Effect of Strain of Layer and Age at Photostimulation on Egg Production, Egg Quality, and Bone Strength 1

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1 Effect of Strain of Layer and Age at Photostimulation on Egg Production, Egg Quality, and Bone Strength 1 F. G. Silversides,* 2 D. R. Korver, and K. L. Budgell *Crops and Livestock Research Centre, Charlottetown, Prince Edward Island, Canada C1A 7M8; Nova Scotia Agricultural College, Truro, Nova Scotia, Canada B2N 5E3; and University of Alberta, Edmonton, Alberta, Canada T6G 2P5 ABSTRACT Bone strength in layers is a concern for computed tomography. Egg production and feed conversion economic reasons and animal welfare concerns. Bone characteristics were investigated in 3 strains of hens: Babcock B-300, a small-bodied commercial white-egg layer; ISA-Brown, a commercial brown-egg layer; and an unselected Brown Leghorn line (BL). After being reared together in a single pen with 8 h of light per day, hens were caged with 14 h of light per day. Half of the hens were caged at 18 wk of age and the other half at 20 wk of age, resulting in a 2-wk difference in the age at photostimulation. Body weights, egg production, feed efficiency, and egg quality were measured throughout production. At 15, 25, 50, and 74 wk of age, hens were euthanized for sampling of the radius and the humerus. Breaking strength of the radius and humerus was measured, and the area and density of trabecular (largely medullary bone) and cortical bone were measured using quantitative of ISA-Brown hens was as good as or better than that of Babcock B-300 hens, and both commercial strains had higher production than the BL. Photostimulation late delayed sexual maturity and improved albumen and shell characteristics but had only minor effects on egg production and did not affect the yolk weight. The delayed photostimulation resulting from caging 2 wk later affected the radius by increasing the area of the trabecular space at 50 wk of age and the density of the bone in the trabecular space at 74 wk of age. Breaking strength of the humerus at 25 wk of age was greater for the birds that were photostimulated late but was not different later in the trial. The humerus, but not the radius, of the BL had a greater breaking strength than that of the commercial strains, suggesting that selection has decreased humeral breaking strength. Key words: laying hen, sexual maturity, strain, egg production, bone strength 2006 Poultry Science 85: INTRODUCTION Bone breakage in layers during production, depopulation, and processing reduces egg production, is an animal welfare concern, and reduces the value of the carcasses of spent hens. McCoy et al. (1996) found that throughout a laying cycle, 35% of mortality was due to cage layer fatigue (an extreme form of osteoporosis), and Roland and Rao (1992) estimated that 15 to 30% of layer mortality in the US is due to weak bones. A European study (Gregory and Wilkins, 1989) found that 29% of spent hens had broken bones at the water bath stunner, with 98% of spent hen carcasses having broken bones by the end of the evisceration line. Budgell and Silversides (2004) found similar results among a smaller group of commercial hens in Canada Poultry Science Association Inc. Received November 1, Accepted February 11, Agriculture and Agri-Food Canada Contribution Number Corresponding author: silversidesf@agr.gc.ca Soon before a pullet starts to produce eggs, estrogen and testosterone initiate the formation of medullary bone (Johnson, 1986). Medullary bone is deposited on the interior surface of the cortex of several bones in laying hens (McCoy et al., 1996). The calcium metabolism in medullary bone is extremely active, with calcium from dietary sources being deposited during the day and removed for eggshell formation during the night. With the onset of sexual maturity and the start of medullary bone formation, deposition of structural bone stops (Whitehead and Fleming, 2000). In birds and mammals, calcium is needed for bone formation and cell metabolism, and in laying hens it is needed for egg production as well. An eggshell contains approximately 2 g of calcium, and over a normal cycle the eggs that a hen lays contain 30 times the total calcium that her body has at any particular time (Johnson, 1986). Calcium metabolism involves an array of factors, including phosphorus, micronutrients, vitamin D 3, other hormones, and respiration, suggesting that improving bone strength will require a multifactorial approach. Nutrition, exercise, and genetic selection have been proposed as methods to reduce bone breakage in layers. 1136

2 STRAIN, SEXUAL MATURITY, AND BONE STRENGTH OF LAYERS 1137 Increased calcium in the diet leads to increased excretion (Keshavarz and Nakajima, 1993), and other nutritional manipulations provide limited (Guinotte and Nys, 1991; Rennie et al., 1997; Fleming et al., 2000) or no (Rennie et al., 1997) benefit. Adequate nutrition is essential, but dietary levels of nutrients in excess of requirements do not improve bone strength (Merkley, 1981; Rennie et al., 1997; Whitehead and Fleming, 2000). Lack of exercise leads to decreased bone strength, and the use of an aviary system increases it (Fleming et al., 1994). However, bone strength is increased only in the limbs that are subjected to the exercise (Nightingale et al., 1972), and increased activity leads to increased risk of accident (Gregory et al., 1990). There are strain differences in susceptibility to bone breakage (Whitehead and Wilson, 1992; Rennie et al., 1997; Budgell and Silversides, 2004), and Bishop et al. (2000) demonstrated differences in bone strength of lines divergently selected for 3 generations. Bone strength at the end of lay depends on the peak structural bone mass and the rate of structural bone loss (Fleming et al., 1998), and the 10 wk following the start of lay is very important in the development of osteoporosis (Whitehead and Fleming, 2000). Gregory et al. (1990) attempted to alter bone strength by delaying sexual maturity using feed restriction and delayed photostimulation. They obtained a difference of 10 d in sexual maturity but found no difference in bone strength. Unfortunately, the effects of feed restriction and lighting treatment were confounded, and any advantage of later sexual maturity may have been eliminated by feed restriction. In the present study, attempts were made to maximize structural bone development in 3 strains of pullets by delaying sexual maturity. The effects of this treatment on production and bone integrity were measured. MATERIALS AND METHODS Approximately 180 chicks of each of 3 strains of birds were raised from 1 d of age in the poultry research facilities of Nova Scotia Agricultural College (NSAC). Commercial white (Babcock B-300; BAB) and brown (ISA- Brown; ISAB) egg-layer chicks were obtained from a commercial hatchery (Cox Brothers Poultry Farm Ltd., Maitland, NS, Canada), and Brown Leghorn (BL) chicks were hatched from fertile eggs obtained from a flock kept at NSAC that has not been selected since 1965 (Crawford, 1981). Only female chicks of the commercial strains were raised. The BL chicks were sexed from secondary sex characteristics at approximately 6 wk of age, and males were discarded. Chicks of the 3 strains were raised together in a single room of 90.6 m 2, providing 1,510 cm 2 per pullet, with a day length of 8 h and were fed diets that were adequate for normal bone development. At 18 wk of age, 72 pullets from each of the 3 strains were housed in cages with a day length of 14 h and light intensity of 5 lx. At 20 wk of age, another 72 pullets per strain were housed, resulting in photostimulation 2 wk later than the first group. Each cage had 2,610 cm 2 of floor space and held 6 birds, initially providing 435 cm 2 per hen with corresponding increases at 25 and 50 wk as birds were removed for sampling of bones. There were 12 cages per strain (3) and photostimulation treatment (2), representing 72 pullets of each combination of strain and treatment, for a total of 432 pullets. The number of birds per cage was reduced over the course of the trial by mortality and sampling. Feed (Table 1) was formulated to provide adequate levels of all nutrients (National Research Council, 1994) and was provided to the hens of the 3 strains as a mash with feed and water available ad libitum. The beaks of the hens were not initially trimmed, and some cannibalism was observed. Subsequently, red lights were used, and beaks were trimmed if cannibalism occurred. Throughout the experiment, care of the hens was consistent with guidelines outlined by the Canadian Council on Animal Care (1993), and the protocol was approved by the Animal Care and Use Committee of NSAC. Egg production was recorded for each cage from 19 to 74 wk of age to allow determination of the age at sexual maturity and the level of production. Sexual maturity was defined as the age in days that the first egg was produced from each cage. The birds were weighed every 8 wk during the laying period. All eggs produced on 1 d when the hens were 30, 49, and 69 wk of age were collected for determination of egg quality. Egg weight, albumen height, and yolk weight were measured. Shells were washed under running water and allowed to airdry for at least 4 d prior to being dried at 100 C for 4 h. The dry shell weights were recorded, and albumen weight was calculated by difference. At 32, 51, and 68 wk of age, feed consumption was measured for 2 cage units and, combined with egg production data, allowed calculation of the feed efficiency for egg production. Twelve birds per strain per treatment (one per cage) were randomly chosen and euthanized by cervical dislocation at 15 (treatments had not yet been applied, and 12 birds per strain were measured), 25, 50, and 74 wk of age. The right radius and humerus were cleaned of flesh and stored at 20 C for later measurement of bone parameters. After being thawed, the area and density of the cortical bone and the bone in the trabecular space of the radius and humerus and the breaking strength were measured as described by Silversides et al. (2006). The data were subjected to an ANOVA using software of SAS (Littell et al., 1991), with strain of hen and age at photostimulation as main effects, along with the interaction between them. The ANOVA for BW and bone measures at 15 wk of age included only the effect of strain and that for egg quality included the effect of week of age along with strain and age at photostimulation. Twoand three-way interactions were included in the models, but nonsignificant interactions were removed. When main effects were significant at P < 0.05, means were separated using Duncan s multiple range test. RESULTS Of the 3 breeds, the ISAB hens were heaviest throughout the trial (Table 2). At 18 and 20 wk of age, the BAB

3 1138 SILVERSIDES ET AL. Table 1. Formulated nutrient composition of diets fed to 3 strains of layers Period (wk of age) Nutrient 19 to to to to to 73 ME (kcal/kg) 2,860 2,860 2,840 2,840 2,840 Crude protein (%) Calcium (%) Available phosphorus (%) hens were heavier than the BL, but weights of the BAB and BL thereafter were not different. At 20 wk of age, birds photostimulated late weighed less than those photostimulated early, but this effect was transitory. Egg production by ISAB hens was not significantly different than that of BAB hens in each of the 3 periods and overall, and production by BL hens was significantly lower than either commercial strain (overall, 84.9% for ISAB, 83.1% for BAB, and 58.9% for the BL). Age at photostimulation did not significantly affect egg production in any period or overall (76.0 and 74.4% for early and late photostimulation, respectively). Sexual maturity was delayed by over a week for BL hens compared with ISAB and BAB hens, who reached sexual maturity at the same age. Photostimulating the hens at 18 instead of 20 wk of age delayed sexual maturity by 3.8 d. Feed consumption of ISAB hens was not different from that of BAB hens early or late in production and was significantly less at 50 wk of age (Table 3). The BL hens consumed less feed than the others throughout production. Data on feed consumption and egg production were combined and showed that BL required more feed to produce a dozen eggs than ISAB hens at 50 wk of age and than both ISAB and BAB hens at 68 wk of age. Feed conversion for ISAB and BAB hens was not different. Age at photostimulation did not affect feed consumption or feed conversion. In the full ANOVA, the interaction between week of lay and age at photostimulation and the 3-way interaction were not significant for egg quality measurements and were eliminated from the analysis, with the resulting model shown in Table 4. Later photostimulation resulted in greater egg, albumen, and shell weight. Shell weight was also affected by the main effect of the strain, with eggs from ISAB hens having the heaviest shells, and those from BL hens having the lightest shells. The interaction between the strain and the age at photostimulation was significant for albumen height. Later photostimulation did not affect albumen height of eggs from ISAB (6.86 vs mm for early photostimulation) or BAB hens (8.84 vs mm for early photostimulation). For eggs from BL hens, later photostimulation resulted in significantly greater albumen height (6.30 compared with 5.64 mm for early photostimulation). The week-by-strain interactions for albumen height and the weight of the egg, albumen, and yolk were significant, but generally with increasing age of the hen, eggs were larger, had lower albumen height, and greater albumen and yolk weight. As well, the ISAB hens laid the largest eggs, and their eggs had the most albumen and shell; Table 2. Body weight, egg production, and age at first egg (6 hens/cage) of 3 strains of laying hens photostimulated (PS) at 18 and 20 wk of age 1 BW (wk of age) Egg production (wk of age) Age at first egg Item to to to 74 (d) (g) (hen day %) Strain ISAB 1,422 a 1,599 a 1,923 a 2,058 a 77.2 a 92.0 a 82.9 a b BAB 1,221 b 1,378 b 1,577 b 1,815 b 74.5 a 90.9 a 80.9 a b BL 1,185 c 1,304 c 1,563 b 1,782 b 45.4 b 69.8 b 57.0 b a SEM Age at PS Early 1,455 a 1,688 1, b Late 1,399 b 1,693 1, a SEM ANOVA P Strain <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 Age at PS <0.01 NS NS NS NS NS <0.01 Strain age at PS NS NS NS NS NS NS NS a c For each measure, means within strains or age at PS having different superscripts are different at P < n is 216 for BW at 18 wk of age, 432 at 20 wk of age, 352 at 28 wk of age, and 266 at 68 wk of age; n is 12 per strain and PS combination for age at first egg and egg production except for ISA Brown hens PS early, for which one observation was missing for late production and those PS late for which one was missing for the mid egg production and 2 for the late egg production. ISAB = ISA Brown, BAB = Babcock B-300, BL = Brown Leghorn.

4 STRAIN, SEXUAL MATURITY, AND BONE STRENGTH OF LAYERS 1139 Table 3. Feed consumption and efficiency of 3 strains of laying hens photostimulated (PS) at 18 and 20 wk of age 1 Feed consumption (wk of age) Feed per dozen eggs (wk of age) Item (g/bird per d) Strain ISA Brown a b a 1,661 1,507 b 2,027 b Babcock a a a 1,679 1,598 ab 2,084 b Brown Leghorn 97.7 b 90.6 c b 1,684 1,758 a 2,436 a SEM Age at PS Early ,631 1,591 2,200 Late ,719 1,659 2,183 SEM ANOVA P Strain <0.01 <0.01 <0.01 NS <0.05 <0.05 Age at PS NS NS NS NS NS NS Strain age at PS NS NS NS NS NS NS a c For each measure, means within strains or age at PS having different superscripts are different at P < Means represent 6 repetitions per strain and age at PS, except that one group of ISAB hens was excluded after 32 wk of age and another after 50 wk of age; both were PS late. eggs from BAB were smaller with less shell and albumen, but they had yolk weights similar to those of eggs from ISAB; and eggs from BL weighed least. Albumen height of eggs from BAB hens was greatest, followed by that of eggs from ISAB, with that of eggs from BL hens being lowest. The week of age by strain interactions are investigated further in Table 5. Albumen height of eggs from BAB hens decreased 0.74 mm over the period of lay, whereas that of ISAB hens decreased 1.18 mm, and that of BL hens decreased 2.05 mm. The increase in egg weight between 30 and 70 wk of age was 2.87 g for ISAB hens, that for Table 4. Quality of eggs of from 3 strains of hens at 30, 50, and 70 wk of age photostimulated (PS) at 18 and 20 wk of age Albumen Egg Albumen Yolk Shell height weight weight weight weight Item (mm) (g) (g) (g) (g) n Strain ISA Brown a Babcock b Brown Leghorn c SEM Age at PS Early b b b Late a a a SEM Week SEM ANOVA P Week <0.01 <0.01 <0.01 <0.01 NS Strain <0.01 <0.01 <0.01 <0.01 <0.01 PS <0.05 <0.01 <0.01 NS <0.01 Week strain <0.01 <0.01 <0.05 <0.01 NS Strain age at PS <0.05 NS NS NS NS (g) BAB hens was 5.48 g, and that for BL hens was 8.70 g. The increase in the weight of eggs from BL hens was due to increases in albumen (4.15 g) and yolk (4.09 g) weights. The weight of yolks of eggs from ISAB and BAB hens increased by 1.66 and 3.53 g, respectively, between 30 and 50 wk of age, and that for albumen increased by 0.9 and 2.2 g, respectively. The difference in albumen weight for eggs from 30- and 50-wk-old ISAB hens was not significant. The BW of birds sampled for bone measurements generally followed those of the larger population (Table 6). At each age, the total and trabecular areas of the radius a c For each measure, means within strains, age at PS, or wk having different superscripts are different at P < 0.05.

5 1140 SILVERSIDES ET AL. Table 5. Some quality attributes of eggs laid by 3 strains of hens at 30, 50, and 70 wk of age 1 Attribute and strain 30 wk 50 wk 70 wk SEM Albumen height (mm) ISA Brown 7.60 c (47) 6.28 d (47) 6.42 d (44) Babcock B a (50) 8.45 b (49) 8.62 b (48) Brown Leghorn 7.18 c (47) 5.52 e (45) 5.13 e (44) 0.17 Egg weight (g) ISA Brown b (107) b (99) a (65) Babcock B c (113) b (89) ab (75) Brown Leghorn f (92) e (63) d (51) 0.53 Albumen weight (g) ISA Brown ab (47) bc (46) a (44) Babcock B d (50) cd (48) bc (48) Brown Leghorn g (47) f (43) e (44) 0.53 Yolk weight (g) ISA Brown c (47) b (46) b (44) Babcock B d (50) ab (48) a (48) Brown Leghorn e (47) c (43) b (44) 0.23 a g For each measure, means with a common superscript do not differ at P < The number of observations is in parentheses after the mean. were greatest for ISAB hens, next largest for BAB hens, and smallest for the BL hens. At 15 wk of age, there was no difference among breeds in the density of the bone in the trabecular space. The density of bone in the trabecular space was lowest for BL hens at 25 wk of age and highest at 50 wk of age, with no difference between ISAB and BAB hens. At 74 wk of age, the density of bone in the trabecular space of the radius of BL and BAB hens was not different, and both were higher than that of ISAB. Cortical density of the radius of ISAB hens was higher than that of BAB and BL hens at 15 and 25 wk of age and higher than BAB hens at 50 wk of age. At 74 wk of age, cortical density was highest for BL, with no difference between the other strains. The radius of ISAB hens had the greatest cortical area at 15 and 25 wk of age but was not greater than BAB at 50 wk of age or than either of other breeds at 74 wk of age. The BL hens had a greater cortical area than BAB hens at 25 and 50 wk of age and a smaller area at 15 wk of age. Breaking strength of the radius of ISAB hens was significantly greater than either of the small-bodied strains at all ages, with no differences between BAB and BL hens. The age at photostimulation had minor effects on parameters of the radius. The total and trabecular areas at 50 wk of age were greater with later photostimulation. At 74 wk of age, the ANOVA showed that hens photostimulated late had greater trabecular density (P = 0.048), but Duncan s test did not. This result was not observed at other ages. At each age, the total area of the humerus was greatest for ISAB, next largest for BAB, and least for the BL hens (Table 7). The trabecular area was greatest for ISAB hens at all times, but it was not significantly larger than that of BAB hens at 15 and 50 wk of age. The trabecular area was smallest for the BL humeri at each time. Of the 247 hens observed, only one, an ISAB, had medullary bone in the humerus, as evidenced by the presence of bone in the trabecular space. Cortical density of the humerus was not different among breeds at 15 wk of age but was highest for the BL at 25 (not significantly different from ISAB), 50, and 74 wk of age. At 25 and 74 wk of age, the cortical density for BL and ISAB hens was not different, and at 50 wk of age the difference between ISAB and BAB was not significant. At 15 wk of age, the cortical area was least for BL hens; at 25 wk of age, it was least for BAB hens; and thereafter it was not affected by the strain. Breaking strength of the humerus was not affected by the strain at 15 wk of age but was highest for BL at 25 (not significantly different from ISAB), 50, and 74 wk of age. At 50 and 74 wk of age, breaking strength of the humeri of ISAB and BAB hens was not different. Age at photostimulation did not affect the total or trabecular areas of the humerus. At 50 wk of age, cortical density of the humerus was higher for hens that received early photostimulation, but other differences among photostimulation times were not significant. At 25 and 74 wk of age, but not at 50 wk of age, humeral cortical area was greater for hens with late photostimulation than for those photostimulated early. Breaking strength at 25 wk of age was higher with later photostimulation, but this effect was not observed at 50 and 74 wk of age. DISCUSSION In this trial, the BAB hens represented the type of lightweight high-producing layers in which bone strength is thought to be most often a problem, and ISAB hens represented a heavier-bodied layer that has also been selected for very high egg production. Commercial white-egg strains are based on the White Leghorn breed and brown-

6 STRAIN, SEXUAL MATURITY, AND BONE STRENGTH OF LAYERS 1141 Table 6. Bone density, area, and breaking strength of the radius from 3 strains of laying hens photostimulated (PS) at 18 and 20 wk of age 1 Trabecular Trabecular Cortical Cortical Age and BW Total area density area density area Breaking strain (g) (mm 2 ) (mg/cm 3 ) (mm 2 ) (mg/cm 3 ) (mm 2 ) strength (kg) 15 wk ** ** 0.07 ** ** ** ** ISAB (12) 2 1,286 a 6.97 a a 1,023 a 4.24 a 4.44 a BAB (12) 1,113 b 6.14 b b 984 b 3.72 b 3.41 b BL (12) 998 c 4.88 c c 998 b 3.39 c 2.87 c SEM wk NS ** ** ** ** ** ** ISAB (24) 1, a 126 a 2.97 a 1,045 a 5.21 a 6.08 a BAB (24) 1, b 145 a 2.14 b 1,020 b 4.53 c 4.69 b BL (22) 1, c 100 b 0.84 c 1,025 b 4.80 b 4.32 b SEM NS NS NS 0.09 NS NS 0.08 Early PS (35) 1, , Late PS (35) 1, , SEM wk ** ** ** ** * ** ** ISAB (24) 2,086 a 7.83 a 156 b 3.25 a 1,051 a 4.67 b 5.64 a BAB (24) 1,844 b 6.77 b 182 b 2.39 b 1,019 b 4.46 b 4.65 b BL (21) 1,803 b 5.73 c 211 a 0.74 c 1,033 ab 5.09 a 4.84 b SEM NS * NS ** NS NS NS Early PS (34) 1, b b 1, Late PS (35) 1, a a 1, SEM wk ** ** ** ** ** NS ** ISAB (24) 2,162 a 8.20 a 157 b 2.58 a 1,029 b a BAB (24) 1,824 b 6.78 b 202 a 1.64 b 1,017 b b BL (24) 1,762 b 5.82 c (21) 212 a (21) 0.63 c (21) 1,073 a b SEM * NS * NS NS NS NS Early PS (36) 1,863 b , Late PS (36) 1,969 a 7.08 (33) 202 (33) 1.73 (33) 1, SEM a c For each measure and age, means within strain or age at PS with a common superscript do not differ at P < ANOVA included main effects of strain, age at PS, and the interaction, but no interactions were significant. Means were separated using least square means if a main effect was significant with ANOVA (*P < 0.05; **P < 0.01). ISAB = ISA Brown; BAB = Babcock B-300; BL = Brown Leghorn. 2 Numbers in parentheses are the sample size and apply to all measures in that row unless noted otherwise. egg lines are derived from the Rhode Island Red, Barred Plymouth Rock, Australorp, and New Hampshire breeds, among others (Crawford 1990). The BL and BAB strains share a common Leghorn origin, but the BL line has not been subjected to intense selection for increased egg production and feed efficiency for many years (Crawford, 1981). This trial succeeded in delaying sexual maturity of layers by delaying their exposure to long days. It is clear that genetic selection has had a large effect on production as demonstrated by the differences between the BAB and BL hens. However, the relationship is distant, and the assumption that differences between them are due only to selection for egg production is likely not entirely true. Even at 20 wk of age, the BL hens were clearly not ready for increased day length, as shown by the age at first egg and their continued growth. The later sexual maturity contributed to lower production by the BL hens, but egg production by the BL hens was low throughout the trial, suggesting other differences as well. Egg production by the ISAB hens was as good as or better as that of BAB hens, and the 2 strains had a similar age at sexual maturity. In spite of being heavier throughout and producing heavier eggs, the efficiency of feed use for egg production by ISAB hens was equal to that of the BAB hens. Feed consumption for the commercial hens was higher than expected (Silversides et al., 2006), which might be explained by feed wastage. Delaying sexual maturity had only minor and early effects on production characteristics and was confounded by the effect of moving the birds from the floor to cages. The cage environment may have been less stressful and it allowed less exercise. The changes in egg quality over the production cycle agree with previous reports (Hill and Hall, 1980; Silversides and Scott, 2001), with greater hen age associated with lower albumen height; higher egg, albumen, and

7 1142 SILVERSIDES ET AL. Table 7. Bone density, area, and breaking strength of the humerus from 3 strains of laying hens photostimulated (PS) at 18 and 20 wk of age 1 Total Trabecular Trabecular Cortical Cortical Breaking Age and area density area density area strength strain (mm 2 ) (mg/cm 3 ) (mm 2 ) (mg/cm 3 ) (mm 2 ) (kg) 15 wk ** ** NS ** 0.07 ISAB (12) a a 1, a BAB (12) b a 1, a BL (12) c b 1, b SEM wk ** ** ** * ** ISAB (24) a a 1,126 a a a BAB (24) b b 1,100 b b b BL (22) c c 1,139 a a a SEM NS NS 0.07 ** ** Early PS (35) , b b Late PS (35) , a a SEM wk ** ** ** NS * ISAB (24) a 200 (1) a 1126 b b BAB (24) b (23) a (23) 1119 b (23) (23) b (23) BL (21) c b 1150 a a SEM NS NS * NS NS Early PS (34) ,141 a (33) (33) (33) Late PS (35) (1) ,122 b SEM wk ** ** ** NS ** ISAB (24) a a 1,151 a b BAB (24) b b 1,125 b b BL (24) c c 1,165 a a SEM NS NS NS * NS Early PS (36) b Late PS (36) a SEM a c For each measure and age, means within strain or age at PS with a common superscript do not differ at P < ANOVA included main effects of strain and age at PS and the interaction, but no interactions were significant. Means were separated using least square means if a main effect was significant with ANOVA (*P < 0.05; **P < 0.01). ISAB = ISA Brown; BAB = Babcock B-300; BL = Brown Leghorn. 2 Numbers in brackets are the sample size and apply to all measures in that row unless noted otherwise. yolk weights and less change in shell weight. The yolk weight was not affected by the time of photostimulation, suggesting that the ovary did not benefit from further development, but albumen and shell weights were increased by late photostimulation, suggesting a benefit to the oviduct from later sexual maturity. Albumen height of eggs from the commercial strains was not affected by age at photostimulation, but delayed maturity resulted in greater albumen height for the BL, likely because the oviduct of BL hens was immature, especially in the earlyphotostimulated group. The heritability of egg weight is relatively high (Fairfull and Gowe, 1990). Thus, it is not surprising that eggs from highly selected ISAB and BAB hens were larger than those from BL, because egg weight is undoubtedly included in selection programs. It was surprising that the eggs from ISAB hens increased less in weight over the production cycle than did those from BAB hens, and that egg weights from BL hens increased much more than those of the other strains. The difference between commercial strains and the BL suggests that the change in egg size over the production cycle has received attention by commercial breeders, and these data suggest that they have been more successful in the ISAB strain than in the BAB strain. The differences in the radius among breeds in trabecular (assumed to correspond to medullary bone; Korver et al., 2004) bone density likely relate largely to the state of egg production. Bone in the trabecular space for all 3 strains was low at 15 wk of age, and it increased dramatically by 25 wk of age. The BL hens reached sexual maturity later than the others, and the low density of bone in the trabecular space at 25 wk of age was a reflection of this. Later in production, BL hens had greater density of bone in the trabecular space, likely because medullary bone reserves were less depleted by egg production than those of the other, higher-producing strains. Late in production, the density of bone in the trabecular space may have been lower for BAB than ISAB hens because their

8 STRAIN, SEXUAL MATURITY, AND BONE STRENGTH OF LAYERS 1143 bones were smaller and had corresponding smaller reserves. Fleming et al. (1998) found that medullary bone reserves increase with a hen s age, and data shown here support this. The difference among breeds in trabecular area relates partly to body size, but it may also be affected by selection for egg production or shell quality because the radii from BAB hens had greater trabecular area than those from BL hens. Trabecular area of the bones measured by QCT is a measure of the area within the cortical shell of the bone. As laying hens do not deposit cortical bone while reproductively active (Hudson et al., 1993), increases in trabecular area probably represent endocortical thinning, which is a predisposing factor for osteoporosis (Whitehead and Fleming, 2000). With increased egg production, a greater reliance on cortical bone calcium to support eggshell formation could lead to endocortical thinning. Late photostimulation of the hens resulted in increases in trabecular area at 50 wk of age. This result was somewhat surprising, as an increase in trabecular area suggested increased endocortical thinning. As the late photostimulation treatment slightly delayed the onset of egg production and total egg production to 50 wk of age, these hens might be expected to have decreased reliance on mobilization of cortical bone to support shell formation. However, at 74 wk of age, density of bone in the trabecular space was increased in the late-photostimuated hens. Therefore, in spite of the transient difference in trabecular area observed at 50 wk of age, the late-photostimulated hens may have had greater medullary bone reserves for shell production than those photostimulated at 18 wk of age, and thus a reduced reliance on cortical bone by the end of production. In most, but not all hens, the humerus does not have medullary bone (Fleming et al., 1994; McCoy et al., 1996; Riczu et al., 2004). One hen of 247 described here did have bone in the interior cavity of the humerus. Radiographic data obtained in other studies suggest that this represents variable infiltration of the air sac into the cavity of the humerus, with the remaining cavity filled with medullary bone (W. E. Clark, 2005, Agassiz, BC, Canada, personal communication). Although medullary bone may contribute to strength (Fleming et al., 1996), bone breaking strength is largely a combination of the density and area of cortical bone. The breaking strength of the radius was greatest for ISAB hens and similar for the BAB and BL hens, suggesting that it is related largely to body size (Whitehead and Fleming, 2000). However, the cortical density and area of the humeri of BL hens appeared to be closer to the ISAB hens than the BAB hens, and despite their small size, the humeri of BL hens had the greatest breaking strength, suggesting that selection of the ISAB and BAB strains for increased egg production has reduced bone strength, especially by the end of production. The age at photostimulation had no effect on breaking strength of the radius, except at 25 wk of age. Delayed photostimulation increased the breaking strength of the humerus at 25 wk of age, but this advantage appeared to disappear later on. Lighting and environment were confounded, and the additional time in a pen may have increased early bone strength because of the increased opportunity for exercise. It is clear that egg production characteristics of the commercial strains were much better than those of the unselected BL line, and that production characteristics of the commercial brown-egg layer were at least equal to those of the commercial white-egg layer. Differences between the unselected BL and the BAB strain suggest that selection for egg production and the associated change in calcium demand for shell formation may have decreased the strength of the humerus but not that of the radius. Later photostimulation successfully delayed sexual maturity and resulted in better albumen height and changes in some bone characteristics. ACKNOWLEDGMENTS The authors thank Rena Currie, Merridy Rankin, and Ron Mekers for care of the birds; Kerry Nadeau for analysis of bones; and Natalie Cole for administrative help. George Ansah of ISA arranged for the gift of the commercial chicks, and Dian Patterson allowed the use of the Brown Leghorns. 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