Hi all,
I'm a newbie but am well experienced with trimming cattle feet :) We have to trim the whole herd at least once a year usually twice and then the showies too during prep.
Yes feed is a major reason, time of year another. Genetics yes too. Your biggest reasons though are conformation and ground type. Conformationally your heifer (i can't remember already ?? :) ) has hard feet i'd guess and you have softer ground. Also you might want to look at the areas your trimming ie heel toe etc you could be making the conformation worse ie long toe, low heels??
Really look at your bull choice when breeding - feet and legs are genetic and can be potentially difficult to improve as they rely on the whole hanging of the shoulders hips etc. Install large areas of concrete around water and feed troughs or fill your laneways with gravel - make the animal walk on rougher, harder ground.
Good Luck