Paper type: Original Research Effect of feed form on performance, gut morphology and cecal microflora on late-phase laying hens under cyclic high ambient temperature Reza Mahdavi * , Ali Hossein Piray, Mehran Torki Department of Animal Science, College of Agriculture and Natural Resources, Razi University, Kermanshah, Iran * Corresponding author, E-mail address: R.mahdavi@razi.ac.ir Received: 15 May 2024, Received in revised form: 17 Jul 2024, Accepted: 27 Jul 2024, Published online: 28 Jul 2024, © The authors, 2024. ORCID Reza Mahdavi 0000-0001-9942-4273 Ali Hossein Piray 0009-0009-2252-4036 Mehran Torki 0000-0001-5660-0273 Abstract Feed form and heat stress (HS) are two major critical issues that affect feed intake (FI) in poultry. Limited research has been conducted on the impact of feed form on the intestinal morphology of laying hens. Additionally, the influence of feed form on the intestinal microbiota of laying hens under HS conditions remains unexplored. One hundred and forty-four laying hens (90 weeks of age) were used in a completely randomized design with two treatments (pellet and mash). Each treatment contained six replicates of 12 hens each. Laying hens were subjected to a cyclic heat stress regimen of 32 ± 3 °C for 6 hours per day at a humidity of 35-40% during the late laying period (weeks 90-97). Feed form had no effect on egg production and feed conversion ratio (P>0.05), but egg weight and mass, FI, and body weight gain increased in the birds fed the pelleted diet (P<0.05). Feeding the pelleted diet decreased the shell weight and thickness, as well as the yolk color score (P<0.05), but did not affect other egg quality parameters (P>0.05). Villus width and villus surface area were higher in hens fed the pelleted diet (P<0.05) than those fed the mash diet, but villus height, crypt depth, and the villus height-to-crypt depth ratio were not affected by the treatments (P>0.05). There were no significant differences between treatments regarding cecal bifidobacteria spp. lactobacillus spp. and coliforms populations (P>0.05). However, feeding laying hens with pelleted diet resulted in greater numbers of cecal Enterococcus spp. compared to those fed the mash diet (P<0.05). This study indicated that feeding the pelleted diet improved the egg mass production and intestinal morphology, reduced the shell weight and thickness, and increased the population of Enterococcus spp. in the ceca of late- laying hens exposed to HS. The economic benefit from performance improvement with the pelleted diet may have offset the higher feeding cost. Keywords: feed form, gut morphology, laying hens, microbial population, pellet Introduction the world's land surface and 60-70% of Iran's land surface During the end of the laying cycle, there is a decline in are arid or semi-arid, facing high ambient temperatures for egg production and quality, including eggshell breaking at least several months of the year. Heat stress (HS) is one strength and albumen height. This necessitates the of the major stressors in laying hen production, adversely development of strategies to enhance both egg affecting the reproductive capability, growth, feed intake production performance and quality parameters in late- (FI), egg quality, gut health, physiological status, and laying hens. These advancements could potentially immune function of laying hens. Additionally, high ambient extend the laying cycle and thus improve the overall temperature increases the mortality rate of laying hens (Luo breeding efficiency (Chen et al., 2021). More than 30% of et al., 2018; Xing et al., 2019). Some studies have showed 2024, 12 (2): 1 2 -29 DOI: 10.22103/JLST.2024.23397.1538 http://lst.uk.ac.ir