Reproductive organ morphology and carcass traits in unselected naturally mating female Bronze turkeys at onset of lay

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1 Reproductive organ morphology and carcass traits in unselected naturally mating female Bronze turkeys at onset of lay R. A. Renema 1, V. L. Melnychuk 1, F. E. Robinson 1,3, H. L. Classen 2, and R. D. Crawford 2 1 Department of Agricultural, Food, and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada T6G 2P5; 2 Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5B5. Received 14 April 1997, accepted 11 December Renema, R. A., Melnychuk, V. L., Robinson, F. E., Classen, H. L. and Crawford, R. D Reproductive organ morphology and carcass traits in unselected naturally mating female Bronze turkeys at onset of lay. Can. J. Anim. Sci. 78: A study was conducted examining the rates of development of components of the reproductive tract during sexual maturation in a flock of naturally mating Bronze turkeys. Birds were processed at 7-d intervals beginning at photostimulation (29.5 wk of age), for the following 5 wk, and following their first oviposition. The relatively low BW of this strain (6.6 kg) was associated with a very low lipid content (16.4%), abdominal fat pad weight (107 g), and breast muscle weight (1.15 kg). At sexual maturity the ovary contained 12.2 large yellow follicles with 59% of these follicles being in a multiple hierarchy arrangement. There was an average of 1.4 unreconciled POF (the remnant of the site of ovulation). The presence of unreconciled POF suggests that this may be a natural phenomenon in turkeys and not limited to modern fast growing strains. Birds that laid their first egg quickly were more likely to have ovulated several times prior to their first oviposition than birds commencing lay later (r = 0.64; P < 0.02). Regression analysis of reproductive tract development revealed that the oviduct reached its mature weight 3 d earlier than did the ovary (28.3 vs d) (r 2 = 0.81; P < ). This strain resembled modern female line strains in regard to reproductive traits. Key words: Bronze Turkey, body weight, ovary morphology, reproductive disorders Renema, R. A., Melnychuk, V. L., Robinson, F. E., Classen, H. L. et Crawford, R. D Morphologie des organes reproducteurs et caractères de carcasse au déclenchement de la ponte chez des dindes Bronzées non sélectionnées et conduites en accouplement naturel. Can. J. Anim. Sci. 78: Nous avons examiné les taux de développement des organes reproducteurs dans la phase de maturation sexuelle chez des dindes Bronzées conduites en accouplement naturel. Les volailles étaient observées une fois par semaine pendant 5 semaines à partir du début de la stimulation photopériodique à l âge de 29,5 semaines, puis encore après la première ponte. Le poids corporel relativement faible de la souche utilisée (6,6 kg) était associé à une très faible teneur en lipides (16,4 %) de la carcasse, au peu de graisse abdominale (107 g) et au poids très bas du muscle pectoral (1,15 kg). À la maturité de ponte, l ovaire contenait 12,2 grands follicules jaunes dont 59 % disposés selon une hiérarchie multiple. On observait en moyenne 1,4 FPO vides, c est-à-dire n aboutissant pas à l oviposition. La présence de ces FPO vides incite à penser que c est peut-être un phénomène naturel chez les dindes et non pas un phénomène limité aux souches à croissance rapide modernes. Les sujets qui commençaient à pondre tôt avaient plus de chance d avoir ovulé plusieurs fois avant leur première oviposition que ceux qui commençaient à pondre plus tard (r = 0,64; P < 0,02). L analyse par régression du développement des organes reproducteurs a permis de constater que l oviducte atteignait son poids normal trois jours plus tôt que l ovaire, soit 28,3 contre 31,2 j (r 2 = 0,81; P < 0,0001). La souche utilisée ressemblait aux souches femelles modernes pour ce qui est des caractères de reproduction. Mots clés: Dinde Bronzée, poids corporel, morphologie de l ovaire, troubles de reproduction Body weight and egg production are negatively correlated in turkeys (Nestor 1971). Selection for growth parameters has caused an increase in the number of large ovarian follicles, which increases the incidence of multiple follicle sets and multiple ovulations (Nestor et al. 1980; Hocking 1992). The fat content of turkeys has increased due to BW selection (Nestor 1982; Emmans 1989). The BW of breeder parent stocks increased by 25% between 1985 and 1989 (Hester and Stevens 1990) and continues to rise. The egg production potential of male-line parent stock hens is now less than half that of their smaller female line counterparts (Hocking 3 Author to whom correspondence should be sent, frobinso@afns.ualberta.ca ). Feed restriction is a standard practice in broiler breeder management and has been shown to be an effective way to improve egg production (Summers and Robinson 1995), but it has not been used with the same success in turkeys (Hester and Stevens 1990). The heavy turkey strains that could benefit most from feed restriction are also most resistant to its effects (Hocking 1992; Renema et al. 1994, 1995). A recent study with male line turkeys (selected almost exclusively for growth characteristics) demonstrated that Abbreviations: BW, body weight; CP, crude protein; LYF, large yellow follicles; ME, metabolizable energy; POF, post-ovulatory follicles; SEM, standard error of the mean

2 182 CANADIAN JOURNAL OF ANIMAL SCIENCE these birds ovulate up to five times prior to the ovulation resulting in their first oviposition (Renema et al. 1995). In female line stocks, in which reproductive traits are more strongly selected for, 1.6 unexplained ovulations occurred prior to the first one resulting in an egg (Melnychuk et al. 1997). The rates of internal ovulation (follicle ovulated into body cavity) were high in these birds, particularly in early production, when excess large yellow follicles (LYF) were present. Melnychuk et al. (1997) suggested that the development of the oviduct and ovary have become unsynchronized with selection for carcass growth rate. If genetic selection programs continue as they are, these problems may continue to worsen. However, details of reproductive development and the effects of current genetic selection programs have not been well characterized in turkeys. It needs to be clarified what normal is before it can be determined if turkey reproduction is threatened or doing well. This experiment utilized a naturally mating Bronze turkey line which had been unselected for 40 yr. Changes in the state of the reproductive morphology and carcass traits were examined throughout sexual maturation, with particular emphasis on the relative rates of development of the oviduct and ovary. MATERIALS AND METHODS Stocks and Management Forty-two female Bronze turkey poults were obtained from a closed flock of naturally mating birds maintained at the University of Saskatchewan (Saskatoon, SK). These rather primitive stocks originated from a rural Saskatchewan seasonal hatchery supply flock. They are thought to be typical of commercial turkeys of 1940 to Upon arrival at the University of Alberta, bird age ranged from 27 to 30 wk of age due to staggered hatch procedures in the source flock. The birds were housed in a light-controlled facility and allowed ad libitum access to both feed and water. Diets were corn-soybean based and formulated according to commercial breeder guidelines. Birds were fed a 2850 kcal kg 1 ME, 12% CP developer diet (Renema et al. 1994) prior to photostimulation and changed to a 2850 kcal kg 1 ME, 14% CP breeder diet (Renema et al. 1994) for the remainder of the experiment. Individual BW data were recorded at weekly intervals. The flock was maintained on short day conditions (8L:16D) until photostimulation at an average age of 29.5 wk (range of 28 to 31 wk of age), when daylength was increased to 14L:10D. Birds were randomly assigned to a study time (Table 1). The mean BW of birds in each study period was similar at the beginning of the experiment. Beginning at photostimulation, five birds were processed each 7 d for a 35-d period (Group A; n = 30 birds total) to characterize the changes in the reproductive tract and carcass organs. At 09:00 h, birds were euthanized by lethal injection with T-61 Euthanasia (Hoechst Canada Inc., Regina, SK, Canada, S4N 6C2) (3 ml dosage). A second group of birds (Group B) (n = 12 birds) was selected to be processed at sexual maturity (first oviposition) to characterize ovarian and carcass traits at this time. Beginning at 30 wk of age, birds were palpated in the morning for the Table 1. Number of turkeys processed in each study period Mean Time post- Group age (d) photostimulation (d) n A A A A A A B 1 d after first 24 to oviposition presence of a hard-shelled egg in the shell gland. If an egg was present, the turkey was placed in a trap nest ( cm) to ensure positive identification of first oviposition. Following oviposition, Group B birds were deprived of feed and water and processed the following morning as described above. The experimental protocol was approved by the Animal Policy and Welfare Committee of the Faculty of Agriculture, Forestry and Home Economics of the University of Alberta. Reproductive Organ Morphology At processing, birds of both Group A and Group B were individually weighed, and the shank length was recorded. The breast muscle, liver, abdominal fat pad, oviduct, and ovary were removed and weighed. The number, weight, and diameter of large yellow ovarian follicles (LYF) (diameter greater than 10 mm), number of POF (the remnant of the site of ovulation), and the weight of the stroma (ovary minus large yellow follicles) were recorded. Unexplained ovulations were defined as ovulations occurring prior to first oviposition and were calculated by subtracting any eggs that were laid or were in the oviduct from the number of POF found at processing. An assessment of the potential for multiple ovulations to occur was determined by assigning LYF of similar size (differing by less than 1 g or 1 mm diameter) to the same position in the hierarchy (Renema et al. 1995). The total number of positions in the LYF hierarchy and the proportion of LYF in a multiple arrangement were determined using the method of Hocking et al. (1992). The incidence of large follicle atresia or internal ovulation (as evidenced by yolk residue in the body cavity) was recorded. Carcass Composition Dissected organs and tissues were returned to the carcass following processing, which was stored at 15 C until carcass composition analysis was performed. The frozen carcasses were cut in half and the halves pressure-cooked for 6 h. Halves were homogenized in a large industrial blender, recombined, and a 100 g homogenate sample taken and freeze dried for 7 d as described by Yu et al. (1990). Corrections were made for moisture loss during homogenization and freeze drying and samples were analyzed for total dry matter, crude protein, lipid, and ash using standard chemical analysis procedures of the Association of Official Analytical Chemists (AOAC 1980).

3 RENEMA ET AL. ONSET OF LAY IN BRONZE TURKEY HENS 183 Table 2. Carcass traits of Bronze turkey females beginning at photostimulation and proceeding throughout sexual maturation (Group A), and at first oviposition (Group B) Body Abdominal Absolute weight (kg) Relative weight (% of BW) weight Breast muscle fatpad Carcass Carcass Carcass Carcass Carcass Carcass Group (kg) (% of BW) (% of BW) protein lipid ash protein lipid ash A a c A ab bc A b c A ab ab A c a A bc ab SEM B SEM a c Means within a column with no common letters differ significantly (P < 0.05). Statistical Analysis Data were evaluated by analyses of variance using the General Linear Models (GM) procedures of the SAS Institute, Inc. (1992). Preliminary analysis of variance using bird age as the main effect indicated that variability in bird starting age had no effect on any of the parameters measured. Sources of experimental variation were age (processing time) and error variation of birds within time period. Differences between means were evaluated with Fisher s protected LSD procedure (Peterson 1985). Pearson correlation coefficients (Steel and Torrie 1980) were computed between reproductive and carcass parameters. Growth curves for the oviduct, ovary, and LYF were compared as actual weights and as percentages of mature weights using Kolmogorov-Smirnoff analysis (SAS Institute, Inc. 1992). Mature organ data were obtained from the Group B birds processed at first oviposition. Regression analysis (SAS Institute, Inc. 1992) was performed on oviduct, ovary, and LYF weight maturation curves between 7 and 35 d after photostimulation. Day 0 values were deleted from this analysis because the rapid phase of development was determined to commence after day 7. Unless otherwise stated, all statements of significance were based on testing at the P < 0.05 level. RESULTS AND DISCUSSION Carcass Traits The turkeys reached sexual maturity 29.8 d after photostimulation on average. This time to mature was slightly longer than the 25.8 d reported for heavily growth selected maleline turkeys (Renema et al. 1994), and the 26.9 and 27.3 d reported for female and male lines, respectively (Melnychuk et al. 1997). In 1968, Bacon and Cherms stated that sexual maturity should occur between 19 and 31 d after photostimulation. A reduced time to sexual maturity could come about through selection for reproductive traits, and a delay in the onset of production may be possible through selection for growth traits (Nestor 1971). Hocking (1992) found a range of 24 to 42 d to sexual maturity between light and heavy strains and suggested the difference to be due to decreased light sensitivity. The birds in the current study came into production in the middle of the range of recently reported values, which supports the existence of reduced days to sexual maturation in modern stocks selected for reproduction and an increased time period for stocks selected for growth. Sexual maturity was reached at a BW of 6.58 kg (Table 2) with no significant changes in BW being seen throughout the study period in Group A birds. Body weight would be expected to increase during this period due to both development of the reproductive tract and to further growth of the bird. This is supported by a significant increase in the weight of carcass ash in Group A birds (Table 2), which suggests that the birds were increasing in frame size. An alternative explanation is that the amount of medullary bone may have been increasing at this time. A 600 g (10%) numerical increase in BW was noted in Group A birds (Table 2) during this period. This lack of statistical significance is contributed to by a small sample size within a variable unselected population. The sexually mature BW of 6.58 kg was about 50% that of values reported for modern female-line birds (Melnychuk et al. 1997), and similar to the 7.4 kg Hocking (1992) reported for the BUT 5 traditional strain at sexual maturity. The breast muscle (pectoralis major and minor), which reflects changes in lean body mass, weighed 1.15 kg on average at sexual maturity. This value is low compared with modern turkey stocks, where breast muscle weights have been reported from 3.06 to 4.67 kg in female line stocks and male line stocks respectively (Melnychuk et al. 1997), which represents 25% and 29% of total BW. The proportion of breast muscle was 17.5% in the Bronze birds. The breast muscle content of modern strains of closer to 30% of BW demonstrates the effects of genetic selection pressure on this trait. Although breast muscle weight correlated strongly with body weight (r = 0.89, P < ), it declined significantly in Group A birds throughout sexual maturation (Table 2). Turkeys lose body mass once egg production commences, and breast muscle has been shown to shrink significantly prior to 40 wk of age in a laying flock (Renema et al. 1994). In the present study, the total carcass protein content was 24.7% of BW and did not change in Group A birds (Table 2).

4 184 CANADIAN JOURNAL OF ANIMAL SCIENCE Table 3. Reproductive traits of Bronze turkey females beginning at photostimulation and proceeding throughout sexual maturation (Group A), and at first oviposition (Group B) Mean LYF Multiple Oviduct Ovary Stroma Number of weight arrangement Group (g) (g) (g) LYF 1 (g/follicle) (% of LYF) 2 A0 2.0d 3.2c 3.2c 0.0b 0.00c 0.0d A7 7.3d 5.1c 5.1c 0.0b 0.00c 0.0d A c 17.7c 8.4bc 2.0b 1.37c 5.0cd A b 78.1b 12.1b 9.8a 6.45b 63.4a A b 119.8a 19.0a 11.0a 9.17a 46.2ab A a 124.8a 20.5a 10.2a 10.22a 29.4bc SEM B SEM LYF = large yellow follicles of > 10 mm diameter. 2 Multiple LYF present at the same positions in the hierarchy (differ by less than 1 g and 1 mm diameter). a d Means within a column with no common letter differ significantly (P < 0.05). The abdominal fat pad weighed g, or 1.6% of BW (Table 2). This lipid depot is the bird s major lipid storage site and is most responsive to changes in energy availability. It can be closely related to total carcass lipids and may thereby indicate changes in carcass lipid deposition. Total carcass lipids represented 16.4% of BW and numerically increased with time in Group A birds (Table 2). This change was due more to ovarian development, as absolute abdominal fat pad weight was unaffected. The lipid content of these birds was considerably lower than that of current turkey stocks. Renema et al. (1994) reported a carcass lipid content of 15% at sexual maturity in a male line heavily selected for feed conversion capabilities, but an abdominal fat pad weight of 269 g, or 1.9% of BW. The four strains examined by Hocking (1992) had a mean carcass lipid content of 24.2%, and the abdominal fat pad of the smallest strain (BUT 5) was about 300% heavier than that of the current experiment. Melnychuk et al. (1997) reported similar values, and stated that female line hens had a higher percentage total carcass fat and a heavier abdominal fat pad (as a percent of BW) than male line hens did. This is supported by Emmans (1989), who found higher carcass lipid levels in heavier BW than in lower BW turkey strains. Selection for increased BW results in an increased abdominal fat pad weight both on an actual and a percentage basis (Nestor 1982). Reproductive Traits At sexual maturity, the average Bronze hen oviduct weighed 83.9 g and the ovary weighed g (Table 3). Between 7 and 14 d after photostimulation both the oviduct and ovary entered a rapid growth phase which culminated in oviposition. Oviduct and ovary weights increased by 283% and 441%, respectively, in the week of 14 to 21 d after photostimulation (Table 3). As large yolky follicles first appeared at 14 d in Group A birds (53% of ovary weight), this rapid growth phase corresponded with the onset of yolk deposition. Bacon and Cherms (1968) reported that rapid yolk deposition commenced between 4 and 11 d after photostimulation. The most rapid period of yolk deposition has been reported to occur between 18 and 21 d after photostimulation in female line birds (Melnychuk et al. 1997). The data of the current study concur with both of these observations. The ovary contained an average of 12.2 LYF at sexual maturity, with 58.6% of the large follicles present in a multiple hierarchy (as evidenced by two follicles within 1 g and 1 mm diameter). The production of multiple follicle sets was more likely to occur with increased follicle numbers (Nestor et al. 1980). Multiple follicle sets are a major cause of decreased egg production through the production of defective eggs (Nestor and Bacon 1972), which have poor hatchability (Stephenson and Krause 1988). Hocking et al. (1992) reported that 24% of the 9.5 LYF in a small BW turkey strain to be in a multiple hierarchy arrangement at sexual maturity. Renema et al. (1995) reported that 95% of 19.1 LYF at sexual maturity were in a multiple hierarchical arrangment in a larhe BW male line hen. The LYF were combined in 8.5 positions on the ovary. This value represents the maximum number of follicles the ovary could potentially maintain without forming multiple follicle sets. Hocking et al. (1992), reported eight positions to be present in 7.5 kg birds having 10 LYF (> 8 mm). As LYF growth is expected to take 11 to 13 d (Bacon et al. 1972; Hocking et al. 1987), the presence of only 8.5 LYF positions suggests that there may be some non-laying days in the birds future. The potential egg production of these birds having 10 to 12 LYF does not appear to be severely compromised. However, in ad libitum fed male line hens with 17.6 LYF, Renema et al. (1995) reported non-laying days to occur 3.8 d per week. Clearly the ovary was not able to accommodate a greatly increased number of LYF without compromising physiological control of egg production. Internal ovulation or follicular atresia are two possible outcomes for follicles in multiple arrangements. Each occurred in 42% of birds at sexual maturity. Internal ovulation has been theorized to be the result of compromised hormonal control over ovulation (Renema et al. 1995), which may lead to a lack of synchronicity of the oviduct with the ovulation process. The incidence is lower in turkey meattype and egg-type strains with lower LYF numbers (23% and 13%, respectively) (Bacon et al. 1972). The ovaries of the Group B birds contained an average of 1.4 unreconciled POF at sexual maturity, indicating that

5 RENEMA ET AL. ONSET OF LAY IN BRONZE TURKEY HENS 185 some early ovulations did not result in ovipositions. A high of 4.9 unreconciled POF has been reported in male line hens (Renema et al. 1995) and 1.6 in female line hens (Melnychuk et al. 1997). The majority of these missing follicles are thought to be lost to internal ovulation (Melnychuk et al. 1997). There was a good correlation between POF and internal ovulation in this experiment (r = 0.67; P < 0.02), which supports this line of reasoning. The fact that unreconciled POF are present at all in this traditional turkey stock suggests that this may be a natural phenomenon in turkeys. The similarity of unexplained POF numbers between these birds and female line birds as opposed to those of male-line birds may indicate that genetic selection for growth parameters has accelerated ovarian relative to oviduct development, or slowed oviductal relative to ovary development, resulting in the loss of initial ovulations due to oviductal incompetence in male line hens. Unreconciled POF were more likely to be present in birds coming into production earlier (r = 0.64; P = 0.02) than in birds that were late in initiating lay. Early onset of lay has been associated with increased numbers of LYF (Hocking 1992; Hocking et al. 1992), which could explain the increase in lost follicles in early maturing birds. In the current experiment, the highest incidence of follicles in a multiple arrangement was seen at 21 d after photostimulation and decreased thereafter (Table 3). Inadequate growth of the oviduct may also be a factor as an increased degree of multiple follicle arrangement was negatively correlated with oviduct weight (r = -0.68; P = 0.01). Oviduct weight continued to increase throughout the experiment. As the birds aged, physiological control of the ovary may also have been improving. The negative correlation between multiple follicle arrangement and large follicle atresia (r = 0.79; P < 0.002) suggests that with age, management of excess LYF is switching to a more controlled process. Renema et al. (1995) observed a similar trend in the ovarian control of male line hens which was found to relate well to body size. Fig. 1. Rate of maturation of oviduct, ovary, and LYF weight for naturally mating Bronze turkeys relative to sexually mature values. Mean age at first oviposition was 29.8 d after photostimulation. To examine the potential for internal ovulation prior to the onset of egg production, maturation rates of the reproductive organs were assessed. The rates of oviductal and ovarian development of Group A birds relative to values for birds processed at sexual maturity (Group B) are presented in Fig. 1. The growth curves of the oviduct and ovary were found to be similar using Kolmogorov-Smirnoff curve comparison analysis. Despite the similarity of the curves, oviduct development was correlated only with ovarian stroma growth (r = 0.57; P = 0.05) and not with total ovary development. Oviduct growth, which is based on cell differentiation and tissue growth, is a very different process than the sequestering of yolk materials associated with LYF growth. It should be pointed out that some of the variability seen was a result of only five different birds being processed each week. Whereas the stroma weight of Group A birds reached the sexually mature weight prior to 28 d after photostimulation, ovary weight never reached the weight achieved by the 12 hens in Group B (Table 3, Fig. 1). Average LYF weight was calculated to compensate for the effects of variability in LYF numbers on ovary weight (Table 3). Oviduct and LYF weight maturation curves both reached the 100% mature size between 28 and 30 d after photostimulation (Fig. 1), which corresponds well with the average 29.8 d to sexual maturity. Regression analysis of oviduct, ovary, and LYF weight maturation curves is presented in Fig. 2 (r 2 = 0.81; P < ). The oviduct and LYF weight regression lines intersected 100% at 28.3 and 31.2 d after photostimulation, respectively, and the ovary line intersected it at 34.6 d. As the oviduct appears to reach mature size before the ovary, substantial loss of initial ovulations due to a lack of maturity would not be expected. Melnychuk et al. (1997) found that female line hens had a 3 d delay in ovary development, whereas male line ovary development was accelerated, possibly leading to the increased reported unreconciled POF. Lilburn and Nestor (1993) found that oviduct weight of egg-

6 186 CANADIAN JOURNAL OF ANIMAL SCIENCE line hens did not change substantially after the onset of lay whereas meat-line oviducts continue to grow through 49 d of production. The 3 d delay in development of the ovary relative to the oviduct in the current study suggests that there should be successful capture and oviposition of follicles by the oviduct. This experiment was performed to characterize the reproductive development of an unselected line of Bronze turkeys. The low BW of this strain was associated with a very low lipid content and breast muscle weight. The ovary, however had an average number of LYF with a fairly high degree of multiple follicle arrangement. The presence of unreconciled POF suggests that this may be a natural phenomenon in turkeys. This strain behaves much like modern female line strains with regard to reproductive traits, especially the 3 d delay in ovary development. The traditional Bronze turkey is a good model for comparison with current turkey stocks, particularly when genetic change brings about unexpected changes in growth or reproductive parameters. ACKNOWLEDGMENTS The donation of turkeys for this project from the University of Saskatchewan is gratefully acknowledged. This project was supported in part by the University of Alberta Poultry Research Centre, through funding provided by Alberta Agriculture, Food and Rural Development (AAFRD) and the four Alberta poultry marketing boards. The authors gratefully acknowledge the assistance provided by F. Dennis and B. Tchir in this project. Association of Official Analytical Chemists Official methods of analytical chemists. 13th ed. AOAC, Arlington, VA. Bacon, W. L. and Cherms, F. L Ovarian follicular growth and maturation in the domestic turkey. Poult. Sci. 47: Bacon, W. L., Nestor, K. and Renner, P Further studies on ovarian follicular development in egg and meat type turkeys. Poult. Fig. 2. Regression analysis of oviduct, ovary, and LYF weight for naturally mating Bronze turkeys relative to sexually mature values. The mature weight threshold was reached at 28.3, 34.6, and 31.2 d for the oviduct, ovary, and LYF weight, respectively. Sci. 51: Emmans, G. C The growth of turkeys. Pages in C. Nixey and T. C. Grey, eds. Recent advances in turkey science. Butterworths, London, UK. Hester, P. Y. and Stevens, R. C. W Feed restriction of turkey breeder hens - a review. Poult. Sci. 69: Hocking, P. M Genetic and environmental control of ovarian function in turkeys at sexual maturity. Br. Poult. Sci. 33: Hocking, P. M., Walker, M. A., Waddington, D. and Gilbert, A. B Observations on the size of the follicular hierarchy in the turkey hen and a case of arrested follicular growth. Br. Poult. Sci. 28: Hocking, P. M., Waddington, D. and Walker, M. A Changes in ovarian function of female turkeys photostimulated at 18, 24 or 30 weeks of age and fed ad libitum or restricted until point of lay. Br. Poult. Sci. 33: Lilburn, M. S. and Nestor, K. E The relationship between various indices of carcass growth and development and reproduction in turkey hens. Poult. Sci. 72: Melnychuk, V. L., Robinson, F. E., Renema, R. T., Hardin, R. T., Emmerson, D. A. and Bagley, L. G Carcass traits and reproductive development at the onset of lay in two lines of female turkeys. Poult. Sci. 76: Nestor, K. E Genetics of growth and reproduction in the turkey 3. Further selection for increased egg production. Poult. Sci. 50: Nestor, K. E The influence of genetic increases in body weight on the abdominal fat pad of turkeys. Poult. Sci. 61: Nestor, K. E. and Bacon, W Production of defective eggs by egg and meat type turkey hens. Poult. Sci. 51: Nestor, K. E., Bacon, W. C. and Renner, P. A The influence of genetic changes in total egg production, clutch length, broodiness and body weight on ovarian follicular development in turkeys. Poult. Sci. 59: Peterson, R. G Design and analysis of experiments. Marcel Dekker, Inc., New York, NY. Renema, R. A., Robinson, F. E., Melnychuk, V. L., Hardin, R.T., Bagley, L. G., Emmerson, D. A. and Blackman, J. R.

7 RENEMA ET AL. ONSET OF LAY IN BRONZE TURKEY HENS The use of feed restriction for improving reproductive traits in male-line Large White turkey hens. 1. Growth and carcass characteristics. Poult. Sci. 73: Renema, R. A., Robinson, F. E., Melnychuk, V. L., Hardin, R.T., Bagley, L. G., Emmerson, D. A. and Blackman, J. R The use of feed restriction for improving reproductive traits in male-line Large White turkey hens. 2. Ovary morphology and laying traits. Poult. Sci. 74: SAS Institute, Inc The SAS System for Windows 3.10 Release SAS Institute Inc., Cary, NC. Steel, R. D. G. and Torrie, J. H Principles and procedures of statistics. 2nd ed. McGraw-Hill Book Co., Inc., New York, NY. Stephenson, A. B. and Krause, G. F Compressed-sided turkey eggs. Poult. Sci. 67: Summers, J. D. and Robinson, F. E Comparative feeding program for poultry reproduction. Pages in P. Hunton, ed. World animal science, C9: Poultry production. Elselvier, Amsterdam, The Netherlands. Yu, M. W., Robinson, F. E., Clandinin, M. T. and Bodnar, L Growth and body composition of broiler chickens in response to different regimens of feed restriction. Poult. Sci. 69: