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Part 4. Selenium (Se): Comments on NRC requirements and geochemistry
Recommended minimum daily Se intakes in Table 2, Part 2 are 1.0 mg and 1.13 mg per day for horses at average maintenance and moderate workload, respectively. NRC recognizes that these levels might be too low for optimal horse health. In addition, the recommended Se intake for exercising horses at moderate and heavy workloads is only slightly greater than those at maintenance. This small increase does not account for Se losses from red blood cells during exercise and greater excretion (urination) following exercise. As indicated for other minerals, supplement Se at 150% minimum NRC requirement; however, higher Se levels are recommended for working horses, pregnant and lactating mares, and growing horses (Table 1, JD Pagan 2000).
There are always some health concerns about elements like Se that are beneficial in small amounts but have a low threshold of toxicity. The maximum total Se per day recommended by most equine nutritionists is based on Dry Matter Intake (0.3 mg/kg DMI), or body weight (1 mg/500 lbs BW).
Recommended minimum daily Se intakes in Table 2, Part 2 are 1.0 mg and 1.13 mg per day for horses at average maintenance and moderate workload, respectively. NRC recognizes that these levels might be too low for optimal horse health. In addition, the recommended Se intake for exercising horses at moderate and heavy workloads is only slightly greater than those at maintenance. This small increase does not account for Se losses from red blood cells during exercise and greater excretion (urination) following exercise. As indicated for other minerals, supplement Se at 150% minimum NRC requirement; however, higher Se levels are recommended for working horses, pregnant and lactating mares, and growing horses (Table 1, JD Pagan 2000).
There are always some health concerns about elements like Se that are beneficial in small amounts but have a low threshold of toxicity. The maximum total Se per day recommended by most equine nutritionists is based on Dry Matter Intake (0.3 mg/kg DMI), or body weight (1 mg/500 lbs BW).
Let’s assume that you are feeding 16 lbs grass hay containing 0.1 ppm Selenium (Se) plus 2 lbs mineral supplement with Guaranteed Analysis 1.5 ppm Se (product label). To determine total Se in your horse’s diet, multiply ppm concentration (= mg/kg) for each feed by the amount consumed in kg, or lbs (1 kg =2.2 lbs). For the grass hay, 0.1 mg/kg x 16 lbs = 0.1 mg/2.2 lbs x 16 lbs = 0.7 mg Se. Mineral supplement will supply an additional 1.4 mg Se (1.5 mg/2.2 lbs x 2 lbs = 1.4 mg Se) for total Se = 2.1 mg per day. In this case, DMI = 18 lbs, or 8 kg, and maximum recommended Se = 0.3 mg/kg x 8 kg = 2.4 mg.
According to NRC estimates, the maximum tolerable level of Se is 2 mg/kg DMI of daily diet, or in our forage-based diet, 20 mg (2 mg/kg x 10 kg). In our grass hay example, if you add 1 mg Se supplement to meet 200% NRC minimum daily requirement (0.1 mg/kg x 10 kg hay + 1 mg supplement = 2 mg), total Se intake is an order of magnitude (10x) less than the tolerable amount.
The lethal dose of Se is expressed in terms of LD50, which is the amount needed to kill 50% of the population of test animals. The LD50 for Se is 3.3 mg/kg per body weight. For example, a 500 kg (1100 lb) horse would have to consume 1650 mg Se per day (3.3 mg/kg x 500 kg).
To determine the correct supplement amount, you must analyze your hay and pasture, or request whole blood Se tests, which measure Se in both plasma and inside red blood cells.
Geochemical maps showing the Se distribution in soils and stream sediments must be interpreted with caution. Figure 1 shows the National Geochemical Survey Map for Se published by the U.S. Geological Survey (USGS). Harrison County, Indiana and the surrounding area, including the Louisville Metro area, are circled in black. You can also view these results by county.
The lethal dose of Se is expressed in terms of LD50, which is the amount needed to kill 50% of the population of test animals. The LD50 for Se is 3.3 mg/kg per body weight. For example, a 500 kg (1100 lb) horse would have to consume 1650 mg Se per day (3.3 mg/kg x 500 kg).
To determine the correct supplement amount, you must analyze your hay and pasture, or request whole blood Se tests, which measure Se in both plasma and inside red blood cells.
Geochemical maps showing the Se distribution in soils and stream sediments must be interpreted with caution. Figure 1 shows the National Geochemical Survey Map for Se published by the U.S. Geological Survey (USGS). Harrison County, Indiana and the surrounding area, including the Louisville Metro area, are circled in black. You can also view these results by county.
Fig. 1. Selenium (Se) distribution map of the U.S. based on results from Atomic Absorption Spectrometry (AA). Harrison County, Indiana and the surrounding area are circled in black. Average Se concentration for Harrison County based on soil and stream sediment analyses is 0.34 ppm ± 0.05. Click on image for larger version.
After inspecting this map, you might conclude that the Se concentration in your hay/pasture exceeds NRC requirements, i.e., 0.34 ppm soils (or stream sediments derived from soil erosion) versus 0.1 ppm NRC. The dominant Se species in acidic soils of Harrison County, Indiana (pH ~ 5-6) is selenite anion (Se-IV), which is strongly adsorbed (bound) to cation sites on surfaces of inorganic compounds, such as clays and hydrous Fe oxides. As a result, it is very immobile and unavailable for plant uptake (Mayland et al. 1991). In acidic soils, plants more readily absorb (take in) the selenate form (Se-VI). The problem is that analytical results in Fig. 1 measure total Se (selenite plus selenate) rather than the more bioavailable selenate. That’s one reason why the average Se concentration in Harrison County soils (0.34 ppm) is greater than the amount measured in our hay sample (~0.1 ppm). There are too many geochemical and biochemical (related to plant physiology and microbial processes) factors that affect the nutrient profile of plants to use soil analyses for diet balancing.
The mineral content of hays is also affected by farming practices, such as fertilization, time of cutting and number of cut. Most hay producers in this area routinely apply lime (pulverized limestone) to their soils to increase pH to near neutral (pH ~ 7). For soils with relatively low organic content (e.g., Crider series in Harrison County) this practice will increase Se bioavailability (Eich-Greatorex et al. 2007). In a recent Finnish study of low-Se soils, application of sodium selenate in commercial fertilizers to cereal and forage crops benefited both human and animal nutrition, and increased plant yields.
Se supplements are usually combined with vitamin E, another potent antioxidant that works in conjunction with Se. It’s a good combination because vitamin E is always deficient in grass hay and must be supplemented (1000 IU/500 lbs BW) if your horse is not on pasture (E Kane, Advances in Equine Nutrition IV, Kentucky Equine Research, p. 61, 2009).
If you are interested in the geochemistry of selenium, download this short PowerPoint presentation (pdf version) “Selenium in Sediment and Soil” by Jonathan Gillip, USGS. For a general introduction to selenium for horse owners, refer to the article "Selenium: A Balancing Act" at theHorse.com. More information on comparisons between organic and inorganic Se sources and the complementary nature of Se and vitamin E can be found here.
Se supplements are usually combined with vitamin E, another potent antioxidant that works in conjunction with Se. It’s a good combination because vitamin E is always deficient in grass hay and must be supplemented (1000 IU/500 lbs BW) if your horse is not on pasture (E Kane, Advances in Equine Nutrition IV, Kentucky Equine Research, p. 61, 2009).
If you are interested in the geochemistry of selenium, download this short PowerPoint presentation (pdf version) “Selenium in Sediment and Soil” by Jonathan Gillip, USGS. For a general introduction to selenium for horse owners, refer to the article "Selenium: A Balancing Act" at theHorse.com. More information on comparisons between organic and inorganic Se sources and the complementary nature of Se and vitamin E can be found here.
Take-home message
1. Se concentration in most Harrison County, Indiana hays meets minimum NRC requirements (0.1 ppm = 0.1 mg/kg).
2. Se levels higher than 150% NRC are recommended for most horse classes.
3. Total Se from all feed rations is normally in the range of ~2-3 mg per day.
4. Local soil maps are poor indicators of Se bioavailability and cannot be used for diet balancing.
5. Supplement amounts must be determined from hay/pasture analyses, or whole blood tests.
1. Se concentration in most Harrison County, Indiana hays meets minimum NRC requirements (0.1 ppm = 0.1 mg/kg).
2. Se levels higher than 150% NRC are recommended for most horse classes.
3. Total Se from all feed rations is normally in the range of ~2-3 mg per day.
4. Local soil maps are poor indicators of Se bioavailability and cannot be used for diet balancing.
5. Supplement amounts must be determined from hay/pasture analyses, or whole blood tests.
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