help me identify the factors which make grass-fed beef tender or tough
Some breeds are more tender.

Aging beef for at least 2 weeks allows the protease enzyme naturally present in beef to relax rigor mortis.

The way the beef is chilled initially can cause something called "cold shortening" in which protein fibers contract, and so are more chewy.

The amount of exercise that the animal gets during life affects the toughness of its muscles. Sedentary animals are more tender.

Older animals are tougher but also more flavorful.

A calcium deficiency can prevent protease enzymes from working as well, and so there is less natural tenderization during aging.