deficiencies and imbalances for herbivores are reported from almost all tropical regions of the world. Phosphorus (P) deficiency has been reported in 25 Latin American countries and deficiencies of calcium (Ca) in 11, sodium (Na) in 15, magnesium (Mg) in 14, cobalt (Co) in 13, copper (Cu) in 21, selenium (Se) in 17 and zinc (Zn) in 16 (McDowell, 1997). Iodine (I) deficiency is reported worldwide.
For many classes of livestock, including pigs, poultry, feedlot cattle
cows, mineral supplements are incorporated into concentrate diets, which generally ensure that animals are receiving required minerals. However, for grazing ruminants
to which concentrate feeds cannot be economically fed, it is necessary to rely on both indirect and direct methods of providing minerals. Self-feeding of ‘free choice’ mineral supplements is widely used for grazing livestock.
Forages often do not satisfy mineral requirements of grazing cattle (McDowell, 1992; 1997; 1999; and Underwood and Suttle, 1999). Plants do not need Se, Co, or I and can grow normally and produce optimum yields even though they contain less Fe, Zn, Mn, Cu and Co than required by livestock. Moreover, normally growing plants may contain excessive levels of Se, Mo and Cu for grazing animals. Thus, it is necessary to provide mineral supplementation to promote efficient and profitable cattle production.
One of the most important trace mineral needs for supplementation is selenium (Se) as the majority of the world’s soils and crops are deficient in this element. As an example, in a review of 15 experiments throughout Florida (USA), 95% of mean forage Se values were deficient at less than 0.1 ppm, and 67% were extremely deficient (0.05 ppm Se or less) (McDowell and Tiffany, 1998).
White muscle disease (WMD) is a degeneration of striated muscles that occurs without neural involvement and is the major clinical sign of Se vitamin
E deficiency in newborn ruminants. White muscle disease, which may develop intrauterine or extrauterine, is seen in young ruminants and characterized by generalized weakness, stiffness and muscle deterioration with affected animals having difficulty standing. Calves with WMD have chalky white striations, degeneration, and necrosis in the skeletal muscles and heart. During some years, incidence of WMD in certain world regions is sporadic, with less than 1% of the herds affected.
In other areas such as Turkey and New Zealand, a 20-30% incidence of WMD may occur regularly. In calves, the tongue musculature may be affected, preventing effective suckling (NRC, 1996). Death often occurs suddenly from heart failure as a result of severe damage to heart muscle. In milder cases in calves, where the primary clinical signs are stiffness and difficulty standing, a rapid and dramatic improvement can result from Se-vitamin E injections.
Less specific clinical signs indicating a lack of Se-vitamin E relate to unthriftiness of livestock, including poor growth, diarrhea and susceptibility to disease. Levels of Se and vitamin E above the generally accepted requirements have been shown to enhance the immune response in several species.
Currently, considerable attention is being paid to the role both vitamin E and Se play in protecting leukocytes and macrophages during phagocytosis, the mechanism whereby mammals immunologically kill invading bacteria. The effects of vitamin E and Se supplementation on protection against infection by several types of pathogenic organisms, as well as antibody titers and phagocytosis of the pathogens, have been reported for calves (Rajaraman et al., 1998) and lambs (Reffett et al., 1988).
The purpose of this paper is twofold: 1) to discuss Se supplementation for grazing livestock, and 2) to mention methods and considerations for mineral supplementation, with emphasis on providing freechoice minerals for ruminants.Selenium supplementation
METHODS OF SELENIUM SUPPLEMENTATION
The principal methods of increasing Se intake by grazing livestock include 1) a free-choice Se mineral supplement, 2) Se fertilization, 3) injections of Se, 4) Se as an oral drench, 5) Se in water, and 6) Se in ruminal pellets (heavy boluses) (McDowell et al., 2002).
Aqueous Se solutions have been successfully used as a periodical oral drench or as an intramuscular or subcutaneous injection (NRC, 1983). Direct subcutaneous injections, usually as sodium selenite, or oral dosing with this compound in doses from 10 to 30 mg for cattle and 1 to 5 mg for sheep are common means of preventing Se-responsive diseases in grazing livestock (Underwood and Suttle, 1999). Barium selenate injections have been shown to have a long lasting effect in ruminants. Barium selenate as a pellet or a subcutaneous injection maintained blood Se for at least 200 weeks in ewes and their lambs (Judson et al., 1991). Use of heavy ruminal pellets (similar to those earlier developed for Co), consisting of 95% finely divided Fe and 5% elemental Se, has prevented the occurrence of WMD in sheep and cattle grazing Se-deficient pastures (Underwood and Suttle, 1999). Selenium provided as a fertilizer is uniformly received by grazing livestock through forage consumption after top-dressing pastures with Se. Plant species differ in response to fertilizer Se. In Florida, Se was elevated significantly higher in fescue (Festuca spp) than for bahiagrass (Paspalum notatum) with Se fertilizer applications (McDowell et al., 2002).
The use of Se-fortified salt mixtures appears to be the most promising procedure for prevention of deficiency of this element (McDowell, 1997). Due to extensive grazing areas such as in Latin America, Se fertilization would not be economically feasible. Likewise, Se injections, drenches and ruminal pellets can be more labor intensive and expensive.
SOURCES OF SELENIUM
Sources of supplemental Se currently in use in the US are sodium selenite (Na2SeO3) and sodium selenate (Na2SeO4), with organic selenium
approved for some species. The organic yeast product has the potential to be a better supplemental source of Se for all species due to its higher bioavailability. A number of reports indicate that organic Se is more effective for cattle than sodium selenite in increasing Se in milk, increasing GSHPX, increasing resistance to stress and disease, improving meat color and reducing drip loss. In pigs (Mahan, 1999; 2000) and cattle (Ortmann and Pehrson, 1999), organic Se increased milk
Se content more than inorganic Se. Organic Se increased blood, milk, and liver Se concentration 2-3 times more than inorganic Se (Knowles et al., 1999). The Se yeast product resulted in a 130% increase in milk Se compared to the control, while the increase due to selenite and selenate was only 20% (Ortman and Pehrson, 1999).
Several researchers have suggested that at least 100 μg of Se/L of whole blood is associated with optimal immune capacity and optimal fertility. Pehrson et al. (1999) compared supplemental Se as selenite and Se in yeast on Se blood levels of calves. Ten of eleven calves had blood Se <100 μg/ L in the yeast group. For the selenite group, seven of nine calves were <100 μg/L and two of nine <50 μg/L. Liver, heart, pancreas and muscle tissues of sheep were significantly higher when fed the same level of dietary Se from wheat compared to selenite (Van Ryssen et al., 1989).Florida experiment comparing injectable selenite and selenate with free-choice mineral mixtures containing organic seleniumMATERIALS AND METHODS
Seventy-five Angus cows (1124 lb average weight, and 5-8 months pregnant as determined by palpation) were utilized in a two-year experiment at the Beef Demonstration Unit of the University of Florida in Chipley, Florida. The duration of the experiment was from December 1996 to December 1998. Animals were randomly allotted to five groups and treatments as follows: 1) a control (no Se supplementation), 2) subcutaneous injection of 5 ml of Mu-Se® (5 mg Se per ml sodium selenite, from label, Burns Biotech Labs, Inc., Oakland, Ca) every 6 months, 3) subcutaneous injection of 9 ml of Deposel® (50 mg Se per ml as barium selenate, from label, Grampian Pharmaceuticals Ltd., Leyland, Lancashire, U.K.) administered only at the initiation of the experiment, and 4) use of mineral mix with organic Se (Sel-PlexTM, Alltech Inc.
Nicholasville, KY) administered in two groups as replicates. The free-choice mineral mixture contained 54.5 g/kg Sel-PlexTM(calculated), which resulted in a Se concentration of 30 mg/kg in the mineral mixture. For both free-choice mixtures, the average daily Se intake was 2.1 mg per cow. Each of the five groups consisted of 15 cows that remained in the experiment for a two-year study.
Blood (plasma), liver biopsy and milk samples were collected for Se analysis. Selenium analyses of pasture and hay averaged 0.045 mg/kg Se.RESULTS AND DISCUSSIONCow plasma selenium (Table 1)
Six months after initiation of the experiment, the Mu-Se® group had lower (P<0.05) plasma Se than the Deposel® and the two free-choice mineral mixture treatments groups. After 12 months, all Sesupplemented groups had higher (P<0.05) plasma Se concentrations than the control. At the end of the experiment (24 months), there were also differences among treatments with all supplemented animals higher than the control (P<0.05). Mean Se concentrations of animals receiving Se were above the critical level of 0.03 μg/ml, while the control group was below, averaging 0.02 μg/ml. For the twoyear period, the Deposel® treatment maintained the Se status in cow plasma while both free-choice mineral mixture groups resulted in higher plasma Se concentrations at the end of the study than the control and Mu-Se® groups. For the present experiment, 2.1 mg Se per day for the free-choice groups was sufficient for maintaining plasma Se.Cow liver selenium (Table 2)
At six months, liver Se was higher (P<0.05) than controls for Mu-Se® and Deposel® treatment cows but lower (P<0.05) than for cattle receiving the two Sel-PlexTMmineral mixtures. Liver Se concentrations for both groups of animals receiving theTable 1. Plasma selenium concentration (μg/ml) of Angus cows given different selenium supplementsa.aLS means. Standard error = 0.015. (n=375 samples)
b cdMeans in a column differ (P <0.05) by LSD test.Table 2. Liver selenium concentration (mg/kg, dry basis) of Angus cows given different selenium supplementsa.aLS means. Standard error = 0.008. (n=250 samples)a.
bcdMeans in a column differ (P <0.05) by LSD test. Sel-Plex® mineral mixtures were considered adequate for status of this element.
One year after the initiation of the experiment, liver Se in control animals (0.04 mg/kg) was lower than all other groups (P<0.05). The control, Mu- Se® and Deposel® treatments were relatively low in Se, whereas the two Sel-PlexTMmineral mixes were adequate (0.25-0.50 ppm). Deposel® and the two free-choice mineral mixtures succeeded in raising liver Se concentrations to adequate levels after two years (1.69, 1.29 and 1.24 mg/kg, respectively), and this was also reflected in adequate plasma Se concentrations in the cows. Research with cattle and sheep has shown injectable barium selenate to be effective for one year in cattle (MacPherson et al., 1988) and 2-4 years in sheep (Judson et al., 1991).Cow colostrum selenium (Table 3)
During the first year of the experiment there were differences among treatments in colostrum Se concentrations. The control treatment had the lowest concentration and was different (P<0.05) from all other treatments. Mu-Se® treated animals were similar to Deposel® but lower (P<0.05) than animals receiving the Sel-PlexTMmineral treatments.
In the second year, highest colostrum Se was found in the cows receiving Deposel® and the two Sel- Plex® mineral mixtures.Table 3. Selenium concentrations (μg/ml) in colostrum from Angus cows in two years given different selenium supplementsa.aLS means. Standard error = 0.007. (n=80 samples)a
bcdMeans in a column differ (P <0.05) by LSD test.
Cow milk selenium (Table 4)
At 60 days after calving (Year 1), the highest milk concentrations were for the Sel-PlexTMmineral mixture treatments (0.03 and 0.04 μg/ml) and the lowest (0.02 μg/ml) were found in the Mu-Se® and Deposel® groups. At 120 and 180 days post-partum, the trend was similar, with the highest concentrations of Se in milk coming from cows receiving the mineral mix treatments. During the second year, milk Se concentrations at 60 days post-partum were 0.02 μg/ml for all supplemented groups and higher (P<0.05) than the control. Ortman and Pehrson (1999) and Suoranta et al. (1993) have indicated that supplementing inorganic Se to the dam to alleviate Se needs in calves is not satisfactory due to the poor capacity of these compounds to increase the Se content of milk. Ortman and Pehrson (1999) also reported that organic Se in the form of selenium yeast for dairy cows resulted in higher concentrations of Se in the milk than supplemental sodium selenite.Table 4. Milk selenium concentrations (μg/ml) of Angus cows given different selenium supplementsa.aLS means. Standard error = 0.002. (n=240 samples)
bcdMeans in a column differ (P <0.05) by LSD test.Calf plasma selenium (Table 5)
During year one (1997), calf plasma Se concentrations at birth were at a critical level (0.03 μg/ml) for the control, and below adequacy (0.07 μg/ml) for the Mu-Se® and Deposel® treatments.
Calves from cows given the two Sel-PlexTM
mineral treatments had an average Se concentration of 0.06 μg/ml, which is borderline to adequate (0.07 μg/ml).
At 60, 120 and 180 days, the control, Mu-Se® and Deposel® calves had plasma Se below critical concentrations, whereas the averages for the Sel- PlexTM
mineral mixtures were at or above adequacy.
During year two (1998), calves from the two Sel- PlexTM
mineral mixtures had higher (P<0.05) plasma Se than all other treatments with average borderline concentrations (0.055 μg/ml) at birth and adequate values (0.065 μg/ml) the rest of the time.Table 5. Plasma selenium concentrations (μg/ml) of calves from cows supplemented with different sources of selenium during 1997 and 1998a.aLS means. Standard error = 0.004.
bcdMeans in a column differ (P <0.05) by LSD test.
Highest levels of selenium in blood, milk and liver resulted from use of organic Se (Sel-PlexTM). Injectable Mu-Se® is only reliable as a short-term therapeutic agent for Se deficiency while Deposel® has value for longer-term provision of Se. Organic Se in a free-choice mixture provides continuous Se, resulting in the highest status of cows and calves.Providing minerals to tropical grazing livestock, emphasizing free-choice supplementation
Although providing minerals to ruminants in a concentrate mixture would be the most efficient method, most grazing ruminants in tropical countries receive little or no concentrates and depend almost entirely on forages to meet their needs. Indirect provision of minerals to grazing livestock includes use of mineral-containing fertilizers, altering soil pH, and encouraging growth of specific pasture species.
Direct administration of minerals to livestock in water, mineral licks, mixtures, drenches, rumen preparations, and injections is generally the most economic method of supplementation. Benefits and disadvantages of both indirect and direct mineral supplementation methods have been reviewed (Underwood, 1981; McDowell, 1992; 1997; 1999; Judson et al. 1991; McDowell and Valle, 2000).FREE-CHOICE (FREE ACCESS) MINERAL SUPPLEMENTATION
Voluntary consumption of individual minerals or mineral mixtures by animals is referred to as freechoice or free-access feeding. This practice has been used for many years to supply needed minerals, but is often based on an erroneous assumption that the animal knows which minerals are needed and how much of each mineral is required. Arnold (1964) cited evidence that most mammals exhibit little nutritional wisdom and that animals will select a palatable but poor quality diet in preference to an unpalatable, nutritious diet, even to the point of death.
Animals that do not receive concentrates are less likely to receive an adequate mineral supply. Freechoice minerals are much less palatable than concentrates and are often consumed irregularly.
Intakes of free-choice mineral mixtures by grazing cattle are highly variable and not related to mineral requirements (McDowell, 1985). Coppock et al. (1972) measured individual daily consumption of dicalcium phosphate by lactating dairy cows and found individual variation to be large, ranging from 0 to more than 1000 g/head daily. Factors that affect the consumption of mineral mixtures have been listed by Cunha et al. (1964) and McDowell (1992; 1997).SOIL FERTILITY AND FORAGE TYPE CONSUMED
Usually, the higher the level of soil fertility, the lower the consumption of minerals. Barrows (1977) reported that for cattle, salt, Ca, P, and Mg each appear to be consumed in relation to the content of the particular element in the grass. A number of reports have shown that cattle on native range consume more mineral supplement than those on improved pastures. Cattle on low quality or overgrazed pastures consume more mineral supplements.SEASON OF THE YEAR
Season of the year affects mineral intake (Cunha, 1987). Mineral intake is often greatest during the winter or dry season when forages stop growing, lose green color and become high in fiber and lignin and low in digestibility and mineral availability. As plants mature, the content of most minerals declines (McDowell, 1985). Mineral supplement intake is lower during the period of the year when forage quality and quantity is optimum. Under drought conditions, mineral supplement intake is increased to counteract the low mineral availability in the forage and the low level of forage intake due to its reduced palatability.AVAILABLE PROTEIN-ENERGY SUPPLEMENTS
The kind and level of protein-energy supplementation will influence mineral supplement intake. Protein and energy supplements that likewise provide minerals will decrease both the need and desire for free-choice minerals. Weber et al. (1992) reported a wide day-to-day variability in free-choice mineralized salt and protein block consumption by British breed beef cows. Variation was much greater for salt-type blocks than for the softer, protein-type blocks, with several cows consuming none of the salt-type blocks for periods of several weeks.INDIVIDUAL REQUIREMENTS
Growth rate, calf crop, and milk production
influence mineral needs. Added requirements of gestation and lactation increase mineral needs and, thereby, consumption. The higher the level of productivity, the more important an adequate level of mineral intake. Barrows (1977) reported that mineral consumption tended to decline as cows increased in age.SALT CONTENT OF DRINKING WATER
Naturally-high salt concentration of drinking water decreases mineral supplement intake. Livestock have a natural craving for salt. However, if that desire is fulfilled from drinking water high in salt, grazing livestock will consume less or none at all of a free-choice mineral mixture based on salt. Where naturally occurring salt content of water is high, mineral supplements cannot be based on salt and should be reformulated with other palatability stimulators such as cottonseed meal and molasses.PALATABILITY OF MINERAL MIXTURE
As previously mentioned, research has shown that ruminants have no particular desire for the majority of minerals, with the exception of common salt. In a review on salt appetite, Denton (1967) noted that all mammals have the ability to taste salt, and there is a universal liking for it. Becker et al. (1944) noted that the attitude of cattle toward salt in a mineral supplement is inversely related to the amount of salt present in feeds and water. Common salt, because of its palatability, is a valuable carrier of other minerals. If mixtures contain 30-40% salt or more, they are generally consumed on a free-choice basis in sufficient quantities to supply supplementary needs of other minerals.
Palatability and appetite stimulators such as cottonseed meal, dried molasses, yeast culture and fat help achieve more uniform, herd-wide consumption. Some of these products not only give the supplement a dust-free, moist, and free-flowing character, but also provide energy, protein and other benefits. Ingredients that increase palatability must be used in moderation or they will cause overconsumption.
A relatively palatable P source other than bonemeal is monosodium phosphate. Coppock et al. (1972) reported that dicalcium phosphate was preferred to defluorinated phosphate by dairy cattle
fed three different diets. Cattle preferred an acidic supplement (pH 3.5) such as dicalcium phosphate to an alkaline supplement (pH 8.5) such as defluorinated phosphate.
When Mg-deficient cattle are provided with a free-choice supply of supplementary Mg, such as magnesium oxide (MgO), they will die of grass tetany rather than consume this unpalatable source of Mg. However, when even high concentrations of MgO (e.g., 25%) are combined with palatable ingredients, grass tetany is prevented.AVAILABILITY OF FRESH MINERAL SUPPLIES
Previous diet or access to mineral supplements is a factor affecting short-term mineral consumption.
When animals are not allowed access to minerals for long periods of time, they may become so voracious that they often injure each other in attempting to reach salt. Under these conditions, they will consume 2-20 times the normal daily quantities of minerals until appetite is satisfied. By overindulging, they may suffer salt poisoning without access to sufficient water.
Rainproof mineral feeders help increase mineral intake by preventing caking, molding, and blowing away during windy weather. The choice of palatability or appetite stimulators is important when considering the keeping value of a supplement.
Cornmeal is a good appetite stimulator when included in a mineral mixture but is more easily fermentable than a proteinaceous product such as cottonseed meal. The use of 20-40% salt prevents molding and blowing.
Mineral feeders will be used more frequently by livestock if they are located near water tanks, shaded loafing areas, back rubbers, and areas of best grazing. Mineral feeders should be constructed low enough so that calves can also use them. They should be located on dry ground accessible to trucks for checking and servicing throughout the year.
Mineral boxes should be filled frequently and not allowed to get empty. Keeping the mineral supply fresh increases its consumption. Feeders should be spaced at intervals of less than one-half mile and be adequate in number for the stocking capacity of the pasture. One suggestion is to have approximately one mineral feeder per 50 head of livestock. Less minerals are consumed if grazing livestock must travel long distances to the mineral box.
In some regions with vast grazing areas, there are great difficulties in locating feeders so that animals have constant access to minerals. This is a particular problem where animals graze over large areas with no central location for drinking water.
Also, in regions that seasonally flood, locating mineral feeders above the water level is sometimes a problem.PHYSICAL FORM OF MINERALS
Mineral consumption is often a minimum of 10-20% less when provided in block versus loose form.
Mineral blocks can be developed on the basis of degree of hardness to take into consideration rainfall, humidity and other environmental conditions. Rain will dissolve too soft a block causing mineral losses, and yet livestock experience difficulty consuming enough of a hard block to fulfill mineral requirements. If the animals remain only a limited time in the vicinity of mineral blocks, then excessive block hardness will result in reduced mineral consumption. Providing a supplement in block form has the advantages of convenience and much greater resistance to rain and dew. Also, the control of excessive intakes by the use of blocks may be a significant advantage.BIOLOGICAL AVAILABILITY OF MINERAL SOURCES
There is considerable difference in the availability of a mineral element among sources. Chemical analysis of a mineral element in a feed or mineral supplement does not provide information on availability of the element for animals (Ammerman et al., 1995). Biological availability may be defined as that portion of the mineral which can be used by the animal to meet its bodily needs. In recent years, trace mineral chelates and organic complexes have become available. Reviews on the significance of these products are available (Kincaid, 1989; Patton, 1990; Spears et al., 1991; McDowell, 1992; Underwood and Suttle, 1999). The bioavailability and percentage of mineral elements in various sources commonly used in mineral supplements are shown in Table 6, however it should be noted that values very much depend on the measurement used.EXPOSURE TIME, PREVIOUS EXPERIENCE AND SOCIAL INTERACTIONS
Livestock exposed to new feeds often exhibit neophobia, or a cautious sampling or rejection of the feed that is not related to palatability (Launchbaugh, 1995). The acceptance and degree of preference by grazing animals for a specific supplement is likely to depend on recognition by the animal of the supplement as a potential foodstuff, prior experience of the animal with the same or similar supplements, social interaction and the degree of preference of the animal for the supplement relative to available forages (Provenza, 1996).
Experience, age and social interactions will influence supplement consumption. Individual supplement intake variation usually decreases with time, as animals progress through the neophobic eating pattern found with unfamiliar supplements (Bowman and Sowell, 1997). Total time spent consuming supplements for inexperienced animals was lower for two-year-old than for three-year-old cows (Sowell et al., 1995). Social interactions play an important role in supplement consumption by cattle and sheep. Dominant animals often consume large amounts of supplement and prevent other animals from consuming desired levels. It may be possible to change dominance patterns by altering feeder design (Bowman and Sowell, 1997). Inexperienced sheep commonly increase supplement intake in the presence of more experienced sheep (Foot et al., 1973).SELECTING A FREE-CHOICE MINERAL SUPPLEMENT
Even though grazing livestock have been found not to balance their mineral needs perfectly when consuming a free-choice mixture, there is usually no other practical way of supplying mineral needs under grazing conditions. As a low cost insurance policy to provide adequate mineral nutrition, ‘complete’ mineral supplements should be available free-choice to grazing livestock (Cunha et al., 1964). A ‘complete’ mineral mixture usually includes salt, a low fluoride-phosphorus source, Ca, Co, Cu, Mn, I, Fe and Zn. Except where selenosis is a problem, most free-choice supplements should contain Se. Magnesium, K, S, or additional elements can also be incorporated into a mineral supplement or can be included at a later date as new information suggests a need.
Calcium, Cu, or Se, when in excess, can be more detrimental to ruminant production than any benefit derived by providing a mineral supplement. In regions where high forage Mo predominates, three to five times the Cu content in mineral mixtures is needed to counteract Mo toxicity (Cunha et al., 1964). As little as 3 ppm Mo has been shown to decrease Cu availability by 50%. Sulfur at 0.4% can have the same effect. Thus, the exact level of Cu needed to counteract Mo or S antagonism is a complex problem and should be worked out for each area.Table 6. Percentage of mineral element and relative bioavailabilitya. To enlarge the image, click here
aFrom Ellis et al. (1988). Recent updates provided by Ammerman et al. (1995) and P.H. Miles (personal communication). bBioavailability percentages computed by P.H. Miles (personal communication). Percentages are not absolute but rather relative comparisons using the source designated as 100 as the standard. Comparative values are estimated for both monogastric and ruminant species, unless otherwise stated.
cValue is for ruminant, 65% for chick.
dValue is for ruminant, 0% for chick.
eSome liberation of free iodine when mixed with trace minerals.
fSome samples are fairly high in availability, but not as available as ferrous sulfate.
gValue is for ruminant, 55% for chick.
A number of authorities feel there is no justification for the use of complete free-choice mineral mixtures that are designed to cover a wide range of environments and feeding regimens and that contain a margin of safety as an insurance against deficiency. These people feel that ‘shotgun’ mixtures are economically wasteful and can also be harmful. This author is in disagreement with this viewpoint. There is little danger of toxicity or excessive cost in relation to the high probability of increased production rates for cattle from administering a complete free-choice mineral mixture following the guidelines in Table 7 (McDowell, 1992).
Copper and Se added at recommended levels would be the minerals of most concern for toxicity. However, cattle, contrary to sheep, are much less sensitive to Cu toxicity, and inorganic forms of Se (e.g. sodium selenite) are less well utilized by livestock when administered in excess of the requirements. In conclusion, it is best to formulate free-choice mixtures on the basis of analyses or other available data. However, when no information on mineral status is known for a given region, a free-choice complete mineral supplement is definitely warranted.INFORMATION REQUIRED FOR MINERAL SUPPLEMENT FORMULATION
Mineral ratios and interrelationships are always important, but not as important as adequate concentrations of individual minerals in the mixtures.
As an example, the Ca:P ratio is of minimal importance to ruminants provided the P level is adequate. However, a narrower ratio of Ca:P is much more important for monogastric species, such as growing poultry and pigs. Likewise, high levels of Ca are extremely detrimental in relation to Zn requirements for monogastrics, but apparently have much less effect in ruminant diets.
To evaluate a free-choice mineral supplement, it is necessary to have an approximation of: (1) grazing livestock requirements for the essential nutrients, which includes the age of the animals involved, stage of current production or reproduction
cycle, and intended purpose for which the animals are being fed; (2) relative biological availability of the minerals in the sources provided; (3) approximate daily intake per head of the mineral mixture and total dry matter intake anticipated for the target animals; and (4) concentration of the essential minerals in the freechoice mixture.
Palatability of the supplement affects intake more than do physiological needs. In formulating mineral mixes, estimating the possible need must coincide with adequate intake. A number of reports conclude that grazing cattle do not always consume mineral mixtures well. Cunha et al. (1964) in Florida, Weber et al. (1992) in Oregon and Rode and Beauchemin (1993) in Lethbridge, Canada have presented data showing a wide monthly variation in the consumption in specific regions. When evaluating mineral supplements where consumption is not known, researchers commonly start with an intake figure of 50 g/day and adjust this figure according to local conditions. For sheep, a starting intake figure would be 15 g/day (McDowell, 1996).
It is virtually impossible to measure accurately the total dry matter consumption of livestock on pasture. However, this is essential since requirements are based on intakes of dry matter.
The quality of a pasture will determine intake to a great degree. Although 2% of body weight is considered a rough estimate of forage dry matter intake by cattle, they may eat much less if the forage is of poor quality. Actual dry matter consumption often becomes a matter of judgment on the part of the researcher or rancher. For grazing mature cattle, often daily dry matter consumption is between 7 and 10 kg. For sheep an estimate of 1.8 kg could be used (McDowell, 1996).
The concentration of each element furnished by the mineral mixture can be compared to total requirements for that element to determine if a significant amount is being furnished by the supplement. It is difficult to determine what constitutes a significant portion of the requirement for each mineral that should be supplied by the mineral mixture, but it is generally believed the figure should be 25-50% for the trace elements. In zones known to have trace mineral deficiencies, 100% of the requirements for these elements should be provided.Calculations for free-choice mineral supplement formulationThe calculations are as follows:
|% element in mix × daily intake of mix (g)|
|total daily DM intake (g)|
|= % element in total diet from mineral mix|
If, for example, copper in the mineral mixture is 0.12 %, daily intake of mineral mixture is 50 g and total daily dry matter intake is 10,000 g, then:
|0.0012 × 50 g
||× 100=0.0006% or 6 ppm|
Note that to convert percentage to ppm, the decimal is moved four places to the right. If approximately 10 ppm is considered the allowance for Cu, then 60% of the Cu requirement would thus be supplied by this particular mixture. For sheep, use estimated intakes of mineral mixtures and total dry matter as 15 and 1800 g, respectively (McDowell, 1996).
Table 8 illustrates the estimated trace mineral requirements and percentages of each element required in a beef cattle mineral mixture to meet 25, 50 or 100% of the requirement. These figures are based on an estimated daily mineral consumption of 50 g. With less consumption, the mineral supplement should contain a higher percentage of each mineral. Likewise, a lower intake of dry matter would reduce the percentage of minerals required in the mixture. Each producer should determine mineral consumption for his herd and change products if higher consumption rates are required (e.g. increase the cottonseed meal, as a palatability factor, in the mixture from 5 to 10%).
Table 7. Characteristics of a ‘good’ complete free-choice cattle mineral supplement.
* For most regions, it would be appropriate to include Se unless toxicity has been observed. Iron should be included in temperate region mixtures but often both Fe and Mn can be eliminated for acid soil regions. In certain areas where parasitism is a problem, Fe supplementation may be beneficial.
- Contains 6-8% minimum total P. In areas where forages are consistently <0.20% P, supplements in the 8-10% P range are preferred.
- Has a Ca:P ratio not substantially over 2:1.
- Provides a significant proportion (e.g. about 50%) of the trace mineral requirements for Co, Cu, I, Mn and Zn. In known trace mineraldeficient
regions, 100% of specific trace minerals should be provided*.
- Includes high-quality mineral salts that provide the most available forms of each mineral element, and avoidance or minimal
inclusion of mineral sources containing toxic elements. e.g. phosphates containing high F should be either avoided or formulated
so that breeding cattle receive no more than 30-50 ppm F in the total diet. Fertilizer or untreated phosphates could be used to a
limited extent for feedlot cattle.
- Is sufficiently palatable to allow near adequate consumption in relation to requirements.
- Is backed by a reputable manufacturer with quality control guarantees as to accuracy of mineral supplement label.
- Has an acceptable particle size that will allow adequate mixing without smaller size particles settling out.
- Is formulated for the region involved, the level of animal productivity, the environment (temperature, humidity, etc.) in which it will be
fed, and is as economical as possible in providing the mineral elements used.
Note that to convert percentage to ppm, the decimal is moved four places to the right. If approximately 10 ppm is considered the allowance for Cu, then 60% of the Cu requirement would thus be supplied by this particular mixture. For sheep, use estimated intakes of mineral mixtures and total dry matter as 15 and 1800 g, respectively (McDowell, 1996).
Table 8 illustrates the estimated trace mineral requirements and percentages of each element required in a beef cattle mineral mixture to meet 25, 50 or 100% of the requirement. These figures are based on an estimated daily mineral consumption of 50 g. With less consumption, the mineral supplement should contain a higher percentage of each mineral. Likewise, a lower intake of dry matter would reduce the percentage of minerals required in the mixture. Each producer should determine mineral consumption for his herd and change products if higher consumption rates are required (e.g. increase the cottonseed meal, as a palatability factor, in the mixture from 5 to 10%).FREE-CHOICE MINERAL SUPPLEMENT EVALUATION
Problems with mineral supplementation programs in diverse regions of the world have been summarized (McDowell, 1997) and include: (1) insufficient chemical analyses and biological data to determine which minerals are required and in what quantities; (2) lack of mineral consumption data needed for formulating supplements; (3) inaccurate and (or) unreliable information on mineral ingredient labels; (4) supplements that contain inadequate amounts or imbalances; (5) standardized mineral mixtures that are inflexible for diverse ecological regions (e.g. supplements containing Se distributed in a Se-toxic region); (6) farmers not supplying mixtures as recommended by the manufacturer (e.g. mineral mixtures diluted 10:1 and 100:1 with additional salt); (7) farmers not keeping minerals available to animals continually; and (8) difficulties involved with transportation, storage, and cost of mineral supplements. Many of these problems in tropical countries are more related to quality control issues for mineral supplements, however others are universal including inadequate quantities of Cu and Zn in mixtures, with some products low in P while others still not providing Se.
Responsible firms that manufacture and sell highquality mineral supplements provide a great service to individual farmers. However, there are companies that are responsible for exaggerated claims of advertising, and some that produce inferior products that are of little value, or worse, those likely to be of detriment to animal production. Table 9 provides an example of an inferior mineral mixture available in Latin America. This particular mineral supplement is recommended for cattle, sheep, pigs, and chickens. It is impossible to adequately meet requirements of both ruminants and monogastric animals with the same mixture. This unbalanced mineral mixture, which is extremely high in Ca (29.4%) and low in P (1.8%), would likely be more detrimental to grazing cattle than having no access to supplemental minerals, and may actually contribute to a P deficiency.Table 8. Trace minerals in an adequate supplementab.aFrom McDowell (1997).
bAssumes average consumption of 50 g/day for cattle and 10 kg total dry feed intake daily.
The user of mineral supplements must rely on the reputation and integrity of the mineral feed manufacturer, who should provide properly balanced essential mineral fortification, taking into account target species, level of production, season of the year and individual variations in requirement. Safe, biologically available and palatable forms of the minerals, at a fair price, allow both the user and the manufacturer to realize a profit from its use.Production responses from mineral supplementation
Many reports from tropical regions of the world dating back to the early part of the century have revealed the beneficial effects of P supplementation on overall performance. Increased reproductive performance due to mineral supplementation is illustrated in Table 10 for 18 locations in Latin America, Africa, and Asia. Averaging the 18 reports resulted in a mean calving percentage of 51.3% for animals receiving salt only versus 73.3% for those receiving salt plus additional supplemental minerals.
Reports of improved weight gains by mineral supplemented cattle have been summarized for various world regions (McDowell, 1997; 1999).Table 9. An inferior mineral mixture available in Latin Americaa-c.To enlarge the image, click here
aFrom McDowell (1997).
bMineral mix is recommended for cattle, sheep, pigs, and chickens. It is assumed that consumption will average approximately 0.5% of the total dietary intake. Based on an estimated intake of 50 g mixture for cattle and 10 kg of total dry feed per head daily.
cCriticisms of mineral mixture: (1) Mixture extremely low in P and exceptionally high in Ca. The Ca:P ratio is 16.4:1. (2) The supplement does not provide a significant proportion (e.g., 50%) of the requirements for Cu, I, Mn and Zn. (3) The majority of the Fe is from ferric oxide, an unavailable form of this element. (4) Since this diet contains 29.4% Ca and only 20% salt (NaCl), it is likely to be of low palatability.
Table 10. Latin American, African and Asian Studies on effects of mineral supplementation on increasing calving percentages.To enlarge the image, click here
1Control animals received only common salt (NaCl). 2Bone meal.
4Complete mineral mixture.
5Dicalcium phosphate + triple superphosphate.
6Dicalcium phosphate + copper sulfate.
aComplete references cited by McDowell (1985) with the exception of Rojas et al. (1994).
The importance of mineral supplementation to overall production of cattle in the llanos regions of Colombia is presented in Table 11. Supplemental minerals dramatically increased all parameters of production. Multiplication of the weaning percentage by the weaning weight gave 88.2 kg of calf produced per cow with complete minerals compared to 44.9 kg with salt alone. For this study, free-choice mineral supplementation resulted in a return of 15.6 Colombian pesos for every peso invested in the supplement (Miles and McDowell, 1983). The improved productivity of a cattle ranch in Venezuela after a minimal change in technology, particularly by providing a high quality mineral supplement to cattle, is illustrated in Table 12.Table 11. Four-year Colombian study evaluating supplemental mineralsaaMiles and McDowell, 1983.
bEvaluation of the complete mineral supplement indicated adequate concentrations of most minerals but suboptimum levels of Zn and Cu, with no added Se and S.
cWeaned calf percent multiplied by weaning date.Table 12. Improved technology, emphasizing mineral supplementation on a cattle ranch in Bolivar State, Venezuelaa.aPersonal communication, L. Rojas.SummaryOrganic Se is more available than selenite and has resulted in higher Se levels in blood, milk and liver.
Injectable barium selenate provides long-lasting effects compared to injectable selenite. The use of selenium yeast in a free-choice mineral mixture provides a continuous supply of Se to grazing livestock. For grazing ruminants to which concentrate feeds cannot be economically fed, it is necessary to rely on self-feeding of mineral supplements.
As a low cost insurance to provide adequate mineral nutrition, nutritionally complete mineral supplements should be provided free-choice to ruminants. A number of factors affect mineral consumption of free-choice mixtures.
Cattle exhibit little nutritional wisdom and will select palatable mixtures in preference to mixtures designed to meet their requirements. Palatability and appetite stimulators are often used to achieve a more uniform, herd-wide consumption.
Calcium, Se and Cu, when in excess, can be more detrimental to ruminant production than any benefit derived by providing a mineral supplement. Mineral supplementation can greatly increase ruminant livestock production at a highly favorable cost-tobenefit relationship.References
Authors: L.R. MCDOWELL, G. VALLE, L.A. CRISTALDI, P.A. DAVIS, O. ROSENDO and N.S. WILKINSON Department of Animal Sciences, University of Florida, Gainesville, FL, USA
Ammerman, C.G., D.H. Baker and A.J. Lewis. 1995. Bioavailability of nutrients for animals. Academic Press, San Diego, CA.
Arnold, G.W. 1964. Some principles in the investigation of selective grazing. Proc. Aust. Soc. Anim. Prod. 5:285.
Barrows, G.T. 1977. Research efforts have lagged in free-choice feeding. Anim. Nutr. Hlth. May 12- 14.
Becker, R.B., G.K. Davis, W.G. Kirk, R.S. Glasscock, A.P.T. Dix and J.E. Pace. 1944. Defluorinated superphosphate for livestock. Bull. 401, Fla. Agric. Exp. Stn., Gainesville, FL.
Bowman, J.G.P. and B.F. Sowell. 1997. Delivery method and supplement consumption by grazing ruminants: a review. J. Anim. Sci. 75:543.
Coppock, C.D., R.W. Everett and W.G. Merrill. 1972. Effect of ration on free-choice consumption of calcium-phosphorus supplements by dairy cattle. J. Dairy Sci. 55:245.
Cunha. T.J. 1987. Salt and trace minerals. Salt Institute, Alexandria, VA.
Cunha, T.J., R.L. Shirley, H.L. Chapman, Jr., C.B. Ammerman, G.K. Davis, W.G. Kirk and J.F.
Hentges. 1964. Minerals for beef cattle in Florida. Fla. Agr. Exp. Stn. Bull. 683, Gainesville, FL.
Denton, D.A. 1967. Salt appetite. Handbook of Physiology I. Am. Physiological Society, Washington, DC.
Foot, J.Z., A.J.F. Russel, T.J. Maxwell and P. Morris. 1973. Variation in intake among group-fed pregnant Scottish Blackface ewes given restricted amount of food. Anim. Prod. 17:169.
Judson, G.H., N.F. Ellis, B.R. Kempe and M. Shallow. 1991. Long-acting selenium treatment for sheep. Aust. Vet. J. 68:263.
Kincaid, R.L. 1989. Availability, biology of chelated, sequestered minerals explored. Feedstuffs 61(11):22.
Knowles. S.O., N.D. Grace, K. Wurms and J. Lee. 1999. Significance of amount and form of dietary selenium in blood, milk and casein concentrations in grazing cows. J. Dairy Sci. 82:429.
Launchbaugh, K.L. 1995. Effects of neophobia and aversions on feed intake: Why feedlot cattle sometimes refuse to eat nutritious feed. In: Symposium Intake by Feedlot Cattle. Okla. Agric. Exp. Sta. P-942. Page. 36.
MacPherson, A., E.F. Kelly, J.S. Chalmers, D.J. Roberts. 1988. The effect of selenium deficiency on fertility in heifers. In: Trace Substances in Environmental Health – XXI (D.D. Hemphill, ed). University of Missouri, Columbia, Mo, USA. p. 551.
Mahan, D.C. 1999. Organic selenium: Using nature’s model to redefine selenium supplementation for animals. In: Biotechnology in the Feed Industry (T.P. Lyons and K.A. Jacques, eds). Nottingham Univ. Press, UK. Page 523.
Mahan, D.C. 2000. Effect of organic and inorganic selenium sources and levels on sow colostrum and milk selenium content. J. Anim. Sci. 78:100.
McDowell, L.R. 1985. Nutrition of Grazing Ruminants in Warm Climates. Academic Press, New York, NY.
McDowell, L.R. 1992. Minerals in Animal and Human Nutrition. Academic Press, San Diego, CA. Page 524.
McDowell, L.R. 1996. Free-choice mineral supplements for grazing sheep in developing countries. In: Detection and Treatment of Mineral Nutrition Problems in Grazing Sheep (D.G. Masters and C.L. White, eds). ACIAR, Canberra, Australia.
McDowell, L.R. 1997. Minerals for Grazing Ruminants in Tropical Regions. University of Florida, Gainesville.
McDowell, L.R. 1999. Minerais para Ruminantes sob Pastejo em Regiões Tropicais Enfatizando o Brasil. University of Florida, Gainesville, FL.
McDowell, L.R. and M.E. Tiffany. 1998. Mineral deficiencies in Florida and supplementation considerations. 1998 Beef Cattle Short Course. University of Florida, Gainesville, FL. Page 127.
McDowell, L.R. and G. Valle. 2000. Major minerals in forages. In: Forage Evaluation in Ruminant Nutrition. CAB International Press, UK. pp. 373- 397.
McDowell, L.R., G. Valle, L. Cristaldi, P.A. Davis, O. Rosendo and N.S. Wilkinson. 2002. Selenium availability and methods of selenium supplementation for grazing ruminants. 13th Annual Florida Ruminant Nutrition Symposium. University of Florida, Gainesville, FL. Page 86.
Miles, W. and L.R. McDowell. 1983. Mineral deficiencies in llanos rangelands of Colombia. World Anim. Rev. 46:2.
NRC. 1983. Selenium in Nutrition (Rev. Ed.). National Academy Press. Washington, D.C. NRC. 1996. Nutrient Requirements of Beef Cattle. 7th revised edition. National Academy Press. Washington, D.C.
Ortman, K. and B. Pehrson. 1999. Effect of selenate as feed supplement to dairy cows in comparison to selenite and selenium yeast. J. Anim. Sci. 77:3365.
Patton, R.S. 1990. Chelated minerals: What are they, do they work? Feedstuffs 62(9):14.
Pehrson, B.G., K. Ortman, N. Madjid and U. Trafikowska. 1999. The influence of dietary selenium as selenium yeast or sodium selenite on the concentration of selenium in the milk of suckler cows and on the selenium status of the calves. J. Anim. Sci. 77:3371.
Provenza, F.D. 1996. Acquired aversions as the basis for varied diets of ruminants foraging on rangelands. J. Anim. Sci. 74:2010.
Rajaraman, V., B.J. Noneche, S.T. Franklin, D.C. Hammell and R.L. Horst. 1998. Effect of dietary selenium and vitamin E on the primary and secondary immune response in lambs challenged with parainfluenza virus. J. Dairy Sci. 81:3278.
Reffett, J.K., J.W. Spears and T.T. Brown, Jr. 1988. Effect of dietary selenium and vitamin E on the primary and secondary immune response in lambs challenged with parainfluenza virus. J. Anim. Sci. 66:1520.
Rode, L.M. and K.A. Beauchemin. 1993. Cattle are poor judges of their mineral needs. Weekly Letter Agric. Canada No. 3082. Lethbridge, Canada.
Rojas, L.X., L.R. McDowell, F.G. Martin and N.S. Wilkinson. 1994. Estado mineral de una finca en el suroeste de los llanos de venezuela. Zootecnia Tropical. 12:161.
Sowell, B.F., J.G.P. Bowman, D.L. Boss and H.W. Sherwood. 1995. Feeding behavior of range cows receiving liquid supplements. Proc. West. Sect. Am. Soc. Amin. Sci. 46:388.
Spears, J.W., E.B. Kegley and J.D. Ward. 1991. Bioavailability of organic, inorganic trace minerals explored. Feedstuffs 27(47):12.
Suoranta, K., E. Sinda and R. Pihlak. 1993. Selenium of the selenium yeast enters the cow’s milk. Nor. J. Agric. Sci. Suppl. 11:215.
Underwood, E.J. 1981. The Mineral Nutrition of Livestock. Commonwealth Agricultural Bureaux, London, England.
Underwood, E.J. and N.F. Suttle. 1999. In: The Mineral Nutrition of Livestock. Third Ed., Midlothian, UK.
Van Ryssen, J.B.J., J.T. Deagen, M.A. Beilstein and P.D. Whanger. 1989. Comparative metabolism of organic and inorganic selenium by sheep. J. Agric. Food Chem. 37:1358.
Weber, D.W., T.O. Dill, J.E. Oldfield, R. Frobish, K. Vandebergh and W. Zollinger. 1992. Daily intake of free-choice salt and protein blocks by beef cows was highly variable. Prof. Anim. Sci. 8(2):15.