The discovery during the 18th century that bone consisted primarily of calcium phosphate led to the use of calcium in the prevention of rickets, a childhood disorder of bone development which had plagued humans for centuries. Similar disorders in farm livestock were quickly linked to calcium deficiency, induced by feeding pigs and laying hens diets low in calcium and prevented in calves by feeding calcium-rich diets. As animal production intensified, energy-rich, grain-based diets were increasingly fed to livestock in protected environments and the incidence of bone disorders multiplied.
Unwittingly, livestock were being fed diets naturally deficient in calcium while simultaneously being ‘starved’ of vitamin D3, essential to the efficient utilization of calcium, by cutting them off from sunlight. Animal breeders ensured that interest in calcium nutrition was maintained by selecting for traits which had high requirements for calcium – growth, milk yield, litter size and egg production. By encouraging reproduction while skeletal growth is still incomplete, producers have ensured that bone disorders in poultry remain commonplace. In the high-yielding dairy cow, an acute calcium deficiency still strikes many animals at calving with no sign of bone disorders and controversy still surrounds the optimum level and pattern of calcium provision for averting such problems.
Forages are generally satisfactory sources of calcium (Ca) for grazing live- stock, particularly when they contain leguminous species. Minson (1990) gives the average published values as 14.2 and 10.1 g Ca kg21 dry matter (DM) for temperate and tropical legumes and 3.7 and 3.8 g Ca kg21 DM for the corresponding grasses. The average temperate grass sward will meet the
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© CAB International 1999. Mineral Nutrition of Livestock (E.J. Underwood and N.F. Suttle)
calcium requirements of sheep, but a contribution from legumes is needed for the dairy cow (see Tables 4.6 and 4.7). Cultivar differences in calcium content can be marked, but it is the maturity of the sward which has the more widespread influences. The leaf generally contains twice as much calcium as the stem, and pasture calcium concentrations are therefore increased by applying nitrogenous fertilizer and decrease with advancing maturity. Slowing of pasture growth by flooding or a seasonal decline in soil temperature increases herbage calcium levels. Selective grazing is likely to result in higher calcium concentrations in ingested forage than in hand- plucked samples. In the UK, conservation as silage will usually result in higher calcium concentrations than conservation as the more mature hay (6.4 vs. 5.6 g kg21DM (MAFF, 1990)). The application of lime or limestone to soils to correct soil acidity has surprisingly little effect on forage calcium concen- trations, possibly because of increases in herbage yield. As far as other forage species and roughage sources are concerned, maize silage commonly contains 2.0–5.0 g Ca kg21 DM (see Table 2.1) and is usually poorer in calcium than herbage, while cereal straws contain around 3 g Ca kg21DM.
Absorbability of forage calcium
With minerals such as calcium, which are absorbed according to need, the full potential of a feed as provider of absorbable calcium can only be tested under conditions were requirements (R) are barely met by intake (I). AFRC (1991) plotted all recorded absorption coefficients for calcium (ACa) in sheep against R/I, found no evidence that they could not raise ACawhen necessary from any feed and suggested an average absorbability of 0.68 for all feeds, including forages (Fig. 4.1). However, much lower ACa have been reported for green-feed oats and ryegrass–white clover (0.24–0.47) given to stags with high requirements for antler growth (Muir et al., 1987), for herbages (0.17–0.19) given to lactating ewes (Chrisp et al., 1989a) and for lucerne hay (0.26) given to high-yielding dairy cows (Martz et al., 1990). Higher values were reported for lucerne–maize silage (0.49; Martz et al., 1990) and for lucerne (Medicago sativa) given to non-lactating goats (0.50; Freeden, 1989), which may have had the capacity to absorb even more. Thompson et al.
(1988) estimated ACa of 0.41–0.64 for different grass species given to lambs and considered that the higher values, obtained with the most nutritious species, were closest to the maximum potential of forages. Why, then, should stags, lactating ewes and milking cows not suceed in absorbing more of their dietary calcium when their needs were maximal? The low ACa may be an indirect consequence of obligatory resorption of bone matrix during seasonal peaks in requirements, which maintains plasma calcium concentrations and lessens the perceived need for dietary calcium (Sykes and Geenty, 1986). It is noteworthy that the stags were not hypocalcaemic (Muir et al., 1987), while ACafor the lactating ewes increased to 0.30 when a protein supplement was given and milk yield (i.e. need) increased (Chrisp et al., 1989a), suggesting that maximal ACa had not been reached. A low ACa in some lucerne crops may be due to the presence of unavailable calcium oxalates. There is some
metabolism of oxalate by rumen microbes, particularly when there has been time to adapt to high-oxalate diets, but the appearance of oxalate crystals in cattle faeces indicates that it is not always complete (Ward et al., 1979).
Oxalates may have greater nutritional significance in grass species, such as kikuyu (for review, see Barry and Blaney, 1987), which contain relatively little calcium (Jumba et al., 1995), and in forage trees, such as the mulga (Acacia aneura). Horses are particularly vulnerable to the calcium-depleting effects of oxalate-rich roughages.
Most concentrates are low in calcium. Cereals rarely contain > 1 g Ca kg21 DM and cereal by-products rarely > 1.5 g kg21 DM. Maize typically contains only 0.2 g Ca, wheat 0.6 g and oats and barley 0.6–0.9 g Ca kg21 DM (see Table 2.1). Supplementation with vegetable protein sources, with the exception of rapeseed meal (8.4 g Ca kg21 DM), will not give adequate calcium levels in mixed rations because these usually contain no more than 2–4 g Ca kg21 DM (MAFF, 1990). Sugar-beet pulp is an excellent supplement for grain-based diets, being rich in calcium (6.0–7.5 g Ca kg21DM) but low in phosphorus. Fish-meal and meat- and bone-meal are also good sources of calcium, levels of 50–100 g Ca kg21DM being commonplace.
Fig. 4.1. Calcium is absorbed according to need: thus pooled data for the efficiency of calcium absorption in lambs plotted against adequacy of dietary supply (net Ca requirement: Ca intake) shows a curvilinear rise to a plateau representing the full potential of the dietary Ca source (from AFRC, 1991): ●, nutritionally balanced diets;
C, nutritionally imbalanced diets.
Concentrates
Absorbability of calcium from concentrates
Cereals and vegetable protein sources are important determinants of ACa to non-ruminants, because the phytate which they contain forms unabsorbable complexes with calcium in feeds, as well as with any added inorganic calcium. Thus animals with high calcium requirements might be prevented from absorbing according to need. The only data for ACa in a foodstuff for non-ruminants, published in a review by Soares (1995), indicated a relative value for sesame seeds only 65% of that of calcium carbonate (CaCO3) when given to rats. The review abounds with relative values for inorganic sources when added to various diets but gives no indication of the quantitative influence of dietary phytate on calcium availability. Hill (1984) suggested that, as a rule of thumb, 1.3 g Ca should be added per gram of phytate phos- phorus (P) present in excess of 2 g kg21 in rations for poultry: the calculation was based on the formula of calcium phytate. Improvements in apparent ACa have been reported when phytase was added to a maize–soybean meal diet low in phosphorus (4.3 g kg21 DM) for young pigs (Pallauf et al., 1992);
further examples are given in Chapter 5. Correction of Pallauf et al.’s data for endogenous faecal calcium (FECa) at 32 mg kg21live weight (LW) (ARC, 1981) gives an ACa coefficient of 0.58 for the basal diet and similar calculations for data from Han et al. (1997) give a coefficient of 0.72. With other papers indi- cating even higher ACa values for cereal/vegetable protein diets, with inor- ganic sources providing most of the calcium (e.g. Kornegay and Qian, 1996), an average absorbability of 0.70 will be used in later calculations of require- ment. As far as absorbability of calcium in concentrates to ruminants is con- cerned, phytate is not a threat, because it is degraded in the rumen. Low absorption, associated with negative calcium balances, has been reported in normocalcaemic lactating ewes given energy-rich diets (Braithwaite, 1983b), but, as with forages, this is unlikely to represent a limitation imposed by the feed (AFRC, 1991).
The priority of all mammals is to maintain calcium concentrations in plasma and extracellular fluids (ECF) close to 2.5 mmol (100 mg) l21 in the face of large fluctuations in demand and lesser fluctuations in supply (Hurwitz, 1996). Such constraints are necessarily relaxed in egg-laying avian species, and the processes for achieving the contrasting objectives of the dairy cow and the laying hen were reviewed by Horst (1986) and Gilbert (1983), respectively.
Homeostasis is achieved partly by the hormonal regulation of absorption.
Calcium is absorbed according to need up to the limits set by the absorb- ability of the mineral in the diet (Schneider et al., 1985; Bronner, 1987); this is close to 90% for milk and probably rarely < 50% of the total calcium supply