Pasto vs. pastizal
In Spanish, both 'pasto' and 'pastizal' derive from the same root and are linked to the concept of grassy fields where livestock feeds. However, they are not entirely interchangeable as they refer to slightly different concepts.
Pasto
The term 'pasto' generally refers to grass or the grass eaten by livestock. It can also refer to a field or an area covered in grass.
Las vacas comen pasto en el campo.
(Cows eat grass in the field.)
El jardín necesita más pasto.
(The garden needs more grass.)
El pasto se ve verde después de la lluvia.
(The pasture looks green after the rain.)
Pastizal
'Pastizal' is a more specific term that is used to describe a type of natural environment, specifically a large area of land mostly covered by grass.
El pastizal es hogar de muchas especies de aves.
(The grassland is home to many bird species.)
En la región hay extensas áreas de pastizal.
(In the region, there are extensive areas of grassland.)
El ganado pasta libremente en el pastizal.
(The cattle graze freely in the grassland.)
Summary
Although both 'pasto' and 'pastizal' relate to grass and areas where animals graze, 'pasto' tends to be used more generally to refer to grass or grazing areas, while 'pastizal' specifically points out a type of natural environment —a large area dominated by grass.