LAUSR

Abstract This study investigated the effects of low-cost dried yeast and multi-strain probiotics on laying hens. A total of 108 Charoen Pokphand (CP) brown hens aged 25 weeks were used in a completely randomised design (CRD) experiment involving 18 cages of six hens per replicate and six replicates per group. The hens were allocated to three groups: basal diet (BD, control), BD plus dried yeast (additive-fed), and BD plus dried yeast and probiotics (probiotic-fed). The hens were fed the BD for 2 weeks, the experimental diet for 4 weeks, and the BD for 4 weeks, and the ends of these periods were termed before feeding (BF), during feeding (DF) and after stopping feeding (ASF). At DF, the additive-fed group showed the highest egg production and % albumen, and at ASF, the Haugh unit (HU) was higher than that of the probiotic-fed group. At DF, the probiotic-fed group exhibited the most intense yolk colour and the highest eggshell thickness. Only the probiotic-fed group produced jumbo and extra-large size eggs. The expression of the immune response-related genes IL-1β, IL-2, IFNγ and HSP90 did not differ among the groups and feeding periods with the exception of the IL-2 level of the additive-fed group at ASF, which was significantly lower than that of the control. Thus, dried yeast delivered probiotics effectively and thereby improved the egg production and quality and the immune response, and the combination of yeast with probiotics induced synergistic increases in the yolk colour and egg size (p < .05, .01). HIGHLIGHTS Yeast supplementation improved egg production and quality and the immune response of 25- to 35-week-old laying hens. The addition of yeast combined with probiotics synergistically increased yolk colour and egg size. Yeast additive reduced the expression of the signalling cytokine IL-2, and the effect lasted for at least 4 weeks.