Penn State Agronomy Guide lists manure production for a beef cow and calf as about 60 lb manure/day/ 1000 lb live weight;  for a steer, that number is bumped up to about 75 lb manure/day/1000 lb live weight. Manure analysis value is listed as 11-7-10 for the cow and calf, and 14-5-8 for the steer.  You can find that info here.   click on table 1.2-13.  (also, check out table 1.2-14 - hopefully that table makes sense to you.)

My question is:  when you say "Mother Nature takes its course", what exactly are you saying?  Is the manure running off the lot?  If so, where is it going to?  Are you able to scrap manure off the lot now?  Have you thought about contacting the local NRCS technicians, have them look at it, and see if there is a potential for EQUIP funds to help address your concerns?  If manure is leaving the lot in storm water runoff, I would expect they would be able to get some cost-share funds for you. At least it works that way here.

Note: figuring an average of 550 lb/animal and 30 animals, I estimate these 30 animals are generating 225 tons manure per year.  That is at least a ball-park figure. From there, you can figure if that is worth an investment.