A dairy farmer wanted to increase the amount of milk produced by his cows. He decides to test the effect of a particular feed additive. He give half of his herd
the additive. He maintains the amount of grazing time, the amount of water the
cows drink per day, and even how often the cow gets chased by the local dog
was the same for all the cows. Each day the farmer milks the cows at the same
time and compares the volume of milk. What is the independent variable?

I think the Independent variable is the amount of feed additive that the farmer gives to his cows.

Explanation:

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.