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Egg production and quality
Yousefi and Karkoodi (2007) investigated the dose-related effect of yeast on laying performance traits and observed similar egg production indices and quality at 0.05%, 0.1%, and 0.15% supplementation levels. This result is in consonance with Yalcin et al. (2008 a, b) who reported similar egg production data in birds on yeast. Chickens fed probiotic yeast supplemented diets have been noticed to produce large egg and yolk (Swain et al. 2011); heavier shell weight, increase shell thickness and reduced yolk cholesterol (Yousefi and Karkoodi 2007; Yalcin et al. 2008a; Swain et al. 2011). The incidences of laying soft shell and broken eggs have recorded in chickens following yeast culture supplementation (Park et al. 2001). Other investigators have also observed increased egg production and egg weight and decreased the concentration of egg yolk in Hyline brown laying aged 22 weeks fed diets containing 0.1-0.4% yeast autolysate (Yalcin et al. 2010). This is similar to the findings of others (Kim et al. 2002; Shivani et al. 2003; Shareef and Al-Dabbagh 2009) who reported that yeast supplementation improves egg production in laying hens. This result is in harmony with Hassanein and Soliman (2010) who reported that yeast improves egg production and quality (egg albumen, egg yolk, and eggshell thickness) in white leghorn layers fed live yeast culture at 0.4% and 0.8%. Similar findings were obtained in egg typed chicken fed yeast diet (Sharmah et al. (2001). Sanaa (2013) evaluated the dose-related effect of yeast supplementation on layers and observed that the group in 0.3% yeast had 4.67% increase in shell thickness and 2.59% reduction in albumen weight. Similarly, Nursoy et al. (2004) recorded an increase in eggshell thickness in laying birds fed S. cerevisiae based diet at 0.8%, while Songsak et al. (2009) observed that addition of cassava yeast probiotic at 1×106, 1×107 and 1×108 microorganism/kg in layer diet for 8 weeks increased egg weight and eggshell thickness and reduced hen day production. However, the observed increase in eggshell thickness of birds fed yeast diet may be due to the ability to enhance calcium (Bradley and Savage 1995) and phosphorus availability (Reed and Naodawithana 1999). In contrast, Sanaa (2013) reported that yeast supplementation at 0.3 and 0.6% in layer diets reduced hen-day production and egg number. Similar results were also recorded by Dizaji and Pirmohammadi (2009) who found that adding yeast inclusion at 0, 200, 300, 400g/ton of diet) for 10 weeks in laying hens aged 46-55 weeks reduced egg weight. The discrepant variations in these studies may be fully explained but can be partly attributed to the strains of birds used, yeast type as well as the rearing environment (Mahdavi et al. 2005).

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