Mineral nutrition considerations

Influence of pH on availability of plant nutrients.
Redrawn from S.S.S.A.P., 1946. 11:305 by K. Williams

Plants are a major source of minerals for the goat because they require all the minerals that goats do exceptfor iodine. However, plant requirements for minerals such as cobalt and selenium may be much lower than the level animals need. Some soils are inherently deficient in some minerals such as iodine and selenium due to soil geology. Plants grown on soils deficient in a mineral are likely to be lacking in that mineral. However, some plants have an ability to concentrate the minerals available in the soil. Maps of mineral-deficient areas of the United States are available. However, consulting local extension agents is a better method of determining soil mineral deficiencies or toxicities that could affect mineral levels in local forages. Soil maps showing deficient areas of selenium, copper, molybdenum and cobalt are located at the end of this article.

Various factors other than soil mineral level can interact to influence the mineral content of forages. Soil pH is one factor that affects mineral uptake by plants, as illustrated by the figure below. Under acidic soil conditions, many trace minerals are less available for plant uptake. Environmental temperature at certain times of the year may also affect mineral uptake. Additionally, interactions among minerals after soil fertilization can have an impact on their availability for incorporation into plant material. The season of the year influences plant mineral concentrations, mainly due to a dilution effect, with decreasing mineral levels as plants mature. Different plant species will also have varying contents. Browse and forb plant species may have higher mineral concentrations than do some grasses. As goats eat a variety of plants, they are less likely to have mineral deficiencies than other species of animals that eat predominantly one plant species.

Mineral relationships.

To determine plant mineral content, a producer can collect and send samples for analysis. Parts of plants that are being consumed throughout the day and growing season should be sampled. Analysis of a sample will cost a minimum of $25. To obtain enough data to formulate a custom mineral supplement would require sampling several times over a growing season and over more than one year, if possible. This could be worthwhile for a large goat herd but too expensive for most producers. The alternative is to use a commercially prepared mineral block or loose supplement. Some mineral mixes are formulated for regions and are more appropriate to use than a mineral formulated for the whole United States. Many state extension specialists know what minerals are likely to be deficient in given areas of a state and what levels of calcium and phosphorus are appropriate for beef cattle production. Those recommendations are a good place to start for goat mineral nutrition.

Mineral supplements should not be overfed. Mineral supplements are formulated for goats to consume a sufficient quantity. Many minerals interact with one another (interactions shown on following page,) and excess consumption of one mineral may decrease absorption and/or utilization of another. For example, it is well known that excess iron depresses absorption of zinc, copper, manganese and selenium. There are several regions of the United States that have high enough levels of iron to depress absorption of these other minerals, requiring that they be supplemented. Feeding a regional mineral with no supplemental iron would be preferable to feeding an all-purpose mineral containing high levels of iron that would further depress absorption of these minerals.

Mineral status.

The range between safe supplementation and toxic levels is narrow for many of the trace minerals. Do not overfeed trace minerals or mix additional minerals in a diet if another source of trace minerals, such as a trace mineral block, is present. Formulation of mineral supplements requires considerable expertise since the addition of high levels of one mineral may depress the utilization of another, causing a deficiency. Also, some trace minerals can be toxic in excess.
Calculation of supplemental levels for feed formulas requires a certain amount of technical expertise and specialized scales for weighing, along with sophisticated mixing equipment. Most common farm mixing methods are inadequate, resulting in pockets of dangerously high mineral levels in a batch of feed.

Choosing a mineral supplement

The most important consideration in choosing a mineral supplement is the level of calcium and phosphorus. Some mineral mixes are designated 12 – 8, which means they contain 12-percent calcium and 8-percent phosphorus. The levels of these two minerals should be the same that is being fed to cattle in your area. Contact your county agent or livestock extension specialist for that information. Phosphorus is expensive, so a 12 – 12 mineral will cost more than one that is 12 – 8. However, most forages are low in phosphorus, making it the most common mineral deficiency.

The mineral supplement should also contain trace minerals that are deficient in the area. Levels of trace minerals used in local cattle supplements can provide a guide for goats. Most mineral supplements are formulated to provide less than half the trace mineral requirements due to toxicity concerns. A mineral supplement should be provided in the loose form to maximize consumption. The salt level in the mineral drives intake; therefore, no other sources of salt should be available. A mineral feeder should be used to protect from rain and keep the supplement clean. Replenish minerals frequently to keep them fresh.

Current approximate wholesale costs for supplying 100 percent of mineral needs of a 150-pound goat for various minerals in one year are as follows:

Feedstuffs will normally provide at least half of all minerals and in some cases all required. It should be noted that phosphorus alone accounts for half the mineral cost.

Diagnosing mineral deficiencies or toxicities

The proper procedure for diagnosing a mineral deficiency or toxicity depends on which mineral is being considered. Secure the assistance of a local veterinarian and extension animal nutritionist in the state who are familiar with minerals in the region.

1. Deficiency or toxicity symptoms usually provide initial indications of the mineral status, for example, “knuckling over” is a symptom of manganese deficiency. However, deficient animals do not always show classic symptoms and the major symptom may only be an animal that is doing poorly.
2. Blood tests are adequate for some minerals such as magnesium, calcium, and phosphorus, and for other blood factors that give an indication of mineral status. Examples of these factors include glutathione peroxidase for selenium, hemoglobin for iron, zinc binding protein for zinc, and thyroid hormones for iodine.
3. Hair analysis has been used for zinc and selenium but in general is a poor diagnostic test.
4. The liver is a good tissue to test for iron and copper adequacy. Liver samples can be obtained via biopsy or from animals that are slaughtered or die.

Copper map.
Map taken from Kubota, Welch, and Van Campen. 1987. Adv. Soil Sci. 6:189-215 with permission

Selium map.
Map taken from Kubota, Welch, and Van Campen. 1987. Adv. Soil Sci. 6:189-215 with permission

Cobalt map.
Map taken from Kubota, Welch, and Van Campen. 1987. Adv. Soil Sci. 6:189-215 with permission

Molybdenum map.
Map taken from Kubota, Welch, and Van Campen. 1987. Adv. Soil Sci. 6:189-215 with permission

Take-home lessons on mineral nutrition

1. The diet should contain adequate levels of calcium and phosphorus and have close to a 2:1 calcium to phosphorus ratio.
2. Provide a free-choice, loose mineral supplement with appropriate levels of calcium and phosphorus and trace minerals deficient in the region.
3. Monitor intake of the mineral to make sure the animals are eating an appropriate amount.
4. Avoid excessive feeding of any supplementation.