Khan, Naheeda
(1995)
Effect of phytase on availability of phosphorus to growing pigs.
PhD thesis, University of Nottingham.
Abstract
Problems caused by excessive phosphate output from the animal production sector were addressed by examining means of lowering phosphate excretion of growing pigs. A review of literature looked at the requirement, dietary allowance, and availability of phosphorus to growing pigs. Variation in estimated phosphorus requirements needed to be resolved before dietary allowances could be matched to requirements. It was recognised that phytate-phosphorus present naturally in cereals would be sufficient to meet the pig’s requirement for phosphorus if it could be hydrolysed prior to or during digestion. The use of Aspergillus niger phytase to improve phytate - phosphorus availability, and thereby lower faecal phosphate output, was explored.
A series of trials assessed the efficacy of phytase from Aspergillus niger (var ficuum) and looked at the phosphorus requirement of growing pigs. The first studied the effects of phytase on apparent ileal and total tract digestibility, and examined whether an alteration in fermentation induced by dietary inclusion of yeast (Saccharomyces cerevisiae) could influence phosphorus availability. Six gilts, cannulated at the terminal ileum, were used in a 3 x 3 latin square balance trial. A high-phytate barley based diet containing 4.7g phosphorus (2.3g present as phytate-phosphorus) was fed (1) as a control, (2) with 2.5g (1000 units) Aspergillus niger phytase/ kg or (3) with 5g Saccharomyces Cerevisiae yeast/ kg. For each period, a 10-day acclimatization to the diet was followed by a 5-day total faeces and urine collection period, and thereafter by a 5-day ileal sampling period. Phytase addition resulted in an increased apparent ileal digestibility of phosphorus from 0.259 to 0.387 (p = 0.094, s.e.d. = 0.054), an increased total tract digestibility from 0.0483 to 0.632( p = 0.074, s.e.d. = 0.070), and a 37% reduction in faecal phosphate output (p = 0.018). Any alteration in fermentation as a result of yeast addition did not influence phosphorus digestibility. It was concluded that the addition of 1000 phytase units/kg feed caused hydrolysis of phytate and therefore an improved availability of phytate phosphorus.
A second balance trial looked at the relationship between level of dietary phytase and phosphorus digestibility, in order to derive an optimum inclusion rate. 12 gilts of 25kg liveweight were used in a triplicated 4x4 latin square trial. Treatments were a maize-soya basal diet (1) containing 5g/kg phosphorus (of which half was present as phytate phosphorus) to which was added phytase at levels of 500 (2), 1000 (3), and 1500 units phytase/kg (4). Calcium to phosphorus ratio was 1.5:1. Each 10 day acclimatization period was followed by a 5 day total faeces and urine collection. Addition of phytase increased apparent phosphorus digestibility (over the control) at all levels of inclusion (p = 0.05, s.e.d. = 0.025). The quadratic response of phosphorus digestibility to dietary phytase was described by the equation y = 0.5832 + 0.000162x - 0.84E'7x2 (p = 0.048; y = apparent phosphorus digestibility, x= phytase units/kg feed). The maximum response was calculated at 1000 units/kg, however, the shape of the curve indicated that 400-500 units/kg was the economic optimum.
Having established repeatable increases in phosphorus digestibility with phytase, two growth trials were used to assess the amount of phytate-phosphorus that could become available through use of the enzyme. In both trials, graded levels of non-phytate phosphorus were achieved by addition of inorganic phosphate to a low phosphorus basal diet. Each phosphorus level was fed ad libitum either with or without phytase added at 1000 units/kg feed. The first trial looked at young pigs growing from 10 to 25kg liveweight. Growth performance, whole body mineral content, bone strength and bone mineral content were used as response criteria. 72 individually penned piglets weighing 10kg were assigned randomly within sex to 2 (basal diet), 2.5, 3, 3.5, 4, 4.5,5,5.5 or 6g non-phytate phosphorus/kg at a constant calcium level (8 g/kg). At 25kg the pigs were killed, and the right femur and right third and fourth metatarsal bones removed for breaking force determination using an Instron detector. Samples of the ground whole body were analyzed for phosphorus content.
Reduced daily gain of pigs receiving the basal (2g/kg) diet was counteracted by the addition of phytase, approaching that of pigs on the 3.5 g/kg diet (p= 0.085, sed = 0.056). Femur strength and phosphorus content of the body were increased both by increasing the level of inorganic phosphorus in the diet and by phytase addition. Linear and quadratic relationships between non-phytate phosphorus intake and growth, bone strength, bone mineral content and carcase mineral content were apparent, but were destroyed by addition of phytase to the diet. A daily intake of 5.5g digestible phosphorus was necessary for maximum growth, whereas 4g/day was sufficient for maximum bone strength. 3.7 g digestible phosphorus/kg diet was recommended for pigs of 10-25kg liveweight. Based on combined criteria it was concluded that adding phytase to a phytate-rich diet at low levels of digestible phosphorus made approximately 70% of the phytate-phosphorus available for bone accretion.
The final trial used pigs growing from 25 to 60 kg liveweight. Levels of non-phytate phosphate were 0.85 (basal diet), 1.25, 1.65, 2.05, 2.45, 2.85, 3.25, 3.65, and 4.05g/kg. 72 individually penned male pigs weighing 25kg were assigned to one of the diets, fed either with or without phytase, at a constant calcium level (8 g/kg).Pigs were slaughtered at 60kg. The left third and fourth metatarsals were removed for breaking force determination and subsequently for mineral analysis.
A lowered daily gain of pigs receiving the basal (0.85g/kg) diet was overcome by the use of phytaseand approached that of pigs receiving the 1.65 g/kg diet (p = 0.015, sed = 50.1). Linear and quadratic responses of bone strength to intake of non-phytate phosphorus were observed. Daily digestible phosphorus requirement was estimated at 6g for optimum feed conversion efficiency and maximum bone strength, and 9g for maximum growth rate. Addition of phytase resulted in an increased breaking strength of both the third and the fourth metatarsal (p=0.045, p=0.011, respectively). Based on growth and bone strength data it was calculated that phytase addition to the basal diet enabled 50% of the phytate phosphorus to be utilised. However, the contribution of liberated phytate-phosphorus to that utilised diminished as the dietary phosphorus level increased towards requirement.
It was concluded that addition of Aspergillus tiiger phytase resulted in hydrolysis of phytates, allowing 50-70% of phytate-phosphorus to be utilised, and if used correctly, could substantially reduce phosphorus output from the growing pig sector.
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