energy density). Recommended values were some 10% lower for brown-egg laying hens, which consume more food, and are much lower than those advocated by Roland (1986; Table 4.10), but they carry the proviso that they may not give maximum eggshell thickness. It has been suggested that calcium requirements should take account of the size of egg produced, increasing from 3.8 g to 4.5 g Ca day21 as egg size increases from 50 to 60 g (Simons, 1986). For pullets entering lay, the optimum transitional feeding regimen for calcium is to increase levels from 10 to 30 g Ca kg21DM 1 week before the first egg is anticipated (Roland, 1986). Thereafter, the need to meet calcium requirements for peak egg production on a daily and even an hourly basis at times is in contrast to the needs of any other class of livestock for any mineral, with the possible exception of calcium for the recently calved cow.
Indeed, the laying hen will voluntarily consume more of a calcium supplement on laying than on non-laying days (Gilbert, 1983).
Calcium is not generally regarded as a toxic element, because homeostatic mechanisms ensure that excess dietary calcium is extensively excreted in faeces. However, doubling the dietary calcium concentration of chicks to 2.05 g kg21 DM caused hypercalcaemia and growth retardation, fast-growing strains being more vulnerable than slow-growing strains (Hurwitz et al., 1995). The adverse nutritional consequences of feeding excess calcium are generally indirect and arise from impairments in the absorption of other elements when the digesta are enriched with calcium; thus deficiencies of phosphorus and zinc are readily induced in non-ruminants. Dietary provision of calcium soaps of fatty acids (CSFA) represents a convenient way of increasing the fat (i.e. energy) content of the diet without depressing fibre digestibility in the rumen. Significant improvements in milk yield can be obtained in dairy cows (Jenkins and Palmquist, 1984) and ewes (Sklan, 1992)
Table 4.10. Requirements of laying hens for dietary calcium and phosphorus (g day21) (Roland, 1986).
Weeks in production
Time in lay (weeks) 21 to 8a 9–16 10–25 26–moult
Ca 3.75 3.75 4.00 4.25
P
Total 0.70 0.70 0.60 0.50
Non-phytate 0.50 0.50 0.40 0.30
aIt is recommended that for this first period figures be used as %DM fed without adjustment for food intake; thereafter, dietary concentrations should be adjusted for estimated food intake to give the desired daily provision.
but not without feeding (by necessity) gross excesses of calcium (c.
10 g Ca kg21DM) and sometimes (by choice) matching excesses of phospho- rus (6 g P kg21 DM). Whether or not these excesses are related to adverse effects of CSFA on conception rate in primiparous cows (Sklan et al., 1994) remains to be investigated. Hypercalcaemia can cause life-threatening tissue calcification but usually occurs as a secondary consequence of phosphorus deficiency or overexposure to vitamin D3 (Payne and Manston, 1967) and its analogues (Whitehead, 1995). Of particular interest is the tissue calcification in cattle caused by ingestion of Solanum malacoxylon, which contains a vita- min D analogue. The first sign of trouble is the development of osteophagia and there may be no alternative but to temporarily destock pastures infested with the water-loving plant. Factors which safely maximize the deposition of calcium in bone (heavy-boned breeds; ample dietary phosphorus; alkaline mineral supplements; grain feeding) are worth exploring as preventive measures where S. malacoxylonis a problem.
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An early indication that a shortage of phosphorus could have serious conse- qences for livestock production came from the pioneering work of Sir Arnold Theiler (1912) early this century. He investigated two debilitating diseases of cattle and sheep grazing pastures at Armoedsvlakte in South Africa’s northern Cape, diseases known locally as ‘styfziekte’ and ‘lamziekte’. They were characterized by high mortality, poor growth and fertility, fragile bones and periodic cravings among survivors for the bones of their less fortunate predecessors. Looking to the bones for a clue as to the possible underlying nutritional deficiency, phosphorus emerged as the prime suspect and, within 20 years, a series of successful preventive measures had been developed (Theiler and Green, 1932). However, the optimal level and pattern of phosphorus supplementation for cattle at Armoedsvlakte is still the subject of investigation (De Waal et al., 1996). Other pastoral areas lacking in phos- phorus were subsequently identified on many continents, particularly those of the southern hemisphere. Indoor experiments were conducted to confirm and quantify the need for phosphorus and the synergism with calcium whereby the skeleton could develop while maintaining its strength. As the story unfolded, it came apparent that phosphorus had equally important roles to play in the soft as well as the hard tissues of the body and that exchanges between them influenced the development of clinical abnormalities, just as much as the dietary supply. Pigs and poultry would continually succumb to phosphorus deficiency if their grain-based rations were not routinely supple- mented with phosphorus. The success of early supplementation studies with mixtures of calcium and phosphorus (bone-meal or dibasic calcium phosphate (CaHPO4)) delayed recognition of the source of the problem (low phosphorus absorbability in grains), the antagonistic role of dietary calcium and the contrasting metabolism of the two elements. In all three respects, non-ruminants differ markedly from ruminants and these contrasts can best be drawn out by breaking with convention and giving phosphorus a chapter
Phosphorus 5 5
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© CAB International 1999. Mineral Nutrition of Livestock (E.J. Underwood and N.F. Suttle)