Except for water, protein is the most abundant substance in your horse’s body. Even the framework for bone is protein. Proteins are constructed by linking individual amino acids into a chain, like beads on a string.
Once the string is assembled, hydrogen and sulfide bonds between individual amino acids make it twist and bend into complicated and beautiful structures that resemble the streamers of coiled ribbon you can buy to decorate packages.
There are 20 amino acids, eight to 10 of which are known to be ”essential” in other species. An essential amino acid is one that the horse cannot construct in his own body and which must therefore be present in the diet. The most important essential amino acids are the ”limiting” amino acids.
In the string of beads, each bead being an amino acid, the sequence of amino acids used to construct the chain is dictated by the horse’s genetic code. When putting together the string, if the next amino acid in line isn’t available, protein building is stopped. The missing amino acid that puts the brakes on protein building is called the ”limiting” amino acid.
Research into the precise amino-acid requirements of horses hasn’t come far since the 1989 NRC book. We have known for quite a while though that lysine is an important limiting amino acid in equine diets. A 2005 study from Virginia Intermont College showed that supplementing both young and old horses in light work with more lysine resulted in improved muscle mass. The new NRC has increased the lysine requirement for horses from 3% of the total crude protein to 4.3%.
Suggested intakes of crude protein have also been changed. Low, average and high categories have been created for horses at maintenance. The NRC defines horses in the low category as those that don’t move around much — either because of temperament or because they are stall-bound — average as horses with average activity levels and high for horses that are nervous, pace, rip around more on turn out. There are other things, however, that could put a horse into a specific category. Horses with low lysine intakes will need more crude protein, because more protein will be wasted (see sidebar). Horses recovering from serious illnesses, injuries or surgery will also need more protein to repair the tissues and support the immune system.
The equations now suggested for calculating a horse’s protein needs are (Note: 1 kg = 2.2 lbs. An 1100-pound horse weighs 500 kg):
1.08 x bodyweight in kg
1.26 x bodyweight in kg
1.44 x bodyweight in kg
For a 500 kg horse at maintenance, this results in a range of protein needs from 496.8 to 662.4 kg, a difference of just over one-third of a pound of protein. The lysine requirement for these intakes ranges from 21.4 grams/day to 28.5 grams.
The suggested protein intake for working horses has also changed. The 1989 NRC recommendations assumed horses in work would get all the additional protein they needed simply by meeting their increased calorie requirements using the same diet they had at maintenance. The new NRC recognizes that horses in work build lean body tissue (muscle) and also have protein losses in sweat that should be considered.
The current recommendations are:
Light Work: Maintenance +
[0.089 g x bodyweight in kg]
Moderate Work: Maintenance + [0.177 g x bodyweight in kg]
Heavy Work: Maintenance +
[0.266 g x bodyweight in kg]
Very Heavy Work: Maintenance + [0.354 g x bodyweight in kg]
In other words, they’re using the baseline maintenance protein requirement plus an allowance for building muscle, as determined by research that has been performed since the 1989 NRC. This amounts to an increase of 88.7 grams for a 500 kg horse in moderate work. Lysine requirements are the same, meaning 4.3% of the total protein should be lysine.
Make It Useful
It’s going to take feed or supplement manufacturers and ration software a while to catch up to these new recommendations. Meanwhile, let’s take a look at how your diet might measure up. The new NRC also contains a convenient table on pages 164 to 166, which lists the average amino acid content of some common feeds. We’ve listed the lysine content of some in our table on this page.
Let’s take as an example a 500 kg horse at maintenance eating 2% of his bodyweight of a 10% protein timothy hay. His intake of protein is 10,000 grams (10 kg) x 0.1 (10% protein) = 1,000 grams of protein, more than enough to meet the average requirement of 630 grams of crude protein. His lysine requirement is 630 grams x 0.043(4.3%) = 27 grams/day. By the NRC averages, his lysine is met since he’s getting 38 grams/day. Other sources report lysine in grass hays range from 0.26 to 0.4% with high quality hays.
Obviously, we could use some clarification regarding exactly how much lysine we can count on from hays, but suffice it to say that if you horse is on a hay-only diet and appears to be losing muscle mass, you may need to supplement with lysine.
Let’s take the same horse in moderate work. He now needs 630 grams + 88.7 grams for work = 718.7 grams of crude protein and 718.7 x .0043 = 30.9 grams of lysine. If he stays on the same amount of hay, he’ll need about 2 kg or 4.4 lbs. of grain per day to hold his weight. His crude total protein needs are still being met by his 20 lbs. of hay, so the protein level per se in the grain portion isn’t a consideration.
How’s he doing for lysine' If (and it’s a big ”if”) his hay meets his maintenance lysine requirement, he needs another 3.9 grams of lysine. Even the lowest lysine grain option, which is corn at 0.27% lysine, would meet this additional lysine requirement.
However, if his lysine needs were not being met this is the time you’re really going to see it. Symptoms such as fatigue, muscle pain/cramping and failure to build muscle bulk properly will show up.
Equine total protein needs can be easily met by a variety of diets, including feeding only quality hay through moderate levels of work. However, variable levels of lysine in hays could cause muscle problems, especially in horses being worked. If you run into these issues, the first thing to try is to supplement your horse’s diet with 10 to 20 grams/day of lysine.
Article by Dr. Eleanor Kellon, Veterinary Editor.