I also typically feed a 13-14% concentrate feed along with forages.It 
  has
been proven that horses under strenuous exercise can benefit from 
  a
slightly higher protein level (13-15%). 
   
   
  Hmmm.  That would disagree with the research 
  conducted by Ralston, et al at Rutgers, whose studies demonstrated a 
  progressively higher incidence of metabolic problems in eventing horses when 
  total protein levels exceeded 12.  Also with work by Glade in Australian 
  racehorses whose acing times increased (that is, got slower) when protein 
  levels were increased above NRC recommendations.  I would be 
  interested in seeing the reference of the research you refer 
  to. 
   
   
  The average protein requirement
for pasture horses or horse under 
  light exercise is 10%.
   
  True, but people tend to get a bit hung up on 
  percentages, rather than absolute amounts.  So a 13-14% grain mix may be 
  barely adequate for one horse, while being excessive for another.  It all 
  depends on the protein provided by other feeds, including hay, other grains, 
  etc.
   
   
   
  Horses that are
not being ridden extensively and are on a high protein 
  ration are just
wasting money, because a lot of it is excreted in its 
  urine. 
   
  All of the excess nitrogen groups will be 
  excreted in the urine, actually (the amount excreted through sweat is actually 
  infinitisimal).  However, the carbon backbones of the amino acids will be 
  retained and used as an energy source.  However, there are other effects 
  of a high protein ration that can be detrimental to horses, especially in hot 
  weather, but may be beneficial in other situations---ie, in cold climates 
  where the increased heat production helps maintain body heat.
   
  However,if you do see a difference in performance or
you are 
  concerned, one option is to supplement with beet pulp. It is an
excellent 
  source of protein, 
   
  Well, no.  I dearly love beet pulp, 
  especially for endurance horses, for a multitude of reasons, but as a protein 
  source isn't one of them.  It has an average crude protein content of 9%, 
  equal or lower than most grass hays.  I would actually consider that a 
  point in its favor, though, as most horses get excessive protein already, and 
  its low protein content means relatively large amounts can be fed without 
  providing excessive protein in the total ration.
   
  do. The other nice thing is that beet pulp is relatively high in 
  fiber,
which will help our horses stay warm through these cold winter 
  months. 
   
  Well, that's another myth, especially about beet 
  pulp.  The fiber in beet pulp is primarily soluble fiber, which is highly 
  digestible, and so provides almost no heat increment to maintain body heat at 
  all.  If you wanted to rely on fiber digestion for winter heat, you would 
  have to rely on an insoluble fiber source, such as regular old hay.  
  Also, while ruminants, such as cattle and sheep, can provide a bonfire of body 
  heat just from digesting fiber, horses aren't all that good at it.  They 
  can produce some heat increment that way, but if you lived in a really cold 
  climate, or there was a bad icestorm coming over the pass and you wanted to 
  jump start the horse's 'furnace', a good protein source will do it better than 
  anything else.  IMO, that's about the best use of alfalfa there is.  
  Good source of fiber (since that does provide *some* heat increement), plus 
  plenty of protein to produce metabolic waste heat to bump up the core 
  temperature for a few hours.
   
  Hope this helps. :-)
   
  Susan G