Mature bucks can lose 30% of their body weight during the 6 weeks of peak breeding.  That's a 200 lb live weight buck falling to 140 lbs in 6 weeks.  I can show plenty of examples of absolute toad mature bucks on trail-cam in mid-October ending up only shells of themselves when killed late in December.  And THAT is why - outside of big agriculture areas - so few bucks live beyond 5-6 years old.  Their bodies can't take the ravages of the rut anymore.