Major Minerals

The major minerals to be discussed are calcium, phospho­rus, magnesium, potassium, sulfur, sodium, and chloride. The literature on mineral nutrition of dairy goats has been reviewed by Haenlein (1980) and Kessler (1991).

Calcium

Approximately 99% of the calcium in the body is found in the skeleton in combination with phosphorus (Ca : P ratio of 2.2 : 1). The remainder of the calcium is very impor­tant for diverse body functions, including muscle contrac­tion, neuromuscular excitation, and blood coagulation. Absorption of calcium in the proximal small intestine involves calcium-binding protein. Absorption is regulated by 1,25-dihydroxycholecalciferol, which is synthesized in the kidney from vitamin D. Parathyroid hormone increases osteolysis, while calcitonin (from the thyroid gland) decreases osteolysis. These hormones are in turn regulated by the blood calcium concentration. The normal serum cal­cium concentration for goats is approximately 9-11.6 mg/dL (4.5-5.8 mEq/L), as discussed in Chapter 4.

Signs of Deficiency or Excess

Young, growing kids are subject to retarded growth and rickets if calcium intake or vitamin D is inadequate, as dis­cussed in Chapter 4. If doelings are bred when too young or too small, especially if they are carrying the weight of mul­tiple fetuses, it is common to have bowing of the limbs and lameness develop because of high calcium requirements and inadequate calcium intake.

Veterinarians must be aware of related and clinically similar problems in the same type of animal (Anderson and Adams 1983) and in young, rapidly growing kids (Baxendell 1984). In this instance, the animals develop metabolic bone disease (epiphysitis) because of excessive calcium intake, often in the form of oyster shell or similar supplement to a legume hay.

A severe imbalance of calcium and phosphorus (rela­tively too much phosphorus from a mostly grain diet) can cause osteodystrophia fibrosa in goats, discussed in Chapter 4.

The occurrence of milk fever (periparturient hypocalce­mia) in goats is discussed below. Lactation diets low in calcium lead to decreased milk production.

Dietary Requirements and Supplementation

Intestinal absorption of calcium is quite efficient for preru­minants but probably less than 40% in adults when forage is the calcium source; realistically, the figure is approxi­mately 30% (Kessler 1991). The most recent NRC (2007) uses an absorption coefficient of 0.45. Absorption rates are increased from inorganic sources such as calcium monophosphate and calcium chloride. When calcium intake is decreased, absorption efficacy increases (though usually not above 45%). Absorption decreases rapidly if intake is high. Legumes and cruciferous plants are typi­cally rich in calcium, as are bone meal and fish meal. Dicalcium phosphate, ground limestone, and oyster shells are alternate sources. Little is known regarding calcium absorption in goats, and thus values established for sheep are used. It is best to be generous when stating mineral requirements in general, and this is particularly true of calcium, which is relatively inexpensive. However, large excesses of calcium may predispose male and castrated male goats to urinary obstruction with calcium carbonate stones (see below and Chapter 12).

The calcium contained in milk is readily absorbed by the kid, but forages and concentrates are less available sources. After weaning, the calcium requirement of kids depends markedly on expected growth rate. For instance, according to the NRC (2007), a 20 kg dairy doeling needs 1.4g calcium/ day at maintenance, but 5.1 g/day if gaining 150 g bw/day.

Adult goats at maintenance need approximately 3.5-6 g of calcium in the diet/day for goats weighing 50-100 kg (Morand-Fehr and Sauvant 1978). This requirement can be expressed on a metabolic weight basis as 0.19 g calcium per kg⁰'⁷⁵ (Kessler 1981). Skeletal growth, mineralization of the fetuses, and milk production all increase the goat's need for calcium. The NRC (1981b) allowed only 2 g/day extra for late pregnancy, while French recommendations have varied from an extra 6 g total dietary calcium per day (Morand-Fehr and Sauvant 1978) to 1-1.1 g additional absorbable calcium (INRA 2007). Current NRC (2007) rec­ommendations take into account the litter size. If dietary sources are inadequate during pregnancy, the goat can draw on its skeletal reserves at this time, as long as an increased intake during lactation permits repletion of the reserves.

Goat milk contains approximately 1.4 g of calcium/kg (NRC 1981b), and the NRC (1981b) suggests feeding 2-3 g/L, depending on the fat content of the milk. This

Table 19.3 Summary of major mineral requirements for dairy goats.

Ca, calcium; P, phosphorus Mg, magnesium; Na, sodium.

assumes a more efficient absorption of calcium than might be expected from forage sources, and thus the French use a value of approximately 4 g/L to calculate requirements. Others have made a similar recommendation of 4.3 g/kg milk (Kessler 1981). See Table 19.3.

Phosphorus

Phosphorus is required for both soft tissue and bone growth. It plays an important part in nucleic acid synthesis, energy metabolism, and acid-base balance. The normal serum phosphorus concentration is approximately 4.2-9.8 mg/dL for adult goats and 8.3-10.3 mg/dL for juve­niles (Sherman and Robinson 1983). Additional reference ranges are presented in Chapter 4.

Signs of Deficiency

Clinical signs of a deficiency include slowed growth, pica (appetite for abnormal substances), an unthrifty appear­ance, and decreased serum phosphorus levels. As with cal­cium, a temporary phosphorus deficiency in the adult can be met from body reserves, but milk production decreases with a prolonged deficiency.

Dietary Recommendations and Supplementation

Phosphorus is absorbed mainly from the small intestine, but also from the stomachs. Phosphorus absorption is much more efficient than and is independent of calcium absorption. The true absorption coefficient of phosphorus for goats seems to be higher than for sheep. The NRC (2007) uses a value of 65%, Meschy (2000) uses 70%, and other texts propose that 64-75% of dietary phosphorus is typically absorbed. Some natural phosphates are relatively insoluble and poorly absorbed. Goats recycle phosphorus efficiently through saliva (NRC 2007).

The maintenance requirement is 2.5-5 g dietary phospho- rus/day for goats weighing 50-100 kg (Morand-Fehr and Sauvant 1978). On a metabolic weight basis, this has been expressed as 0.14 g phosphorus per kg⁰'⁷⁵ (Kessler 1981). For late pregnancy, an additional 1.5 g per day is in order. Goat milk contains 0.84-2.1 g P/L, which can be supplied by add­ing to the diet 1.4-2.1 g P/L milk produced.

Animals fed a large quantity of concentrates, which typi­cally contain 3-5 g P/kg, usually have adequate phospho­rus intake, and in this instance a relative calcium deficiency may occur. In general, the Ca : P ratio in the diet should not be less than 1.2: 1 (2 : 1 for young males). Grazing goats rarely develop a phosphorus deficiency because of their tendency to browse varied and phosphorus-rich plants.

Magnesium

Magnesium has received far less study than have calcium and phosphorus, although the metabolism of these miner­als is interrelated. Approximately 62% of body magnesium is deposited in bone, 37% in cells, and 1% in extracellular fluid (Stelletta et al. 2008). Magnesium is required for many enzyme systems (including those necessary for energy metabolism and for synthesis of RNA and DNA) and for normal neuromuscular function (Hoffsis et al. 1989).

Signs of Deficiency

The normal serum magnesium concentration for goats is 2.8-3.6 mg/dL (see Chapter 4). Hypomagnesemic tetany typically occurs if the serum level drops below 1.1 mg/dL (Stelletta et al. 2008). An average serum concentration of 0.79 mg/dL was reported in five goats with uncharacterized clinical signs of hypomagnesemia in India (Vihan and Rai 1984). Serum calcium may be low because magnesium is required for the release and action of parathyroid hor­mone (Hoffsis et al. 1989).

A magnesium deficiency may lead to anorexia or hyper­excitability. Hypomagnesemia is sometimes referred to as grass tetany. Although review articles such as Martens and Schweigel (2000) report clinical signs in dairy animals that include decreased production, teeth grinding, salivation, tetany, seizures, recumbency, and death, actual case reports in goats are lacking.

Dietary Recommendations and Supplementation

The percent magnesium absorption probably varies con­siderably with the diet and is not regulated by a hormonal feedback system (Martens and Schweigel 2000). NRC (2007) has proposed using an absorption coefficient of 0.20. Much is thought to be absorbed from the forestom­achs (Poncelet 1983; Martens and Schweigel 2000), and transit time that is too rapid or a potassium excess may hin­der magnesium absorption. A wide margin of safety is desirable. The possible contribution of a magnesium excess to urolithiasis in males, however, must not be ignored. Grasses growing rapidly in cool, wet weather or after heavy fertilization are often low in magnesium and high in potas­sium. Forages containing less than 0.2% Mg on a DMI basis have been associated with hypomagnesemic syndromes in ruminants (Hoffsis et al. 1989). Whole goat milk also occa­sionally supplies inadequate magnesium, and kids might develop tetany if not supplemented (Hines et al. 1986).

There is little goat-specific data available from which to calculate dietary requirements. AFRC (1998) and the French suggest using the levels indicated for sheep. For an animal that weighs 60 kg, then, total daily intake of magne­sium is 1 g for maintenance (approximately 0.045 g/kg⁰'⁷⁵), plus 1.5 g for late gestation. The approximate magnesium supplement needed per kg of milk is 0.7 g (Kessler 1981). When additional magnesium is needed in the diet, it is often supplied at MgO, although MgCO₃, MgSO₄, and MgCl₂ are other possibilities, although less palatable or more laxative. On spring pastures, provision of NaCl in loose or block form helps to counter high dietary potassium and maintain absorption of magnesium from the rumen. A magnesium source can also be mixed with the salt.

Potassium

Potassium is the major intracellular cation in the tissues of the goat. Although large amounts of potassium are required for health, this mineral is normally present in abundance in forage-based diets and is well absorbed.

Signs of Deficiency

Possible signs of potassium deficiency include reduced feed intake, poor growth, and reduced milk production. Severe deficiencies may cause emaciation and muscle weakness. Dietary potassium is also important in the main­tenance of plasma sodium concentration.

Dietary Recommendations and Supplementation

Potassium levels of 0.5% of the diet for growing kids and 0.8% for lactating goats have been recommended, based on requirements for other ruminant species. Lush for­ages with high potassium content can contribute to the development of hypocalcemia and hypomagnesemic tetany by antagonizing magnesium absorption from the rumen (Underwood and Suttle 1999).

Salt, Sodium Chloride

Ordinary diets are more apt to be deficient in sodium than in chloride.

Signs of Deficiency

Goats that lack sodium may lick dirt. They also may show reduced growth and feed intake and reduced milk produc­tion with increased butterfat concentration (Schellner 1972). Goat milk from European breeds contains approximately 0.4 g/kg sodium.

Dietary Recommendations and Supplementation

Dietary sodium recommendations of 0.045 g/kg⁰'⁷⁵ and 0.5 g/kg milk have been specified (Kessler 1981). The goat normally will eat salt to supply its sodium requirement, then stop. Sodium chloride can be provided free choice or mixed with the feed at 0.5% of the complete ration DM. A higher inclusion rate in feed is often used to pro­mote water intake and diuresis to prevent urolithiasis in males. Either blocks or loose salt can be used. Consumption is typically increased with a loose salt, but the goat that needs more salt is willing to chew on a block to get it. A mild excess only leads to increased water consumption and urination. However, high consumption of salt results in high salt content in the manure, which in turn can inhibit plant growth when the manure is used as a soil amendment. Excessive consumption of salt prepartum might contribute to udder edema, but this theory has fallen out of favor for dairy cattle.

When salt is used as a vehicle for trace minerals or medicaments and is fed free choice, it is important that the goat have no other source of sodium (plain salt or bicarbonate of soda) to satisfy its cravings. Goat keepers who offer a smorgasbord of supplements are ascribing greater nutritional wisdom to the goat than it actually possesses.

Sulfur

Sulfur is important in body proteins because it occurs in sulfur-containing amino acids such as methionine and cys­tine. The sulfur content in hair is especially high. Dietary supplementation of cashmere goats with dl-methionine increased guard hair yield much more than it increased cashmere yield (Ash and Norton 1987).

Signs of Deficiency

Salivation, lacrimation, alopecia, and emaciation are reported with marked sulfur deficiency (NRC 1981b, 2007). A less severe deficiency may cause reduced feed intake and poor growth rate.

Dietary Recommendations and Supplementation

An S : N ratio of 1 : 10 is commonly recommended (NRC 2007). This is necessary to meet the growth require­ments of rumen microorganisms and thereby assure increased production by the goat. The presence of increased levels of tannins in certain range plants may interfere with sulfur availability (Gartner and Hurwood 1976). Likewise, if feeds are grown on certain deficient soils or if a high proportion of NPN is fed (Bhandari et al. 1973), supplementation of the ration with sodium sulfate or ammonium sulfate may be required. A sulfur concentration of 0.15% of ration DM with classic rations and 0.20% with rations high in NPN is appropriate. More recently, the estimated sulfur requirements have been increased to 0.22% for maintenance, gestation, and growth of goats and 0.26% for lactating goats, with an S : N ratio of 1 : 10.4 (NRC 2007).

Large excesses of sulfates interfere with trace mineral (copper and selenium) absorption and have been impli­cated in the development of polioencephalomalacia (Rousseaux et al. 1991; Gould 1998). Extrapolating from recommendations for cattle and sheep, the maximum tolerable dietary sulfur level in high-concentrate diets is 0.30%, and 0.50% if the diet contains at least 40% forage. Drinking water should contain less than 600 mg sulfate/L for high-concentrate diets, whereas 2500 mg sulfate/L is acceptable with higher forage intake (NRC 2005).