Forgive me if I have underestimated your level of knowledge.
Presumably you have a database table where each row is one bison? And one of the columns is the weight? And another column indicates whether this bison is one that will gain weight or not (ie adult or not)? Or maybe, instead, a date of birth?
So you need a loop to work through adding the required weight to the relevant column in each row of the bison that are not adults?
So you could run a SELECT query to obtain the animalCode# of the animals that would gain weight. You could then set up a PHP WHILE loop working through the results to run a SQL UPDATE query on one animal at a time.
Apologies if you knew all this already.
It occurs to me that they may be an object oriented programming approach to this but I don’t have the experience/knowledge to advise about that.