the milk produced (Muschen et al., 1988). In the early stages of a deficiency or where the deficiency is moderate, the animal is able to draw on its skeletal reserves to maintain yield; several successive lactations may be necessary before bone defects and other clinical signs become obvious and milk production is impaired. In parts of South Africa where phosphorus depriva- tion was severe and prolonged, increases in milk production of 40 to 140%
were recorded from the use of bone-meal supplements. These increases could not be attributed solely to phosphorus, since the bone-meal also provided protein (Bisschop, 1964), but improvements of up to 27.5% in weaning weight to specific phosphorus supplements have subsequently been reported (Read et al., 1986a, b). In an experiment with beef cows given adequate dietary protein, phosphorus deficiency imposed during late pregnancy and early lactation lowered milk yield (Fishwick et al., 1977). In lactating goats, transfer to a low-phosphorus diet after 6 weeks’ lactation was sufficient to suppress milk yield (Muschen et al., 1988). Cows pregnant with their second calf, given diets low in phosphorus (2.4 g P kg21 DM) from the seventh month of gestation and for 7.5–10 months of lactation, showed maximum depression of milk yield (32–35%) between 18 and 34 weeks of lactation and proportionate reductions in food intake (Call et al., 1986).
Depression of milk yield may be a secondary consequence of loss of appetite or reduced synthesis of rumen microbial protein. On the whole, improve- ments in milk yield or weaning weight (an index of milk yield) have rarely been found but are most likely to occur during the first lactation, when animals are still growing (Winks, 1990; Karn, 1997).
Phosphorus deprivation is manifested by a decline in egg yield, hatchability and shell thickness, but it is much less likely to occur than calcium deficiency, because of the far smaller requirement for phosphorus. Egg production and hatchability of fertile eggs from caged hens receiving all-plant diets, contain- ing 3.4–5.4 g P kg21DM, decreased rapidly until 0.9 g Pikg21DM was added, but the same amount of phosphorus added as phytate in hominy was ineffec- tive (Waldroup et al., 1967). Supplementation of diets of commercial type with inorganic phosphorus may thus be necessary for maximum egg yield and quality, but the effects are essentially an induced calcium deficiency resulting from the failure to build up the skeletal reserve of calcium needed to sustain egg production.
The only definite evidence that lack of phosphorus has critically restricted livestock performance comes from improvements seen when appropriate supplements have been given. The following indices provide only rough guidance.
Reduction in egg yield and quality in poultry
Although serum Pi concentrations can fall spectacularly, particularly in the young growing animal depleted of phosphorus, their diagnostic value is limited. In the absence of tight homeostatic control, further factors, such as recent feeding (+), milk yield (2) and handling stress (2), can each significantly influence serum Piin the direction indicated (Teleni et al., 1976;
Forar et al., 1982). Concentrations are also influenced by the site of sampling (coccygeal and mammary vein > jugular vein; Teleni et al., 1976) and interval between sampling, clot formation or separation of plasma and analysis. Use of buffered trichloroacetic acid (TCA) as a protein precipitant can reduce postsampling differences (Teleni et al., 1976), but results obtained with colorimetric assay kits, which require no protein precipitation, will remain vulnerable to postsampling changes. For these and other reasons, it is a mistake to interpret phosphorus levels using a single threshold, and the recognition of a marginal band for serum Pi is crucial. Serum concentrations of 1.25–1.75 mmol Pil21 are ‘marginal’ for ruminants because there are many instances in which values of this order have been recorded in healthy grazing livestock, which do not respond to phosphorus, and others where responses to supplementation have been obtained at the upper limit (e.g. Wadsworth et al., 1990; Coates and Ternouth, 1992). Marginal serum Pi concentrations are most likely to precede clinical aphosphorosis in young, rapidly growing stock or lactating animals receiving diets of high nutritive value. Older beef cattle on dry-season pastures of low nutritive value can have mean serum Pivalues around 1.0 mmol l21 and not benefit from phosphorus supplementation (Engels, 1981; Wadsworth et al., 1990). The equivalent marginal range for young pigs is higher (Fig. 5.2) and probably narrower than for ruminants – 2.5–2.8 mmol l21 is suggested; other papers confirm that growth retardation can occur with serum Pi as high as 1.9 mmol l21 (Koch and Mahan, 1985).
Since bone contains the reserves of calcium and phosphorus and calcium cannot be removed without changes in or relating to bone phosphorus, abnormalities are only indicative of a phosphorus deficit if calcium is non- limiting (compare Table 5.1 with Table 4.2). Protein depletion can also lead to poor mineralization of bones through the resorption of bone matrix or failure to form that matrix. The adequacy of phosphorus (or calcium) nutri- tion can be reflected by the degree of mineralization of the skeleton in sheep (Table 4.3), cattle (Little, 1984; Williams et al., 1991a), pigs (Table 5.1) and turkeys (Fig. 1.3), but there are examples of differences in production attributable to phosphorus deprivation not being reflected by significant differences in bone quality (Wan Zahari et al., 1990; Coates and Ternouth, 1992). The inconsistency is to be expected, given that the primary deter- minants of health and performance are factors that influence satiety (both ruminal and systemic), rather than bone strength, and that the bone sample is a minute portion of a heterogeneous reserve. Having acknowledged the
Serum inorganic phosphorus
Bone criteria in general
limitations of bone indices, it is important to choose the most informative from a bewildering array of possibilities.
Rib biopsy has been widely used since its introduction for cattle (Little and Minson, 1977) and recently applied to pigs (Combs et al., 1991a). The rib biopsy is particularly useful in experimental studies of patterns of change in skeletal mineralization over time, provided that the selected site is consistent (Beighle et al., 1993) and repeated biopsies of the same rib are performed dorsally to earlier biopsies after a reasonable interval (> 3 months; Little and Ratcliff, 1979). Having obtained a disc of bone by trephine (1.5 cm diameter in cattle), a number of analytical options are available, of which the best is a measure of bone density, such as weight (specific gravity), ash or phosphorus (or calcium) per unit volume. Mineral concentrations per gram of ash have been advocated, because they show minimum variation in normal animals (Beighle et al., 1994). However, the mineral composition of bone is not greatly changed by depletion of phosphorus (or calcium) (Qian et al., 1996) and P (or Ca) g21 ash provides little or no indication of phosphorus (or calcium) status (Williams et al., 1991a). Fluctuations in the mineralization of rib biopsy samples in lactating animals can be marked, particularly on phosphorus-deficient pastures (Fig. 5.6; Read et al., 1986c), and they can give a different assessment of phosphorus depletion from that given by plasma Pi. Phosphorus depletion tends to increase Ca : magnesium (Mg) ratios in the skeleton of cattle (Fig. 5.6) as well as lambs (Table 4.3). Tail-bone biopsies can be used, but they contain less ash and are probably less responsive to changes in phosphorus status than rib samples. In the past, experimental stations with access to X-ray equipment have monitored cortical bone thick- ness and recently dual photon absorptiometry (PA), radiographic photometry (RP) and ultrasound (US) have been used to obtain in vivomeasurements of bone mineralization (Ternouth, 1990; Williams et al., 1991b). There are also biochemical tests on serum or urine which indicate the extent to which bone matrix is being broken down; they include assays of hydroxyproline (McLean et al., 1990) and pyridinium X-links in urine. These biochemical measures of bone demineralization in vivoare prognostic rather than diagnostic tools, and they have more relevance to racehorses than commercial livestock, since avoidance of stress injury is crucial to racing performance (Price et al., 1995).
In dead animals, the X-ray, morphometric or mineral analysis of bones may be the only diagnostic option. There are, however, considerable variations between species and also within and between particular bones in the degree of mineralization at a given age (Fig. 5.4). Furthermore, most indices increase with age, irrespective of mineral nutrition (Fig. 5.1; Beighle et al., 1994;
Chapter 4). Mineral : or ash : volume ratios in the entire bone are the simplest and best biochemical indices of bone mineralization (Field et al., 1975;
Williams et al., 1991a). Guideline values are given in Table 5.3, which In vivobone assessment methods
Post-mortem assessment of bone mineralization
summarizes the contribution that each parameter can make towards a diagnosis.
Natural shortages of phosphorus in livestock usually develop in different circumstances from those of calcium, and situations in which the two minerals are both limiting are rare.
Phosphorus deprivation is predominantly a chronic condition of grazing cattle, arising from a combination of soil and climatic effects on herbage phosphorus concentrations. The presence of soils low in plant-available phosphorus (< 10 mg kg21DM) resulted in herbage low in phosphorus (Kerridge et al., 1990). As pasture productivity and milk yield rise with high nitrogen use, the critical soil phosphorus value can rise to as much as 30 mg P kg21 DM (Davison et al., 1997a). Acid, iron-rich soils are particularly likely
Table 5.3. Marginal bandsafor interpreting the most diagnostically useful biochemical indices of phosphorus status in livestock.
Bone ashb Plasma Pic (% dry weight) (mmol l21)
Cattle Calf 60–70 (R,d)d 1.3–1.9
48–56 (R,c)
Cow 50–60 (R,c) 1.0–1.5
Pigs Starter (10–25 kg) 45–48 (d) 2.6–3.2
Grower (25–65 kg) 52–55 (d) 2.3–2.6
Finisher (65–80 kg) 56–58 (M,d) 2.3–2.6
Sheep Lamb 20–30 (V) 1.3–1.9
Ewe 30–36 (T) 1.0–1.5
Broiler Chick – < 4 weeks 40–45 (T,d)
Chick – > 6 weeks 45–50 (T,d) 2.0–2.2 50–55 (Tc,d)
Hen Laying – –
Turkey Poult 38–40 (T)
aValues below bands indicate a probability of impaired health or performance; values within bands indicate possibility of future losses if phosphorus supplies are not improved.
bBone ash can be converted approximately to specific mineral concentrations on the assumption that ash contains 0.18 g P, 0.36 g Ca and 0.09 g Mg kg21; ash/unit volume is roughly similar to ash/unit dry fat-free weight.
cMultiply by 31 to obtain units in mg l21.
dSource and pretreatment of bone sample: R, whole rib; M, metacarpal/tarsal; V, lumbar vertebra; T, tibia; d, defatted bone; c, cortical bone.