ABSTRACT We conducted two studies to determine the bioavailability and apparent digestibility of P in a low-phytate corn hybrid (.28% total P, .10% phytate P) genetically modified to be homozygous for the 1pa1-1 allele and a nearly isogenic corn hybrid (normal) (.25% total P, .20% phytate P). Additionally, we conducted an in vitro assay involving a peptic and pancreatin digestion to estimate P availability. The first study used 50 individually penned pigs (initial body weight 9 kg) and 10 treatments in a randomized complete block design. A cornstarch-soybean meal basal diet (.6% Ca, .2% P) was used. Treatments consisted of the basal diet and the basal diet plus .05, .10, or .15% P from monosodium phosphate (MSP), low-phytate corn, or normal corn. After a 35-d feeding period, pigs were killed to collect the fourth metacarpal for measurements of ash and breaking load. Breaking load was regressed on added P intake, and the bioavailability of P was determined by the slope ratio method. The bioavailabilities of P (relative to MSP) for low-phytate and normal corn were 62 and 9%, respectively. These were similar to the determined in vitro values of 57 and 11% for low-phytate and normal corn, respectively. In the second study, 20 pigs (initial BW 20 kg) were used in a randomized complete block design with a 2 × 2 factorial arrangement of treatments. Two corn lines (low-phytate and normal) and two levels of supplemental P (0 and .2%) from dicalcium phosphate were used. Diets with no added P were formulated to contain .9% lysine, .6% Ca, and .34% P. Apparent nutrient digestibilities were calculated from total collection of urine and feces for 5 d. There were no differences among treatments for energy and nitrogen digestibility. Pigs fed low-phytate corn with no added P had increased digestibility and retention of P and reduced total P excretion (P < .05). We conclude that low-phytate corn contains at least five times as much available P as normal corn. The use of low-phytate corn greatly reduced the amount of P excreted by the pig and increased the N:P ratio in the manure.
Implications
These studies are the first to report on the nutritional value of a genetically modified low-phytate corn containing the lpa1-1 allele and its effect on phosphorus excretion in pigs. Phosphorus in this low-phytate corn is approximately five times more available than in a nearly isogenic equivalent normal-phytate corn hybrid and can significantly reduce the amount of phosphorus excreted in swine waste. Diets containing low-phytate corn with no added phosphorus from an inorganic source significantly increased the nitrogen:phosphorus ratio in the swine waste, making it more environmentally suitable for land application. An increase in calcium digestibility was seen in swine diets formulated with low-phytate corn, and it may be possible that the digestibilities of other cations or amino acids may be enhanced by the use of this corn hybrid.
Key Words: Pigs, Phosphorus, Phytic Acid, Pig Manures, Maize
© 2000, by the American Society of Animal Science. All rights reserved.
J. Anim. Sci. 2000. 78:675-681
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