Dietary Mineral Requirements for the Growing Horse

Mieke Holder, PhD, assistant professor in the University of Kentucky Department of Animal and Food Sciences, gave a talk on dietary minerals in the growing horse during the second session of UK’s 10th annual Equine Research Showcase Jan. 19. The session highlighted nutrition and pasture topics appropriate for weanling to yearling horses. Presenting sponsors for the event included BET, Kentucky Performance Products, McCauley’s, Merck, Rood & Riddle Equine Hospital and Tribute Equine Nutrition.

Holder shared the importance of looking at dietary minerals in the growing horse for the benefit to both horse owner and environmental health. She also described the complications of researching this age group because of how fast their mineral uptake can change. Participants were given information to advocate for better mineral research in establishing biomarkers for future studies, resulting in horse owners not having to overpay for minerals they don’t need as well as to help protect the groundwater from eutrophication (process by which a body of water becomes enriched in dissolved nutrients).

Holder began by establishing the definition of growth in horses, which she said is the increase of both the body size and body weight as the young horse matures. Growth rate, however, is another way to define growth, according to Holder, and that is the amount of growth that occurs over a specified period of time.

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“There are many factors that can affect growth rates, some of which have to do with genetics – breed – or even the environment -nutrition, management, season.”

Holder then displayed actual data of some of the foals being housed at the UK Maine Chance Equine unit. Their growth rates gradually got lower the older the foal became.

“As expected, growth rates of the very young foals that are just a week old are quite high, and as these horses mature and have an increase in their body size, the average daily gain slowly decreases,” Holder said, “There are many factors that can affect growth rates, some of which have to do with genetics – breed – or even the environment -nutrition, management, season.”

Comparing the requirements of a young, growing horse to a mature horse hanging out in the pasture, it is evident that the younger of the two has higher nutritional requirements per kilogram of body weight. Holder explained that when thinking about mineral requirements specifically, we have a general understanding that as the skeleton changes, there is going to be a higher requirement for minerals such as calcium and phosphorus.

“But in truth, it’s not just calcium and phosphorus. It is all the other minerals that also play a role in how these foals grow and develop,” Holder said.

To further explain why dietary minerals are so important, Holder gave attendees an example.

“So, if we have a 12-month-old yearling, and this guy has a predicted adult weight of 1,100 pounds (or 500 kilograms), he is going to be consuming approximately 17 pounds (or 7.8 kilograms) of dry matter per day. And that amount of dry matter is going to come from the concentrate portion as well as the forage portion of what that yearling is being fed,” she said. “If we envision that as a pile of food in front of the horse, it’s quite impressive that at the end of the day, if we take this concentrate/forage and break it down into all of these nutrients, only about five ounces (or 150 grams) of that pile is actually going to represent the minerals in that diet.”

Breaking down the minerals and their responsibilities, Holder first differentiated macro minerals versus micro minerals. Macro minerals tend to be needed in grams per day, and they’re primarily responsible for bone and teeth strength/integrity, muscle contraction, nutrient transfer and intra/extra cellular fluid distribution. Micro or trace minerals are needed in milligrams per day and are part of or co-factors to enzymes through which they affect: metabolism, hormones, connective tissue, antioxidant status and the nervous system.

Holder explained that however important minerals are, more is not always better.

“Some minerals are actually toxic at relatively low levels, whereas other minerals can become toxic if you feed too much of them,” Holder said. “From an environmental point of view, because horses are not capable of storing large quantities of minerals, those excess minerals that you provide to the horse become part of the fecal matter and urine and end up eventually contaminating our waterways.”

“All of this is already very tricky, but now if you throw a growing horse into the mix, you are dealing with a dynamic model.”

Knowing how much to feed a horse is incredibly important. For many years, scientists have been trying to determine what the actual mineral requirements of horses are, she said. There are a couple of different ways this can be determined, but one of the traditional ways is to do a balance study.

“Basically, you feed the horse a certain amount of mineral, and you collect all the feces and urine from that horse and calculate what disappeared. In theory this sounds pretty simple, but in the real world this is a very difficult study to do. You normally have to start out with an adaptation period of 14 days to stabilize the intake of that specific mineral. You then have a five to seven-day period of collection of urine and feces. And then if you want to determine endogenous (or internal) losses, you typically need to include multiple levels of that mineral,” Holder said.

According to Holder, what seems like a simple experiment becomes very difficult, expensive and resource intensive. Evaluating trace minerals can be challenging. As they are already prescribed in small amounts, the accuracy in which they can be recovered in feces and urine becomes difficult.

“All of this is already very tricky, but now if you throw a growing horse into the mix, you are dealing with a dynamic model,” Holder said, “The rate at which a 6-month-old horse is growing at the beginning of your adaptation period is going to be different than the growth rate at the end of the collection period.”

Another way to determine requirements is through using biomarkers. The biomarker will tell you when you are feeding the optimal amount of the specific mineral in question. Holder used selenium as an example.

“We know selenium has this biomarker of glutathione peroxidase. In theory, as you feed more selenium, there is an increase in glutathione peroxidase activity. So if you were to set this up in a study, you would have several different levels of the mineral of interest that you feed, and then you could see some kind of pattern,” Holder said.

The biggest problem that we have is that selenium is one of the only minerals that has a good reliable biomarker available. Not just an issue for horse research, the lack of reliable biomarkers available affect human and other animal research. Holder said that there is a lot of research going on across different species to establish biomarkers that can be used.

Begging the question, “How do we know how much to feed our horses then?” Holder went into detail establishing the route that equine nutritionists take.

“As nutritionists, we like to refer to the recommendations made by the NRC [Nutritional Requirements of Horses]. The NRC basically contains the nutrient requirements for horses and the most recent publication was put out in 2007. The NRC recommendations are based on available scientific literature, and wherever the literature is lacking, they will apply mathematical equations and use that to make recommendations. In some cases where data is completely lacking, they will look at other species’ available data to make recommendations,” Holder said.

An important thing to note about the NRC guidelines, Holder said, is that they are based on specified average growth rates. This means that if a horse is maturing at a different growth rate, you may need to make adjustments that will take into account the growth rate variance.

Holder explained that we don’t really know if all mineral requirements decrease similarly or linearly with age and decreased growth rates. It is important to try to find new ways to solve this puzzle.

She highlighted the need for future research on identifying biomarkers for minerals that can be used to monitor status and better characterize mineral requirements. Doing this work can benefit the financial bottom line for horse owners by helping them not pay for excess dietary minerals in addition to contributing to overall environmental health. She also stressed the importance of involving the community in research efforts.

~ reprinted with permission https://equine.ca.uky.edu/