Effects on phosphorus digestibility
Exogenous phytases are either 3-phytases (EC 126.96.36.199) or 6-phytases (EC 188.8.131.52), which is grouped according to the specific position of the phosphomonoester group on the phytate molecule at which hydrolysis is initiated . Traditionally, phytases of microbial origin are generally considered 3-phytases, whereas phytases from plant origin are 6-phytases ; however, 6-phytases from E. coli, P. lycii, and the bacterial phytase used in this study are clear exceptions. Thus, previous assumptions regarding the evolutionary distribution of 3- and 6-phytases may be of limited relevance . Exogenous phytases have also been isolated from a variety of sources, expressed in a wide range of hosts, purified, and refolded using various biochemical methods . Depending on the source and expression host, commercially-available phytases have distinct physical and biochemical properties [10, 11, 24, 25] and as a result, they exhibit varying efficacies in pigs and poultry [13, 23, 26, 27]. It is, therefore, important to evaluate the efficacy of new sources of phytase in improving phosphorus utilization for effective use in commercial practice. The phytase used in this study is a 6-phytase from a proprietary strain of bacteria and expressed in a strain of A. oryzae. Currently, there are no data on the effects of this novel bacterial 6-phytase on phosphorus utilization by pigs.
In the present study, phosphorus digestibility of the negative control diet was 40.5% and 39.8% for weanling and growing pigs, respectively. These values were within the range determined in previous studies using low-phosphorus, corn-soybean meal-based diets fed to weanling (17.4% to 46.4%; [28–30]) and growing pigs (16.6% to 39.7%; [13, 27]). The relatively wide range in phosphorus digestibility of the negative control diets across these studies may be related to the inherent variability of phosphorus digestibility in corn and soybean meal. Previous studies have reported that ATTD of phosphorus in corn ranged from 16.1%  to 28.8% , whereas in soybean meal, values from 27.6%  to 46.5%  have been reported. As expected, the phosphorus digestibility values of the negative control diets were less than in the positive control diets. Thus, the amounts of phosphorus absorbed from the negative control diets were reduced compared with the positive control diets, which is mainly an indication of the reduced digestibility of phytate-bound phosphorus in corn and soybean meal compared with inorganic phosphates. Even with the addition of 4,000 FYT to the negative control diet, absorption of phosphorus was not at levels that were similar to the positive control diet. Thus, if one assumes that the positive control diet was at the requirement for phosphorus, this indicates that inorganic phosphorus must be also included in corn-soybean meal diets in combination with phytase.
Values for the ATTD of phosphorus that were observed for weanling pigs fed the diets containing phytase are similar to values reported from previous nursery pig studies in which A. niger phytase [28, 35] or E.coli phytases [9, 29, 36] were used. Likewise, values for the ATTD of phosphorus obtained in growing pigs fed the phytase containing diets are close to or slightly greater than values reported for pigs fed corn-soybean meal diets containing E. coli, A. niger, or P. lycii phytases [13, 37, 38]. Thus, the responses observed in this experiment for this phytase, is similar to what has been reported for other commercially-available phytases.
As a result of greater phytate hydrolysis, fecal phosphorus excretion was markedly reduced in weanling and growing pigs fed low-phosphorus diets containing the bacterial 6-phytase compared with pigs fed the positive or the negative control diets. This observation is also in agreement with results of previous experiments [9, 28, 30, 35, 38, 39]. Thus, the novel 6-phytase used in this experiment is expected to reduce fecal phosphorus excretion to the same degree as other phytases that are currently marketed to the swine industry. Likewise, the increase in the digestibility of phosphorus that was observed by including the novel 6-phytase to the diets is in agreement with results from previous experiments using weanling [9, 29, 30, 36, 37] or growing-finishing pigs [13, 27].
The use of a broken line model in this experiment may have underestimated the phytase levels that maximises the ATTD of phosphorus and calcium, and a quadratic regression curve could have been a more accurate fit to this data . However, it has been suggested that fitting a quadratic regression curve is preferable when the data consists of at least 4 data points below the breakpoint, which was not the case in this experiment . Results of dose–response experiments using A. niger phytase have indicated a curvilinear relationship between phytase level and phosphorus digestibility [40–43], and the maximum response is usually achieved at approximately 1,000 FYT/kg. However, Dungelhoef and Rodehutscord  reported that if a fungal phytase is used, improvements in phosphorus digestibility may be minimal if doses greater than 750 FYT/kg of phytase are used. Braña et al.  also observed that when using G:F as the response criteria, the maximum response to an E. coli phytase was achieved at 738 FYT/kg. Thus, the observation that the response to increasing levels of the bacterial 6-phytase that was used in the present experiments is dose-dependent is in agreement with results obtained with other commercially-available phytases.