THE EFFECTS OF FEED PROCESSING ON THE FORMATION OF STARCH DAMAGE IN SOFT AND HARD WHEAT AND ITS IMPACT ON STARCH DIGESTIBILITY AND APPARENT METABOLIZABLE ENERGY IN BROILER CHICKENS

Abstract

Wheat is classified into soft, hard and durum classes. Previous research has shown that durum and hard wheat have the highest apparent metabolizable energy corrected to nitrogen (AMEn) in broiler chickens and the reasons for this is not known. It was hypothesized that starch damage is caused by hammer milling and pelleting but hard or durum wheat will have the highest levels due to hard endosperm structure and this increases starch utilization resulting in increased AMEn. The objective was to investigate the effect of wheat hardness and feed processing on the incidence of starch damage (SD) and its impact on starch digestibility and (AMEn) in broiler chickens. Digestibility studies in broiler chickens were conducted using soft, hard and durum wheat treated with varying degrees of processing and the impact on SD, starch utilization and AMEn was measured. Grinding with a hammer mill caused SD but the level was higher in hard than soft wheat (P < 0.0001). SD increased with processing intensity as affected by screen size in the hammer mill and pelleting temperature, where hard wheat had significantly higher SD than soft wheat overall. Water absorption capacity followed a similar trend to SD, where processing intensity increased water absorption and hard wheat had higher water absorption capacity than soft wheat. Scanning electron microscope (SEM) analysis revealed that after grinding the endosperm of hard durum had irregular shapes and the starch granules appeared to have evidence of fractures but they remained mostly embedded in the endosperm matrix. In contrast, the starch granules in soft durum wheat appeared to have smoother edges and the starch granules were clearly visible, distinct and mostly separate from the endosperm matrix. After being processed using low processing intensity (coarse hammer milling), soft wheat had lower starch digestibility and AMEn than hard wheat. However, after processing using high intensity processing (ground through a 3.6 mm screen size and hot pelleted), soft wheat had higher AMEn levels than hard wheat (13.23 versus 12.62 MJ/kg DM, respectively, P < 0.0001). Examination of SEM images of the freeze dried ileal digesta suggests starch granules were digested in high intensity processing diet for both wheat types as no intact starch granules were evident. However, after application of low intensity processing (coarse hammer milling) starch granules were still evident in soft wheat but not hard wheat. Starch digestibility at proximal and distal ileum was higher in birds fed hard wheat compared to soft wheat regardless of processing (P <0.0001). Starch digestibility at proximal and distal ileum and SD were positively correlated, which appears to explain some of the difference in AMEn in soft and hard wheat. The AMEn was positively correlated with ileal starch digestibility (r = 0.61, P = 0.0002). In conclusion, feed processing causes SD in both wheat types. Pelleting produces higher levels of SD than grinding. SD is higher in hard wheat than soft wheat after processing. SD increases starch digestibility and AMEn with the greatest change occurring during high intensity processing.