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Minerals play a vital role in bone and egg formation. Rapid broiler growth may lead to bone deformities resulting in suffering for the animals, lower production performance and economic losses. Similarly, inadequate mineral deposition, especially of calcium in the egg, may end up in weaker shelled eggs, thus adversely affecting egg quality. Such insufficient mineral supply may cause a higher incidence of egg breakage and ultimately reduce layer productivity and profitability. The poultry industry uses various techniques to produce feed with high quality and maximum sanitation. Long and short term thermal treatments and acid supplementation of feed are considered as efficient strategies to achieve high feed safety standards. However, the effects of these treatments on mineral digestibility are not well documented. Broilers are usually fed with pellets having least differences in particle size and shape. The feed form and particle size is of specific interest in laying hens, which are offered feed either as mash or expandate. The effect of particle size and feed form in layers has previously been studied, however their impact on mineral digestibility and retention in egg contents as well as egg quality was not considered in detail. Keeping in view the importance of various feed treatment strategies distinctly in broilers and layers, the investigations presented in this thesis were designed in two different studies.
The first study was conducted to investigate the effect of thermal processing of feed including pelleting (P), long-term conditioning at 85°C (L) and expanding at 130°C (E) without or with 1.5 % of an acid mixture containing 64 % formic and 25 % propionic acid on the apparent ileal absorption of calcium, phosphorus, magnesium, potassium, sodium, iron, copper, manganese and zinc, their concentrations in liver and tibia as well as various tibial quality parameters in broilers. In total, 480 day-old Cobb broiler chicks were assigned using a completely randomized design with a 3 × 2 factorial arrangement. The ileal digesta, liver and tibia were collected at d 35. AIA of calcium and sodium was improved in group E compared to L. Group P and E showed higher AIA for potassium than L. Bone ash content was increased in group E compared to L. The body weight to bone weight ratio was lower and tibial zinc content was higher in group P compared to E. Tibial iron content was higher in group L than E. Acid addition did not affect AIA, mineral content in tibia or tibial quality parameter. Both thermal treatment and acid did not affect mineral concentrations in the liver, except an inconsistent interaction effect for dry matter content and sodium.
The second study dealed with the impact of roller (R) and hammer (H) mills, mash (M) and expandate (E) with fine (F) and coarse (C) particle sizes, on apparent ileal absorption (AIA) and apparent total digestibility (ATD), egg quality and retention of calcium, phosphorus, magnesium, zinc, manganese, copper and iron, in yolk, albumen and shell. A total of 384 hens (Lohmann Brown), 19 weeks old, were assigned using a randomized design with a 2×2×2 factorial arrangement. Eight experimental diets were offered ad libitum during the whole experimental period and one week before for diet adaption. At 23 weeks of age, digesta from the ileum and rectum and eggs were collected. AIA of magnesium, zinc, copper and iron was higher in treatment R in comparison with treatment H. The ATD of copper and iron was higher in treatment R than treatment H. The shell membrane weight and percent shell membrane weight was lower whereas yolk index and yolk height were higher in treatment H as compared to treatment R. The AIA of magnesium was higher in treatment M than treatment E and in treatment C than treatment F. Thermal treatments displayed higher percent shell membrane weight in treatment E than treatment M. The ATD was higher for phosphorus and lower for iron in treatment F than treatment C. The copper concentration in yolk and albumen was higher in treatment C than treatment F. The shell thickness, shell weight and albumen height were higher and shell density was lower in treatment C as compared to treatment F.
In conclusion, for both broiler and layer studies, the minor impact of feed treatments was limited to ileal and total digestibility of minerals in general and trace elements in particular. The retention of minerals and trace elements in tibia, liver and egg contents as well as quality of tibia and egg was not altered at most. It may be assumed that mineral concentrations in the feed were sufficient to compensate minor differences in digestibility induced by various feed treatments used in the present study. Therefore, different thermal treatments, acid supplementation, milling method and particle sizes used in the present study seem to be of lower importance for feed production regarding optimized tibia and egg quality and therefore may be used for broiler and layer feed formulation.