Ruminants can tolerate a wide range of Ca : P ratios when their vitamin D status is adequate and the dietary supply of each mineral is adequate. In a trial with sheep, the high Ca : P ratio of 10 : 1 had no adverse effect with a diet containing 2.6 g P kg21 DM, but severe bone disorders arose when the diet contained only 0.8 g P kg21 DM (Young et al., 1966). There are other instances of the addition of calcium to phosphorus-deficient diets exacerbat- ing or inducing a state of phosphorus deficiency in sheep (Field et al., 1975;
Wan Zahari et al., 1990). In an experiment with calves given three levels of dietary calcium (2.7, 8.1 and 24.3 g kg21 DM) and three of phosphorus (1.7, 3.4 and 6.8 g kg21DM), nine Ca : P ratios, ranging from 0.4 : 1 to 14.3 : 1, were
‘tested’. Dietary ratios between 1 : 1 and 7 : 1 all gave satisfactory and similar results, but, with Ca : P ratios below 1 : 1 and over 7 : 1, growth and feed efficiency decreased significantly (Wise et al., 1963). No adverse effect was apparent with heifers fed for 2 years on diets with Ca : P ratios ranging from 7 : 1 to 9 : 1 (Call et al., 1978). None of these experiments critically examined the effects of the Ca : P ratio independently of calcium and phosphorus supplies. In a subsequent critical experiment, Field et al. (1983) showed that the Ca : P ratio per se was unimportant over the range 0.6–3.6, although increases in dietary calcium from 3.4 to 5.4 g kg21DM decreased the efficiency of absorption of added inorganic phosphorus by 18% at the higher of two supplementation levels. Calcium had no such effect on phosphorus absorption from the basal diet (1.5 g P kg21 DM). The greater responses to phosphorus reported in South African cattle on calcareous soils than in Australian beef cattle grazing on non-calcareous soils were explained by Cohen (1979) in terms of the influence of calcium on phosphorus metabolism. It is worth noting that tropical legumes often have extreme Ca : P ratios (> 10.0: N.F. Suttle, unpublished data).
Pigs and poultry appear to be less tolerant of high dietary Ca : P ratios than ruminants, but the problem is again one of sensitivity to high calcium when phosphorus supplies are marginal (Koch and Mahan, 1985; Reinhart and Mahan, 1986; Mohammed et al., 1991; Lei et al., 1994). Unfortunately, pre- occupation with dietary Ca : P ratios has dominated the design and reporting of many later experiments, especially those dealing with the phosphorus nutrition of pigs (Koch and Mahan, 1985; Reinhart and Mahan, 1986). The extensive data already cited from one of these experiments (Figs 5.1 and 5.2;
Reinhart and Mahan, 1986) can be used to illustrate the specific and dominant effect of calcium over Ca : P ratio on phosphorus metabolism (Fig. 5.7). Newly weaned pigs were taken through ‘starter’, ‘grower’ and ‘finisher’ phases on diets 5% below or 10% above the then phosphorus standards (NRC, 1979) for each phase (0.5 and 0.7, 0.4 and 0.6, 0.35 and 0.5 g P kg21DM, respectively), while maintaining Ca : P ratios of 1.3 : 1, 2 : 1, 3 : 1 or 4 : 1 at each phosphorus level. For a given phase, any gap between the two lines in each graph indicates a phosphorus effect, any slope a calcium effect and any difference Influence of calcium in ruminants
Influence of calcium in non-ruminants
Fig. 5.7. Effects of dietary phosphorus and calcium concentrations on growth, feed conversion efficiency (FCE) and serum inorganic phosphorus (Pi) in ‘grower’ pigs (data from Reinhart and Mahan, 1986).
in slope a Ca 3 P interaction. The lower phosphorus level was inadequate throughout, particularly for growth (Fig. 5.7) and bone ash (Fig. 5.1), provid- ing a sensitive test of both the National Research Council (NRC, 1979) standard and any further influence of dietary calcium. Growth and serum Pi (Fig. 5.7) both showed similar linear patterns of decline as dietary calcium increased. In each growth phase, anyaddition of calcium was detrimental to each index of phosphorus status. The most likely explanation is that the smallest addition of calcium was sufficient to reduce phosphorus absorption by a fixed amount, probably by completing the conversion of PP in the maize–soybean meal diet (0.25–0.34 g PP kg21 DM) to non-absorbable complexes. The explanation is supported by the much smaller effects of calcium addition seen when a diet with maize starch rather than PP-rich maize grain was used (Koch and Mahan, 1985). Similar adverse effects of dietary calcium on the absorption of plant phosphorus are seen in poultry (Mohammed et al., 1991). Addition of Pi to a PP-rich diet can dramatically increase the apparent absorption of total phosphorus (e.g. from 15 to 35%;
Cromwell et al., 1995: from 27.6 to 57.2%; Kornegay and Qian, 1996), but, when calcium is slavishly added in equal measure, no improvement may been seen (Simons et al., 1990). If any addition of calcium has a fixed effect on phytate phosphorus absorption, irrespective of the amount of added Pi, the preservation of added Ca : added Pi ratios serves no useful nutritional purpose, as Eeckhout et al. (1995) concluded, but merely drives up the phosphorus requirement in pigs and growing poultry. For laying hens, any acceptable ratio must be considerably higher than 2 : 1, due to their far greater requirements for calcium than for phosphorus.
Early experiments with lambs, aged 5–6 months, indicated that 1.3 g P kg21 DM was borderline, 1.0 g P kg21 DM inadequate and 1.7 g P kg21 DM adequate, as judged by growth and serum Pi, for growing sheep (Beeson et al., 1944). These values were soon confirmed (Mitchell, 1947), but they are low when compared with the NRC (1985) recommendations for growing lambs of 1.7–3.8 g P kg21 DM. The discrepancy is due to the fact that field observations deduce requirements which allow skeletal reserves to be depleted, while scientific estimates of requirement allow for the complete mineralization of bone at all times and therefore have a built-in safety factor (AFRC, 1991). The NRC (1985) estimates of requirements were 2.0–2.9 g P kg21 DM for pregnant and lactating ewes. The derivation of factorial estimates of phosphorus requirements for ruminants is particularly difficult, due to the problems of defining the maintenance requirement at a level commensurate with satisfactory bone mineralization and production (Braithwaite, 1984b; Challa et al., 1989; Scott et al., 1995). The factorial method for estimating net requirements proposed by AFRC (1991) will be used here, since there is evidence that the faecal endogenous losses for whole-roughage diets was appropriately estimated (Ternouth, 1989; Scott et al., 1995). However, a higher efficiency of phosphorus absorption (0.74 vs.
Phosphorus requirements for sheep
0.64) will be used to derive gross requirements, based on the mean of 11 recent estimates for roughages by previously cited authors (see ‘Dietary Sources of Phosphorus’) (0.74 ± 0.09 SD). The resultant requirements are given for convenience alongside calcium in Table 4.6. It must be emphasized that sheep will often consume far less phosphorus than that recommended and remain healthy, particularly when concentrates are fed; these are guide- lines for feed formulation, not diagnostic criteria.
Studies with Holstein calves (Wise et al., 1958) and Hereford bullocks (Tillman et al., 1959) indicated minimum growth requirements of 2.1 g P kg21 DM between 3.5 and 11 months of age. Much higher needs (up to 3.2 g P kg21 DM) were suggested for young calves by Miller et al. (1987), but these may be peculiar to the semipurified diet used. Australian investigations with grazing beef cattle indicated that 1.8 g P kg21 DM in pastures is sometimes adequate for all except lactating cows and young fast-growing animals and that 1.2 g P kg21 DM is sometimes adequate for animals of 200 kg LW growing at 0.5 kg day21 (Cohen, 1975). The latest NRC (1984) recommenda- tions are similar to those given previously (NRC, 1975), ranging from 2.2 to 4.3 g P kg21DM for calves and 1.9 to 3.9 g P kg21DM for lactating cows, but there is growing evidence that they remain generous for growing and breeding beef cattle (Ternouth et al., 1996; Ternouth and Coates, 1997) and for dairy cows. In earlier work, no reduction in weight gain (0.45 kg day21), feed efficiency or appetite was observed in Hereford heifers fed 66% of NRC (1975) recommendations for 2 years (1.4 as opposed to 3.6 g P kg21 DM) (Call et al., 1978). Similarly, feeding phosphorus at 3.2 g kg21 DM (80% of NRC (1975) requirement) to lactating cows for 7.5–12 months did not reduce milk yield or induce hypophosphataemia. Again, there is a place for factorial estimates, but an important adjustment is necessary to the AFRC (1991) model; using an absorption coefficient of 0.75 rather than 0.58 to convert net to gross requirements lowers the estimates by 23% (Table 4.7). Requirements are greatest for young, rapidly growing calves and decline markedly with age.
The use of high-quality feeds does not increase requirements stated as concentrations, but they do increase as milk yield increases. These trends are shared by the estimated sheep requirements (Table 4.6); again, their limita- tions must be stressed and they need not be met on a day-to-day basis. If fulfilled during periods of supplementary feeding, these recommendations should provide a skeletal reserve sufficient to sustain the herd until the next period of supplementation. No adjustment has been made to AFRC (1991) maintenance requirements, despite suggestions by Australian workers that they were grossly overestimated (Coates and Ternouth, 1992; Ternouth et al., 1996; Ternouth and Coates, 1997). Their recent studies appear to confirm that FEP is related to DMI and plasma Pi, but, if the target plasma Pi is 1.5 mmol (45 mg) l21, the FEPvalues used by AFRC (1991) are close to those observed in growing and pregnant beef cattle in Queensland. Maintenance needs for lactation may be only 50% of those used by AFRC (1991), according to
Phosphorus requirements for cattle
Ternouth and Coates (1997), but more evidence is needed before further adjustments are made.
According to Miller et al. (1964), baby pigs up to 6–8 weeks of age required 4.0 g P kg21 DM to permit normal growth and feed utilization, 5.0 g P kg21 DM to maintain normal serum Pi and alkaline phosphatase values and an adequate rate of skeletal development and 6.0 g P kg21 DM for maximum bone density and breaking strength (Table 5.1). Using the most sensitive criterion, optimum bone strength, empirical estimates of requirement for phosphorus still vary (ARC, 1981) and, for reasons argued in Chapter 4, averaging the literature values for feeding trials will overestimate minimum requirements. The preferred approach is that of factorial modelling, using the components of net requirement for growing pigs given by the Agricultural Research Council (ARC, 1981). The choice of absorption coefficient is critical, and the use of a single value to cover diets varying in concentration of phy- tate phosphorus and calcium is unrealistic. Ranges of phosphorus require- ment are therefore advocated to accompany absorption coefficients ranging from 0.50 to 0.70 (Table 4.10), assuming that calcium will not be overfed (see Chapter 4). The lower requirements would apply to diets with wheat and animal protein components or added phytase and the higher requirements to maize–soybean meal diets for young pigs.
The need to minimize the amounts of phosphorus accumulating in animal wastes has introduced a new perspective into what constitutes a mini- mum requirement (Eeckhout et al., 1995). Quotas have been imposed on farmers in the Netherlands to limit the amounts of minerals which are dis- persed on land as animal wastes, and they are so restrictive that whole enter- prises may have to halt production. In these circumstances, the supply of sufficient phosphorus to obtain maximum bone strength in pigs grown for meat production is a luxury. Although small deviations from NRC (1979) requirements (these were not changed much in the subsequent NRC (1988) report) caused losses of performance, phosphorus and calcium were increased or decreased together (Combs et al., 1991b). Eeckhout et al.’s recent (1995) study of the effects of small but independent changes in phos- phorus and calcium provision in wheat–soybean meal diet supports the esti- mates given in Table 4.10 for heavier pigs given a high-phytate diet. Since bone strength is important in boars, they should always be fed phosphorus at the higher level. Gilts should also be fed generous amounts of phosphorus (and calcium) during the first reproductive cycle. Nimmo et al. (1981b) found that a high proportion of gilts (30%) became lame when fed 5.5 g P (and 7.2 g Ca) kg21DM during gestation, whereas those given 8.3 g P (and 10.7 g Ca) kg21DM did not. There were associated differences in bone strength but no residual effects on offspring with common levels (5.0 g P and 7.5 g Ca kg21 DM) fed during lactation (Nimmo et al., 1981a).
Phosphorus requirements for pigs
Unlike calcium requirements, those for phosphorus have declined over the years. According to the NRC (1966), 6.0 g P kg21 DM was adequate for growth and normal bone formation in chicks, so long as at least 4.5 g Pikg21 DM was present. The latest report (NRC, 1994) recommends 4.0 g non- phytate (np)P kg21 DM for the youngest Leghorn-type chicks, decreasing to 3.0 g kg21 DM by 12–18 weeks; values for the youngest broilers remain at 4.5 g kg21DM (Table 4.8). In contrast, the requirements for growth in turkeys have increased, mainly because the type of bird has changed and rate of growth has increased considerably, as has efficiency of feed use. The latest recommendation from NRC (1994) is a sliding scale, which decreases from 6.0 to 4.2 g between 4 and 8 weeks of age and to 2.8 g npP kg21 DM by weeks 20–24. However, Qian et al.’s (1996) results for turkey poults between 1 and 21 days (Fig. 1.3) suggest that 5 g npP kg21DM was optimal for all cri- teria of phosphorus status. National Research Council provisions for the laying hen plummeted from 429 in 1966 to 250 mg npP day21 in 1994. A more cautious approach is to reduce the phosphorus provision over a similar range as lay progresses (Table 4.9; Roland, 1986). A high rate of egg-laying increases phosphorus needs more than would be expected from the small amount of phosphorus in the egg (about 120 mg), because the increase in phosphorus catabolism associated with egg production increases endogenous losses. In two experiments with caged layers (Owings et al., 1977), it was concluded that diets providing about 200 mg available phosphorus day21 maintained egg production but that at least 300 mg was necessary to maintain the ‘livability’ of the hens, i.e. to prevent the ‘cage layer fatigue syndrome’
(see Chapter 4). Phosphorus requirements are further affected by environ- mental conditions. High temperatures reduce feed consumption and hence raise phosphorus requirements, when these are expressed as dietary propor- tions (NRC, 1994). A similar reasoning will, of course, apply to other minerals and nutrients. Floor-housed birds are able to recycle phosphorus through coprophagy and their requirements are therefore lower than those of caged hens (Singsen et al., 1962).
It is difficult to give precise estimates of the requirements of growing horses, because the optimal growth rates for maximum performance are unknown and the rate of growth, like that of other domestic livestock, has increased over the years. Experiments with young growing ponies (Schryver et al., 1974) and with mature horses (Schryver et al., 1970, 1971) indicated relatively high requirements for younger animals. Estimates were derived from the mineral composition of the growth increment, the obligatory losses of minerals and an intestinal absorption efficiency of 30–50%. From the average feed consumption observed in these experiments (H.F. Hintz, 1979, personal communication), it can be calculated that the phosphorus requirements of foals would be met by diets containing 5.0 g P kg21DM and those of mature horses from diets containing 1.5 g P kg21DM.
Phosphorus requirements for poultry
Phosphorus requirements for horses
Phosphate is intrinsically a well-tolerated ion and, for this reason, animals can allow circulating levels of phosphate to fluctuate widely. This, coupled with ready excretion of excess phosphate via the urine, means that livestock generally tolerate excessive intakes of phosphorus. Problems arise through adverse interactions or cumulative effects with other minerals. High phos- phorus intakes predispose animals to urinary calculi in sheep, this is more likely to happen when the diet also provides excess magnesium, because for- mation of magnesium phosphates is integral to growth of the phosphatic cal- culus (Suttle and Hay, 1986). Dietary excesses of phosphorus (5.5–8.3 g npP kg21DM) predispose broilers to TD (see Chapter 4), but the damage can be greatly restricted by concurrently feeding excess calcium (15–17 g Ca kg21 DM) (Edwards and Veltmann, 1983). Excesses of acidogenic sources, such as monobasic phosphate, disturb the acid–base balance in laying hens (Keshavarz, 1994; see also Chapter 6). In the horse, the feeding of excess phosphorus in diets low in calcium can cause secondary hyperparathy- roidism.
AFRC (1991) A reappraisal of the calcium and phosphorus requirements of sheep and cattle. Technical Committee on Responses to Nutrients, Report Number 6.
Nutrition Abstracts and Reviews (Series B)61, 573–612.
Andrews, T.L., Damron, B.L. and Harms, R.H. (1972) Utilisation of various sources of plant phosphorus by the turkey poult. Nutrition Reports International6, 251–257.
ARC (1981) The Nutrient Requirements of Pigs. Commonwealth Agricultural Bureaux, Farnham Royal, Slough, UK, pp. 215–248.
Bar, A. and Hurwitz, S. (1984) Egg shell quality, medullary bone ash, intestinal calcium and phosphorus absorption and calcium-binding protein in phosphorus-deficient hens. Poultry Science63, 1975–1981.
Beeson, W.M., Johnson, R.F., Bolin, D.W. and Hickman, C.W. (1944) The phosphorus requirement for fattening lambs. Journal of Animal Science3, 63–70.
Beighle, D.E., Boyazoglu, P.A. and Hemken, R.W. (1993) Use of bovine rib bone in serial sampling for mineral analysis. Journal of Dairy Science76, 1047–1072.
Beighle, D.E., Boyazoglu, P.A., Hemken, R.W. and Serumaga-Zake, P.A. (1994) Determination of calcium, phosphorus and magnesium values in rib bones from clinically normal cattle. American Journal of Veterinary Research55, 85–89.
Biehl, R.R. and Baker, D.H. (1997a) 1a-Hydroxycholecalciferol does not increase the specific activity of intestinal phytase but does improve phosphorus utilisation in both caecotomised and sham-operated chicks fed cholecalciferol-adequate diets.
Journal of Nutrition127, 2054–2059.
Biehl, R.R. and Baker, D.H. (1997b) Utilisation of phytate and non-phytate phosphorus in chicks as affected by source and amount of vitamin D3. Journal of Animal Science75, 2986–2993.
Biehl, R.R., Baker, D.H. and DeLuca, H.F. (1995) 1a-Hydroxylated cholecalciferol compounds act additively with microbial phytase to improve phosphorus, zinc