Written By Dr. Kris Hiney

This month we will review research concerning pastures and foraging behaviors in horses. Most horsemen would agree that horses grazing at pasture represent the most natural way to feed a horse.  Certainly it represents the most economical and the least labor intensive method of feeding.  However, many owners have questions related to what or how much a horse’s is consuming when its primary source of feed is pasture grass.

This ambiguity of how much grass a horse may consume makes selecting additional concentrates or supplements more of a challenge.  In addition, many horses clearly volunteer to consume pasture grass well over their nutritional needs making regulation of body condition score very difficult.   The range of dry matter intake of horses on pasture has been reported to be as wide as 1.5 to 3.1% of their body weight in a 24 hour period. Usually young horses and lactating mares will be on the upper range of intake which would make sense due to their nutritional demands.   Mature horses are reported to typically consume 2-2.5% of their body weight in dry matter.  However, it does appear that many of our equine friends have failed to adhere to book values when given the opportunity.  A recent study looking at weight gain in pastured ponies found that on average the ponies consumed 3.8% of their body weight in dry matter, with ranges of 2.9 to 4.9%.  Others have also reported horses consuming as much as 5% of their body weight in dry matter! It is rather easy to see why horses can quite easily gain weight on pasture.

But what about horses which are only turned out for part of the day in an attempt to control feed intake? Is this an effective technique or do they simply manage to eat faster in their allotted grazing time?  In a study which attempted to determine how much a horse can consume in an 8 hour period, horses were individually assigned to small paddocks, allowed to graze for four hours, then switched to a new paddock for an additional 4 hours. The small paddocks were then harvested to determine how much the horses consumed in the given time period. In this experiment horses were able to consume 1.3% of their body weight within an 8 hour period.  In addition, their consumption rate was twice as high in the initial four hours the horses were allowed access to grazing. Therefore the horses were able to consume almost 1% of their body weight in just four hours!  Thus even limited grazing can easily result in weight gain.  From this data the authors concluded that for these particular grazing horses, only 9 hours of grazing was necessary to meet their energy needs.

While we know that good quality pasture can easily meet a horse’s maintenance requirements, does it provide additional benefits to the horse?  In a study designed to look at the effectiveness of pasture turnout in maintaining fitness, horses which has been ridden 1-2 hours per week, 5 days per week for 12 weeks were then stalled, continued to be ridden or were turned out in a large pasture.  After a 14 week period, all horses participated in a standard exercise test.  This proved that the horses allowed free access to wander through a relatively large pasture maintained their fitness compared even to the horses ridden 5 days per week.  Thus pasture turnout seems to be a reasonable solution during down time when the horse is not ridden for maintaining fitness. The pastured horses in this study traveled on average 10 km a day compared to 5 km per day in the horses which were ridden.  This study again lends support to the value of pasture turnout.

So what if we want the fitness benefit of pasture turnout without the obesity inducing over consumption?   Often the traditional answer has been to employ a grazing muzzle. In a study looking at intake rates in ponies wearing muzzles compared to their non-muzzled counterparts, muzzling resulted in an 83% decrease in overall intake. However, in just three hours, the non-muzzled ponies were able to consume 0.8% of their body weight in dry matter.  This is quite similar to the observations in the previous studies.  In addition, the same team of researchers found that the ponies “grew wise” to their limited access to grass and learned to increase their consumption rates during their restricted period.  Therefore limited time on pasture may not be as effective for foxy ponies once they learn what you are up to!  So what is our take home message?  Allowing horse’s time to graze is very beneficial, not only for their mental health, but also for their physical health.  However, in order to control intake and thus body condition score in our enthusiastic eaters, we made need to employ additional measures such as limited turnout or grazing muzzles.