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New developments in the production and processing of feed for the use in the poultry sector are associated with changes in the feed structure, which is characterized by the particle size and the particle size distribution as well as the physical form of the diet, i.e. mash, pellets or expandate. Different grinding methods, grinding intensities as well as thermal treatment processes are used in order to achieve more energy-efficient feed production and to increase the quality and safety of feed. The aim of this study was to investigate whether gastrointestinal function and consequently animal performance and health were affected by the feeding of differently structured diets. For this purpose, investigations were focused on morphological, histological and immunological parameters and on the jejunal glucose transport of the digestive tract in laying hens.
The eight experimental diets were produced using two different mills, the roller mill and the hammermill. By changing the grinding intensities, coarsely and finely ground feed was produced either as a mash or it was processed further to expandate. In total, the following eight diets were produced, which differed in their feed structure, but showed an identical botanical and chemical composition (30 % corn, 29 % wheat and 22 % extracted soybean mash): Coarsely and finely ground mash diets, coarsely and finely ground expanded diets, each of them produced by one of the two mills. In eight consecutive trial runs, a total of 384 laying hens (Lohmann Brown), aged 20 weeks were randomly allocated to eight different experimental groups (in total 48 hens per feeding group) with six animals per floor pen each representing a single experimental unit. The hens had ad libitum access to feed and water and were kept in barn systems. Each experimental group received differently structured diets over a period of 21 days. During the experimental trial, the laying performance, i.e. the egg production and the egg weight of each feeding group was recorded daily, and the feed intake and body weight recorded weekly. At the end of each trial run, the hens, aged 23 weeks, were slaughtered. In order to examine the effect of different feed structures on the development of the digestive organs, the pancreas, proventriculus, gizzard and the three segments of the small intestine of hens of each feeding group were extracted and subsequently weighed. For morphometric analysis, segments from the duodenum, jejunum and ileum were removed, processed histologically and finally the villi lengths and crypt depths were determined. In order to investigate the effect of the different diets on the active glucose transport in the jejunum, Ussing chamber experiments were conducted. On the basis of flow cytometric investigations, the intraepithelial lymphocytes (IELs) of the duodenum and jejunum of laying hens were characterized, and the effect of different feed structures on their distribution and relative frequency examined. All collected data were analyzed by using a three factorial analysis of variance in order to control for interaction and single factor effects with respect to the factors milling method, physical form and particle size of the diet.
Concerning the collected animal performance data, the physical form of the diet had an impact on the overall feed intake. Hens fed with mash diets showed significantly higher feed intakes than those given the expandate. Regarding the total experimental period, neither the particle size of the feed nor the milling method used affected the overall animal performance. However, the results showed that both the physical form as well as the particle size of the diet influenced the development of the digestive organs. Laying hens that received mash instead of expandate had significantly higher gizzard and proventriculus weights as well as increased pancreas weights. Hens fed the coarsely ground diets showed significantly higher relative gizzard weights than those fed the finely ground diets. Regarding the microscopic structure of the small intestine, the results demonstrated that only the physical form of the diet had an impact on the villus lengths and crypt depths. Laying hens fed with mash diets showed significantly longer duodenal and shorter ileal villi as well as increased duodenal villus height to crypt depth ratios than those given the expandate. Moreover, the results of the Ussing chamber experiment showed that the active glucose transport in the jejunum, mediated by the SGLT-1, was influenced by the factor physical form of the diet. Mash-fed hens had a higher glucose transport rate than expandate-fed hens. The protocol for the flow cytometric analysis, which was established in order to determine the different lymphocyte subsets from the gut-associated lymphoid tissue of laying hens, led to reliable and reproducible results. With regard to the effect of the feed structure, the results showed no difference in the distribution and relative frequency of the IELs.
To conclude, the results of this study illustrated that in general the feed structure had no impact on the animal performance, with the exception that the physical form of the diet (mash vs. expandate) influenced the FI. The feeding of mash diets compared with the feeding of expanded diets showed positive effects regarding the development of gastrointestinal organs and the intestinal microstructure of epithelium of laying hens, which may have led to the observed heightened glucose absorption of the jejunum. Furthermore, the feeding of coarsely ground diets also resulted in increased gizzard weights, but did not influence the intestinal microstructure of the epithelium and the jejunal glucose absorption. This study provides a characterization of different lymphocyte subpopulations from the gut-associated lymphoid tissue of Lohmann Brown hens. However, the feed structure did not seem to influence the distribution and relative frequency of IELs in healthy hens of this age. The milling method used for the feed production had no effect on the investigated parameters.
Finally, it should be noted that based on the results of the present study, neither intensive grinding nor the use of thermal treatment processes led to improved animal performance. In contrast, the feeding of coarsely ground mash diets seemed to have positive effects on the gastrointestinal function and thus may lead to improved animal health and welfare. From an environmental and economical point of view, it should be emphasized that the production of coarsely ground mash diets using energy-saving milling methods is resource-efficient and therefore contributes to climate protection.