion of the land is covered with grasses. The dominant vegetation in grassland are the grass which are not much tall. The number of tall trees are proportionally very low. Although a grassland is a place where abundance of food is available for animals, there is limiting factor too. Lack of rainfall and source of water can be taken as limiting factor. The existing source of water might dry-out unless there is abundance rainfall. So, rainfall is limiting factor in grasslands.