Effect of Feeding Different Crude Protein and Threonine Levels on Performance, Egg Quality and Intestinal Histological Alterations of Laying Hens
This experiment was designed to evaluate the effects of different levels of dietary crude protein and threonine on performance, egg quality and histological structure of jejunum in laying hens. One hundred and eighty 48-week old white leghorn hens (Hy-Line, W-36) were randomly assigned to a 3×3 factorial arrangement of treatments based on a completely randomized design that consisted of 9 dietary treatments with 4 replicates of 12 birds each. Experimental diets were included three levels of crude protein (90, 95 and 100% of Hy-Line W36 recommendations) and three levels of dietary threonine (100, 110 and 120% of Hy-Line W36 specifications). The experimental period lasted for a total of 10 weeks and egg quality and performance parameters of laying hens were assessed two times at the end of 5th and 10th weeks of experiment. At final day of trial, two hens per cage were slaughtered to investigate the influence of dietary treatments on jejunal histological structure. Our results showed that decrease in dietary crude protein percent up to 10% and increase of dietaty threonine percent up to 20% had no significant effect on average daily feed consumption, egg production, egg mass, feed conversion ratio, Haugh unit, yolk index and eggshell thickness and hardness. Nonetheless, feeding low-crude protein diets caused to significant decrease on the average of egg weight (P<0.05). On the other hand, although dietary crude protein reduction had no remarkable effects on villus height, crypt depth and the ratio of villus height to crypt depth, fortification of low-crude protein diets (90% of recommendations) decreased the relative jejunal goblet cell numbers; however, supplementation of diets with 120% threonine could compensate these alterations. In conclusion, although reduction of dietary crude protein level up to 10% had no adverse impact on performance, egg quality and jejunal histological structure of laying hens, the highest relative goblet cell numbers observed when the birds fed high-threonine diets.