Efecto de la concentración de proteína en la dieta sobre rendimiento productivo, características de la canal y composición química de carne de pollos de engorda en el trópico seco Effect of protein concentrations in the diet on productive performance, carcass characteristics, and meat chemical composition of broiler chickens in the dry subtropics

Introduction: Current diets of broiler chickens tend to increase protein levels to obtain high productive variables that are achieved in combination with genetic, management, and sanitary improvements, among others. An increase in dietary crude protein levels does not always accompany an increase in broiler productive efficiency due to multiple factors involved in the production system. The objective of this study was to evaluate the effect of increasing levels of dietary crude protein (CP) on productive performance, carcass characteristics, and chemical composition of breast and thigh meat in broiler chickens raised in the dry subtropics of northeastern Mexico. Method: The study used 200, 1-day-old male Ross broiler chicks. In a completely randomized design, birds were allocated to the four treatments with five replicates (floor pens) of ten birds each. The trial was divided in two phases (starter and finisher) of 21 days each (42 days total). Treatment diets (T) for starter and finisher phases had crude protein concentrations (CP; %) of 21 and 18.1 (T1), 21.4 and 18.5 (T2), 21.8 and 18.9 (T3), and 22.2 and 19.3 (T4), respectively. Within each feeding phase, the four treatment diets were formulated to similar levels of apparent metabolizable energy. Infante-Rodríguez, F. et al. No 25, Vol. 12 (2), 2020. ISSN 2007 – 0705, pp.: 1 18 3 Results: Protein concentrations had no effect (P > 0.05) on weight gain, while feed intake was greater in T1 (P < 0.05) than in T2 and T3. Feed conversion was better in T2 and T4 (P < 0.05) than in T1. There was no influence of treatment on carcass weight or carcass cuts (P > 0.05). Carcass yield was greater in T1 than in T3 (P < 0.05). Breast and thigh dry matter and ether extracts were similar (P > 0.05) between treatments. Breast crude protein was greater (P < 0.05) in T2 than in T3. The lowest (P < 0.05) CP concentration in thigh meat was in T3. Discussion or Conclusion: These results indicated that in the dry subtropics area of northeastern Mexico increases in CP above the level of T2 (21.4% and 18.5% CP in starter and finisher diets, respectively) did not improve broiler chicken productive performance, carcass characteristics or meat chemical composition.


Introduction
The poultry industry in 2018 generated 63.3% of animal production in Mexico; of the total, 34.9% was broiler chickens. Chicken meat is an important food for Mexican consumers. Animal protein intake was 39% from chicken, compared to 16% from beef and 8% from pork. In Mexico, per capita consumption in 2018 was 23 kg of broiler chicken meat (UNA, 2019).
Soybean meal is the preferred vegetable protein source for poultry diets. It contains 40 to 48% crude protein (CP), depending on the amount of hulls removed and the oil extraction procedure used. Adequately processed, soybean meal is an excellent source of protein for all kinds of poultry; however, soybean meal is expensive when compared with other protein rich ingredients of vegetal source. In Mexico, approximately 90% of broiler production costs are represented by the cost of feed plus the cost of the chicks (UNA, 2019). Therefore, it is necessary to identify alternatives for optimal protein rich ingredients to include in the broiler diet, and also to optimize the quality of the broiler meat produced.
Considering warmer environments, Laudadio et al. (2012a) and Amiri et al. (2019) reported no effect of dietary CP levels on broiler growth performance. However, Zulkifli et al. (2018) observed reduced growth performance of broilers under heat stress and fed low-CP diets.
Carcass characteristics and meat composition of broiler chickens have also been quite variable after feeding different levels of crude protein.
The genetic development of broiler chickens is continually generating better performing animals. Aftab (2019) documented reduction in time at slaughter weight (2.3 kg) from 52 d in 1995 to 36 d in 2017; breast yield also has been increased. The author identified nutrient density increase in diets of broiler chicken, particularly protein (amino acids). As discussed, nutrition is directly related to this development, so revisions and updates must be constantly undertaken in order to define new protein, energy, and nutritional requirements for each particular productive zone (Applegate and Angel, 2014). There are no conclusive reports about the productive performance of broiler chickens in the tropical hot and humid environments when fed diets with different crude protein levels. In these environments, CP level has not influenced growth of birds (Laudadio et al., 2012a;Amiri et al., 2019) or the productive performance has decreased when chickens were fed low-CP diets (Awad et al., 2014). Therefore, the objective of this study was to assess the productive performance, carcass yield of major cuts, and chemical composition of breast and thigh meat of broiler chickens fed diets with increasing crude protein levels in the dry subtropical area of northeastern Mexico.

Management and diets
All procedures involving animal care and management were in accordance with and approved by the Bioethics Committee of the College of Veterinary Medicine and Animal Science of the Autonomous University of Tamaulipas.
Two hundred, 1-day-old male Ross broiler chickens weighing 42 g on average were obtained from a commercial hatchery. Each treatment (diet) included 50 birds randomly assigned with five replications of ten animals each. During the entire experiment, birds were housed in 20 floor pens with wood shavings as litter. Twenty-four hours of light per day were provided during the entire trial. Each pen had an automatic drinker and a manually-filled feeder. Space allocation was at ten birds per square meter. Water and feed were offered ad libitum. Birds were vaccinated on day 7 of the trial against fowl pox (wing puncture) and against Newcastle (ocular) using the La Sota strain.
The chickens were raised following standard commercial practice. Two feeding phases

2.
Body weight and feed intake were measured weekly on an electronic scale with a capacity of 40 kg (±2 g; Rhino Model Bar-8, México) and feed conversion ratio (FCR; feed intake, g/weight gain, g) was calculated. At the end of the trial, two birds per cage, selected at random, were sacrificed by cervical dislocation (Mexican official law NOM 033-ZOO-1995) for carcass determinations and accessing chemical composition of breast and thigh meat. Carcass weight without viscera was used to estimate hot carcass yield. Then the carcass was dissected for main cuts: breast, thighs plus drumsticks, wings, back, trimmed back fat, liver, heart, and gizzard. These weights were recorded on an electronic scale with a capacity of 10 kg (±1 g; Rhino Model BAPO-10, México). A sample of approximately 150 g from the left breast and thigh from each carcass was obtained and frozen (-20 ºC) in nylon bags for 7 days, and then, laboratory analyses were performed. the samples were thawed at 4ºC for 24 h and subsequently at room temperature until thawed. Muscle samples were analyzed for dry matter (oven at 105 °C for 24 h), ash (furnace at 550 °C for 3 h), ether extraction and crude protein (AOAC, 1990).

Statistical analyses
A completely randomized design with four treatments and five replicates was used in the statistical analysis. Dietary crude protein levels were the treatments compared. For growth performance (weight gain, feed intake, and feed conversion ratio) the replicate was the average of all broiler chickens in each pen. For carcass evaluation and meat chemical composition, the replicate was the average of two birds (selected at random) per pen. The percentage of dressed carcass yield was determined as carcass weight (g)/live weight (g)). An analysis of variance with Tukey's test for mean comparisons was applied. Statistical significance was declared at P < 0.05. For statistical analyses, the GLM procedures of SAS (2007) were used.

Growth performance
Results of productive performance of broiler chickens are shown in Table 3. For the starter phase (1-21 days), there was no effect (P > 0.05) of dietary CP concentration on weight gain. Broilers in T2 had lower (P < 0.05) feed intake than those in T1 and T3. Feed conversion in T2 and T4 was similar (P > 0.05) and was better (P < 0.05) than that in T3. For the finisher phase (21-42 days), there was no effect (P > 0.05) of CP level on weight gain. Broilers in T3 had lower feed intake and better feed conversion than those in T1 (P < 0.05

Carcass characteristics
Carcass characteristics of broiler chickens are shown in Table 4. There was no influence of treatment on weights of carcass, breast, drumsticks plus thighs, wings, or back fat (P > 0.05).
Carcass yield was higher (P < 0.05) in T1 than T3. Chickens in T2 and T4 showed intermediate values.
Likewise, there were no effects (P > 0.05) of dietary CP on the weights of liver, heart, or gizzard. Table 3. Effect of protein concentration on the productive performance 1 of broilers chickens. Tabla 3. Efecto de la concentración de proteínas en el rendimiento productivo 1 de pollos de engorde.

Proximate composition of meat
Results of the chemical composition of broiler chicken breast and thigh meat are shown in Table   5. Breast meat had similar (P > 0.05) percentages of water, ash, and ether extract; however, crude protein was greater (P < 0.05) in breast meat of birds in T2 than in T3. Chickens in T1 and T4 showed intermediate values. Thigh meat between treatments had similar (P > 0.05) water and ether extract percentages. Crude protein was highest (P < 0.05) for T2 and lowest (P < 0.05) for T3. With respect to ash, treatments T2 and T3 had higher values (P < 0.05) than in T1.  Mean values (n = 5) with different letters in the row indicate significant differences (P < 0.05) between treatments (T). 2 For starter and finisher diets crude protein (CP; %) levels were: 21.0 and 18.1 (T1); 21.4 and 18.5 (T2); 21.8 and 18.9 (T3); 22.2 and 19.3 (T4), respectively.

Growth performance
Both the starter and finisher diets had increasing crude protein levels from T1 to T4. Broiler weight gain was not influenced by the treatment; however, birds of T1 exhibited increased feed intake that negatively influenced feed conversion. Animals in T2, T3, and T4 showed roughly similar feed conversion ratios. Reports on productive performance of broiler chickens have been variable when comparing different levels of dietary crude protein. Houshmand et al. (2012) observed better growth performance of broiler fed diets containing 23.4% and 20.2% CP for starter and finisher diets, respectively, than birds fed the low protein diets. Abdel-Maksoud et al. (2010) reported maximum body weight and feed efficiency in starter broilers at 21% CP, particularly when Efecto de la concentración de proteína en la dieta sobre rendimiento productivo, características de la canal y composición química de carne de pollos de engorda en el trópico seco Nº 25, Vol. 12 (2), 2020. ISSN 2007 -0705, pp.: 1 -18 -12 -supplemented with essential amino acids; broilers fed 19% and 23% CP diets showed lower productive parameters. Boonsinchai et al. (2016) observed no effect on growth performance in broilers (1-42 days) when dietary crude protein had a small reduction (5%) from the recommended requirement; however, reductions of 10% and 15% negatively affected feed efficiency.
Contrary to the observations of the present study, Ghazanfari et al. (2010) observed that in broilers that incremental changes in dietary protein levels did increase feed intake and improved weight gain and feed conversion, compared with a low level of protein in the diet. Jafarnejad et al.
(2010) also reported improved weight gain and feed conversion in broilers (1-21 days) with high dietary protein levels (23% vs. 21% CP). Using mathematical models, Faridi et al. (2012) found that 24.1% dietary CP was the optimal level for feed efficiency in starter broilers. With a metaanalysis, Faridi et al. (2015) established 24.5% CP in the diet as the optimal level for feed efficiency for starter broilers 1-21 days of age. However, Laudadio et al. (2012a) reported that CP protein levels at 18.5%, 20.5%, and 22.5% did not affect growth performance of broilers during hot temperature conditions. In a three-week study with broiler chickens raised under a tropical hot and humid environment, Awad et al. (2014) observed reduced growth performance of broilers fed low-CP diets.
Improved productive performance of broiler chickens has been observed when increased over NRC recommendation the levels of lysine (Oliveira et al., 2013;Bernal et al., 2014;Quadros et al., 2019;Zarghi et al., 2020) and methionine plus cystine (Goulart et al., 2011). In the current study, the increase in crude protein increased the lysine concentration in the diets over the NRC recommendations; however, the crude protein increase in diets did not have evident increase on the levels methionine plus cystine in the used diets over the NRC recommendations. It is possible that methionine plus cystine were not in the level required to optimize the productive performance of broiler chickens in the present study. The crude protein increase in diets was made by the increase in soybean meal that increased lysine; nevertheless, the increase in methionine plus cystine was less evident. Further research considering these adjust in amino acids should be made in this

Carcass characteristics
In agreement with the results from the present study, Mandal et al. (2010) and Oliveira et al. (2013) reported that carcass characteristics and breast yields were not influenced by dietary CP level. Also, Miranda et al. (2015) did not report effects on carcass yield of broilers fed diets with different CP levels supplemented with L-valine and L-isoleucine; however, the proportion of abdominal fat was reduced without dietary supplementation of these amino acids.
Contrary to the information observed in the present study, Gheorghe et al. (2013a) observed greater carcass yields in broilers fed a diet with high vs. low protein levels. Gheorghe et al. (2013b) found that the increase in dietary protein did not affect carcass yield, but did improve breast and thigh yields and reduced abdominal fat. In agreement, Laudadio et al. (2012b) obtained higher muscle yields (breast and thigh) in broilers with a high protein diet than those with a medium protein or low protein diet. Likewise, Hada et al. (2013) and Dehghani and Jahanian (2016) reported reduced carcass and breast yields using low protein diets; however, abdominal fat was increased with low protein diets. Dozier III et al. (2008) recognized the importance of lysine in breast muscle formation; higher lysine levels than NRC recommendations promoted breast meat yield (Bernal et al., 2014;Zarghi et al., 2020). In the current study all diets were supplemented with the same lysine level and covered the NRC recommendation and the increase in CP level in diets had small increases in lysine or methionine plus cystine. Further research with higher levels of these amino acids in diets for broiler chickens to establish optimum productive efficiency and carcass yield is necessary in the dry subtropics of Mexico.

Proximate composition of meat
Contrary to the observations from the present study, increased dietary protein levels in other studies boosted protein and reduced lipid deposition in broiler chicken muscle (Marcu et al., 2012;Marcu et al., 2013). Nagata et al. (2011) reported that high dietary energy also requires adequate CP to ensure protein deposition in the muscles of birds. In contrast, Gómez et al. (2011) found no effect of dietary CP on carcass protein deposition, although low crude protein diets were supplemented with amino acids that were used for muscles growth in the birds.

Conclusions
In the present study, conducted in a subtropical area of northeastern Mexico, in the entire study (1-42 d) broiler chickens fed 21.4% and 18.5% CP (T2) in starter and finisher diets, respectively, had better feed conversion ratio than broilers fed the lower CP diet (21.0% and 18.1% (T1), respectively). Increases in CP over T2 did not improve broiler feed conversion ratio. Carcass characteristics showed minimal influences of dietary CP level; however, crude protein concentration in thigh was optimal in T2.