As a reminder, the main source of information for my posts is Nutrient Requirements of Horses, Sixth Revised Edition.
There is no formula for carbohydrate requirements for horses. This is a huge gray area that depends on the horse's metabolism, its environment, what else it is eating, and its work load. It's kind of frustrating to learn that the requirements are so variable, especially because "[c]arbohydrates are the principal sources of energy in horse diets" (p. 34). So basically, carbohydrates are the most important thing your horse will eat, but you're kind of on your own when figuring out how to feed them.
There are different categories of carbohydrates, and some of them are considered better than others depending on the situation. The different types include:
- Monosaccharides - Examples are glucose and fructose.
- Disaccharides - The most relevant example for horses is lactose.
- Oligosaccharides - The examples given in the book are unrecognizable and mostly unpronounceable, so I'm skipping them here.
- Polysaccharies - Examples are starch, cellulose, and pectin.
As a little bit of context, Nutrient Requirements explains that,
For many years, the amount of nonstructural carbohydrate (NSC) in a feed was determined by subtracting the amount of NDF, protein, ether extract, and ash from total dry matter (DM). More recently, the term "nonfibrous" (or nonfiber) carbohydrates (NFC) has been used to represent this difference, whereas "nonstructural carbohydrate" has been used to describe a chemically analyzed fraction of a feed...The NFC fraction is comprised of all carbohydrates not found in the NDF component of a feed. The NSC fraction includes mono- and disaccharies, oligosaccharides (including fructan) and starch...Few commercial feed analysis laboratories completely fractionate the carbohydrates that make up NSC, but in most feeds, the amount of NSC can be approximated by summing the amount of starch and the amount of water-soluble carbohydrates (WSC). (pp. 34-35)I think this information borders on relevant because the NSC value of a feed is now included on most feed labels that I see and lower NSC values are considered more appropriate, particularly for horses with metabolic issues. I say borders on relevant because at this point, all we're getting is a general definition, which is not that meaningful, along with the information that feed companies may not be precisely measuring the value, but rather estimating it. (Estimates may not be bad, but I think it is always good to know if something is measured exactly or estimated and if the NSC value matters to you, you may want to contact the feed company directly to get clarification on how it was determined.)
However, the NSC value alone is not going to tell you how a food's energy is being absorbed by your horse. Nutrient Requirements goes through a variety of studies that assess things like volume of feed, type of feed, combinations of feed, where the feed is digested (sooner in the process versus later in the process), and the type and volume of bacteria in the digestive tract.
I managed to glean a couple of useful pieces of information from this section. First, feeding smaller volumes of a high-starch feed (i.e. high NSC value) may result in greater energy absorption from the feed than if a higher volume was fed (p. 38). Regrettably, what is considered a low versus high is not identified. But, presumably, if you feed three meals of two pounds of oats, your horse will get more energy than if you fed one meal of 6 pounds. This knowledge has been around for awhile, but it's always nice to see a scientific basis. Second, oat starch is considered more digestible in the small intestine than corn or barley starch (p. 38). Previously, Nutrient Requirements mentioned that an increase in digestion in the small intestine led to greater energy absorption than if the digestion occurred later in the process in the large intestine. Again I think it is common knowledge that oats is the most digestible grain, but I didn't realize it had to do with where the digestion occurred until I read this chapter.
Another thing that I want to at least mention here is the application of the glycemic index to horse feed. You can read the details on pp. 39-41, but I'll relay what I thought was most interesting.
In humans, the term "glycemic index" (GI) has been used to characterize the magnitude of the blood glucose increase to various foods. The primary purpose of the GI in human nutrition was to provide a means of comparing carbohydrate sources in order to manage hyperglycemia... Recently, several studies have attempted to apply the GI concept to horse feeds. The methods that have been used in horses have been extremely variable, making it difficult to interpret the results across studies... With appropriate standardization, there may be some application of the GI to horse nutrition; however, many factors will have to be considered, including the age, breed, and physiological state of the horse, as well as the physical form of the feed. In addition, a GI system must account not only for differences among feeds when they are fed separately, but also when they are mixed with other ingredients as is common in the horse industry. For example, Pagan et al. (1999) reported that the glycemic response of horses consuming sweet feed was reduced when vegetable oil was added to the feed.What I had trouble understanding was how the GI might be different and potentially more valuable from the NSC value already used. I have been thinking of the NSC value as something like a GI value, but that appears to not be the case. That said, my takeaway from this section is that a person probably wants to exercise some caution when it comes to the claims of certain feed companies that a particular feed is best for a horse with a insulin- or metabolic-related issue that requires more careful monitoring of blood sugar. I'm not an expert in this area, of course, but there are definitely feeds that appear to be labelled for use in metabolically-challenged horses based at least in part on starch values. I suspect at least some of this may be more marketing than reality, and I would encourage anyone with a horse that needs a special diet due to concerns about blood sugar levels to do some research on their own, preferably using sources not sponsored by a feed company (or even a vet because many - not all! - vets seem to parrot what the feed company tells them). In particular, I noted that age, breed, and physiological state of the horse may play a role in how a horse processes and reacts to different types of feeds. Additionally, it appears that it might be possible to control a blood sugar spike by adding fat to the food at feeding time (I think this is also true with people, but don't quote me on that). Anyway, this information is just another indication that feeding your horse is more than buying a certain feed and dumping it in the feed bucket.
The final thing that I want to point out here is that there is some information on things like how exercise impacts glycogen stores as well as the idea of "carbohydrate loading" and other ways in which feeding practices can affect the utilization of glucose during exercise as well as post-exercise recovery. For example, Nutrient Requirements reports that, "When horses exercised for 1 hour at 500 m/min or for 4 hours at 300 m/minute, muscle glycogen stores were depleted by approximately 60 percent" (p. 40). If you're like me and you have no idea how fast those paces are, I looked up the conversions. 500 meters per minute is equivalent to 18.64 miles per hour, which is pretty fast, although not unheard of, for endurance racing. 300 meters per minute is equivalent to about 11 miles per hour, which is probably closer to the pace that medium-paced endurance riders use and the 4 hour time in the example is a good fit for endurance conditioning and riding. Nimo can absolutely trot at 11 mph, although I've never pushed him to 4 hours straight of that pace. If I did, though, it looks like I would be severely taxing his glycogen stores and pushing him toward extreme fatigue.
Which is why I might be tempted to consider "carbohydrate loading." For those unfamiliar with the concept, it basically involves significantly cutting back on carbs while continuing to exercise for a few days before a major physical effort (like a marathon) and then significantly increasing carb consumption while resting just before the event. Apparently there is evidence that this practice does increase glocogen stores in humans and may assist in delaying fatigue. But, Nutrient Requirements goes on to state that "there is no clear evidence that carbohydrate loading has any significant benefits to equine athletes" (p. 40). So carb loading for horses is probably not helpful.
However, feeding a grain meal 2-3 hours prior to exercise may or may not be helpful, depending on a variety of factors like whether it is accompanied by hay. I didn't find this analysis that useful, because in the endurance world, we are feeding our horses all the time, particularly in the few hours prior to a competition, and we may not be feeding just grain or even grain at all. Because endurance feeding practices differ so significantly from what I see commonly done in other disciplines (like not feeding for a couple of hours before the competition to make sure the digestive tract is able to handle a significant, but short-term, physical effort), I think more research is really needed in terms of pre-competition feeding practices.
One study discussed did address the consumption of carbohydrates during the exercise. This study engaged in the practice that everyone can follow (heavy sarcasm here) of intravenous infusion of glucose. The study did show increased time to fatigue, but the authors of the study noted that "the intermittent feeding of horses during long-term exercise might not duplicate the effects of intravenous glucose infusion" (p. 41). Great, so I guess we keep doing the intermittent feeding thing during our conditioning and competitions, but in terms of the specific benefits or guidance on how we might best accomplish increasing time to fatigue with food, there is still really no information.
That said, I do think another study discussed might shed some light on exactly how serious glycogen depletion is for horses in endurance events and why intermittent feeding may only be partially helpful. This study looked at how fast glycogen is restored to the body after exercise. The study found the fastest rate repletion occurred during the first few hours after exercise and it was faster in horses fed hay and grain versus just hay. "At 28 hours post-exercise, muscle glycogen concentrations were 90 percent of pre-exercise values when horses were fed hay and grain, but only 71.7 percent...when they received only hay" (p. 40). I admit to being shocked that at 28 hours post-exercise (and this was probably not an endurance event type of exercise), glycogen stores had still not fully recovered. Although I am not surprised that adding grain made a difference. I often want something starchy after exercise, and I assume that is for a reason.
So there you have my attempt at looking at carbohydrate requirements for Nimo. My conclusion is almost to throw my hands up in the air. There is just enough information in this chapter to tantalize me, but nothing definite jumped out at me to give me a solid path for feeding Nimo. I do feed him a small amount of oats, and I already intermittently feed during longer conditioning rides and post work-out, but in terms of specifically what I should feed and in what amounts, I still feel like I'm in the dark.
That won't stop me from blindly stumbling along toward Fat and Protein requirements in future posts, though:)