Background and Objective: Genetic selection for rapid growth and increased meat yield in a short period of time has greatly improved feed efficiency. However, the increased pressure for growth has prompted unforeseen consequences of other stressors on the broiler bird. These stressors utilize energy and nutrients that otherwise would be utilized for growth. This study was designed to evaluate how two genetic lines would perform while consuming less protein as a potential model to evaluate feed additives that may improve digestibility or immune and oxidative status. Materials and Methods: A field study was conducted and a reduced protein model was used in these two experiments. In experiment 1, Cobb 500 male broiler chicks were randomly allocated to one of two dietary treatments: Nutrient adequate basal diet and the basal diet with reduced crude protein. In experiment 2, Cobb 700 male broiler chicks were randomly assigned to the experimental diets used in experiment 1. Results: Body weight was decreased in Cobb 500 broilers compared with Cobb 700 broilers when dietary crude protein was reduced. Furthermore, Cobb 500 broilers consumed less feed compared with Cobb 700 broilers when dietary crude protein was reduced. Conclusion: As a producer, it is essential to find the point of maximum economic efficiency for the strain of broilers being reared. These data indicated differences among broiler genetic lines and dietary crude protein need to be considered when formulating diets.
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Genetic selection has been used to maximize growth rate and feed efficiency in modern broiler chickens at an unprecedented rate1-3. Research has shown the diverse production potentials offered by gender, genotype and environment and how nutrition, particularly energy and nutrient levels, can influence those potentials4-6. Geneticists have selected the breeds for improved body weight with the aim of increasing the salable product (e.g. carcass weight) with the same or reduced inputs (e.g. feed)4. Genetic differences in growth rate, feed intake and feed efficiency have been reported between genotypes4,6-8. In addition, genetic differences influence the response of chicks to varying levels of dietary crude protein4. Amino acids fed to broilers through dietary crude protein are the most advantageous in terms of meat production. The manipulation of dietary protein can have various effects on broiler performance. Frap9 observed differences in body weight, feed intake and carcass composition. Although, this qualitative relationship has been well known for many years, it is still not widely used in profit maximization models.
A study completed by Parsons and Baker10 showed a significant linear reduction in both rate and efficiency of gain when dietary protein was decreased from 24-16%, but minimal effect was observed when dietary protein was decreased from 24-20%. Whereas Summers et al.11 found no significant differences in weight gain, feed intake, or feed conversion ration with the feeding of various dietary protein levels. Genotype may also impact this model, as Smith and Petsi4 showed that a “high yield” broiler strain required a higher dietary protein level to maximize body weight and feed efficiency when compared with a “fast growing” broiler strain. Genetic selection has primarily focused on increased meat yield with less inputs, this may contribute to unforeseen consequences on broiler health12. Development, maintenance and the use of the immune system utilizes energy and nutrients13, diverting these from growth. The objective of the current experiment was to evaluate how amino acids are utilized when crude protein is reduced and how different genetic lines impact this model of development.
MATERIALS AND METHODS
This study was approved by the Animal Care and Use Committee, Texas A&M University (TAMU) Institutional Animal Care and Use Committee (AUP # 2018-0181) and were consistent with the Guide for the Care and Use of Animals in Research and Teaching Guidelines14.
Experimental design: Two experiments were conducted to evaluate the effects of dietary crude protein (CP) on growth performance of Cobb broiler chickens. In experiment 1, a total of 360 day-of-hatch Cobb 500 male broiler chicks were used. Birds were equally housed at 18 birds per pen, with ten replicate pens per treatment, for a total of 20 pens (0.91 m× 1.83 m). Birds were allotted to pens to achieve equal pen weights across treatment groups. Each pen contained pine shavings as bedding material and equipped with one bell feeder and nipple drinking system. Birds were allowed ad libitum access to feed and water. Birds were housed in an environmentally controlled tunnel ventilated broiler house, with a lighting regime of 22L: 2D from 1-14 days of age and 20L: 4D from 15-42 days of age. In experiment 2, a total of 288 day-of-hatch Cobb 700 male broiler chicks were used. Birds were equally housed at 18 birds per pen, with eight replicate pens per treatment, for a total of 16 pens (0.91 m×1.83 m). Birds allotted to pens to achieve equal pen weights across treatment groups. Each pen contained pine shavings as bedding material and equipped with one bell feeder and nipple drinking system. Birds were allowed ad libitum access to feed and water. Birds were housed in an environmentally controlled tunnel ventilated broiler house, with a lighting regime of 22L: 2D from 1-14 days of age and 20L: 4D from 15-42 days of age.
Experimental diets: All diets were corn and soybean meal based. Calculated nutrient content of diets fed to broilers in experiment 1 and 2 is presented in Table 1. Analyzed nutrient content of the experimental diets is presented in experiment 1 (Table 2), experiment 2 (Table 3). Pens were blocked within and treatments were assigned at random to one of two dietary treatments. The experimental diets included a nutrient adequate basal diet (BD) and one reduced basal diet (RD). The BD diet was formulated to total amino acid and energy levels of that found in a typical industry diet. The RD was the BD reduced by 1.5% crude protein (CP). Birds were fed a three-phase diet consisting of a starter (day 1-14, crumble), grower (day 15-28, pellet) and finisher (day 29-42, pellet). Pelleting temperature was maintained at 70°C.
Growth performance: Mortalities were collected, recorded and weighed daily. Birds and feed were weighed weekly on day 7, 14, 21, 28, 35 and 42 (at the end of each dietary phase) for the determination of body weight (BW), body weight gain (BWG), feed intake (FI) and calculation of BW-corrected feed conversion ratio (FCR).
Statistical analysis: The data were analyzed using Statistical Analysis System (SAS, SAS Institute) to determine if variables differed between treatment groups. The feed intake, feed conversion ratio, body weight, body weight gain was compared between groups using the GLIMMIX procedure of SAS. Probability values of less than 0.05 (p<0.05) were considered significant.
RESULTS AND DISCUSSION
Table 4 and 5 shows the performance data for experiment 1 and 2, respectively. In experiment 1, RD fed birds decreased BW (p<0.05) when compared with BD fed birds on day 21 and day 28. The BW of RD fed birds was decreased by 4.3 and 4.2% when compared with BD fed birds, respectively. No significant differences in BW (p>0.05) were observed on day 42, however, RD fed birds had numerically reduced BW (2.97 vs. 3.15) when compared with BD fed birds. In experiment 2, RD fed birds increased BW (p<0.05) when compared with BD fed birds on day 7, 14 and 21. The BW of RD fed birds was increased by 3.7, 7.4 and 3.0% when compared with BD fed birds, respectively. No significant differences in BW (p>0.05) were observed on day 42, however, RD fed birds had numerically greater BW (3.07 vs. 3.05) when compared with BD fed birds. The results from these experiments are consistent with previous research. A study conducted by Si et al.15 reported Cobb 500 broilers fed reducing crude protein diets below 20% while providing recommended levels of indispensable amino acids resulted in a significant reduction in BW and further growth retardation was observed when dietary crude protein was decreased lower than 18%. Wang et al.16 observed similar effects as Cobb 500 broilers reared on fresh litter fed RD decreased BW by 2.7% (p<0.05) when compared with those fed BD. These results indicate BW is decreased when dietary crude protein is reduced in Cobb 500 broilers when compared with Cobb 700 broilers, as BW decreased in RD fed birds in experiment 1 and BW increased in RD fed birds in experiment 2. Thus, protein level and genotype may have profound effects on BW.