Heteropogon contortus, commonly spear grass or assegaaigras, can form dense stands where runoff water from a tarred or blacktop road makes its life very easy.
The gathering of impala posing with the grass does not necessarily mean they will eat all of this soon. Impala diet includes shortish grass but also much lower leaf browsing from trees. Winter herds may exceed 100 when food is scarce. Very few males bearing the high horns can be spotted in this herd of predominantly females and young.
Sight, hearing and smell are all well developed in impala. Such a herd is super-aware when those senses are applied in unison as here, the concerted herd acting as one organism. Still, the carnivores might take the slowest one on a bad day for impalas.
There is more about H. contortus in the Grasses Album and the Mjejane Album; on impalas as accessed via the Search Box (Riëtte, 2016).