Crude protein levels in diets for laying hens

The aim of this study was to evaluate dietary crude protein (CP) levels for laying hens. 180 Hy-Line W-36 laying hens, with ages ranging from 27 to 43 weeks old, were distributed according to a randomized block design into three treatments with 10 replicates of six birds per cage. The dietary nutritional levels were adequate, except for CP. The treatments were 152, 172, and 182 g/kg. The following parameters were evaluated: feed intake (FI, g/day), egg production (EP, g/kg), egg weight (EW, g), egg mass (EM, g/day), feed conversion ratio (FCR, kg/kg and kg/dozen), yolk (g/kg of EW), eggshell (g/kg of EW), and albumen (g/ kg of EW). Feed intake was not influenced by dietary CP levels (p>0.05). Egg weight, EM, FCR (kg/kg), and albumen were not influenced (p>0.05) by the treatments. Birds fed on diets with 172 g/kg CP showed an improvement (p<0.05) in EP of 2.35 % compared with the level of 152 g/kg CP. Increasing dietary CP levels showed better (p<0.05) FCR (kg/dozen) compared to the lowest CP level. Yolk and eggshell weight were influenced (p<0.05) by increasing dietary CP levels. The dietary CP levels of 172 and 182 g/kg increased yolk weight by 6.33 % and 10.1 % compared to the lowest CP level respectively. The eggshell weight showed an average improvement of 6.31 %, increasing the dietary CP levels. The CP level of 172 g/kg is recommended in diets for white-eggs laying hens from 27 to 43 weeks of age.


INTRODUCTION
Dietary crude protein (CP) levels can be reduced if crystalline amino acids are added to the feed.This allows the application of the ideal protein concept in feed formulation, enhancing amino acid utilization and reducing nitrogen excretion (Keshavarz and Austic, 2004).Currently, this higher utilization in feed formulation is accomplished due to the market availability of crystalline amino acids at affordable prices (Andrade, 2004).
Preliminary studies have shown the possibility to reduce dietary protein levels when crystalline AA are added to feed (Tavernari et al., 2013;Lelis et al., 2014) but this practice for laying hens is influenced by the bird's feed intake, being necessary adjust the crude protein level according to the hen feed intake.
Thus, the objective of the present study was to evaluate the effect of crude protein levels in diets for laying hens.

MATERIALS AND METHODS
The trial was performed at the Poultry Farm of the Department of Animal Science of the Universidade Federal de Viçosa.A total of 180 Hy-Line W-36 laying hens from 27-to 43-wk-old, were evaluated for an experimental period of 16 weeks.The study comprised 4 evaluation periods of 28 days each.The study began on day 0.
Birds were distributed according to a randomized block design into three treatments with 10 replicates of six birds per cage (60 birds for each treatment).Experimental blocks were determined according to egg production, totaling 10 blocks.
Diets were formulated to supply nutritional requirements for hens, except for crude protein, according to the Brazilian tables for poultry and swine (Rostagno et al., 2011), table I.
eggshell weight (g/kg of EW), and albumen weight (g/kg of EW).
The main effects of the study are the three dietary crude protein levels.The statistical model used was Where Y ijk is the observation k of treatment i in period j; µ is the overall mean; τ i is the effect of treatment "i"; βj is the effect of period j; (τβ) ij is the interaction of treatment i × period j; δ k is the effect of block k; ε ijk is the residual random error.
All data were analyzed using the GLM procedure of SAS statistical package (SAS Institute, 2004) and means were compared through the test of Tukey at a 5 % probability level.

RESULTS AND DISCUSSION
There was no interaction between crude protein levels and experimental periods (p>0.05).
Feed intake was not influenced (p>0,05) by the dietary CP levels (table II).Thus, in the present study the average FI of birds observed was 94.1 g/day.The nutritional requirements of laying hens are based on birds intake (NRC, 1994;Rostagno et al., 2011), being necessary to know the FI to formulate hen diets.
Egg weight, EM, FCR (kg/kg), and albumen were not influenced (p>0,05) by dietary crude protein levels.Preliminary studies have been shown that dietary crude protein levels ranging between 140 and 170 g/ kg did not influence the performance of laying hens at their production stage (Costa et al., 2004;Pavan et al., 2005).These results suggest that variations on crude protein levels from 140 to 170 g/kg cannot produce deficiency on the hens.Probably, a higher dietary CP variation than that used in these preliminary studies should be necessary.
In the present study, birds fed on diets with 172 g/kg CP showed an improvement (p<0,05) on EP by 2.35 % compared with the lowest CP level (152 g/ kg CP).Increasing dietary CP levels showed better (p<0,05) FCR (kg/dozen) compared with the treatment containing 152 g/kg CP.Preliminary studies have reported the level of 175 g/kg CP as the adequate for laying hens in production phase (Costa et al., 2004;Carioca et al., 2010).
When dietary crude protein was reduced until 152 g/kg CP, crystalline AA were added to meet bird requirements but hens fed on a diet with the lowest level of CP showed the worst results compared to those fed on diets containing dietary CP levels over than the nutritional recommendations as preconized by Rostagno et al. (2011).These results suggest that more studies are needed to update the nutritional recommendations for these AA.Preliminary studies reported the importance of updating the percentage ratios of amino acids to Lys for laying hens and broilers due to the genetic improvements on birds utilized on poultry production (Tavernari et al., 2013;Campos et al., 2012;Lelis et al., 2014).
The yolk and eggshell weight were influenced (p<0.05) by dietary CP levels.Yolk weight showed an improvement by 6.33 % when comparing CP level of 172g/kg with the lowest level.At the same, the highest CP level led to an improvement by 10.1 % compared with the level of 152 g/kg CP.The eggshell weight showed an improvement by 6.31 % with an increase on the dietary CP levels.Several studies have shown that an increase in dietary CP levels may improve egg contents (Costa et al., 2004;Pavan et al., 2005;Novak et al., 2006).Main benefits were reported in yolk (Pavan et al., 2005), yolk protein percentage (Novak et al., 2006), eggshell (Novak et al., 2006), and albumen (Costa et al., 2004;Pavan et al., 2005).In fact, laying hens are capable Table II.Performance and egg quality of layer hens fed on diets with different crude protein levels from 27to 43-week-old (Rendimiento y calidad de los huevos de gallinas ponedoras alimentadas con dietas con diferentes niveles de proteina bruta en la fase de 27 a 43 semanas de edad).

Item
Crude Means followed by different capital letters in the same row are different by the Tukey test at 5 % probability level; sample size (n) = 120, for performance and egg traits.ns: not significant.CV ( %) = coefficient of variation. of transferring nutrients to the eggs and increasing on dietary crude protein levels may influence directly on egg quality.

CONCLUSION
The dietary crude protein level of 172 g/kg is recommended to improve egg production and egg quality of white laying hens from 27 to 43 weeks of age.