Effect of selection for productive traits in broiler maternal lines on embryo development

Effect of selection for productive traits in broiler maternal lines on embryo development

Schmidt GS^I; Figueiredo EAP de^I,II; Ledur MC^I; Alves HJ^II

^IResearcher from EMBRAPA Suínos e Aves

^IIScholarship from CNPq

^{Correspondence}

ABSTRACT

This study used 300 females and 30 males with 36 weeks of age from the selected PP and control PPc maternal broiler lines. PP has been selected for heavy body weight (PC) and high egg production for eight generations. Fertile eggs were collected and weighed individually for 4 periods of 5 consecutive days at two-week intervals. In each period, a total of 960 eggs/line were identified and separated in groups of 240 eggs, and stored for later incubation. Embryo weight (PE) was evaluated at 9 (P9), 11 (P11), 13 (P13), 15 (P15), 17 (P17) and 21 (P21) days of incubation. The objective was to estimate the effect of selection on embryo development. Egg weight (PO) was similar between the two lines. The differences in PE were significant from P15 on, resulting in 1.9g of difference in the chick weight, indicating correlated genetic changes in the embryo development, which can be credited to the selection for PC. Changes in PE while PO was kept unaltered modified the correlations between these two traits. Differences were significant from P13 on and estimated correlations for P21 were 0.72 and 0.70 for PP and PPc, respectively. Chick weight corresponded to 70.91% (PP) and 68.48% (PPc) of egg weight. The estimated increase in P21 that resulted from the increase of 1.0g in PO was 0.71 in PP and 0.68g in PPc.

Keywords: correlation, egg weight, embryo weight, genetic gain, meat lines.

INTRODUCTION

Embryo development is considered to last, in chronological order, from the fertilization (onset) to hatching (end). Many factors affect embryo development, determining the efficiency of incubation and chick quality. A series of endogenous or genetic components contribute for development variability and might interact with environment variables. Egg physical quality, embryo stage at oviposition, conditions and time lag from oviposition to egg storage, egg storage conditions and incubation conditions, and incubation time are important factors affecting quality of chicks. At the beginning of embryo development, egg size is positively correlated to the number of embryo cells and negatively correlated to cell size (Wiley, 1950). Schmidt et al. (1997) reported that the number of somites – precursor cells of muscle development – was higher in lines selected for meat than in a control line.

The correlation between egg weight and embryo weight increases as a function of the stage of embryo development. The correlation is non-significant in the first half of the incubation period and reaches a maximum at hatching (0.50-0.95) (Hassan & Nordskog, 1971; Yannakopoulos & Tserveni-Gousi, 1987). Zervas & Collins (1965) performed a regression analysis of embryo weight on the 14^th day as a function of egg weight and concluded that only 1% to 3% of the variation could be attributed to egg weight. Chick weight is primarily determined by initial egg weight, corresponding to approximately 68% to 78% (Whiting & Pesti, 1983; Shanawany, 1987; Yannakopoulos & Tserveni-Gousi, 1987; Wilson & Harms, 1988), and secondarily determined by weight loss during incubation, shell weight, residual weight, line, period and conditions of incubation, breeder age and chick gender.

Individual genetic traits (Jull & Heywang, 1930) or line genetic traits (Henderson, 1956) affect egg weight and chick weight correlation. Whiting & Pesti (1983) reported that chick weight corresponded to 67.3% of egg weight in dwarf lines, whereas in normal lines it corresponded to 68.4%. The authors suggested that embryos from larger eggs might be more efficient in utilizing egg nutrients. Such hypothesis might be explained, in part, by the positive correlation that is observed between egg weight and the egg weight:chick weight ratio (Whiting & Pesti, 1983; Skewes et al., 1988; Joubert et al., 1981; Yannakopoulos & Tserveni-Gousi, 1987).

The impact of egg weight on the performance of broilers has been widely studied. Results have been highly variable, mainly because it is difficult to control all the factors affecting the involved traits. The correlation between egg weight and body weight from the 5^th to the 8^th week of age is significant in many meat lines and might vary from 0.3 to 0.5 (Proudfoot & Hulan, 1981; Joubert et al., 1981; Proudfoot et al., 1982; Whiting & Pesti, 1984; Wyatt et al., 1985; Hearn, 1986).

Proudfoot et al. (1982) estimated that an increase of one gram in egg weight affected body weight at 49 days, with gains of 8.9 and 7.6 g in males and females, respectively. When breeder age was evaluated, larger gain was seen in younger flocks at the laying peak (8.2 to 8.3 g) than in older flocks at the end of the laying period (2.6 to 2.1 g). Wilson (1991) estimated gains of 8.3; 2.1; 5.9 and 4.5 g in the weight of broilers at slaughter, when using eggs from in young breeders and old breeders, and for females and males, respectively. Such effect is related to the reduction in slaughter age and, as a result, egg weight becomes important from the economical and technical point of view.

Some results showed that egg weight and chick weight affected mortality negatively (Wyatt et al., 1985; Hearn, 1986), especially in younger breeder flocks, whereas some results show no such effect (Proudfoot & Hulan, 1981; Proudfoot et al., 1982). There are many conflicting results if feed conversion is considered. Both negative (Proudfoot et al., 1982) and positive correlations (Hearn, 1986) with egg weight have been shown.

The objective of the present work was to estimate the effect of selection for productive traits in female meat lines on embryo development during the incubation period.

MATERIAL AND METHODS

The experiment was performed at Embrapa Suínos e Aves, in Concórdia, SC, Brazil. Two maternal meat lines (PP and PPc) were used, which were originated from the same base population in 1988 and have been kept on the genetic improvement program of meat chicken lines from Embrapa. Line PP was selected for fast growth for 8 generations, with greater emphasis on the selection for body weight at 42 days (massal selection), egg production until the 40^th week of age (family selection) and mean egg weight on the 32^nd and 38^th weeks of age (massal selection). A mean of 40 male families were used, each male mated to 8 females during population expansion and a mean selection intensity of 2% for males and 16% for females. Line PPc was the unselected randombred control used as an indicator of the genetic changes resulting from artificial selection.

Birds were individually housed in cages at the age of 36 weeks and 300 females and 30 males were used per line. Fertilization was carried out using two artificial inseminations per week and a ratio of 1 male:10 females. Fertile eggs were collected in four periods of five consecutive days at two-week intervals and four incubations were performed. In each period, a total of 960 eggs/line were individually weighed, identified and separated according to weight in 4 groups (repetitions) of 240 eggs each, and later stored in a colling room. Eight hours before incubation, the eggs were transferred to a pre-heating room and then incubated at 38ºC and 64% humidity.

Live embryos were removed from 160 eggs incubated in each development period (4 repetitions of 40 eggs) at the 9^th, 11^th, 13^th, 15^th and 17^th days of incubation. After being removed, the embryos were blot-dried and individually weighed. On the 18^th day, the remaining eggs were transferred to hatching baskets, where they were kept individually until the 21^st day. Newly hatched chicks were weighed separately.

The genetic gain of the evaluated traits was estimated using the deviation between the selected and control populations, since both originated from the same genetic basis. Egg weight (PO) and embryo weight (PE) were analyzed according to the following mathematical model:

Y_ijklm = m + R_i + L_j + I_k + LI_jk + A_l + LA_jl + IA_kj + LIA_jkl + e_ijklm,

Where: Y_ijklm is the evaluated trait Y, m is the overall population mean; R_i is the effect of repetition i (i = 1, 2, 3 and 4); L_j is the effect of line j (j = 1,2); I_k is the effect of incubation order k (k = 1, 2, 3 and 4); LI_jk is the interaction between line and incubation order, A_l is the effect of incubation time l (l=9, 11, 13, 15, 17 and 21), LA_jl is the interaction between line and incubation time, IA_kl is the interaction between incubation order and incubation time, LIA_jkl is the interaction among line and incubation order and incubation time, and e_ijklm is the random error.

The phenotypic correlations between egg and embryo weights and the deviation between the selected (PP) and control (PPc) populations were determined for both traits. The development curve of the embryo as a function of incubation time was estimated using a regression analysis, fitting to a cubic function. Analysis were performed using the statistical procedures of SAS (SAS, 1990).

RESULTS AND DISCUSSION

Results of the analysis of variance of the collected data and the means of egg weight (PO) and embryo weight (PE) at the different incubation periods (P9, P11, P13, P15, P17 and P21) are shown in Tables 1 and 2, respectively.

Egg weight was not significantly affected by line and incubation order, or the interaction between them in any of the evaluated periods, which indicates that there is no significant difference between lines for this trait. Mean egg weight was 66.31 and 66.03 g for PP and PPc, respectively, varying from 66.97 (P21) to 65.80g (P11) for PP and from 66.47 (P21) to 65.45g (P11) for PPc. The similar egg weight between lines is a result from the fact that the objective of the selection process performed for 8 generations was to maintain mean egg weight constant.

PE differences between lines were significant from P15 (Figure 1). PP mean was higher than PPc mean in 5.41 (P15), 6.69 (P17) and 4.32% (P21), which resulted in a difference of 1.97g in the weight of chick at hatching. Considering that PO was similar between the lines and that both had the same genetic background, the change in embryo development (Figure 2) can be attributed to the selection for body weight. Schmidt et al. (1997) reported higher number of somites – precursor cells of the skeletal muscle in the begining of the embryo development (55-60h) – in the lines selected for body weight, results that might explain in part the changes in PE. Differences in the chemical composition and/or efficiency in nutrient utilization by the embryo, as shown by Whiting & Pesti (1983) and Yannakopoulos & Tserveni-Gousi (1987), might also explain part of the variation. The reduction in the advantage of PP on P21 might be related to two factors (Wiley, 1950): 1) physical egg space, which was similar for the two lines, and 2) limitation of the nutrient absorbing rate.

Alterations in PE while PO was kept constant changed correlations between these traits (Table 3) and significant differences were seen after P13. Estimated correlations were 0.25 and 0.21 (P13); 0.28 and 0.27 (P15); 0.35 and 0.20 (P17) and 0.72 and 0.70 (P21) for PP and PPc, respectively. These values are lower than the estimated correlations reported by Hassan & Nordskog (1971), which were close to zero until P14, 0.50 at P16 and 0.90 at P19. On the other hand, the estimated correlations reported here are within the range reported in the literature, i.e. 0.50 and 0.95 at P21 (Wiley, 1950; Henderson, 1956; Yannakopoulos & Tserveni-Gousi, 1987).

Chick weight corresponded to 70.91 and 68.48% of egg weight for PP and PPc, respectively (Table 3), corroborating results reported by Shanawany (1987). After the 13 ^th day, increases of one gram in egg weight resulted in increases of 0.15 (13^th), 0.28 (15 ^th) and 0.43g (17^th) in embryo weight and 0.71g in chick weight for line PP; and 0.15 (13 ^th), 0.26 (15 ^th) and 0.41g (17^th) in embryo weight and 0.68g in chick weight for line PPc. The increase of 3.56% on the chick weight: egg weight ratio was due to the genetic differences between PP and PPc, and resulted from the selection for body weight that changed correlations without changing egg weight. Genetic differences in chick weight: egg weight ratio were reported by Whiting & Pesti (1983), who observed that chicks from dwarf broiler lines showed lower ratio (63.7%) when compared to normal broiler lines (68.4%).

In the regression analysis, the cubic model explained 95% of the variation of embryonic development (Figure 2). Curve differences between lines were significant after the 15^th day of incubation. Other models have been proposed, such as non-linear models (Freitas et al., 1983).

CONCLUSION

The selection for body weight and egg production whereas egg weight is kept constant change the embryo development curve and the correlations between egg weight and chick weight, and between egg weight and the ratio egg weight:embryo weight, from the 14^th day of incubation.

The correlation between egg weight and embryo weight increased with incubation time and was maximum at the final third of incubation.

Arrived: october 2002

Approved: march 2003

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Publication Dates

• Publication in this collection
  12 Nov 2003
• Date of issue
  May 2003

History

• Received
  Oct 2002
• Accepted
  Mar 2003