Introduction

Genetic improvement for rapid growth, feed efficiency and meat yield has resulted in changing the capacity of broiler birds and their requirements. Therefore, optimal nutrient requirements should be provided for best performance and economic viability. Feed cost represents ~70% of the total cost of intensive poultry production. Protein feeds are usually much more expensive than energy feeds and wasteful usage increases the cost of production. Moreover, excess protein feeding may increase elimination of nitrogenous compounds in faeces and urine, which has environmental implications. Therefore, economically as well as nutritionally it is imperative that balanced diets should be provided during the brooding, rearing and laying stages. Broiler breeder birds are fed on a controlled diet from 3-5 weeks onwards to attain a target body weight (BW) at 20 weeks of age. During restricted feeding, energy models are employed to predict the metabolizable energy (ME) and crude protein (CP) intake taking the targeted BW gain into consideration. Furlan et al. (2004) reported that the feeding of high-protein diets to broilers was not recommended in tropical areas, because protein has the highest heat increment among dietary nutrients. Consequently, low-protein diets were recommended, to decrease the amount of heat produced and its harmful effects on bird’s performance. On the other hand, when low feed intake of heat-stressed birds is associated with low-protein content of diets, there is a reduction in amino-acid intake. The information on growth and body conformation traits of broilers is scanty which are also influenced by breed, strain, system of rearing and climatic conditions. With this backdrop, the present study was carried out to evaluate the effects of different levels of dietary CP during starter and grower phases on growth performance and body conformation traits in coloured synthetic male line (CSML) broiler birds.

Materials and Methods

Experimental Birds, Feeding and Management

The experiment was conducted in two phases. The phase-I was conducted during chick period (0-5weeks). During this period, 360 day-old female broiler chicks of CSML strain were distributed into 3 groups (18-SCP, 19-SCP and 20-SCP) and fed ad libitum with iso-caloric (2850Kcal ME/Kg) starter rations containing 18, 19 or 20% SCP (Starter CP) up to 5th week. The phase-II was conducted during growing period (6-20 weeks). During this period, out of 120 chicks raised on each starter CP level, 96 grower pullets with higher BW were selected. The birds of each group (96) were further divided into two groups i.e. high body weight (HB) and low body weight (LB) groups with 48 chicks per group each. The pullets of each BW group were randomly distributed in 6 replicate groups with similar mean BW. Three of the replicate groups were allotted to iso-caloric (2750Kcal ME/Kg) grower rations with 10 or 14% GCP (Grower CP). In total, there are 12 dietary treatments (18-HB-14 i.e. 18% starter CP-high BW-14% grower CP, 18-HB-10, 18-LB-14, 18-LB-10, 19-HB-14, 19-HB-10, 19-LB-14, 19-LB-10, 20-HB-14, 20-HB-10, 20-LB-14, 20-LB-10). Restricted feeding was practised from 6-20weeks according to the energy model of Sakomura et al. (2003). From 21-23weeks, a pre-breeder ration with 16% CP and 2850Kcal ME/kg and from 24-50weeks, a breeder ration with 16% CP and 2750Kcal ME/kg was fed to the birds.

Daily maximum and minimum temperature forecast data were collected. The daily effective temperature was calculated by using the following formula and this effective temperature was used in energy models.

Effective temperature = [(daytime high temperature × 2) + (night low temperature)] /3

The birds were housed in floor pens up to 20 weeks. After 20 weeks onwards, birds were shifted to cage house. All the birds had access to clean and fresh drinking water round the clock. The chicks were exposed to 23 h of lighting and a dark period of 1 h per day from 0-5 weeks. From 6^th week onwards, the birds were maintained under natural day light till 20^th week of age. Then lighting hours was gradually increased to 17 h at 24^th week. Routine vaccination and medication program was carried out regularly for the experimental stock. Starting from 8^th week, deworming of the stock was carried out at every two months interval. The birds were beak trimmed during 10^th week of age.

Protocol Design

The weekly target BWs were set by plotting a linear graph targeting a BW of 2200g at 20^th week. By subtracting the target BW at 20^th week from the recorded 5^th week BW and dividing it by the number of weeks, the weekly gain required was calculated. Weekly ME requirement/bird/day was calculated by energy model of Sakomura et al. (2003). During breeder period, ME requirement was calculated using the energy model of Sakomura (2004). Feed consumption of the birds was recorded replicate-wise weekly. The CP and ME intakes were calculated from the feed intake values, taking the CP and ME content of the feed into consideration. From the weekly BW, feed CP and ME consumption data, feed conversion ratio (FCR), protein efficiency ratio (PER) and energy efficiency ratio (EER) were calculated. Body conformation traits such as shank length and keel length were measured according to McGovern (2002) at 5^th and 20^th weeks.

Statistical Analysis

Completely randomized design was followed. All data were analyzed using the General Linear Model (GLM) procedure of SPSS 16.0. Significance was declared at P≤0.05.

Results and Discussion

Phase-I

BW, BW Gain and Body Conformation Traits

The 5th week BW of 18% and 19% SCP groups were higher (P≤0.05) than the 20% SCP group (Table 1). The BW gain (0-5weeks) also followed a similar trend. Smith and Pesti (1998) reported that when broiler birds were fed diets with CP 18-24% at 12.96MJ/kg, the weight gain was maximum at 19.4% CP intake. In another study with similar CP levels at 13.38MJ/kg, they reported that with increase in CP levels, the maximum gain was recorded at 22% CP. Nguyen and Buchasak (2005) reported that CP level >17% did not show any significant effect on growth and BW. Roy et al. (2010) reported that birds fed 19% CP diet had significantly higher BW than 18 and 20% CP fed groups. Variations in response to dietary CP intake by different strains have been reported by several workers. Berrhe and Gous (2008) reported that BW gain was greater in Cobb than Ross broiler. Smith and Pesti (1998), by feeding 3 levels of dietary CP (16, 20 and 24%) to Ross×Ross 208 and Peterson×Arbor Acre crosses observed that Ross birds were heavier at each age. They opined that strain has highly significant effect on BW. The findings of the present experiment is in agreement with the findings of other workers who reported that linear increase in CP level is not necessarily associated with a linear increase in BW. Growth is a function of gene and nutrition interaction. Therefore, not only nutrient levels but the genetic potential of the birds is also a factor controlling growth. The BW and BW gain of birds at 18% CP level was probably adequate to support growth which showed similar performance with 19% CP level. At 20% CP level, the growth reduction could be associated with metabolism of high CP intake. The 5th week shank length (SL) ranged between 7.35-7.58cm (Table 1). The feed intake and 5th week BWs were higher in 19% SCP group than other groups which might have contributed to higher keel length (KL). Similar findings were recorded by Romero-Sanchez et al. (2007).

Table 1: BW, BW gain, shank and keel length of chicks at 5^th week

^a,bMeans within a column without superscript differ significantly (P≤0.05)

Nutrient Intake and Efficiency

There was no significant difference in CP and ME intakes and FCR, PER and EER among treatment groups (Table 2). Sterling et al. (2003), Kermanshahi et al. (2011) and Folorunso and Onibi (2012) using CP levels from 12.3-25% reported no significant difference in feed intake. It has been reported by Kamran et al. (2008) and Steiner et al. (2008) that, feed intake decreased with increase in dietary CP levels while taking a protein range from 16.5-23%. Berhe and Gous (2008) using 12.4-26.8% CP diets reported that feed intake remained unchanged with CP levels from 15.3-21%, when the CP level decreased below 15.3% and increased above 21%, the feed intake decreased. The CP intake was a product of feed intake and CP% of diet and hence influenced by these factors. Kamran et al. (2008) and Widyaratne and Drew (2011) using CP levels from 12.3-24% reported that PER decreased with increase in the level of dietary CP indicating better efficiency of protein utilization in diets with higher CP levels. From the results of the present study, where a narrow range of variation in the CP level was incorporated, the PER values seemed to be unaffected by the dietary CP intake. Sterling et al. (2003), Nguyen and Bunchasak (2005), Berhe and Gous (2008), Kamran et al. (2008) and Steiner et al. (2008) have reported that the FCR increased with decrease in dietary protein level. Berhe and Gous (2008) reported that CP levels above or below 15-21% range increased the FCR values indicating poor feed efficiency. In contrast to the present findings, Kamran et al. (2008) in a study on broiler finisher using four different diets with CP levels 17-20% and ME levels 2635-3100Kcal/kg diet with constant ME:CP ratio observed that the energy efficiency decreased with decrease in dietary CP and ME.

Table 2: Nutrient intake and nutrient efficiency of chicks at 5^th week

Phase-II

BW and BW Gain

The 5th week BW of birds did not differ among BW groups within starter CP level (Table 3).

Table 3: BW of birds during different periods (g)

^a-d Means within a column without superscript differ significantly (P≤0.05)

The 5th week BW of HB groups belonging to 18 and 19% SCP were similar and significantly higher than the 20% SCP HB group. The 5th week BW of LB groups also followed a similar trend. The 20th week BW of different groups ranged between 1876.88-2104.37g. The 20th week BW of all the 14% GCP groups had numerically higher values than the 10% BW group within the same SCP and same BW groups.

During growing period, BW gain of 20-LB-14 group was highest (1417g) among all treatment groups (Table 4). BW gain of 18-HB-10 and 19-HB-10 were significantly lower than all other groups. Within SCP and BW groups, 14% GCP groups had higher BW gain than 10% GCP groups.

Table 4: BW gain of birds during different periods (g)

^a-eMeans within a column without superscript differ significantly (P≤0.05)

In all 14% GCP groups, the cumulative CP intake (0-20wk) was higher than 10% GCP groups. Starter and grower protein levels had significant (P<0.01) effect on BW gain during this period. The BW gain from 0-50wk exhibited wide variations, ranging from 3241-3767g/bird. The 5th week BW and GCP intake interaction had significant (P<0.05) effect on BW gain during this period. Feed was offered to all the groups to attain the target BW of 2200g. However, the target BW could not be achieved in any of the groups. This could be due to severe summer stress coupled with high relative humidity during the growing period. Reduction in BW due to low CP intake during growing period has been reported by several workers. Robey et al. (1988) reported that pullet receiving 12% and 14% show significant lower BW at 20th week than pullet receiving 16% or 18% CP diet. However, Wolde et al. (2011) in a study on RIR chicks, fed 5 levels of CP diets (14, 16, 18, 20 and 22%) and reported that BW of chicks were not significant. From the earlier reports, it is envisaged that where the dietary CP levels were more than 14%, there is hardly any significant change in BW. However, when dietary CP levels ≥14% are compared with <14% CP diets, then significant decrease in BW is recorded with decrease in dietary CP level. The BW of the birds at 24th, 30th, 40th and 50th weeks did not differ significantly. The groups having low 20th week BW might have compensated the BW gain during this period leading to insignificant difference between the groups. Similar compensatory growth has been reported by de Beer and Coon (2006).

Body Conformation Traits

The 5th week SL and KL of treatment groups were influenced by 5th week BW (Table 5).

Table 5: Shank and keel length of birds during different periods

^a-eMeans within a column without superscript differ significantly (P≤0.05)

The SL and KL of all HB groups were similar and were significantly higher than all LB groups. Similarly, the SL and KL of all the LB groups did not differ significantly. During 20th week, the SL of 19-LB-14 group was higher (11.55; P<0.05) than other groups. The 20th week KL of treatment groups did not differ significantly from each other. Lilburn et al. (1990) reported that the frame size or skeletal growth in broiler breeder is primarily a function of BW gain. Difference in dietary protein and energy can influence the rate of gain and in this way indirectly influence skeletal growth. The correlation between BW and SL and KL are high, positive and significant. In the present experiment, the 5th week SL and KL of all HB groups were significantly higher than that of LB groups indicating a positive correlation between them. Yaissle and Lilbun (1998) reported that variation in 5th week BW due to dietary treatments was minimized at 20th week due to compensatory growth as observed in the present study.

Nutrient Intake

Highest feed intake of 9.49kg was recorded in 18-LB-10 group which was similar to other groups except 20-HB-14, 20-HB-10 and 20-LB-14 groups (Table 6).

Table 6: Nutrient intake of birds during different periods

^a-dMeans within a column without superscript differ significantly (P≤0.05)

SCP and GCP had highly significant effect (P<0.05) on cumulative feed intake. The data on cumulative CP intake from 0-20wk reveals that all the 14% GCP fed groups irrespective of SCP and BW had more than 1300g cumulative CP intake. Similarly, in all the 10% GCP groups, the cumulative CP intake ranged between 1075-1108g and were lower (P<0.05) than 14% GCP fed groups. GCP had significant effect (P<0.01) on cumulative CP intake. The cumulative ME intake of treatment groups ranged between 24730-26080Kcal. The ME intake of 18-LB-10 group was similar to other groups except 20-HB-14, 20-HB-10 and 20-LB-14 groups. The ME intake of treatment groups was influenced by SCP (P<0.01) and GCP (P<0.05).

Nutrient Utilization Efficiency

The FCR value of 18-HB-10, 18-LB-10, 19-HB-10 and 19-LB-10 groups were similar and higher (P<0.05) than other groups (Table 7).

Table 7: Nutrient efficiency of birds during different periods

a-fMeans within a column without superscript differ significantly (P≤0.05)

The FCR value of 18-HB-14, 18-LB-14, 20-HB-14 and 20-LB-14 groups did not differ significantly from each other but lower (P<0.05) than other groups indicating better feed efficiency. The FCR values were significantly influenced by the SCP levels. The feed efficiency of groups for 6-20wk and 0-20wk decreased with decrease in GCP level and decrease in cumulative CP intake. Smith and Pesti (1998) have reported that, with higher CP levels; some genetic strains maintain consumption levels but grow more, resulting in improved feed utilization efficiency whereas, other genetic strains consume less feed but maintain growth levels resulting in improved feed utilization efficiency. Wolde et al. (2011) also reported that pullets on low CP diets require lower amount of feed for unit body mass gain than those on higher CP groups. Therefore, better feed efficiency obtained in 14% GCP groups in the present experiment may be justified.

Low PER values were obtained for 20-LB-10, 20-HB-10, 20-HB-10 and 20-LB-10 groups. PER value of 19-LB-14, 19-HB-14, 18-HB-14 and 18-LB-14 groups were higher (P<0.05) than other groups. The PER values were influenced by SCP and GCP levels (P<0.05). The protein utilization efficiency of all 10% GCP groups was found to be better as compared to 14% GCP groups. Similar to the present findings, Nguyen and Bunchasak (2005), Kamran et al. (2008) and Widyaratne and Drew (2011) reported that protein efficiency was better for low CP diets. From 0-20wk period, low EER value indicating better energy utilization was recorded in 18-HB-14, 18-LB-14, 20-HB-14 and 20-LB-14 groups. SCP and GCP levels had significant effect (P<0.01) on EER. In low CP groups, reduced CP intake, poor feed efficiency as well as low BWs were recorded which might have influenced EER adversely. Similarly, Kamran et al. (2008) reported that energy efficiency decreased with decrease in dietary CP and ME intakes.

Conclusion

From the present study, it was concluded that birds fed starter diet with 19% CP and grower diet with 14% CP had better body conformation, growth and feed efficiency.

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