Everything is heritable: the terrifying power of selection on additive population variation; "Growth, efficiency, and yield of commercial broilers from 1957, 1978, and 2005", Zuidhof et al 2014 https://pdf.yt/d/bEVFEdtFm8DFf9r6 / https://www.dropbox.com/s/c77nz9emhrvhht0/2014-zuidhof.pdf?dl=0 / http://libgen.org/scimag7/10.3382/ps.2014-04291.pdf . One of the intriguing parts is that while pure selection for weight/feed-efficiency has had some unintended consequences (of the sort it seems every layman immediately jumps to when topics of embryo selection come up), the unintended consequences have been fairly minor even at enormous amplification of body weight (as evidenced by the various metabolic and physiological criteria they look at) and have been ameliorated by somewhat more sophisticated selection.
Excerpts:
"The effect of commercial selection on the growth, efficiency, and yield of broilers was studied using 2 University of Alberta Meat Control strains unselected since 1957 and 1978, and a commercial Ross 308 strain (2005). Mixed-sex chicks (n = 180 per strain) were placed into 4 replicate pens per strain, and grown on a current nutritional program to 56 d of age. Weekly front and side profile photographs of 8 birds per strain were collected. Growth rate, feed intake, and measures of feed efficiency including feed conversion ratio, residual feed intake, and residual maintenance energy requirements were characterized. A nonlinear mixed Gompertz growth model was used to predict BW and BW variation, useful for subsequent stochastic growth simulation. Dissections were conducted on 8 birds per strain semiweekly from 21 to 56 d of age to characterize allometric growth of pectoralis muscles, leg meat, abdominal fat pad, liver, gut, and heart. A novel nonlinear analysis of covariance was used to test the hypothesis that allometric growth patterns have changed as a result of commercial selection pressure. From 1957 to 2005, broiler growth increased by over 400%, with a concurrent 50% reduction in feed conversion ratio, corresponding to a compound annual rate of increase in 42 d live BW of 3.30%. Forty-two-day FCR decreased by 2.55% each year over the same 48-yr period. Pectoralis major growth potential increased, whereas abdominal fat decreased due to genetic selection pressure over the same time period. From 1957 to 2005, pectoralis minor yield at 42 d of age was 30% higher in males and 37% higher in females; pectoralis major yield increased by 79% in males and 85% in females. Over almost 50 yr of commercial quantitative genetic selection pressure, intended beneficial changes have been achieved. Unintended changes such as enhanced sexual dimorphism are likely inconsequential, though musculoskeletal, immune function, and parent stock management challenges may require additional attention in future selection programs.
Between 1960 and 2004, the US consumer price index for poultry products increased at half the rate of all other products (USDA, Economic Research Service, 2004), due to improvements in growth and efficiency. This has likely been a major factor contributing to higher per capita consumption of chicken meat between 1950 (9.4 kg) and 2005 (39.2 kg; USDA, Economic Research Service, 2014). Early on, limited statistical capabilities forced geneticists to focus on economically important parameters that were easily measured and highly heritable, such as BW, feed consumption, feed conversion ratio (FCR), and yield (Hunton, 2006). In response to changing consumer demands, product development has driven genetic selection, with concomitant unintended effects, including increased skeletal defects (Lilburn, 1994; Rath et al., 2000), metabolic disorders (Scheele, 1997; Olkowski, 2007), and altered immune function (Cheema et al., 2003). In 1962, 83% of broilers were marketed as whole birds, 15% as cut-up or parts, and 2% as further processed products (National Chicken Council, 2011). In 2005 only 11% of broilers were marketed as whole birds, 43% as cut-up or parts, and 46% as further processed products (National Chicken Council, 2011). As statistical capabilities expanded, more balanced selection programs became achievable (Emmerson, 1997).
Three treatments (strains) were used in a completely randomized design.
Strains
The current experiment included 2 University of Alberta Meat Control strains unselected since 1957 and 1978 (AMC-1957 and AMC-1978, respectively) and a 2005 commercial Ross 308 broiler (Aviagen North America Inc., Huntsville, AL). The AMC-1957 and AMC-1978 strains have been maintained unselected at the University of Alberta Poultry Research Centre, Edmonton, Alberta, since 1989. Source parent flocks were all 46 wk of age.
At hatch, mixed-sex broilers from each strain (n = 180 per strain) were individually wing banded and grown in 12 floor pens (n = 4 per strain) to 56 d of age. Birds were housed at a stocking density of 4.07 birds per m 2 at the beginning of the experiment, decreasing to 1.90 birds per m 2 at 56 d as birds were removed for sampling. All birds were fed nutritionally complete commercial-type diets ad libitum, based on contemporary recommendations for the Ross 308 (Aviagen Inc., 2003). A starter diet (3,068 kcal/kg; 23% CP) was fed from 0 to 14 d; a grower diet (3,152 kcal/kg; 20.15% CP) was fed from 15 to 28 d; and a finisher diet (3,196 kcal/kg 19% CP) was fed from 29 to 56 d.
A front profile pictorial record at 0, 28, and 56 d of age along with average mixed-sex BW for each strain is presented in Figure 1. Although all chicks were from breeder flocks of the same age, chick weights at hatch were lowest in the AMC-1957 strain and highest in the 2005 strain (Figure 2). The AMC-1978 strain was intermediate to, and different from both other strains. By 4 wk of age, the relative BW of the AMC-1978 strain reached a plateau at approximately 43% of the 2005 strain BW, and the relative BW of the AMC1957 strain reached a plateau at approximately 21% of the 2005 strain BW (Figure 2). Although nutrient requirements for the 3 strains may be different, genetic selection accounted for 85 to 90% of the increase in BW from 1957 to 1991 (Havenstein et al., 1994a). The 42-d BW of the AMC-1957 strain was 586 g, which was approximately 45 g less than the 1959 average of the male and female BW reported by Merritt and Gowe (1962). The 42-d BW of the AMC-1978 strain was 1,205 g, which was approximately 130 g less than the 1,336-g average of the male and female BW in the progenitor strain reported by Chambers et al. (1984). However, the 56-d weights of the AMC-1957 broilers in the present study were consistent with ACRB broiler weights reported by Havenstein et al. (2003b). Gompertz growth parameters for a nonlinear mixed model (Table 1) predicted mature weights (W m ) of 2,373, 3,602, and 7,110 g for the AMC-1957, AMC1978, and 2005 strains, respectively.
From 1957 to 2005, broiler growth rates increased by over 400%, with a concurrent 50% reduction in FCR. The claim that broiler 42-d live BW was increasing at a compounded rate of 3.1% (Barton, 1994) has held up to scrutiny. The current study confirms that the rate of increase in 42-d live BW from 1957 to 2005 was 3.30% per year, compounded for 48 yr. Similarly, FCR to 42 d of age has decreased by 2.55% per year, also in a compounding manner. The net result was that over a period of almost 50 yr, the broiler industry has been able to reduce the amount of feed required to produce chicken meat by one-half, and breast meat by 67%. Because feed accounts for approximately two-thirds of the cost of producing chicken, the resulting savings to consumers is substantial.
To counter a surprisingly widespread popular misunderstanding of the underlying mechanisms behind rapid broiler growth, it is important to note the basis for this transformative change in productivity. Modern chickens grow quickly because they have tremendous genetic potential to grow. Traditional selection methods— breeding efficient and robust birds with high growth rates—have been a particularly successful strategy in poultry because of high reproductive rates and short generation times. There have been lessons along the way. Unintended consequences to selection have proven challenging for the broiler industry, and will likely continue to emerge in spite of a high level of diligence manifested through comprehensive balanced selection programs. Many unintended changes such as increasing sexual dimorphism are not likely to become problematic, but musculoskeletal biomechanics, changes in immune response, and implications of huge growth potential for the welfare of breeding stock will undoubtedly challenge primary meat-type poultry breeders for the foreseeable future."
Excerpts:
"The effect of commercial selection on the growth, efficiency, and yield of broilers was studied using 2 University of Alberta Meat Control strains unselected since 1957 and 1978, and a commercial Ross 308 strain (2005). Mixed-sex chicks (n = 180 per strain) were placed into 4 replicate pens per strain, and grown on a current nutritional program to 56 d of age. Weekly front and side profile photographs of 8 birds per strain were collected. Growth rate, feed intake, and measures of feed efficiency including feed conversion ratio, residual feed intake, and residual maintenance energy requirements were characterized. A nonlinear mixed Gompertz growth model was used to predict BW and BW variation, useful for subsequent stochastic growth simulation. Dissections were conducted on 8 birds per strain semiweekly from 21 to 56 d of age to characterize allometric growth of pectoralis muscles, leg meat, abdominal fat pad, liver, gut, and heart. A novel nonlinear analysis of covariance was used to test the hypothesis that allometric growth patterns have changed as a result of commercial selection pressure. From 1957 to 2005, broiler growth increased by over 400%, with a concurrent 50% reduction in feed conversion ratio, corresponding to a compound annual rate of increase in 42 d live BW of 3.30%. Forty-two-day FCR decreased by 2.55% each year over the same 48-yr period. Pectoralis major growth potential increased, whereas abdominal fat decreased due to genetic selection pressure over the same time period. From 1957 to 2005, pectoralis minor yield at 42 d of age was 30% higher in males and 37% higher in females; pectoralis major yield increased by 79% in males and 85% in females. Over almost 50 yr of commercial quantitative genetic selection pressure, intended beneficial changes have been achieved. Unintended changes such as enhanced sexual dimorphism are likely inconsequential, though musculoskeletal, immune function, and parent stock management challenges may require additional attention in future selection programs.
Between 1960 and 2004, the US consumer price index for poultry products increased at half the rate of all other products (USDA, Economic Research Service, 2004), due to improvements in growth and efficiency. This has likely been a major factor contributing to higher per capita consumption of chicken meat between 1950 (9.4 kg) and 2005 (39.2 kg; USDA, Economic Research Service, 2014). Early on, limited statistical capabilities forced geneticists to focus on economically important parameters that were easily measured and highly heritable, such as BW, feed consumption, feed conversion ratio (FCR), and yield (Hunton, 2006). In response to changing consumer demands, product development has driven genetic selection, with concomitant unintended effects, including increased skeletal defects (Lilburn, 1994; Rath et al., 2000), metabolic disorders (Scheele, 1997; Olkowski, 2007), and altered immune function (Cheema et al., 2003). In 1962, 83% of broilers were marketed as whole birds, 15% as cut-up or parts, and 2% as further processed products (National Chicken Council, 2011). In 2005 only 11% of broilers were marketed as whole birds, 43% as cut-up or parts, and 46% as further processed products (National Chicken Council, 2011). As statistical capabilities expanded, more balanced selection programs became achievable (Emmerson, 1997).
Three treatments (strains) were used in a completely randomized design.
Strains
The current experiment included 2 University of Alberta Meat Control strains unselected since 1957 and 1978 (AMC-1957 and AMC-1978, respectively) and a 2005 commercial Ross 308 broiler (Aviagen North America Inc., Huntsville, AL). The AMC-1957 and AMC-1978 strains have been maintained unselected at the University of Alberta Poultry Research Centre, Edmonton, Alberta, since 1989. Source parent flocks were all 46 wk of age.
At hatch, mixed-sex broilers from each strain (n = 180 per strain) were individually wing banded and grown in 12 floor pens (n = 4 per strain) to 56 d of age. Birds were housed at a stocking density of 4.07 birds per m 2 at the beginning of the experiment, decreasing to 1.90 birds per m 2 at 56 d as birds were removed for sampling. All birds were fed nutritionally complete commercial-type diets ad libitum, based on contemporary recommendations for the Ross 308 (Aviagen Inc., 2003). A starter diet (3,068 kcal/kg; 23% CP) was fed from 0 to 14 d; a grower diet (3,152 kcal/kg; 20.15% CP) was fed from 15 to 28 d; and a finisher diet (3,196 kcal/kg 19% CP) was fed from 29 to 56 d.
A front profile pictorial record at 0, 28, and 56 d of age along with average mixed-sex BW for each strain is presented in Figure 1. Although all chicks were from breeder flocks of the same age, chick weights at hatch were lowest in the AMC-1957 strain and highest in the 2005 strain (Figure 2). The AMC-1978 strain was intermediate to, and different from both other strains. By 4 wk of age, the relative BW of the AMC-1978 strain reached a plateau at approximately 43% of the 2005 strain BW, and the relative BW of the AMC1957 strain reached a plateau at approximately 21% of the 2005 strain BW (Figure 2). Although nutrient requirements for the 3 strains may be different, genetic selection accounted for 85 to 90% of the increase in BW from 1957 to 1991 (Havenstein et al., 1994a). The 42-d BW of the AMC-1957 strain was 586 g, which was approximately 45 g less than the 1959 average of the male and female BW reported by Merritt and Gowe (1962). The 42-d BW of the AMC-1978 strain was 1,205 g, which was approximately 130 g less than the 1,336-g average of the male and female BW in the progenitor strain reported by Chambers et al. (1984). However, the 56-d weights of the AMC-1957 broilers in the present study were consistent with ACRB broiler weights reported by Havenstein et al. (2003b). Gompertz growth parameters for a nonlinear mixed model (Table 1) predicted mature weights (W m ) of 2,373, 3,602, and 7,110 g for the AMC-1957, AMC1978, and 2005 strains, respectively.
From 1957 to 2005, broiler growth rates increased by over 400%, with a concurrent 50% reduction in FCR. The claim that broiler 42-d live BW was increasing at a compounded rate of 3.1% (Barton, 1994) has held up to scrutiny. The current study confirms that the rate of increase in 42-d live BW from 1957 to 2005 was 3.30% per year, compounded for 48 yr. Similarly, FCR to 42 d of age has decreased by 2.55% per year, also in a compounding manner. The net result was that over a period of almost 50 yr, the broiler industry has been able to reduce the amount of feed required to produce chicken meat by one-half, and breast meat by 67%. Because feed accounts for approximately two-thirds of the cost of producing chicken, the resulting savings to consumers is substantial.
To counter a surprisingly widespread popular misunderstanding of the underlying mechanisms behind rapid broiler growth, it is important to note the basis for this transformative change in productivity. Modern chickens grow quickly because they have tremendous genetic potential to grow. Traditional selection methods— breeding efficient and robust birds with high growth rates—have been a particularly successful strategy in poultry because of high reproductive rates and short generation times. There have been lessons along the way. Unintended consequences to selection have proven challenging for the broiler industry, and will likely continue to emerge in spite of a high level of diligence manifested through comprehensive balanced selection programs. Many unintended changes such as increasing sexual dimorphism are not likely to become problematic, but musculoskeletal biomechanics, changes in immune response, and implications of huge growth potential for the welfare of breeding stock will undoubtedly challenge primary meat-type poultry breeders for the foreseeable future."
