CHAPTER I: LITERATURE REVIEW - Repository.lib.ncsu.edu

Intrinsic Phytase. Corn and wheat contain similar phytate P levels (Nelson, 1967;Choct, 1997); however, bioavailability of phosphorus differs between these two cereals.Phosphorous bioavailability estimated for wheat was five-fold higher than soft winter wheat(12 versus 58%) in chickens (Hayes et al., 1979) and growing pigs (Miracle et al., 1997).The greater phosphorus availability in wheat is apparently due to elevated endogenousphytase (Pointillart, 1994; Eeckhout and De Paepe, 1994) and acid phosphatase activities(Viveros et al., 2000) as compared to corn (Table I-3).According to Eeeckhout and De Paepe (1994), feed ingredients should be considered tohave phytase activity when this is higher than 100 U/Kg. Thus, wheat, but not corn, hasintrinsic phytase activity.Endogenous wheat phytases were more resistant to proteolysis in the stomach (Jongbloedet al., 1992) and more active at low gastric pH than extrinsic phytases (Ranhotra and Loewe,1975). Moreover, in a laying hen experiment, wheat-soybean meal and corn-soybean mealdiets were analyzed for phytase activity and found to contain 20 and 267 U/Kg, respectively.Hens fed wheat based diets had higher tibia ash compared to those fed corn based dietsindicating the importance of the elevated activity of native phytase (Liebert et al., 2005).In addition, there is evidence that phytate can be utilized from poultry fed diets containinggrains with high levels of intrinsic phytase (Leytem et al., 2007). In contrast, the use of cornsoybeanmeal diets including 15% wheat middlings increased phosphorus utilization andreduced fecal phosphorus; however, average daily gain, FCR and crude protein utilizationwere reduced by supplementation of intrinsic phytase coming from wheat middlings (Gong etal., 2006). This suggests that the positive response from endogenous wheat phytase mightdepend on the NSP content and other antinutritional factors in wheat.15