At one time two reputable people told me 
  that one should pull one's horse off of grass pasture the week before a ride 
  and put him/her in a dry lot with hay as grass caused a horse's pulse not to 
  drop as quickly at a vet check. 
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  I sure am confused by all this and would like opinions on the permanent 
  pasture question.  I have two horses that I would like to see a bit more 
  weight on.  They are both fit and muscular, but not much covering over 
  the ribs (can't see ribs but sure can feel them)  They look good right 
  now, but I would like to see more weight going into the ride season. Living in 
  Calif, they got moderate exercise all winter, free choice of hay
  (mixes of alfalfa, oat and grass), and about 10 to 15 GALLONS a day 
  between them of beetpulp/pellets, dry cob and corn oil.  Worming and 
  teeth all checked. They looked good,but never plump,and now are on green 
  annual grass pasture (in addition to the above).
   
  My friend has her horses on permanent irrigated pasture.  She has a 
  horse that keeps gaining weight, despite miles and miles of riding. He "looks" 
  really fat, but is extremely fit, and does in miles what I do getting all 5 of 
  my horses out.  She has concerns of taking him to an endurance ride and 
  having the transition from green grass to dry hay.
  ?She can't get him to drop any weight.
   
  Meanwhile, I am scurrying weather wise to put in my own irrigated 
  pasture, about 3600
  sq feet, all I can keep watered over the summer.
   
  Any opinions appreciated.
  Karen