Every once in a while, there will be a calf born that has large front knee(s). Why is this?
It may be normal for some calves to be born with "large knees." Some large-framed beef calves do have large carpal joints (the joint commonly called the front knee and comparable to your wrist). The long bones of the front leg grow from growth plates just above and below this joint, and on large-framed calves these may appear exaggerated until the calf "grows into them." If this is the case, then both joints should appear to be the same size, and the calves should not be lame on either front limb.
If a calf is born with one joint larger than the other, then the cause should be investigated by a veterinarian. Sometimes calves develop one or more swollen joints shortly after birth due to a navel infection (navel-ill). This is a serious disease condition caused by bacteria entering the navel and entering the bloodstream. The resulting septicemia or blood poisoning may localize in the joints (commonly the carpus or the hock). This is a debilitating bacterial infection that must be treated aggressively to prevent death or crippling arthritis. Consult with your veterinarian about an appropriate treatment for navel-ill. Better yet, to prevent navel-ill, ensure that calves are born in a sanitary environment, and thoroughly apply strong iodine to the navel at birth.