Written By Dr. Kristina Hiney

This month I will begin a new series which tries to summarize some of the new information which has been gathered in equine nutrition.  I will be grouping similar topics together and trying to summarize how this information might be relevant to you and your horse.  We will discuss if this new information means you should change what you have been doing, or you can feel reassured that you are right on track!  And remember not all information may be relevant for your horse.  There is no need to feed your mature gelding who is trail ridden on the weekend like an endurance horse preparing for a 100 mile ride!

This month we will focus on some new information on protein nutrition in the horse.  Certainly this is the time of year when many of us are busy procuring our hay supplies for the upcoming year.  Often we want the very best for our horses, and typically look for high quality alfalfa hays.  But is that necessary, especially in a year where the weather does not cooperate and hay selection may be more limited?  In a study using mature idle geldings, the digestibility and usefulness of protein from a variety of hays was tested.   Horses were fed diets of either  mixed grass hay alone, the mixed hay with increasing amounts of oats, or alfalfa hay that was either early bloom, mid bloom or late bloom.  As the maturity of alfalfa hay increases, typically its protein content decreases.   Therefore, many horsemen prefer earlier bloom alfalfa.  But is this necessary? In this particular study the horses were also fed at just 1.6% of their body weight as fed, which is typically a little lower than most people feed.  Thus these horses might have been fed at a lower rate than the average horseman would feed.    As expected, the protein intake of the horses increased as they were fed the alfalfa hay, with increased protein intake the earlier the stage of maturity. The digestibility of the protein in the diet also increased when fed straight alfalfa compared with the mixed hay, and digestibility was greater with less mature alfalfa.  That does reconfirm our knowledge that forages of later maturity are indeed less digestible.  However, the nitrogen retention between the groups of horses was not different.  Nitrogen retention refers to how much nitrogen remains in the horse’s body.  So if the horse’s nitrogen intake (which is reflective of protein intake) was higher, but the nitrogen did not remain in the horse’s body, where did it go?  The extra nitrogen was actually excreted in the urine.  You may remember from our earlier series on protein nutrition that excess protein consumed cannot be stored in the horse’s body. Instead, the nitrogen is removed from the amino acid, and the remainder of it can be used for energy or stored as fat.  Overall, for mature idle horses, there is no need to feed these higher “octane” hays, as it all that extra protein just ended up back on the ground!  There was no value to the horse in these high protein hays.

However, what if you are not feeding a mature, idle horse, but instead are feeding mares and foals?  Their protein requirements are undoubtedly quite higher.  But it is not just protein quantity we must consider, but also the amino acid profile of the diet.  You may remember from previous articles that equine nutritionists have only described the requirements for lysine in the horse.  This is in stark contrast to other species in which the complete amino acid requirements have been well defined for  both growth and lactation. In other species, lysine is also known to be the first limiting amino acid, followed by threonine and methionine.   It is presumed that this may be true in horses as well.   In a study looking at pregnant mares, their subsequent foals and the mare’s themselves during lactation, researchers posed the question if plasma amino acid concentrations would differ after eating.   Theoretically, plasma amino acids which increase the least after eating immediately following a fast indicates the limiting amino acids.  In the weanlings, the amino acids which increased the least were methionine and lysine, for lactating mares it was methionine and for pregnant mares on this particular diet the amino acid which increased the least was leucine, one of the branched chain amino acids.  This study supports the idea that methionine may be the second limiting amino acid for nursing mares and weanlings, but leucine may also need to be considered.  However, this study did not provide information on how much of these amino acids may actually be needed in the diet, but stresses the need for additional studies.

The final study we will look at did try and examine the question of methionine needs in growing horses.   In a study which looked at the growth rate and plasma metabolites of weanling horses fed differing amounts of methionine, growth rate did not change with addition of methionine. However, weanlings were only fed the diet for 56 d which way not have been long enough to observe differences.    Addition of methionine did result in a decrease in plasma urea nitrogen.  But what exactly does that mean?   Remember that any extra amino acids must be catabolized and the amine group is removed as urea. The urea is synthesized in the liver, but excreted by the kidney.  Urea circulates though the blood prior to its removal.  An increase in plasma urea N indicates an increase in amino acid catabolism, which takes place if protein synthesis is limited by the availability of amino acids.  If we assume that an increase in methionine in the diet allowed more protein synthesis to occur, this would result in more N retention, and less amino acid catabolism.  In this study, the authors did not observe a linear decrease in plasma urea nitrogen as  methionine was increased beyond 0 .2% of the concentrate. In this example, the weanlings were fed at a rate of 1.25% of their body weight in concentrate, or about 8.4 g of methionine.  You may have noticed that many feed companies now include the levels of methionine in their product.  Using this study as an indicator of methionine requirements, at least for weanling horses would indicate that methionine should at least be at the level of .2% of the concentrate if fed in comparable amounts. If less concentrate is fed, than the concentration of methionine should be higher.

To summarize what we can take from these three studies, we have reaffirmed that mature idle horses don’t really need high protein hays. While their protein may be more digestible, those amino acids remain largely wasted.  For horses with higher protein needs, it may be time for us to turn our attention to more than just protein quantity, but quality as well.  Hopefully soon we will have better knowledge on exact amino acid requirements, but at least we are now somewhat closer to knowing about methionine!