Forages
Selective grazing
use of 32P to correct for contamination by salivary phosphorus (Langlands, 1987). Such values sometimes show reasonable agreement with hand- plucked samples (Coates et al., 1987; Coates and Ternouth, 1992). If the entire stand is virtually removed during a grazing period, the fact that selection influences phosphorus intake early on is of little consequence in terms of total phosphorus supply, and hand-plucked samples can be standardized to give samples of known maturity for particular species (Kerridge et al., 1990). It is therefore recommended that herbage phosphorus concentrations be measured, wherever possible, by consistent, defined methods to record changes in the supply of available phosphorus to plants from the soil from year to year.
Cereals contain relatively uniform and apparently adequate phosphorus concentrations (2.7–4.3 g P kg21 DM) and vegetable protein sources even more (5–12 g P kg21 DM). Most of this (50–80%) is present as phytate (PP), which is well utilized by ruminants, high AP of 0.78–0.81 having been reported for two cereal and three vegetable protein sources for sheep (Field et al., 1984). Phosphorus deficiency should never arise in ruminants given significant amounts of concentrate feeds. The situation with non-ruminants has, until recently, been quite different. Comparisons with inorganic standards (Pi) of high availability (sodium and potassium phosphates) indicate that cereals and vegetable proteins generally provide phosphorus with only 20–45% of the absorbability of mineral sources for pigs and only slightly more (25–50%) for chickens (Soares, 1995). Limited evidence for turkeys indicates that they may be able to utilize plant phosphorus much more efficiently than pigs or chickens, relative values of 80% being found for maize and cottonseed meal (Andrews et al., 1972). Sources such as sunflower meal, palm-kernel cake and groundnut meal were used very inefficiently by pigs (< 15% relative availability, each in single studies). Cromwell (1992) reported that availabilities of phosphorus in cottonseed meal, maize, dehulled soybean meal, oats and wheat were 0, 12, 20, 23 and 51%, respectively, and that the average relative value for a maize–soybean blend was 15%.
Availability is not simply a reflection of the proportion of PP present, however. The apparent absorption of phosphorus in pigs is much higher in wheat and barley than in maize (47 and 39 vs. 17%), despite similar propor- tions of PP(70.7 and 63.6 vs. 65.6% of total P) (Jongbloed and Kemme, 1990).
The superiority of wheat was related to a high phytase content and the attribute was shared by wheat middlings. To avoid uncertainties regarding availability, a convention was widely adopted whereby only non-phytate phosphorus was regarded as available to pigs and poultry. The convention is unduly pessimistic as far as absorbability of PP is concerned, particularly for pigs. Values for AP from published studies with pigs, obtained by correcting faecal excretion for an endogenous (FEP) contribution of 20 mg kg21 live weight (LW) (ARC, 1981), commonly fall between 0.5 and 0.7 for diets in which most of the phosphorus is provided by phytate and excesses of
Absorbability of phosphorus in concentrates
calcium are avoided. With growing concerns about the amounts of phos- phorus which are discharged via farm wastes into the environment and the costliness of finite global supplies of phosphorus, the profligate convention no longer goes unquestioned. Animal protein sources, other than feather meal, are rich in terms of both concentration and availability of phosphorus (Jongbloed and Kemme, 1990). Meat and bone-meals and fish-meals contain
> 30 g P kg21 DM with high relative availabilities of > 85% for pigs and poultry (Soares, 1995).
The value of PPas a source of phosphorus is influenced by many factors but particularly by the phytase activity in the feed (Pointillart et al., 1987), the levels of added calcium and phosphorus and the age of the animal.
Attainment of high temperatures (> 84°C) during pelleting can inactivate phytases present in the feed (Simons et al., 1990). Soaking the grain can enhance the beneficial effects of added phytase (Liu et al., 1997), and the high relative availabilities of phosphorus in moist maize and wheat (41–53%:
Soares, 1995) may be attributable to enhanced phytase activity. Nelson et al.
(1971) were the first to study purified phytase as a dietary supplement, using chickens. Addition of 850 U phytase released 1 g PP (about 50% of that present) in a recent study with broiler chicks (Schoner et al., 1993), and simi- lar releases were achieved in pigs with 400 U (Hoppe et al., 1993) and 246 U (Kornegay and Qian, 1996) of phytase. Small supplements of phytase (250–750 U kg21 DM) can fully replace Pi supplements at all stages of pig production, from the nursery (Murry et al., 1997) through growing (Liu et al., 1997) to finishing (O’Quinn et al., 1997) stages, with cereals as diverse as pearl millet, maize and sorghum. Bran is rich in phytase and addition of 250 U in this form can also avoid the need to add Pito diets for pigs (Han et al., 1997). Addition of phytase concurrently improves the absorption of calcium in both pigs (Kornegay and Qian, 1996; Qian et al., 1996; Liu et al., 1997; O’Quinn et al., 1997) and poultry (Simons et al., 1990), but this may be due in part to the increased need for calcium which resulted from the improved phosphorus supply from deficient basal diets. Another effective approach has been to give large supplements of 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) (Edwards, 1993). Biotechnology may facilitate both approaches, recombinant phytase being successfully used in pigs (Cromwell et al., 1995) and the 1a-analogue of vitamin D3 in chicks (Biehl et al., 1995;
Biehl and Baker, 1997a, b); combining the two approaches gives additional benefits (Biehl et al., 1995). Early assumptions that hydroxylated vitamin D3 had increased intestinal phytase activity have not been substantiated, and the improved utilization of PP may be a secondary effect following the increased absorption of calcium from the gut (Biehl and Baker, 1997a). Utilization of PP can also be improved by feeding calcium strictly to minimum requirement standards. Edwards and Veltmann (1983) found that the availability of PP to broiler chicks varied from 11 to 39% as dietary calcium and phosphorus varied (from 0.63 to 1.67% and from 0.53 to 1.01%, respectively), low utilization Utilization of phytate phosphorus
being associated with high concentrations of each mineral. Added calcium also reduces the efficiency of phytase additions to pig rations (Lei et al., 1994).
The control of phosphorus metabolism is necessarily quite different from that of calcium. Provided that phosphorus is present in absorbable forms in the diet, it is, like other anions, extensively absorbed, even when supplied well in excess of need, and absorption from milk is almost complete (Challa and Braithwaite, 1989). The kidney and gut are both routes for the excretion of phosphorus that is surplus to requirement, the gut (by way of saliva) being the major route for excretion in grazing ruminants or those fed roughage diets, whereas the kidney is of greater importance in monogastrics and in ruminants fed diets that promote low salivary flow rates (processed, pelleted or concentrate diets). There is no tight hormonal control of inorganic phosphorus concentrations in the bloodstream and serum values range widely above and below the renal threshold of 2–3 mmol l21 in perfectly healthy animals.
The relationship between absorbed and ingested inorganic phosphorus is linear over wide ranges of intake and the slopes indicate AP of about 0.74 in weaned sheep (Braithwaite, 1986) and 0.83 in weaned calves (Challa et al., 1989). Furthermore, these high levels of AP can be attained with indigestible natural sources, such as straw (Ternouth, 1989) and dry, summer forage (Coates and Ternouth, 1992), though less so with bran (0.63; Field et al., 1984). The PP present in the form of inositol tetra-, penta- and hexa- phosphates in straw and seeds is extensively hydrolysed in the rumen by microbial phytase and phosphate is thus released (Reid et al., 1947; Nelson et al., 1976; Morse et al., 1992). There are marginal but quantitatively important increases in AP in the phosphorus-deficient animal (Scott et al., 1984, 1985;
Coates and Ternouth, 1992) and small decreases at excessive phosphorus intakes (Braithwaite, 1984b; Challa et al., 1989). Phosphorus is absorbed principally from the proximal small intestine (Bown et al., 1989; Yano et al., 1991).
Non-ruminants are dependent on the presence of phytases found either in the grains themselves or in intestinal secretions to utilize phytate phosphorus.
Undegraded phytate can be extensively converted to insoluble mineral–
phytate complexes before the site of phosphorus absorption is reached, thus lowering the amount absorbed. Older animals have a greater ability than younger to digest phytate. Adverse effects of calcium–phytate interactions on the absorption of manganese and zinc have been widely demonstrated (Biehl et al., 1995) and are considered in later chapters.