Wyvern are actually really great at gathering cactus sap with their pushback attack. (Clicking R3, AKA click the right stick, while on the ground) Doing this gathers tons of sap from desert cacti. Bit late posting this but haven't seen anything here about it.
To know how much time need to the next feeding just do the next. Look the Food number# then that number multiply x 10, then divide /3600 and the result will be the number of hours until the next time need to be fed. F.E the food number is 1850.5 1859.5x10=18595 /3600 = 5.1 hours
A fire wyvern is one of the best you can have because they are resistant to fire damage so alpha wyverns are much easier to kill same with all the wyverns they can't hurt each if they are the same element but the bite attack still works on all wyverns keep this in mind