AGRICULTURE & NATURAL RESOURCES
Basic Mineral Nutrition in Forages Frequently Fed to Horses
This Timely Information Sheet provides an overview of mineral nutrition of horses (Ca, P, K, Mg, Na, Cl, Fe, and Cu) and the importance of the forage nutritional analysis.
With pasture and hay being the cornerstone of the equine diet, knowing its composition is vital to assuring our horse’s needs are being met. Unfortunately, horse owners are learning the importance of hay testing after their horse’s health has deteriorated. The nutrient content of forage is comprised of a complex interaction of factors, which include: soil fertility, texture and pH, environmental conditions during growth and hay curing, stage of growth when cut, species, and variety of forage.
An important component of the forage analysis often overlooked is the quantity of minerals. Minerals in forages can vary significantly with soil mineral content, plant species, stage of maturity at harvest, harvest conditions etc. Consequently, forages should be sent to a laboratory for mineral analysis. The forage analysis will include the following minerals required by horses: Ca (calcium), P (phosphorus), K (potassium), Mg (magnesium), Na (sodium), Cl (chlorine), Fe (iron), and Cu (copper). Providing minerals at less than the recommended quantities can result in a dietary deficiency whereas providing minerals in excessive quantities can result in toxicity. Mineral requirements of horses can be found in Tables 1 and 2. For further reading and requirements for special cases, the National Research Council (NRC) 2007 has a publication entitled Nutrient Requirement of Horses available for purchase (https://www.nap.edu/catalog/11653/nutrient-requirements- of-horses-sixth-revised-edition).
Macromineral Content in Forages
It is useful to know approximate mineral percentages of legumes versus grass hays when making a decision on the type of forage to be fed. Ca content is 1.3% and 0.6% in legumes and grasses, respectively. P content of most forages is approximately 0.25-0.35%. Mg is approximately 0.25-0.3% and 0.20-0.25% in legumes and grasses, respectively. K ranges 2.0-3% and 1.5-3% in legumes and grasses, respectively. There is less than 0.5% Na in all forages. Cl is between 0.5 to 1.0% in legumes and grasses, respectively. Note: Fe and Cu are reported in ppm, or mg/kg. Fe is approximately 150 mg/kg in legumes and 50 mg/kg in grass. Cu is very low in many forages and ranges from 9-11 mg/kg.
Deficiencies and Toxicities
High quality forages are typically adequate in Mg, K and S. Magnesium is a vitally important ion in the blood; it participates in muscular contraction and it is also a cofactor in several enzymes. In rare cases, deficiency of Mg may lead to hypomagnesaemia, which is associated with loss of appetite, nervousness, sweating, muscular tremors, convulsions, and mineralization of pulmonary artery caused by deposition of Ca and P salts.
While K deficiency in adult horses is rare, young foals may become deficient in K as a result of persistent diarrhea and this in turn tends to precipitate acidosis. Adult horses that are exercised strenuously are also at risk of deficiency due to the resulting spontaneous changes in plasma K. The Na and Cl requirements are met by providing salt. However, a performance horse that sweats a great deal may require K, Na and Cl supplementation to replace excess mineral loss via the sweat.
A critical consideration in mineral nutrition of the horse is the calcium to phosphorus (Ca:P) ratio. The functions of calcium and phosphorus are considered together because of their independent role as the main elements of crystal apatite, which provides the strength and rigidity of the skeleton. The elements of bone are in a continual state of flux with Ca and P being removed and redeposited by a process that facilitates the reservoir role and enables growth and remodeling of the skeleton to proceed during growth and development.
The Ca:P ratio should be approximately 2:1 with twice as much Ca as P; with acceptable ratios ranging from 1.5 to 3:1 in the young growing horse and 1:1 to 6:1 in the mature horse, assuming that adequate levels of P are provided. A ratio less than 1:1 where the P content actually exceeds that of the Ca content, even if the absolute amounts of Ca and P are adequate, will result in interference of the bioavailability of Ca which can cause orthopedic or bone disorders, especially in young, rapidly growing horses. The Ca and P content in forages can be variable. Legumes typically have higher Ca concentrations than grasses whereas grains are usually high in P and low in Ca. Consequently, it is important to know the Ca and P content of all feedstuffs to ensure that the appropriate Ca:P ratio is achieved.
Copper is involved in cartilage formation and development and is of particular concern in growing horses. Since 1989, several studies have found that low Cu diets are associated with an increased incidence of developmental orthopedic disease in growing horses. Iron is used in formation of hemoglobin, which involved in oxygen transport in the body. Fe deficiencies are rarely reported and most forage sources meet the daily requirements of horses.
|Table 1. Daily Nutrient Requirements of Horses (adapted from 1989 NRC for horses)|
|Table 2. Daily Nutrient Requirements of Growing Horses (adapted from 1989 NRC for horses)|
Prepared by: Courteney Holland, Extension Equine Specialist, Department of Animal Sciences, Auburn University, AL. January 2018. CMH – 2018.
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