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.
to provide insufficient phosphorus. The occurrence of a dry period, when plants mature and seed is shed, accentuates any soil effect. In the veld country of South Africa, where the classical studies of bovine aphosphorosis Fig. 5.6. Effects of lactation and phosphorus status on (a) P concentrations in rib biopsy; (b) plasma Pi; (c) rib magnesium (Mg); and (d) rib Ca : Mg ratio. Samples taken from two groups of beef cows, one receiving supplementary phosphorus (●) and one not (C), grazing phosphorus-deficient pasture: note that consistently low plasma Pi from the sixth month did not reflect the skeletal changes (data from Engels, 1981). PH or PC, pregnant heifer or cow; EL, ML and LL, early, mid- or late lactations; PR, pregnant.
were made, herbage concentrations fell typically from 1.3–1.8 g to 0.5–0.7 g P kg21 DM between the wet summer and the dry winter and remained low for 6–8 months. Similarly, in the Mitchell grass (Astrebla spp.) pastures of northern Australia, values fell from 2.5 to as low as 0.5 g P kg21 DM (Davies et al., 1938). Forages from the northern Great Plains of the USA are regarded as marginal in phosphorus (Karn, 1997). The introduction of tropical legumes (e.g. Stylosanthes spp.), tolerant of low available soil phosphorus, lifted the constraint on herbage DM production in northern Queensland (Jones, 1990), but merely exacerbated deficiencies in livestock by producing highly digestible forage, exceedingly low in phosphorus (often < 1 g P kg21 DM);
the problem has required extensive research (Miller et al., 1990). The protein, Fig. 5.6.Continued
digestible energy and sulphur concentrations in the herbage also fall with maturity (Grant et al., 1996; Karn, 1997; for sulphur, see Chapter 9), so that other deficiencies often contribute to the malnutrition of livestock in phosphorus-deficient areas. This was apparent from early studies of zebu cattle in East Africa (Lampkin et al., 1961), of the savannah pastures during the dry winter season in South Africa (Van Niekerk and Jacobs, 1985) and in subtropical Australia (Cohen, 1975, 1979). Responses to phosphorus supple- ments are unlikely to be obtained while other nutritional deficits prevent growth (Miller et al., 1990).
Phosphorus deprivation is less common and usually less severe in grazing sheep than in grazing cattle. One reason suggested for this was that sheep (and also goats) have a higher feed consumption per unit of body weight.
However, the maintenance requirement for phosphorus increases with DMI (AFRC, 1991), putting smaller ruminants at a disadvantage. There are other possible explanations: sheep (and goats) also have a smaller proportion of bone to body weight than cattle and therefore smaller growth requirements;
and, because of their different methods of prehension, sheep are probably more able to select from mixed herbage those plants that are less phosphorus-deficient. The most important species difference, however, is the much shorter period of the annual reproductive cycle during which sheep are lactating (Read et al., 1986a, b); this allows time for depleted skeletal reserves to be replenished. Early Australian studies failed to reveal any benefit from phosphorus supplements to sheep in some areas where a deficiency of phosphorus restricted cattle (Underwood et al., 1940); a similar situation has been confirmed in South Africa (Read et al., 1986a, b).
Phosphorus deprivation would be common in poultry and pigs if inorganic phosphorus supplements were not routinely added to cereal-based diets, due to the poor absorbability of phytate phosphorus. For example, mortality was increased in broiler chicks given a maize/mixed vegetable protein diet containing 4.5 g P kg21 DM, unless it was supplemented with phosphorus (Simons et al., 1990). Phosphorus-responsive bone deformities (rickets) and fragilities will eventually develop, but they are usually preceded by life- threatening reductions in food intake. Nevertheless, a histological study of the proximal tibiotarsus from lame 35-day-old broiler chicks from Holland, where there is great pressure to reduce phosphorus levels in feeds, revealed a high (44%) incidence of hypophosphataemic rickets, which may increase susceptibility to tibial dyschrondroplasia (TD) (see Chapter 4) and bacterial chondronecrosis (Thorp and Waddington, 1997). Growing pigs are less vulnerable than poultry because they need less phosphorus and are normally fed lower concentrations of dietary calcium (see Chapter 4). Skeletal abnormalities are therefore again less likely to occur than in ruminants, but growth of pigs is poor on grain-based rations without phosphorus supplements.
Other grazing livestock
Poultry and pigs
Phosphorus deficiencies in grazing livestock can be prevented or overcome by direct treatment of the animal through supplementing the diet or the water-supply, or indirectly by appropriate fertilizer treatment of the soils. The choice of procedure depends on the conditions of husbandry.
In climatically favoured and intensively farmed areas with sown pastures, phosphate applications, designed primarily to increase herbage yields, also increase herbage phosphorus concentrations (Falade, 1973). Minson (1990) calculated that, on average, pasture phosphorus was increased from 1.7 to 2.4 g kg21 DM with 47 kg fertilizer P ha21 applied, but the ranges in the literature surveyed were wide (increases of 0.2–3.5 to 0.5–3.9 g P kg21 DM from 9–86 kg fertilizer P). Phosphorus can be applied as rock phosphate (RP), as single or triple superphosphate or combined with nitrogen (N) and potassium in complete fertilizers, but it is impossible to generalize about recommended rates (Jones, 1990). For example, lactation milk yields were increased from 3930 to 4310 and 4610 kg cow21 (P< 0.05) in animals on tropical grass pasture when 22.5 and 45.0 kg P ha21, respectively, were applied with 300 kg N ha21 (Davison et al., 1997a, b). However, with 100 kg N or none, there was no response to fertilizer phosphorus. The responses were attributed to increases in the amount of green leaf on offer, rather than an increase in herbage phosphorus concentration (see also Fig. 2.6).
Although herbage phosphorus increased approximately twofold (from 0.6 to 1.3 g P kg21 DM) (Davison et al., 1997a), no benefit was gained from providing an additional dietary phosphorus supplement (Walker et al., 1997).
Pastures treated with phosphatic fertilizers at rates that maximize herbage yield do not necessarily meet the requirements of grazing animals at all times and the process is inefficient on acid, iron-rich soils with a high sorption capacity. On sparse, extensive phosphorus-deficient native pastures, other methods are necessary because transport and application costs are high and herbage productivity is usually limited by climatic disadvantages.
Phosphorus can be provided directly to grazing livestock in phosphatic salt- licks and blocks and also in the water-supply. Typical levels of provision for beef cattle are at least 5 g P head21 day21 for growing cattle and 10 g P head21 day21 for breeding stock (Miller et al., 1990), but intakes > 8 g P day21in growing cattle can retard growth (Grant et al., 1996). The easiest and cheapest procedure is to provide a phosphatic lick in troughs or boxes that afford protection from rain and are situated near the watering-places. A simple 1 : 1 mixture of dicalcium phosphate (DCP, CaHPO4) and salt, with a small proportion of molasses, is well consumed by most animals. Field experience has shown that voluntary consumption of such licks is often acceptable on a herd or flock basis, although consumption varies greatly