| To optimize the dietary metabolizable energy (ME) and protein levels for layers under high temperature condition, the present study investigated the effect of dietary protein and ME level on performance, egg quality and nutrients apparent digestibility of layers. 28-weeks-old Hy-Line Brown layers (n = 1620) were randomly assigned to 9 dietary treatments (4 replicates of 180 birds per treatment) in a 3×3 factorial arrangement. The dietary treatments fed for 28d contained 2550, 2700, 2800 kcal/kg ME of diet each in combination with 16.5, 17.4 and 18.0% protein. Egg production and the number of layers were recorded daily. 20 eggs per treatment were used to assess egg quality. Faeces were collected in the last week for nutrients apparent utilization measurement. The main results were as follows.Performance: The egg production/day of each treatment increased significantly as dietary protein level increased from 16.5 to 18.0% (P<0.05). And as dietary ME level increased from 2550 to 2800 kcal/kg, the egg production/day increased significantly (P< 0.01), whereas no interaction was observed between protein and ME. The diet contained 18.0% protein and 2800 kcal/kg ME provided the highest egg production/day, while the lowest egg production/day was observed in treatment received diet contained 17.4% protein and 2550 kcal/kg ME. Although, neither dietary ME nor protein had a significant effect on egg production ratio (P>0.05), the egg production ratio of all the dietary treatments were above 85%. As both protein and ME levels increased in diet, egg weight increased significantly (P<0.05), and there was also no interaction between protein and ME. The biggest egg size was observed in 18.0% protein×2800 kcal/kg ME treatment (59.26g), whereas the smallest egg size was observed in 16.5% protein×2700 kcal/kg ME treatment (59.26g). In addition, there was interaction for feed intake and feed/egg ratio between protein and ME, and as increased of protein or ME level, the feed intake increased significantly (P<0.01).Egg quality: No significant difference was observed for egg-shape index, Haugh unit, eggshell thickness, relative eggshell weight, P and ASH in eggshell, while dietary ME had a significant effect on eggshell strength (P<0.05). Both dietary ME and protein affect the calcium content in eggshell (P<0.01), and interaction between ME and protein was also observed. While no significant difference was observed for whole-egg water, albumin water, protein content of egg white and fat content of egg yolk, significant effects (P>0.05) of dietary protein on albumin mass fraction, yolk mass fraction, albumin/the whole egg were observed. In addition, As both protein and ME levels increased in diet, yolk cholesterol decreased significantly (P<0.05), and the interaction between protein and ME was also observed. In detail, the diet contained 16.5% protein and 2550 kcal/kg ME provided the highest yolk cholesterol (19.76 mg/g), while the lowest yolk cholesterol (16.61 mg/g) was observed in treatment received diet contained 18.0% protein and 2800kcal/kg.Nutrients apparent utilization: No significant difference was observed for apparent utilization of P, Cu and Mn, while the apparent utilization of protein increased significantly (P<0.01) as dietary ME level increased from 2550 to 2800 kcal/kg. Moreover, the apparent utilization of Fe and Zn increased as dietary ME level increased (p < 0.05).Conclusions: Different levels of ME and protein diets could influence layers performance, egg quality and apparent utilization of nutrients under heat stress. For the purposes of egg production, the diet contained 2800 kcal/kg ME and 18.0% protein provided the better performance, while for the purposes of egg production ratio that contained 2700 kcal/kg ME and 17.4% protein provided the better performance. |
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