Effects of altering growth curve and age at photostimulation in female broiler breeders. 2. Egg production parameters

Effects of altering growth curve and age at photostimulation in female broiler breeders. 2. Egg production parameters R. A. Renema 1, F. E. Robinson 1,3, P. R. Goerzen 1, and M. J. Zuidhof 2 1 Department of Agricultural, Food, and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada T5G 2P5; and 2 Alberta Agriculture, Food and Rural Development, 7000-113 St., Edmonton, Alberta, Canada T6H 5T6. Received 13 November 2000, accepted 3 July 2001. Renema, R. A., Robinson, F. E., Goerzen, P. R. and Zuidhof, M. J. 2001. Effects of altering growth curve and age at photostimulation in female broiler breeders. 2. Egg production parameters. Can. J. Anim. Sci. 81: 477 486. An experiment was conducted to evaluate the effect of differences in growth curve and age at photostimulation on egg production parameters and carcass traits at 61 wk of age in broiler breeder hens. Pullets were grown on one of three growth curves: STD (standard), LOW (150 g lighter than STD) and HIGH (150 g heavier than STD), and photostimulated at either 19 wk of age (19WK) or 21 wk of age (21WK). The egg production and BW of 36 birds per interaction were individually monitored from photostimulation to 61 wk of age. Individual, daily egg production records were analyzed for total, settable, and defective egg production, rate of production, sequence length, and egg weight. Eggs were incubated for determination of fertility, hatchability, and embryonic mortality parameters. All birds remaining at 61 wk of age were processed for determination of carcass and reproductive morphology. Unless otherwise stated, all significance was assessed at the P < 0.05 level. Earlier photostimulation age advanced age at sexual maturity, but reduced the first egg weight. Weight of the first egg did not differ among growth curves. Settable egg production to 61 wk was not affected by growth curve, with settable egg production ranging from 157.5 eggs in STD birds to 164.1 eggs in HIGH birds. The HIGH birds produced more double-yolked eggs than LOW birds. Photostimulation age did not affect egg production. Eggs of the LOW birds underwent more embryonic loss during 1 to 7 d of incubation than either STD or HIGH eggs. At 61 wk of age, the shank length of birds on the LOW profile was less than that of the STD birds or the HIGH birds. Abdominal fatpad as a percentage of BW differed among growth curve treatments, comprising 4.46% (LOW), 5.06% (STD), and 5.69% (HIGH) of hen BW. Ovary weight of HIGH birds (44.7 g) was higher than that of LOW (39.2 g) or STD birds (37.7 g), but contained a similar number of large yellow follicles. The proportion of large yellow follicles in multiple follicle sets at 61 wk was correlated with unsettable egg number (r = 0.265), and was negatively associated with chick number (r = 0.269). There was no production advantage to early photostimulation, and egg and chick production were not negatively affected by a moderate raising or lowering of the growth curve. Key words: Broiler breeder, growth curve, egg production, sequence length, body weight Renema, R. A., Robinson, F. E., Goerzen, P. R. et Zuidhof, M. J. 2001. Modification de la courbe de croissance et de l âge à la photostimulation et leur incidence sur les pondeuses de poulets de chair : 2. Paramètres de ponte. Can. J. Anim. Sci. 81: 477 486. Les auteurs ont effectué une expérience pour déterminer quels effets une modification de la courbe de croissance et de l âge à la photostimulation aurait sur les paramètres de ponte et la conformation de la carcasse des pondeuses d œufs d incubation âgées de 61 semaines. Les poulettes ont été réparties en trois groupes de courbe de croissance différente soit STD (standard), FAIBLE (150 g de moins que STD) et ÉLEVÉE (150 g de plus que STD), puis photostimulées à l âge de 19 (19 S) ou de 21 semaines (21 s). La production d œufs et le poids corporel de 36 oiseaux par interaction ont été surveillés individuellement de la photostimulation à l âge de 61 semaines. Ensuite, on a analysé la production quotidienne de chaque sujet afin d établir la quantité totale d œufs, d œufs d incubation et d œufs défectueux de chaque pondeuse, le taux de ponte, la durée de la séquence et le poids des œufs. Ensuite, on a incubé les œufs pour établir leur fertilité, l éclosabilité et les paramètres de la mortalité embryonnaire. Les pondeuses âgées de 61 semaines ont été abattues, ce qui a permis de déterminer la conformation de la carcasse et la morphologie de l appareil reproducteur. Sauf indication contraire, le seuil de signification était de P < 0,05. Une photostimulation précoce permet à l oiseau de parvenir plus tôt à maturité sexuelle, mais le poids du premier œuf est plus faible, bien qu il ne varie pas avec la courbe de croissance. Le nombre d œufs d incubation produits à 61 semaines n est pas non plus affecté par la courbe de croissance et varie de 157,5 œufs pour le groupe STD à 164,1 œufs pour le groupe ÉLEVÉE. Les poules de ce dernier groupe ont donné plus d œufs à deux jaunes que les oiseaux du groupe FAIBLE. L âge à la photostimulation n a aucune incidence sur la ponte. Les œufs des poules du groupe FAIBLE enregistrent plus de pertes lors des 7 premiers jours d incubation que ceux des poules STD ou ÉLEVÉE. À 61 semaines, le métatarse des oiseaux du groupe FAIBLE était plus petit que celui des pondeuses STD ou ÉLEVÉE. Le poids du coussin de graisse abdominal varie avec la courbe de croissance et représente 4,46 % (FAIBLE), 5.06 % (STD) et 5,69 % (ÉLEVÉE) du poids corporel de l oiseau. Les poules du groupe ÉLEVÉE avaient des ovaires plus lourds (44,7 g) que celles du groupe FAIBLE (39,2 g) ou STD (37,7 g), mais on y a dénombré autant de 3 To whom correspondence should be addressed (e-mail: frank.robinson@ualberta.ca). 477 Abbreviations: CP, crude protein; HIGH, high BW; LOW, low BW; STD, standard BW.

478 CANADIAN JOURNAL OF ANIMAL SCIENCE gros follicules jaunes. La proportion de gros follicules jaunes dans les jeux de follicules à 61 semaines est corrélée avec le nombre d œufs défectueux (r = 0,265) et présente une association négative avec le nombre de poussins (r = 0,269). Une photostimulation précoce n apporte aucun avantage sur le plan de la production, mais une légère hausse ou baisse de la courbe de croissance ne nuit pas à la production d œufs et de poussins. Mots clés: Pondeuses d œufs d incubation, courbe de croissance, ponte, longueur de la séquence, poids corporel The modern broiler breeder female is continually changing in response to selection pressure for desirable reproductive and growth traits. Research with modern broiler breeder females is needed in order for management programs to keep pace with the changing parent female. In recent years, later photostimulatory ages have been used in an attempt to improve the uniformity of sexual maturation and to improve reproductive performance. Yuan et al. (1994) found that when broiler breeder pullets were allowed to attain greater than recommended BW targets while being photostimulated early, total egg production was not improved. Walsh and Brake (1999) altered the pullet growth curve to a convex or concave shape and reported reduced fertility in the convex program, presumably due to reduced rate of feed intake increases late in rearing. Robinson et al. (1996) compared photostimulation ages ranging from 120 to 160 d of age and demonstrated a negative effect of earlier age at lighting on chick production. Fattori et al. (1991a) reported that broiler breeders on BW targets above standard recommended BW targets produced more double-yolked eggs, while those with lower targets took longer to reach sexual maturity. Wilson et al. (1995) reported that broiler breeders reared on an early slow growth curve laid fewer eggs through to 62 wk of age than birds reared on a linear or on an early fast growth curve. The primary objective of this experiment was to examine the long-term effects of rearing broiler breeder pullets on different growth curves. The effects of this, in combination with different photostimulation ages, was examined on egg production, laying patterns, egg traits and fertility parameters, and on carcass and ovarian morphology at 61 wk of age. Unlike previous studies, the different growth curves would be maintained beyond photostimulation and throughout production to examine both the long-term effects of growth curve and photostimulation age on egg production parameters. In addition, the short-term effects of growth curve and photostimulation have been examined in a companion paper (Renema et al. 2001). The production and incubation data provided in this paper will provide information supporting management decisions on photostimulation age and growth curve. MATERIALS AND METHODS Stocks and Management Detailed descriptions of the experimental design, stocks, rearing feeding regimens, rations, housing facilities, and photostimulatory programs were described in a companion paper (Renema et al. 2001). Briefly, 450 broiler breeder pullets were grown on three different growth curves (150 birds each): standard breeder curve (STD), lower BW target (LOW) and higher BW target (HIGH). The LOW and HIGH treatments were 150 g below and above the STD treatment at 20 wk of age, respectively. Feed allocations for the breeding period (from 30 wk of age) are shown in Table 1. Feed allocations prior to this are presented elsewhere (Renema et al. 2001). The breeder diet consisted of 15.5% CP and 2750 kcal kg 1 ME. At 18 wk of age the 116 birds within each growth curve that were closest to their respective targets were individually caged and assigned a photostimulation age of 19 wk of age (19WK) or 21 wk of age (21WK). The photostimulation program is described elsewhere (Renema et al. 2001). By 25 wk of age, all birds were exposed to 15L:9D days, which continued for the duration of the study. There were 58 birds in each of the six treatment combinations (LOW19, STD19, HIGH19, LOW21, STD21 and HIGH21), which were randomly assigned to a processing group. Group A birds (12 per treatment interaction) were processed at photostimulation and Group B birds (10 per treatment interaction) were processed at sexual maturity (first oviposition). Data for Group A and B birds are presented elsewhere (Renema et al. 2001). The current study examines the 36 birds per growth curve photostimulation age interaction assigned to Group C for individual assessment of egg production parameters until 61 wk of age. Individual BW were recorded on a weekly basis. 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. Laying Records The day of sexual maturity (first oviposition), weight of the first egg, and BW at first egg were recorded for each bird. Individual oviposition records were maintained, with eggs categorized by type (normal, soft-shelled, shelless, double yolk, broken, abnormal shell, or pecked). The total number of eggs and the number of normal and settable (above 52 g) eggs were calculated for each bird. The proportion of defective (unsettable) eggs was also calculated. All eggs from a hen were weighed on the day of oviposition until two consecutive eggs of 52 g or more were laid, after which one settable egg per week was weighed. Hen age at the time of the second consecutive 52-g egg was recorded and designated as the settable egg age. Hen-day egg production was compared using weekly production means generated from daily production values. Sequence length, inter-sequence pause length, and sequence profiles were determined from oviposition records. A sequence was defined as the number of days on which an egg was laid before a non-laying or pause day. Sequence lengths were calculated for the entire laying period and on a weekly basis (Robinson et al. 1998). Mean sequence length was the average of all sequences for the

RENEMA ET AL. REARING EFFECTS ON BREEDER EGG PRODUCTION 479 Table 1. Feed allowances (g bird 1 d 1 ) from 30 to 61 wk of age z for pullets reared on a LOW, STD, or HIGH growth curve Growth curve LOW STD HIGH (g bird 1 d 1 ) Age (wk) 30 to 31 154 158 167 31 to 32 154 158 167 32 to 33 152 156 165 33 to 34 152 154 165 34 to 35 150 152 163 35 to 36 148 150 161 36 to 37 146 148 159 37 to 38 144 146 157 38 to 39 143 145 156 39 to 40 141 143 154 40 to 41 139 141 152 41 to 42 137 139 150 42 to 43 136 138 149 43 to 44 136 138 149 44 to 45 134 136 147 45 to 46 132 134 145 46 to 47 131 133 142 47 to 48 131 133 142 48 to 49 131 133 142 49 to 50 133 135 144 50 to 51 133 135 144 51 to 52 133 135 144 52 to 53 132 134 143 53 to 54 132 134 143 54 to 55 133 135 144 55 to 56 133 135 144 56 to 57 131 133 142 57 to 58 130 132 141 58 to 59 128 130 139 59 to 60 127 129 138 60 to 61 127 129 138 z Feed allowances prior to 30 wk of age presented elsewhere (Renema et al., 2001). entire laying period. Weekly sequence lengths were calculated by assigning every day a sequence length value based on the length of sequence a birds was in that day, and averaging the daily values for each week. Pauses in egg production were averaged into the average weekly sequence length as 0s. The prime sequence was defined as the longest egg sequence around the time of peak production (Robinson et al. 1990). Fertility, Hatchability, and Embryo Viability Hens were artificially inseminated with 0.05 ml pooled undiluted semen on the day they laid their first egg and subsequently inseminated on a weekly basis. The 36 birds within each treatment were assigned to one of two egg lots. Eggs were collected daily, sorted by lot, and stored at 16 C. Starting at 27 wk of age, all eggs were shipped to a commercial hatchery 2 on a weekly basis. Unhatched eggs from each group were broken open and scored macroscopically as: infertile, embryonic death occurring during Stage 1 (0 to 7 d incubation), Stage 2 (8 to 14 d incubation), or Stage 3 (later than 14 d incubation) of development, dead in shell 2 Lilydale Co-operative Ltd., Edmonton, Alberta, Canada T5C 1R9. (pipped but dead), or cull chick. Percent fertility, hatchability, and embryo viability were determined on an individual hen basis. Fertility was calculated as the proportion of fertile eggs of the eggs set. Hatchability was defined as the proportion of viable chicks obtained from the eggs set. Hatchability of fertile eggs set was defined as the number of viable chicks hatched divided by the number of fertile eggs set. Total chick number was calculated using the mean hatchability for each hen multiplied by its settable egg number. Bird Processing at 61 wk of Age Birds were maintained until 61 wk of age to approximate the egg production period. At 61 wk of age, the 201 birds remaining were killed by cervical dislocation, and weighed. Breast muscle, abdominal fatpad, oviduct and ovary weights were determined. Ovarian small yellow follicles (5 to 10 mm diameter) and large yellow follicles (>10 mm diameter) numbers were determined. The number of large yellow follicles in a multiple hierarchical arrangement was calculated as in the Group B birds (Renema et al. 2001). The F1 (largest large yellow follicles) weight, if present, was subtracted from the ovary weight and multiple large yellow follicles set calculations to standardize ovarian morphology calculations. The body cavity was examined for the incidence of internal ovulation or internal ovulation, and the ovary examined for the combined incidence of atresia of small yellow follicles and large yellow follicles. Statistical Analysis Data were subject to a two-way analysis of variance using the General Linear Models (GLM) of SAS (SAS Institute, Inc. 1996). Sources of variation were growth curve, photostimulation age, and the interaction of growth curve photostimulation age. When significant differences were determined for growth curve, photostimulation age, or the interaction of growth curve and photostimulation age, comparison of means was performed using the least significant difference procedure. The error variation consisted of the variation among birds within growth curve photostimulation age. Egg production and sequence length profiles were compared using Kolmogorov-Smirnoff 2-sample test (SAS Institute, Inc. 1996). Pearson correlation coefficients (Steel and Torrie 1980) of the interrelationships among individual traits were computed across all treatments. Correlations between prime sequence length and week 40, week 44, week 48, week 52, week 56 and week 60 mean sequence length were performed to determine the relationship of early egg production rate with persistency of lay. Unless otherwise stated, all significance was assessed at the P < 0.05 level. RESULTS AND DISCUSSION Body Weight and Sexual Maturity The BW profiles for the LOW, STD, and HIGH growth curves between 18 and 61 wk of age are presented in Fig. 1. Birds of the LOW and HIGH groups were 150 g lighter or

480 CANADIAN JOURNAL OF ANIMAL SCIENCE Fig. 1. Body weights from 18 to 61 wk of age for pullets reared on a LOW, STD, or HIGH growth curve. Birds were photostimulated at 19 or 21 wk of age. Table 2. Time of sexual maturity and settable egg production for broiler breeders reared on a LOW, STD, or HIGH growth curve and photostimulated at 19 or 21 wk of age Age at Days from First egg Settable Days from SM SM PS to SM z BW at SM weight egg age y to settable egg Source (d) (d) (g) (g) (d) (d) Growth curve x LOW 179.0a 39.0a 2444b 45.0 191.6a 12.6 STD 178.5a 38.5a 2652a 45.6 190.2a 11.5 HIGH 173.4b 33.4b 2747a 44.3 187.2b 13.8 SEM 1.1 1.1 41 0.6 1.1 1.2 Photostimulation w 19WK 174.5b 41.7a 2547b 43.9b 189.2 14.7a 21WK 179.4a 32.2b 2681a 46.0a 190.0 10.6b SEM 0.9 0.9 34 0.5 0.9 1.0 z Days from photostimulation to sexual maturity (first oviposition). y Settable egg age = age at second consecutive egg weighing over 52 g. x STD = target BW birds; LOW and HIGH BW targets = 150 g lighter or heavier, respectively, at 20 wk of age. w Photostimulation age of 19 (19WK) or 21 (21WK) wk of age. a, b Means within a column and within a source with no common letter differ significantly (P < 0.05). heavier on average, respectively, than the STD birds by 20 wk of age. To maintain these differences for the duration of the study, feed allocation to HIGH birds was consistently 9 g greater than that of STD birds from 30 wk of age and, by 33 wk of age, feed allocation to LOW birds was 2 g below that of STD birds (Table 1). The BW of 19WK birds was similar to that of 21WK birds, except at 22, 26, 27 and 28 wk of age, when 19WK birds were significantly lighter than 21WK birds (data not shown). Feed allocation to the 19WK birds during this period of rapid ovarian development and early egg production may have been insufficient to maintain similar BW gains to the 21WK birds due to the increased nutrient demands by processes relating to sexual maturation. The effects of both growth curve and photostimulation age on the timing of sexual maturity and BW at sexual maturity (Table 2) were similar to those reported in the companion paper for birds processed at sexual maturity (Renema et al. 2001). Body weight at sexual maturity of the HIGH and the STD birds were greater than the BW of the LOW birds. Age at sexual maturity, days from photostimulation to sexual maturity, and settable egg age data indicate that HIGH birds were able to respond to the photostimulatory cue more quickly than the LOW or STD birds (Table 2). Previous studies have shown that increasing BW during rearing advances age at sexual maturity in broiler breeder hens (Robbins et al. 1986, Yu et al. 1992, Yuan et al. 1994). However, the differences in BW targets between rearing growth curves used in the current study were smaller than that of the previous studies. Whereas the HIGH birds entered lay 5.6 d sooner than LOW birds, the effects of growth curve were already diminished by settable egg age,

RENEMA ET AL. REARING EFFECTS ON BREEDER EGG PRODUCTION 481 Table 3. Egg production parameters (sexual maturity to 61 wk of age) of broiler breeders reared on a LOW, STD, or HIGH growth curve and photostimulated at 19 or 21 wk of age Egg production traits Sequence analysis Total Settable Normal Defective Prime Mean sequence eggs eggs eggs eggs z sequence length y Source (#) (#) (#) (%of total) (d) (d) Growth curve x LOW 180.4a 160.4 171.0 5.21 14.7 3.06 STD 171.4b 157.5 166.3 2.97 16.8 2.89 HIGH 182.3a 164.1 174.9 4.06 16.4 2.97 SEM 2.6 2.7 2.8 0.73 1.4 0.09 Photostimulation w 19WK 177.8 158.9 170.8 3.93 15.09 2.85 21WK 178.3 162.4 170.6 4.32 16.79 3.09 SEM 2.1 2.2 2.3 0.60 1.11 0.09 z Defective eggs = soft shell, shell-less, double yolked, and abnormal shell eggs. y Mean length calculated as mean of all sequences occurring in each bird. x STD = target BW birds; LOW and HIGH BW targets = 150 g lighter or heavier, respectively, at 20 wk of age. w Photostimulation age of 19 (19WK) or 21 (21WK) wk of age. a, b Means within a column and within a source with no common letter differ significantly (P < 0.05). which differed by only 3.4 d between HIGH and LOW birds (Table 2). The earlier onset of egg production in HIGH growth curve birds could still be advantageous if egg production and feed consumption results are also favorable. Use of the 19WK photostimulation age resulted in a lower BW and age at sexual maturity (Table 2), which concurred with previous work (Yuan et al. 1994). The 19WK birds were delayed in responding to photostimulation, as indicated by their increased time from photostimulation to sexual maturity (19WK = 41.7 d, 21WK = 32.2 d). Whereas the 2.1 g greater initial egg weight of 21WK compared to 19WK birds may be partly due to the 4.9 d earlier age at sexual maturity and lower BW of 19WK birds, Renema et al. (2001) suggest that the 11.5% lighter liver weight of 19WK birds at sexual maturity may limit the bird s ability to generate yolk precursors, and ultimately affect egg size. In contrast, Robinson et al. (1996) reported no difference in the weight of the first egg between a wide range in photostimulation ages. Both the 19WK and 21WK groups began producing settable eggs (52 g) at the same age (mean = 189.6 d), indicating no long-term benefit of an early photostimulation program on egg size. Earlier photostimulation age not only advanced age at sexual maturity, but also reduced the weight of the first egg and delayed the attainment of a settable egg after sexual maturity. The increased rate of sexual maturation and more rapid increase in the plasma estradiol-17β profile following photostimulation (Renema et al. 2001) both indicate that the 21WK birds responded more uniformly to photostimulation. Whereas both growth curve and photostimulation age affected sexual maturation parameters, there were no significant interactions between these effects. Based on carcass and reproductive morphology data collected at photostimulation and sexual maturity, Renema et al. (2001) reported no advantage to a particular growth curve being used with a specific photostimulation age. They indicated that this allows the possibility of using the long-term alteration of the rearing growth curve as a tool for reaching specific BW or conditioning goals by a predetermined photostimulation age, thereby allowing for more precise flock management. Egg Production Despite similar initial egg weights among the three growth curves, by the 27- to 30-wk egg weight summary period, HIGH eggs (56.4 g) weighed 1.1 g more, on average, than LOW eggs. Egg weights in these treatments differed for the remainder of the trial, to a maximum of 1.9 g during the 55 to 58 wk of age period (data not shown). The 300 g higher BW profile of HIGH compared to LOW birds (Fig. 1) was enough to significantly increase egg weight. Egg components were not examined to determine if the increased size was due to changes in yolk and/or albumen content, however. Fattori et al. (1991a) found no significant difference in weekly egg weight measurement among BW targets of 8% below standard BW target, standard BW target, and 8% above standard BW target, although they appeared to have a higher error variation. As with Robinson et al. (1996) and Yuan et al. (1994), there was no effect of photostimulation age on mean egg weight after sexual maturity. In both photostimulation age groups, egg weight increased at a similar rate with age and hen BW (data not shown). Photostimulation age did not affect total egg numbers to 61 wk of age, averaging 177.8 in 19WK birds compared to 178.3 in 21WK birds (Table 3). Whereas normal egg production was identical between photostimulation groups, settable egg production was reduced by four eggs in the 19WK group due to production of small (< 52 g) eggs, although this difference did not reach statistical significance. Production of eggs with shell defects or double yolked eggs was not affected by photostimulation age (Table 4). Yuan et al. (1994) noted that despite a longer time in lay, total eggs produced to 64 wk were the same between birds photostimulated early (14 and 17 wk of age) or later (20 wk of age). Robinson et al. (1996) indicated that delayed photostimulation resulted in a quicker sexual maturation and, ultimately, a similar total egg production. Birds photostimulated later

482 CANADIAN JOURNAL OF ANIMAL SCIENCE Table 4. Defective egg production (sexual maturity to 61 wk of age) of broiler breeders reared on a LOW, STD, or HIGH growth curve and photostimulated at 19 or 21 wk of age Soft shell eggs Shell-less eggs Double yolked eggs Abnormal shell eggs Number Percentage z Number Percentage z Number Percentage z Number Percentage z Source (#) (%) (#) (%) (#) (%) (#) (%) Growth curve y LOW 6.66a 3.58a 0.71 0.40 0.10b 0.05b 1.98 1.11 STD 3.12b 1.85b 0.75 0.44 0.27ab 0.15a 1.02 0.62 HIGH 4.06ab 2.22ab 0.74 0.41 0.46a 0.25a 2.18 1.21 SEM 0.96 0.50 0.18 0.10 0.07 0.04 0.41 0.22 Photostimulation x 19WK 4.27 2.36 0.80 0.46 0.32 0.17 1.62 0.92 21WK 4.96 2.74 0.67 0.37 0.23 0.12 1.83 1.03 SEM 0.78 0.42 0.14 0.09 0.06 0.03 0.31 0.17 z Percentage of total eggs laid. y STD = target BW birds; LOW and HIGH BW targets = 150 g lighter or heavier, respectively, at 20 wk of age. x Photostimulation age of 19 (19WK) or 21 (21WK) wk of age. a, b Means within a column and within a source with no common letter differ significantly (P < 0.05). generally compensate for a later age at sexual maturity with an increased rate of lay. The current study also demonstrates that delaying the photostimulation of broiler breeders by 2 wk does not reduce egg numbers. Settable egg production was statistically similar among the growth curves, ranging from 157.5 eggs in STD birds to 164.1 eggs in HIGH birds (Table 3). Whereas defective egg production (calculated as a percentage of total egg production) did not differ (Table 3), LOW birds produced both double the number (6.66) and double the proportion (3.58%) of soft-shelled eggs compared to STD birds (3.12 and 1.85%, respectively) (Table 4). These eggs account for much of the significant difference in total egg production between STD hens (171.4) and LOW (180.4) hens (Table 3). Whereas total egg production of HIGH hens (182.3) was also greater than that of STD hens, the difference was not based on defective egg production (Table 4), as settable egg production was similar (Table 3). The incidence of double-yolked eggs increased with increased BW targets (0.10, 0.27, and 0.25% of eggs produced to 61 wk of age in LOW, STD, and HIGH birds, respectively) (Table 4). Fattori et al. (1991a) found that broiler breeder hens grown on lower BW targets laid fewer double yolked eggs (8% below standard BW = 1.23%) than birds grown on higher BW targets (8% above standard BW = 0.7%). In early production (sexual maturity to 33 wk of age), when double-yolked eggs are more likely to occur, the HIGH birds of the current study had a higher incidence of double-yolked eggs (0.77%) than the STD birds (0.36%), and the LOW birds (0.17%) (data not shown). Doubleyolked egg production to 61 wk of age was also affected (Table 4). The higher degree of multiple follicle pairing of ovarian large yellow follicles on the ovary of HIGH birds compared to STD or LOW birds at sexual maturity (Renema et al. 2001) likely relates to the higher double-yolked egg production of this group. A higher percentage of paired large yellow follicles increases the potential for multiple ovulations in a day. However, double-yolked egg production in this trial was low. A higher incidence of eggs with shell defects and double-yolked eggs suggests that a state of erratic oviposition had been initiated, as has been characterized in the ad libitum fed broiler breeder (Yu et al. 1992). The current study indicates that all growth curves resulted in similar egg production. Clearly, the use of the HIGH rearing and breeder BW profile did not adversely affect egg production parameters (Tables 3 and 4). The breeder strain tested appeared able to tolerate the 19 or 21 wk photostimulation age, as well as the variation in target BW profile. It is not clear at what BW or feeding level the settable egg production of this strain may be adversely affected. With similar rates of egg production, differences in overall efficiency may relate to feed intake. Fattori et al. (1991b) concluded that BW restriction below recommended levels was more economical than standard feeding practices. In the current study, the cumulative feed intakes for the rearing and breeder periods were calculated from the feed allocation records from Renema et al. (2001) and Table 1. Feed for the breeder period accounted for 84.1% of total feed allocated, and totaled 37.5, 38.4, and 41.1 kg per hen for the LOW, STD, and HIGH treatments, respectively. Breeder period feed per settable egg was calculated as 234, 244, and 250 g/egg for LOW, STD, and HIGH birds, respectively. Feed savings appear to be possible with the utilization of a lower target BW profile. However, an adequate feeding level may be more critical in some of the more recent high breast yield broiler breeder strains. Due to potentially increased maintenance requirements of a greater muscle mass, a higher minimum plane of nutrition may be necessary for the long-term maintenance of egg production. Sequence Length There was no significant effect of growth curve on the average sequence length (mean = 2.97 d) or the prime sequence length (mean = 15.9) (Table 3). However, the difference in mean sequence length of the 19WK (2.85 d) and 21WK (3.09 d) hens approached significance (P = 0.054) (Table 3). In combination with this numerically reduced sequence

RENEMA ET AL. REARING EFFECTS ON BREEDER EGG PRODUCTION 483 Table 5. Fertility, hatchability, embryonic mortality traits, and chick production (sexual maturity to 61 wk of age) of broiler breeders reared on a LOW, STD, or HIGH growth curve and photostimulated at 19 or 21 wk of age Hatchability Hatchability Embryonic mortality z Dead in Chick Fertility z of fertile eggs of all eggs z 1 7 d 8 14 d 15 21 d shell z Production Source (%) (%) (%) (%) (%) (%) (%) (#) Growth curve y LOW 90.4 87.0c 79.1b 4.54a 3.21a 3.01b 0.15b 127.7 STD 91.0 90.5a 82.7a 2.77c 2.56b 2.21a 0.22ab 130.7 HIGH 90.5 88.3b 80.2b 3.48b 3.03ab 2.85b 0.31a 132.1 SEM 0.4 0.4 0.5 0.22 0.19 0.19 0.05 3.4 Photostimulation x 19WK 90.8 88.7 81.0 3.53 3.07 2.48 0.18 129.3 21WK 90.4 88.4 80.3 3.66 2.80 2.90 0.28 131.0 SEM 0.3 0.4 0.4 0.17 0.16 0.16 0.04 2.8 z Percentage calculated from settable eggs incubated. y STD = target BW birds; LOW and HIGH BW targets = 150 g lighter or heavier, respectively, at 20 wk of age. x Photostimulation age of 19 (19WK) or 21 (21WK) wk of age. a c Means within a column and within a source with no common letter differ significantly (P < 0.05). length, the 19WK birds generated significantly more egg laying sequences (63.1) than the 21WK birds (59.8) (data not shown). An increased number of first of sequence eggs can be detrimental to hatchability due to a higher incidence of embryonic death (Robinson et al. 1991; Fasenko et al. 1992). In the current study, the advantage to delaying age at photostimulation appears to be in the efficiency of egg production and reduction in number of first of sequence eggs. The relationship between prime sequence length and persistency of lay was examined. The correlation between prime sequence length and week 40, week 44, week 48, week 52, week 56 and week 60 average sequence lengths were significant and positive, ranging between r = 0.264 and r = 0.383 (P = 0.0001). Mean sequence length at 48 wk correlated most strongly with sequence lengths at all other ages measured, averaging r = 0.438 (P = 0.0001). Correlations did not weaken as birds aged, indicating that a burn-out condition was not occurring in birds with high early egg production rates (data not shown). Under the parameters of this study, hens with a long prime sequence length were not prone to having reduced persistency of lay. Prime sequence length was correlated with both total egg production (r = 0.485) and to settable egg production (r = 0.478), concurring with previous research by Robinson et al. (1990), which indicated a correlation r = 0.399 between prime sequence length and total egg production. The relationship between prime sequence length and egg production can be a useful criteria for the early selection of hens with high egg production potential. Chick Production There was no difference in the numbers of chicks produced among the three growth curves (127.7, 130.7, and 132.1 for LOW, STD, and HIGH birds, respectively) or the two photostimulation ages (129.3 and 131.0 for 19WK and 21WK birds, respectively) (Table 5). Egg fertility was similar, averaging 90.6% among the growth curve treatments (Table 5), and did not decline with age, remaining near 90% for the duration of the study. Previous work has shown a reduced fertility with older hens, although this was an effect of male libido (O Sullivan et al. 1991). In artificially inseminated flocks, fertility generally does not decrease with time. Walsh and Brake (1999) reported reduced fertility in birds reared on a convex pullet growth curve (reduced feed allocation increases late in rearing), particularly when the total CP intake level by photostimulation was also reduced. Birds in the current study were maintained on parallel growth curves, although total CP intake by photostimulation would have been affected. Eggs from STD birds had a higher hatchability (82.7%) and hatchability of fertile eggs set (90.5%) than LOW (79.1%, 87.0%) or HIGH birds (80.2%, 88.3%) (Table 5). The STD birds had a lower proportion of Stage 1, Stage 2 and Stage 3 embryonic losses than the LOW birds and a lower proportion of Stage 1 and Stage 3 embryonic losses than the HIGH birds (Table 5), explaining much of their improved hatchability and hatch of fertile values. It is unclear why the STD birds underwent less embryonic loss. Eggs of the LOW birds underwent more embryonic loss during Stage 1 than both STD and HIGH eggs (Table 5). As the LOW birds also had a higher proportion of soft-shelled eggs (Table 4), the increased Stage 1 losses may be indicative of a shell quality issue, perhaps relating to their decreased plane of nutrition. All groups in the current study were given the same feed to avoid introduction of variability due to diet formulation. However, if a lower feed allocation profile is being used, balancing the diet for adequate provision of the minor dietary components and essential amino acids may be necessary. There was no effect of photostimulation age on incubation traits (Table 5), concurring with the results of Robinson et al. (1996), who photostimulated birds at points between 120 and 160 d of age. However, they reported that chick production was reduced in birds photostimulated at 120 or 130 d of age compared to 140, 150, or 160 d of age, possibly due to an additive effect of non-significant, numerical differences in fertility and embryonic loss. The production of defective eggs by individual birds appeared to have implications for incubation traits. Despite no defective eggs being incubated, defective egg production

484 CANADIAN JOURNAL OF ANIMAL SCIENCE Table 6. Body weight, carcass, and ovary morphology traits at 61 wk of age for broiler breeders reared on a LOW, STD, or HIGH growth curve and photostimulated at 19 or 21 wk of age Ovarian follicles z Body Breast Abdominal Oviduct Ovary Multiple weight muscle weight fatpad weight weight weight SYF LYF LYF sets Source (g) (% of BW) (% of BW) (g) (g) (#) (#) (% of LYF) Growth curve y LOW 3415c 15.31 4.46c 67.9 39.2b 12.0ab 5.07 2.43 STD 3724b 15.33 5.06b 65.9 37.7b 10.2b 4.88 1.70 HIGH 3947a 15.20 5.69a 68.4 44.7a 12.2a 5.32 7.39 SEM 38 0.16 0.19 1.9 1.6 0.7 0.15 2.27 Photostimulation x 19WK 3731 15.26 5.21 65.5 41.9 10.9 5.17 3.16 21WK 3660 15.30 4.92 69.4 39.2 12.0 5.00 4.52 SEM 31 0.13 0.15 1.5 1.3 0.6 0.12 1.86 z SYF = small yellow follicles (5 to 10 mm diameter); LYF = large yellow follicles (> 10 mm diameter); Multiple sets = Percentage of LYF arranged in groups differing by < 1 g. y STD = target BW birds; LOW and HIGH BW targets = 150 g lighter or heavier, respectively, at 20 wk of age. x Photostimulation age of 19 (19WK) or 21 (21WK) wk of age. a c Means within a column and within a source with no common letter differ significantly (P < 0.05). was negatively correlated with chick numbers (r = -0.560), hatchability of settable eggs (r = 0.615) and hatchability of fertile eggs set (r = 0.727). There appears to be a relationship between defective egg production and the hatchability of settable eggs within individual birds. Whereas shell quality clearly appears to be a part of this negative relationship, the extent of the contribution of embryo viability to this relationship is not clear. The implications of this relationship are that the hens producing the majority of the defective eggs are reducing their production efficiency further with the reduced potential of their settable eggs to hatch. 61 wk Carcass Traits and Ovarian Morphology The growth curve treatments resulted in significant BW differences among all growth curves at 61 wk of age (Table 6). The shank length of birds on the LOW profile (104.9 mm) was shorter than that of the STD birds (106.9 mm) and the HIGH birds (107.9 mm). Renema et al. (2001) reported a shank length of 103.1 mm in LOW birds compared to 104.9 mm in HIGH birds at photostimulation. The higher degree of feed restriction appears to have limited the skeletal growth of the LOW birds from an early age, which concurred with Fattori et al. (1993). Despite the BW differences, breast muscle weight as a percentage of BW was not different (Table 6). Abdominal fatpad as a percentage of BW increased with increasing BW target, comprising 4.46% (LOW), 5.06% (STD), and 5.69% (HIGH) of hen BW at this time. Abdominal fatpad weights as a percentage of BW at photostimulation and at sexual maturity were 0.42 and 1.56%, respectively (Renema et al. 2001), but demonstrated a similar trend in the effects of growth curve to the 61 wk data. Interestingly, HIGH birds in the 21WK photostimulation age group had a significantly higher breast muscle weight as a percentage of BW (15.6%) than HIGH-19WK birds (14.9%). In contrast, the abdominal fatpad weight HIGH-19WK birds represented 6.17% of BW compared to 5.22% in HIGH-21WK birds. It is not clear if these differences originated from the period of sexual maturation or during lay, as this type of interaction difference was not reported at photostimulation or sexual maturity (Renema et al. 2001). The implications are that when the BW profile and associated feeding level are moderately increased, birds are able to generate a greater amount of breast muscle when a later photostimulation age is used, and the increased maintenance requirements of this muscle appear to result in reduced lipid stores at the end of the egg production period. Oviduct weight was similar in all growth curve and photostimulation age groups (Table 6), indicating that the oviduct maintained a relatively uniform functional size regardless of BW. The ovary weight of HIGH birds (44.7 g) was larger than that of LOW (39.2 g) or STD birds (37.7 g), although the number of large yellow follicles or the incidence of large yellow follicles arranged in multiple follicle sets did not differ significantly (Table 6). The difference in ovary weight was likely due to mean large yellow follicle weight. The production of a heavier yolk would explain part of the 1.9 greater egg weight observed for HIGH compared to LOW birds for the 55 to 58 wk of age period. The number of small yellow follicles was 12.2 in HIGH birds compared to 10.2 in STD birds (Table 6). The LOW birds had an intermediate value of 12.0 small yellow follicles due to an interaction with photostimulation age. The LOW-21WK birds had 14.1 small yellow follicles compared to 9.9 in LOW-19WK birds. A reduced number of small yellow follicles may be indicative of a reduced ability of the ovary to support maximal rates of egg production in the near future, or an ovary already in the process of slowing its rate of production. No other effects of photostimulation age on reproductive traits at this time were noted. The range in incidence of follicular atresia at 61 wk among the HIGH (10.8%), STD (7.4%) and LOW (1.4%) birds did not differ (P = 0.085), but does demonstrate a BWbased trend towards a lower incidence in the smaller birds. Incidence of atresia was positively correlated with BW at 61 wk (r = 0.247) and BW at sexual maturity (r = 0.162). Hocking (1993) concluded that the proportion of atresia

RENEMA ET AL. REARING EFFECTS ON BREEDER EGG PRODUCTION 485 among yellow follicles and the incidence of internal ovulation increases with BW at sexual maturity. The current results suggest that the proportion of atresia among yellow follicles remains high among birds with high BW to the end of lay. It is unclear whether the rate of atresia among yellow follicles at 61 wk was responsible for the reduced egg production in heavy birds, causing further increases in BW, or if factors relating to increasing BW negatively affected ovarian follicle support, thereby increasing incidence of atresia. Atresia is common among smaller follicles as an alternative to growth, yet among the large yellow follicles (> 8 mm) atresia is rare (Gilbert et al. 1983). The incidence of internal ovulation did not differ significantly (P = 0.065) among the HIGH (6.1%), STD (1.5%) or the LOW (0.0%) birds. Production Effects on BW and Ovarian Morphology It has been suggested that reduced rates of lay may be due in part to reduced follicular recruitment into the large yolky follicle pool, and a shortage of large yellow follicles necessary for high rates of egg production exists late in lay (Williams and Sharp 1978; Palmer and Bahr 1992). The reduction of the numbers of large yellow follicles from peak production to end of lay may be due to the gradual onset of photorefractoriness (Sharp et al. 1992). Hens no longer as sensitive to long day lengths would be expected to have reduced rates of follicular recruitment, thus reducing egg production rates. As fewer nutrients are required for egg production, these birds would be expected to become both heavier and fatter than their more frequently laying counterparts. The number of large yellow follicles remaining at 61 wk of age was positively correlated with total eggs (r = 0.193), prime sequence length (r = 0.146) and average sequence length (r = 0.243). Robinson et al. (1990) also reported a link between prime sequence length and laying potential because of its positive correlation with egg production totals. The trend towards increased follicular atresia and internal ovulation in the HIGH compared to the LOW growth curves in the current study may further be indicative of reduced reproductive fitness in treatments with higher BW targets and higher feed allocations. Although carcass and reproductive morphology at the end of lay may be affected by treatment incurred early in the production period, the interaction between rate of lay and BW accretion may have more effect on carcass and ovarian traits at 61 wk of age. Ultimately, BW at 61 wk of age was negatively correlated with total egg number (r = 0.370), unsettable egg number (r = 0.115), prime sequence length (r = 0.156), mean sequence length (r = 0.268) and chick number (r = 0.237). The proportion of large yellow follicles in multiple follicle sets, which correlated with BW (r = 0.220), also correlated with unsettable egg number (r = 0.265), and was negatively associated with chick number (r = 0.269). This indicates that the increased presence of multiple follicle sets near the end of production is likely to occur in hens that have not produced well, and have generated more unsettable eggs. Birds unable to effectively manage their ovarian follicles in proper hierarchies would be expected to have a shorter prime sequence length and, if the rate of egg production is reduced, gain a greater amount of BW throughout lay. CONCLUSIONS The results of this study indicate that egg production to 61 wk was not significantly affected by growth curve, or photostimulation age. In particular, the extra BW of the HIGH growth curve did not negatively affect egg production. There was no advantage of early photostimulation, as the reduced first egg weight and impaired ability of the 19WK birds to increase egg size after sexual maturity indicates that photostimulation at 19 wk of age may have advanced age at sexual maturity to a point where birds may not have been fully mature. The efficiency of egg production in 19WK hens was reduced, resulting in similar egg numbers to 21WK hens. If earlier photostimulation ages are to be used, more work will have to be done to discern the appropriate growth curve to maximize egg production. The HIGH birds were able to respond to the photostimulatory cue faster and lay a settable egg sooner than the LOW or STD birds, although the efficiency of egg and chick production would have been negatively affected by moderate increases in growth curve due to increased feed costs. Carcass and reproductive morphology at the end of lay may be affected by treatment incurred early in the production period, but the interaction between rate of lay and BW accretion in individual birds may have more effect on carcass and ovarian traits at 61 wk of age. This study also characterized part of the contributions of the individual bird to overall egg production efficiency. The prime sequence length of the early production period was correlated with both total egg production and settable egg production. Birds with a long prime sequence length were not prone to having a reduced persistency of lay. The relationship between the production of defective eggs with incubation traits was also demonstrated. Even with only settable eggs undergoing incubation, defective egg production was negatively correlated with chick numbers, hatchability of settable eggs, and hatchability of fertile eggs set. Hens producing the most defective eggs were also reducing their production efficiency through reduced fertility and potential for their settable eggs to hatch. At the end of production, a higher proportion of large yellow follicles in multiple follicle sets on the ovary was correlated with unsettable egg production and BW, and with reduced chick production. Birds unable to effectively manage their ovarian follicles in proper hierarchies would be expected to produce more defective eggs and, if the rate of egg production is reduced, gain a greater amount of BW throughout lay. These relationships have implications for the genetic selection of female breeding stock, particularly under conditions where monitoring of individual egg production is possible. Based on carcass and reproductive morphology data collected at photostimulation and sexual maturity, Renema et al. (2001) suggest that moderate changes to the rearing growth curve may be a useful tool for reaching specific BW or conditioning goals by a predetermined photostimulation age. The use of gradual changes over a long time period

486 CANADIAN JOURNAL OF ANIMAL SCIENCE avoids the potential for negative effects on ovarian morphology, carcass fatness, and egg production. It may therefore be possible to increase or decrease the entire target BW profile by 150 g without serious implications for egg production and incubation traits, regardless of photostimulation age. ACKNOWLEDGMENTS This project was financially supported by the Canadian Broiler Hatching Egg Marketing Agency, the Alberta Agricultural Research Institute, Shaver Poultry Breeding Farms, Ltd., and the Natural Sciences and Engineering Research Council (NSERC) of Canada (Industrial Postgraduate Scholarship). Additional support was provided by Lilydale Co-operatives Ltd. (Edmonton), who donated the use of its hatchery facilities. Infrastructure support was provided by the University of Alberta Poultry Research Centre through funding provided by Alberta Agriculture, Food and Rural Development and the four Alberta Poultry Marketing Boards. Fasenko, G. 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