Upto about 85% of the total phosphorus found in feedstuffs of vegetable origin, particularly the cereals, cereal by-products and oil cakes is present in form of phytic acid (inositol hexa-hosphate). Under most dietary conditions the phytate phosphorus is poorly utilized by monogastrics including poultry and, consequently, excreted via the feces. The bird's need for phosphorus, therefore, has to be met through supplementing diet with inorganic phosphorus sources such as defluorinated phosphate or the dicalcium phosphate. The traditional practice of adding inorganic phosphorus in poultry ration adds to the cost of feeding besides constraints in finding the right kind of stuff for regular use.
Presently in India, about 12 million tones of compounded poultry feed are being manufactured annually. Considering the minimum level of l% addition in diet, about 0.12 tones of dicalcium phosphate of worth USD400, 000 is required.
Besides being an indigestible constituent the phytate phosphorus tends to act as an anti-nutritional factor in diet in that it possesses strong chelating properties thereby markedly reducing the bioavailability of several multivalent cations mainly the Ca2+, Mn2+, Zn2+, Fe2+ and Fe3+ by forming insoluble, phytate-metal, complexes. Moreover, the phytate also decreased protein digestibility through formation of indigestible protein-phytate complex and strongly inhibit the activity of amylases.
Most feed ingredients of plant origin contain from 55-85% of the total phosphorus in phytate form (Table 2). The cereal by- products like wheat or rice brans and oilseed meals usually possess higher levels of total phosphorus than that in whole grains and with a larger proportion bound as phytate.
The practical poultry rations are largely composed of vegetable oil cakes, meals, grains, and their byproducts. Phosphorus bioavailability from such feedstuffs is only about 30%. The hydrolysis and absorption of phytate phosphorus by monogastric animals are complex processes that are affected by certain factors namely- dietary calcium and inorganic phosphorus (or available phosphorus) levels, vitamin D3, age and type (genotype) of birds, types of dietary ingredients, sources of fibre in diet and feed processing etc. The values reported in the literature on the availability in most of the single and many feed mixtures are variable. In feedstuffs of plant origin, the P occurs in association with phytates and the variation in its availability may party be due to the presence of the endogenous phytase which hydrolyses phytate and affects P bioavailability.
High dietary levels of calcium have been reported to have a slightly negative effect on phytate P utilization in pullets and laying hens. It has been reported that a chick's ability to retain phytate P increases when the diet contained inadequate levels of non-phytin phosphorus (NPP) compared to normal phosphorus levels. Phytate P is more efficiently retained when chickens are fed suboptimal levels of NPP. It is generally conceded that older birds hydrolyze phytate phosphorus to a greater extent than chicks, the basis of this being that there is more of the phytase activity present in the gastrointestinal tract of older birds.
Limited reports indicate that there may be breed and strain differences in the utilization of phytate phosphorus. Study on the relative ability of three genotype (White Leghorn, Kadaknath and Aseel) on phytate phosphorus utilization from diets varying in calcium and phosphorus has indicated that the chicks of different genotypes differed in their response to utilize phytate phosphorus.
In view of such undesirable effects of phytic acid, it is preferred to either remove it altogether or reduce its amount in poultry feed or ingredients. Efforts have been made in different laboratories to either eliminate or reduce phytic acid content in plant feedstuffs through chemical methods, solid state fermentation technology, autolysis or by the use of phytase enzyme in diet. Of these, supplementation of diet with microbial phytase appears to be more promising. In early seventies, Dr.T.J.Nelson and his group at International Minerals and Chemical Corporation, Libertyville, Illinois were among the first to report that inclusion of fungal phytase from Aspergillus ficuum in diet of broilers resulted in a marked improvement in the utilization of phytate phosphorus and also in bone mineralization. However, because of the high-anticipated production cost of enzyme and a relatively lower cost of di-calcium phosphate or the de-fluorinated rock phosphate, results of their work could not be commercialized.
Considering an increased awareness amongst the masses and the environmentalists against high phosphorus in animal wastes in many of the developed countries and a rapid advancement in fermentation technology, interest in application of phytase enzyme in improving phosphorus utilization by poultry has further been renewed. In this direction, a great number of phytase, hydrolyzing microorganisms have been isolated. The strains of Aspergillus capable of producing phytase enzyme can greatly meet expectations of the nutritionists. Moreover, steps have been taken towards economic production of phytase through conveying phytase gene in the original wild Aspergillus strain on to the production strain thereby resulting in a considerable increase in production efficiency.
Considerable work has been done in India in recent years on the effectiveness of supplemental microbial phytase in improving the P bioavailibity in poultry. Microbial phytase supplementation @ 250-500 units/kg in diet increased body weight gain and feed intake in broiler chicken. However, as a result of simultaneous increase in body weight gain and feed intake, several reports have indicated that microbial supplementation had no significant effect on feed to gain ratio in broiler chicken. Improvement in chick performance, however, has been related to an improved utilization of dietary phosphorus. The phytase supplementation of diet was found associated with a significant increase in plasma inorganic phosphorus concentrations, tibial ash percentages, as well as improved absorption and reduced excretion of dietary phosphorus. However, contrary to these reports, the addition of dietary phytase to corn-soya diet containing less phosphorus than the NRC (1994) recommendation did not improve either body weight gain or feed intake, but it did increase toe and tibia ash and plasma inorganic phosphorus in broiler chickens, growing quails and guinea fowl. The apparent cause of dissimilar results may partly be attributed to the complex nature of factors influencing the bio-efficacy of phytase and utilization of phytate phosphorus.
Nonetheless, the phytase supplementation to a standard broiler diet can allow the reduction of the usual addition of inorganic phosphate in the diet and also reduce the amount of P excretion into the environment. Enhanced level of vitamin D3 can also be used as an alternative to phytase to improve phytate availability in low Ca and NPP diets.
As layer feed differs from broiler feed in respect of among other things, a very high calcium content, the supplemental phytase may just not be as effective in presence of high dietary calcium (3.5%) as in case of broilers (1.0%). Experiments carried out in this direction indicated that phytase supplemented layer diets @ 250-300 units/kg improved laying performance, tibial bone ash and phosphorus absorption. The phosphorus excretion also decreased significantly in birds fed the phytase-supplemented diet. Quite contrary to these findings. Some workers did not find any improvement in the production performance of laying birds. However, it has been demonstrated that phytase supplementation @ 250 units/kg in layer eliminated inorganic P supplementation without affecting laying performance.
Phytic acid has strong chelating potential and forms a variety of complexes with cations and proteins, rendering, these nutrient biologically unavailable. Theoretically, when phytic acid is hydrolysed by microbial phytase, all minerals bound to it should be released.
Relatively, very little work has been done in India on the effect of phytase supplementation on the availability of minerals. Reports on this aspect suggested that contents of ash and minerals in tibia such as Ca, P, Mg, Zn and Fe were significantly higher in phytase supplemented layer diets.
Phytase not only improves mineral and trace element utilization, but may also enhance the protein digestibility and energy utilization. Some of the recent reports conducted abroad have clearly shown that the supplemental phytase improved apparent ileal digestibility of nitrogen and amino acids (lysine, methionine,cysteine, threonine) and dietary energy.
At the present time, the benefit from adding microbial phytase to poultry diets on the utilization of minerals and trace elements is undisputed, because many studies conducted abroad and in India have shown consistent effects with differing diets. Thus, it is widely accepted that microbial phytase is an effective means of improving the economics of broiler/layer production. The use of phytase has become accepted all over the world, especially in countries with intensive animal production on limited acreage, where reduction in phosphorus output from animal manure is of ecological concern. Phytase contributes to alleviating excessive P output.
Table 2. Phytate phosphorus contents and phytase activities of plant feed ingredients
Ingredient |
Phytate P (%) |
Phytate P (as % of total P) |
Phytase activity (units/kg) |
CEREAL | |||
Maize |
0.24 (0.17-0.29) |
72 (66-85) |
15(0-46) |
Barley |
0.27 (0.19-0.33) |
64(56-70) |
582(408-882) |
Wheat |
0.27 (0.17-0.38) |
69(60-80) |
1193(915-1581) |
Oats |
0.29 (0.22-0.35) |
67 (59-78) |
42(0-108) |
Rye |
0.22(0.20-0.23) |
61(56066) |
5140(4132-6127) |
Sorghum |
0.24(0.21-0.28) |
66 (64-69) |
24(0-76) |
Foxtail millet |
0.19 |
70 |
- |
Finger millet |
0.14 |
58 |
- |
Rice |
0.27 (0.25-0.28) |
77 (74-81) |
- |
Rice polished |
0.09 (0.04-0.17) |
51 (49-55) |
- |
CEREAL BY-PRODUCT | |||
Rice bran |
10.31 (10.02-10.09) |
80 (72-86) |
122(108-135) |
Wheat bran |
0.92 (0.88-0.96) |
71 (70-72) |
2957(1180-5208) |
Rice polishing |
20.42 |
89 |
|
OILSEED MEAL | |||
Soybean meal |
0.39 (0.37-0.42) |
60 (57-61) |
8(0-20) |
Cottonseed meal |
0.84 (0.75-0.90) |
70 (70-71) |
- |
Peanut meal |
0.48 |
80 |
3(0-8) |
Rapeseed meal |
0.70 (0.54-0.78) |
59 (43-70) |
16(0-36) |
Sunflower meal |
0.89 |
77 |
62(0-185) |
Coconut meal |
0.29 (0.26-0.33) |
49 (43-56) |
24(0-80) |
Sesame meal |
10.18 (10.03-10.46) |
81 (77-84) |
- |
