BREEDING AND GENETICS

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1 BREEDING AND GENETICS Growth Curves for Body Weight and Major Component Parts, Feed Consumption, and Mortality of Male Broiler Chickens Raised to Maturity M. Goliomytis, E. Panopoulou, and E. Rogdakis Department of Animal Breeding and Husbandry, Agricultural University of Athens, 75 Iera Odos, 11855, Athens, Greece ABSTRACT Body weight; yield of the major carcass mum growth rate is attained, was predicted at 44.4, 47, component parts of breast, leg, thigh, drumstick, breast meat, thigh meat, and drumstick meat; feed consumption; feed conversion; and mortality of male broiler chickens from two commercial strains were measured from hatching to 154 d of age. As no differences were observed between the two strains, for any of the traits measured, the statistical analysis was made using pooled data. Growth curves for BW, breast weight, and leg weight were calculated. The Richards function was chosen to fit the data. The type of the curves predicted was typically sigmoid. Asymptotic weights for BW, breast weight, and leg weight were estimated at 6,870.2, 1,744.2, and g, respectively. Age at point of inflection, at which maxiand 49.1 d, respectively. The percentage of breast and breast meat increased with age, whereas percentages of leg, thigh, and drumstick remained roughly constant. Weekly feed conversion was determined, and polynomial functions were applied to relate feed consumption and feed conversion to the age of the birds. Cumulative mortality increased with age, especially beyond 70 d of age, rising to 50% by the end of the experiment. Mortality was related to high incidence of leg weakness observed in the same period. The results of the current study provide information on the growth potential of contemporary, genetically improved broiler chickens by means of a mathematical model. (Key words: broiler, growth curve, carcass component, feed consumption, mortality) 2003 Poultry Science 82: INTRODUCTION Tremendous improvement in commercial broiler performance has taken place since the early 1950s. This change has occurred via selection for rapid early growth, combined with improved environmental factors affecting broiler performance, such as nutrition and housing (Prescott et al., 1985; Havenstein et al., 1994a). The slaughter age of contemporary broiler chickens is reduced to 42 d and slaughter weight exceeds 2 kg (Havenstein et al., 1994a; Leterrier et al., 1998). Because most of the published studies are of birds from 42 to 70 d of age, little information is available beyond this age on the overall growth potential of today s broiler chicken. Growth curve functions are the most adequate means for describing the growth pattern of BW or body parts, because they summarize the information into a few parameters that may be interpreted biologically. Several growth functions are available for the description of growth such as Brody s, logistic, Gompertz, von Bertalanfy, and the four-parameter Richards function, which summarizes all the above growth functions into one 2003 Poultry Science Association, Inc. Received for publication October 24, Accepted for publication February 6, To whom correspondence should be addressed: mgolio@in.gr. (Fitzhugh, 1976; Grossman and Bohren, 1982; France and Thornley, 1984). Few published studies are of meattype chickens raised to maturity. Prescott et al. (1985), Rogers et al. (1987), and Hancock et al. (1995) reared broiler chickens to adult weight to obtain growth curves. Knizetova et al. (1985, 1991) estimated growth curves for pure breeds with different type of performance, i.e., egg-type, meat-type, combined performance, and purebred broiler lines. Grossman and Koops (1988) examined the multiphasic growth theory using two randombred control populations of meat- and egg-type chickens. Grey et al. (1982) measured whole carcass yield and the yield of component parts of broiler chickens on an extended growth period of 364 d. The aim of the present study was to investigate the growth pattern of broiler chickens beyond the common slaughter age of 42 d to maturity. Growth curves for BW and important consumer parts such as breast and leg were calculated, and comparisons with previous studies were made to investigate any change in the pattern of development for high value component parts. Feed consumption, feed conversion, and livability were also measured. MATERIALS AND METHODS Three hundred sixty male broiler chickens from two commercial strains were reared on a floor system from 1061

2 1062 GOLIOMYTIS ET AL. TABLE 1. Composition percentages of the diets used Ingredients Corn Wheat Wheat bran Soya bean meal 45% Fish meal 4 Methionine Lysine Limestone Dicalcium phosphate Salt Trace minerals vitamins Laboratory analysis Dry matter Ash (% DM) Fat (% DM) Crude fiber (% DM) Crude protein (% DM) ME 2 (kcal/kg) 2,796 2,916 2,988 2,940 Diet 1 1 Diets 1, 2, 3, and 4 were fed from 1 to 3, 4 to 5, 6 to 7, and 8 to 22 wk, respectively. 2 Metabolizable energy was calculated with Sibbald s equation (Labrier and Leclercq, 1994). 1 to 154 d of age. The two strains used were Cobb 500 and Shaver Starbro. Each bird was identified by a wing band on the left wing. The 180 birds from each strain were randomly allocated to six, equally sized pens of 2.5 m 2. Wheat straw was used as bedding. Each pen housed 30 chickens and contained one feeder and one bell drinker. For the first 21 d of age birds were reared on a 23 L:1 D regime. Thereafter the lighting regime consisted of 18 L:6 D. The air temperature was 30 C at 1 d of age and was gradually reduced to 20 C by21d of age. Water and feed were supplied for consumption ad libitum. All birds received a starter diet in a crumbled form until 21 d of age; thereafter the feed was pelleted. Compositions of the four diets used are shown in Table 1. At weekly intervals to 8 wk and every 2 wk thereafter, six birds from each strain were randomly selected, and live BW was recorded. Then the birds were slaughtered by dislocation of the neck and were eviscerated by hand. The feathers were removed after immersion in water at 65 C for 1 min. The feet and shanks were removed at the tibiotarsus joint. Carcasses were weighed, placed in polystyrene bags, and chilled to 4 C until the following morning when dissection took place. The carcass was cut into breast, legs, wings, and rest of the carcass. The breast was separated from the back at the shoulder and along the junction of the vertebral and sternal ribs. The wings were separated from the carcass at the shoulder. The legs were separated from the carcass by cutting through the iliofemoral joint and included the thigh and drumstick. The right leg was divided into drumstick and thigh at the tibiofemoral joint. The breast, drumstick, and thigh were dissected into meat, skin, and bone, and their weights were recorded. The Richards function (Richards, 1959) was chosen to fit the data for live BW and weight of the major carcass component parts, as it summarizes the most important growth functions into one. The Richards function for the current study was in the following form: W t = A(1± be kt ) 1/n for n > 1, n 0, A, k > 0, e = if n < 0 W = A(1 be kt ) 1/n n > 0 W = A(1+ be kt ) 1/n The biological interpretation of the parameters in the model is as follows: W t = weight of body or carcass component at age t; A = upper asymptotic weight as age approaches infinity (an estimation of mature weight); k = rate of maturing and refers to growth rate relative to mature weight and measures the relative rate at which asymptotic value is attained; n = shape parameter determining the position of the inflection point at which the auto acceleration growth phase passes into the auto retardation phase, and the growth rate is maximum; b = integration constant with no biological significance. Absolute growth rate was estimated by the first derivative of the Richards function. The age (t i ) and weight (W i ) at the inflection point were calculated as follows: t i = 1/k [ln (n/b)] W i = A(n+1) 1/n The data were analyzed with the general linear models procedure of SAS software (SAS Institute, 1990). The nonlinear regression procedure, with the Marquardt algorithm, was used to fit the Richards function to the observed data. BW RESULTS AND DISCUSSION No differences were observed between the two strains for any of the traits measured (P > 0.05). For this reason

3 GROWTH CURVES OF MALE BROILER CHICKENS 1063 FIGURE 1. Growth curve and absolute growth rate for body weight, estimated growth curve ( ); observed mean ( ); estimated absolute growth rate ( ); observed absolute growth rate ( ). the statistical analyses were conducted using the pooled data. The predicted growth curve and growth rate for live BW are shown in Figure 1. The shape of the growth curve predicted was typically sigmoid. Live BW was rapidly increasing until age at the inflection point (44.4 d), at which maximal growth rate (90.4 g per d) was attained. Live BW at this age was estimated at 2,516.4 g. Beyond this age, growth rate declined and approached zero at maturity. The estimated parameters for the Richards function, weight and age at the inflection point, and the coefficient of determination for live BW are shown in Table 2. The high value of coefficient of determination (R 2 = 0.98) indicates the excellent fit of the Richards function to the observed data. Mature BW was estimated at 6,870.2 g, which was higher than the values reported by Knizetova et al. (1985) for cockerels of pure breeds with different levels of performance. In the study of Knizetova et al. (1985), BW for meat-type White Cornish, egg-type White Leghorn, and dual-purpose New Hampshire cockerels were estimated at 6,059, 2,354, and 3,720 g, respectively. The differences observed are explained by the different genetic origins of the flocks used. Mature BW values for the chickens in the present study were even higher than the values reported for commercial strains and purebred broiler lines in earlier comparable studies. Prescott et al. (1985), Rogers et al. (1987), Knizetova et al. (1991), and Hancock et al. (1995) estimated mature weights at 6,430, 5,984, 4,723 to 5,853, and 5,171 to 6,145 g, respectively. The higher value reported in the present study was primarily due to genetic changes that have been brought about by selection for rapid early growth rate. Mature BW is highly correlated with rapid early growth rate (r = 0.5) (Marks, 1995), and as a consequence, the modern adult broiler chicken is heavier than its ancestors (Prescott et al., 1985). Rate of maturing, i.e., maturation index k, for BW was estimated in the present study at 0.036, which was higher than the values reported by Knizetova et al. (1985) for pure breeds (k ranged from to ). The higher values estimated indicate that the modern broiler chicken matures earlier than purebred chickens, regardless of the genetic strain s performance. Almost similar values for the maturation index k were estimated by Tzeng and Becker (1981) (k = 0.037) for pedigreed male broilers and by Hancock et al. (1995) (k ranged from to ) for cockerels from of six commercial strains. Rogers et al. (1987) estimated lower values for k (0.031 to 0.032) probably because it referred to both sexes. The point of inflection for chickens in the present study was estimated to be at 44.4 d of age, much earlier than for purebred chickens of unselected populations. Knizetova et al. (1985) estimated the inflection point at 63.7, 79.8, and 81.5 d of age, for White Cornish, White Leghorn, and New Hampshires cockerels, respectively. A greater age (48.2 to 55.7 d) at the inflection point was estimated in another study by Knizetova et al. (1991) for nine purebred broiler lines in which no selection for earliness of growth was applied. The later study supported the standpoint that a higher maturation index k showed younger age at inflection point, as lines with the highest k values reached the inflection point at a younger age. The phenotypic correlation reported, ranged from 0.01 to The results of the present study combined with the results of Knizetova et al. (1985, 1991) confirms the above relationship. The k value for the chickens of the present study was higher than that reported by Knizetova et al. (1985, 1991), whereas age at the inflection point was earlier. In agreement with Knizetova et al. (1991), Barbato (1991) estimated a negative phenotypic correlation between maturation TABLE 2. Parameters 1 of Richards growth function for live body weight and the major component parts (mean ± standard error) Variable A (g) k (d 1 ) n R 2 t i (d) w i (g) Body weight 6,870.2 ± ± ± ,516.4 Breast 1,744.2 ± ± ± Leg ± ± ± Thigh ± ± ± Drumstick ± ± ± Breast meat 1,449 ± ± ± Thigh meat ± ± ± Drumstick meat 242 ± ± ± A = mature weight; k = rate of maturing; n = shape parameter; t i = age at the inflection point; w i = weight at the inflection point.

4 1064 GOLIOMYTIS ET AL. FIGURE 2. Growth curve and absolute growth rate for breast weight, estimated growth curve ( ); observed mean ( ); absolute growth rate ( ); observed absolute growth rate ( ). FIGURE 3. Growth curve and absolute growth rate for leg weight, estimated growth curve ( ); observed mean ( ); absolute growth rate ( ); observed absolute growth rate ( ). index k and the age at the inflection point (r = 0.93) for male and female progeny. Barbato (1991) also reported a negative phenotypic correlation ( 0.58) between weight at 42 d of age, at which selection for high BW occurs, and age at the inflection point, suggesting that selection for rapid early growth reduced age at the inflection point. As the data in Table 2 show, the shape parameter n for BW tended to zero. Given that the proportion of BW at the inflection point to mature weight was predicted at 36.64% [n tended to zero, and w i /A = 36.8% for the Gompertz function (Fitzhugh, 1976)], the Gompertz function could also be used to describe the growth of the chickens. In agreement with the present study, several other investigators have also suggested that the Gompertz function is the most appropriate equation for describing growth rate of chickens (Tzeng and Becker, 1981; Knizetova et al., 1985, 1991; Prescott et al., 1985; Ricklefs, 1985; Rogers et al., 1987; Barbato, 1991; Hancock et al., 1995). Growth of the Major Components Growth curves and growth rate of the more important components, i.e., breast and leg, are given in Figures 2 and 3. The growth curves predicted were, just as for BW, of the typical sigmoid shape, and high R 2 indicated the excellent fit of the Richards function to the data. Weight gain for breast and leg increased until the inflection point at 47 and 49 d of age, respectively. Maximal growth rate occurred at these ages and was estimated at 24.4 g/d for breast and 10.8 g/d for leg. Thereafter growth rate declined toward zero at maturity. Table 2 gives the growth parameters as well as the derived biological parameters for leg, breast, thigh, drumstick, breast meat, thigh meat, and drumstick meat. In the case of the above parts, the Gompretz function can describe the growth potential given that the n parameter tends to 0. Maturation index k values for leg, breast, thigh, drumstick, breast meat, thigh meat, and drumstick meat were not different from that estimated for BW. From the biological parameters defined, age at inflection point is of the greatest interest because it is related to the period in which maximum growth rate is attained. It is not accidental that the age at inflection point for live BW coincides with slaughter age. The breeding of broilers takes place during the phase in which growth rate is increasing. The points of inflection for breast and leg, which are of great economic interest, were estimated at 47 and 49.1 d, respectively, later than for BW (44.4 d). Perhaps this finding indicates that increasing the growing period of broilers, which are eventually marketed as component parts, may improve the economic efficiency of production. This hypothesis is strengthened by the increase with age of the portions of the parts that have high commercial value such as breast and breast meat. The percentages of the major component parts of the carcass by age are presented in Table 3. The percentage of breast meat increased even after slaughter at 42 d of age, whereas breastbone, which is of no commercial value, decreased (results not shown here). Knizetova et al. (1991) were led to a similar conclusion, and suggested developing two types of broiler chickens. The first type would be marketed for its breast and thigh meat to the food service industry. The second type would be marketed as whole birds, probably at the current 42 d age. A comprehensive economic study is needed to evaluate the possible advantage of marketing highvalue component parts from a broiler bred at a later age. As shown in Table 3, there was an effect of age on the percentage of breast (P < 0.05). The percentage of breast increased rapidly during the early stage of life. Intensive growth occurred during the first 28 d. Growth rate then decreased until 70 d of age. The percentage of breast remained constant from the 70 through the 154 dofage(p > 0.05). Breast yield values from the present study were higher than what has been reported in a number of studies. Hayse and Marion (1973), Merkley et al. (1980), and Broadbent et al. (1981) reported breast percentages of 28.2, 27.5, and 29.5%, respectively, after 56-d experiments. Orr et al. (1984) observed values simi-

5 GROWTH CURVES OF MALE BROILER CHICKENS 1065 TABLE 3. Mean yield (percentage) of the major component parts as a proportion of carcass weight by age 1 (mean ± standard error) Age (d) Component part Breast 21 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.9 Legs 30.5 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.4 Thigh ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.5 Drumstick ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.3 Breast meat 12.4 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 1 Thigh meat ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.4 Drumstick meat ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± There was a significant effect of age on all component parts (P < 0.05) except for drumstick (P > 0.05). 2 Values are for both legs. lar (32.1 to 33.2%) to those of the present study at 49 d of age. The fact that the above authors included the whole ribs in breast portion, whereas in the current study only a part of the ribs was included, indicates that when comparing on the same basis the percentage of breast in carcass is higher in the present study than that reported by Orr et al. (1984). Breast meat development was similar to that observed for the percentage of breast (Table 3). These results for breast meat development were in agreement with those reported by Acar et al. (1993). The relative amount of breast meat was higher in the present than in previous studies reported by Frischknecht and Jull (1946) at 84 d of age; Hayse and Marion (1973) and Broadbent et al. (1981) at 56 d of age; Grey et al. (1982) at 21, 35, 56, and 76 d of age; Bilgili et al. (1992) and Jones (1995) at 42 d of age; Brake et al. (1993) at 28, 35, 42, and 49 d of age; and Havenstein et al. (1994b) at 43, 57, 71, and 85 d of age. The differences observed may be attributed to different genetic origins of the flocks because relative yields of breast and breast meat are influenced by strain (Merkley et al., 1980; Orr et al., 1984; Acar et al., 1993; Moran, 1995). The higher values obtained in the present study indicate that contemporary broiler chickens, as reported by Brake et al. (1993) and Havenstein et al. (1994b), have a greater proportion of breast meat than earlier broiler types. The increasing market demands for white meat has compelled poultry breeding companies to focus on selection for high percentages of breast meat (Brake et al., 1993; Pollock, 1997), resulting in chickens with a higher portion of breast than few years ago. The heavier BW of contemporary broiler chicken also explains the higher relative yield of breast, because heavier birds produce a greater portion of breast (Frischknecht and Jull, 1946; Marks, 1995; Moran, 1995). Age had an effect on the percentage of legs (P < 0.05) (Table 3). The values determined in the present study were lower than those reported by Hayse and Marion (1973), Merkley et al. (1980), and Broadbent et al. (1981) at 56 d and by Orr et al. (1984) at 49 d of age. The values reported were 36, 34.5, 33.6, and 33.4 to 33.8%, respectively. The decrease in the leg portion of contemporary broilers, represented by chickens in the present study, may be explained by the increase in breast portion, as mentioned above, which resulted in a decrease in the other parts on a relative basis (Fletcher and Carpenter, 1993). As shown in Table 3, age had an effect on the percentages of thigh, thigh meat, and drumstick meat (P < 0.05), whereas the percentage of drumstick remained constant throughout the entire growth period (P > 0.05). Feed Consumption Figure 4 summarizes feed consumption values through 154 d of age and the polynomial function that relates feed consumption to the age of the chickens. The values shown represent both strains because the strains

6 1066 GOLIOMYTIS ET AL. FIGURE 4. Feed consumption by age and the polynomial function that relates weekly feed consumption (WFC) to the age (T) of the birds. FIGURE 5. Feed conversion by age and the polynomial function that relates feed conversion (FC) to the age (T) of the birds. were not different at any age (P > 0.05). Feed consumption increased until 84 d of age, declined until 112 d of age, and then stabilized until the end of the experiment. Decrease in feed consumption has been observed during extensive growth period studies by other authors as well. Prescott et al. (1985) and Whitehead and Parks (1988) observed a decline in feed consumption after 70 and 98 d of age, respectively. The decreasing appetite beyond 84 d of age in the present study is related to a high incidence of leg problems observed during the same period. Although feed consumption decreased, growth rate was not affected significantly. On the other hand, a significant decrease was observed for abdominal fat weight and skin weight, which included subcutaneous fat, beyond 98 and 112 d of age, respectively (results not shown). When feed consumption is lowered beyond maintenance requirements, fat is mobilized from adipose tissues, which results in a subsequent reduction of their amount and relative weight (Nir et al., 1988). The reduction in appetite observed in the present study evidently induced fat mobilization from the fat depots, such as abdominal fat pad and subcutaneous adipose tissue. Feed Conversion Feed conversion expresses the individual s efficiency of feed utilization and is presented as the ratio of feed consumed to gain. Figure 5 summarizes cumulative feed conversion and the polynomial function that relates cumulative feed conversion to the age of the birds. The values predicted were similar to the production goals of the breeding companies from 1 d to 70 d of age (Cobb 500, 1995; Shaver, 1996). Feed conversion was predicted at 1.78 at slaughter age, at 42 d of age, rising to 3.1 by the end of the experiment. In close agreement, although slightly higher than in the present study, were the results obtained by Edwards et al. (1973), 1.99 to 2.25 at 56 d of age; Marks (1979), 2 to 2.25 at 56 d of age; Broadbent et al. (1981), 2.04 at 56 d of age; Bilgili et al. (1992), 1.42 at 21 d of age; Acar et al. (1993), 1.8 at 56 d of age; Xin et al. (1994), 2.2 at 56 d of age; and Leterrier et al. (1998), 1.75 to 2.28 at 42 d of age. The general agreement with previous studies confirms the viewpoint that feed conversion should generally be looked at on a constant BW basis rather than on a constant age basis (Marks, 1995). Improvement of feed conversion is achieved primarily by reducing the growing period, which has been accomplished by selection for growth rate and feed conversion (Marks, 1995). Faster-growing birds reach slaughter age earlier when the proportion of feed consumed for maintenance is lower. Mortality Figure 6 presents cumulative mortality as a percentage of the birds placed. Mortality increased significantly beyond 70 d of age, with leg problems as the principal cause. The birds had reduced walking ability and subsequently, because of lameness, reduced access to feed and water, which led to debilitation and subsequent death. Many comparable studies are in general agreement with the present one. Knizetova et al. (1991) reported 33% cumulative mortality for nine purebred broiler lines over a 26-wk growth period. According to FIGURE 6. Cumulative mortality by age (% birds placed).

7 GROWTH CURVES OF MALE BROILER CHICKENS 1067 the authors, the main cause of deaths was a very high incidence of leg weakness that affected mostly males. Hancock et al. (1995) also observed leg weakness beyond 15 wk of age for six broiler strains, especially in males. Hancock et al. (1995) did not report mortality percentages, but the problem was so intense that growth curves were analyzed up to only 11 wk instead of 26 wk of age. Havenstein et al. (1994a) reported a significantly higher incidence of leg weakness for contemporary broiler chickens in comparison with the 1957 Athens- Canadian Randombred Control strain of broilers. In addition, the incidence of leg weakness increased as the birds increased in age between 42 and 84 d of the experiment. They concluded that faster growth rate and heavier BW combined produced higher incidence of leg problems and poorer walking ability. Hulan et al. (1980) reared commercial broiler chickens to roaster weight (3.2 kg) and observed higher incidence of mortality in comparison with broilers raised to a BW of 1.9 kg, with leg weakness being the main cause of deaths. Rapid growth rate and disproportionate breast muscle development, resulting from intense genetic selection for increased BW and breast meat yield, are the principal causes of leg problems (Hulan et al., 1980; Hester, 1994; Lilburn, 1994; Sullivan, 1994; Kestin et al., 1999; Sorensen et al., 1999). Early feed restriction and intermitted lighting programs are suggested as the most efficient means of reducing skeletal problems in meat-type birds (Hester, 1994; Lilburn, 1994; Kestin et al., 1999; Sorensen et al., 1999). ACKNOWLEDGMENT We thank Antonios Kominakis for his helpful advice and comments. REFERENCES Acar, N., E. T. Moran, Jr., and D. R. Mulvaney Breast muscle development of commercial broilers from hatching to twelve weeks of age. Poult. Sci. 72: Barbato, G. 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