The effect of photoperiodic history on egg laying in dwarf broiler hens

No evidence that selection for egg production persistency causes loss of bone quality in laying hens

BACKGROUND

The physiological adaptations that have evolved for egg laying make hens susceptible to bone fractures and keel bone damage. In modern laying hen breeds, longer periods of egg laying could result in a greater risk of poor bone quality, and selection for increased egg production has frequently been stated to be a cause. However, the existing literature does not support this hypothesis. To test the hypothesis that egg production is associated with quality, breaking strength and density of bone, genetic correlations between these traits were estimated in White Leghorn and Rhode Island Red breeds. Genetic correlations of cortical and medullary bone material chemical properties with bone quality were also estimated, in order to identify methods to improve bone quality with appropriately targeted measurement of key traits.

RESULTS

Estimates of heritability for bone quality traits were moderate (0.19-0.59) for both White Leghorn and Rhode Island Red breeds, except for the keel bone trait, which had a heritability estimate equal to zero. There was no evidence for genetic or phenotypic relationships between post-peak egg production and bone quality. In the White Leghorn breed, the estimate of the genetic correlation between pre-peak production/age at first egg and bone quality was significant and negative (- 0.7 to - 0.4). Estimates of heritability of thermogravimetric measurements of tibial medullary bone mineralisation were significant (0.18-0.41), as were estimates of their genetic correlations with tibia breaking strength and density (0.6-0.9).

CONCLUSIONS

The low genetic correlation of post-peak egg production with bone quality suggests that selection for increased persistency of egg production may not adversely affect bone quality. Onset of puberty and mineralisation of the medullary bone, which is a specialised adaptation for egg laying, were identified as important factors associated with the quality of the skeleton later during egg production. These are traits for which genetic, as well as environmental and management factors can positively impact the overall quality of the skeleton of laying hens.

Explanations for keel bone fractures in laying hens: are there explanations in addition to elevated egg production?

The current article served to provide the most up-to-date information regarding the causes of keel bone fracture. Although elevated and sustained egg production is likely a major contributing factor toward fractures, new information resulting from the development of novel methodologies suggests complementary causes that should be investigated. We identified 4 broad areas that could explain variation and increased fractures independent of or complementing elevated and sustained egg production: the age at first egg, late ossification of the keel, predisposing bone diseases, and inactivity leading to poor bone health. We also specified several topics that future research should target, which include continued efforts to link egg production and bone health, examination of noncommercial aves and traditional breeds, manipulating of age at first egg, a detailed histological and structural analysis of the keel, assessment of prefracture bone condition, and the relationship between individual activity patterns and bone health.

Evidence of a chemopreventive effect of progestin unrelated to ovulation on reproductive tract cancers in the egg-laying hen

Epidemiologic, laboratory, and animal evidence suggests that progestins and vitamin D may be potent ovarian cancer preventives. Our objectives were to evaluate progestins as reproductive tract cancer chemopreventives in the chicken, determine whether restricted ovulation affected the incidence of reproductive tract tumors, and assess whether vitamin D would confer cancer protection either alone or in addition to progestin. A total of 2,400 two-year-old Single Comb White Leghorns were randomized into six groups (400 each) with hormonal and dietary manipulation for 2 years as follows: (i) no intervention, regular feed/caloric intake, (ii) control, (iii) vitamin D, (iv) the progestin levonorgestrel, (v) vitamin D plus levonorgestrel, and (vi) the progestin Provera (medroxyprogesterone acetate). Groups 2 to 6 were caloric restricted to inhibit ovulation. Our results indicated that caloric restriction decreased egg production by more than 60%, and was associated with a greater than 70% decrease in reproductive tract cancers. Ovulatory events did not differ among the caloric-restricted groups (groups 2-6), except for the group receiving levonorgestrel, which had fewer ovulatory events than controls (P = 0.046). After correcting for egg production, birds receiving progestins had significantly fewer reproductive tract cancers [OR, 0.61; confidence interval (CI), 0.39-0.95; P = 0.03], with similar proportionate reductions in tumors arising in either the ovary or oviduct. Vitamin D did not significantly affect cancer incidence overall, or add to the cancer preventive effect of progestins. This study suggests a protective effect of progestins against ovarian and oviductal cancers. These data support the concept that progestins provide a chemopreventive effect unrelated to ovulation.

Effects of body weight at, and lighting regimen and growth curve to, 20 weeks on laying performance in broiler breeders

1. A total of 4000 Ross broiler breeders were reared to a 20-week target body weight of 1.55, 2.16 (standard), 2.50 or 2.84 kg, using either a convex or concave growth curve. Each treatment group was either exposed to a conventional broiler breeder lighting regimen, with a series of weekly one-hour increments in photoperiod from 8h at 19 weeks to a 16-h maximum, or maintained from 4d on a 17-h photoperiod. 2. Each 100 g increment in body weight at 20 weeks was associated with a significant increase of 0.55 kg in cumulative food intake, and a 1.5-d advance in sexual maturity. However, leaner body weights at 20 weeks resulted in smaller body weight gains between 20 and 60 weeks (-2.4 g/d/kg body weight at 20 weeks), and an increased production of double-yolked eggs. Neither egg production nor mean egg weight was affected by 20-week body weight. 3. Birds fed to produce more rapid early growth had higher feed intakes to 20 weeks, but reached sexual maturity 7 d later than birds permitted accelerated growth from 15 weeks. Growth curve did not influence body weight gain in lay, egg production or mean egg weight. 4. Birds maintained on 17-h photoperiods reached sexual maturity 27 d later, produced 7 fewer eggs to 60 weeks, and had a mean egg weight 1.2 g heavier than birds photostimulated at 19 weeks. Lighting treatment did not affect food intake to 20 weeks, the proportion of double-yolked eggs or body weight gain between 20 and 60 weeks. 5. Birds fed to have a faster growth early in the rearing phase and maintained on 17-h photoperiods produced 11 fewer eggs than those fed to have accelerated growth at the end of the rearing phase, yet there was only one egg difference between the growth-curve groups for the conventionally lighted birds, which was not significant. 6. The earlier sexual maturity of the conventionally lighted birds compared with those maintained on 17-h photoperiods either indicates that broiler breeders require an increment in photoperiod to stimulate rapid gonadal development or that broiler breeders exhibit juvenile photorefractoriness that takes longer to be dissipated when birds are not given a period of short days. 7. The findings suggest that a nutritional stimulus late in rearing is only necessary for satisfactory egg production if birds have not received a concurrent increment in photoperiod.

Model to predict age at sexual maturity in broiler breeders given a single increment in photoperiod

1. Data from 9 experiments in which broiler breeder pullets had been photostimulated at two or more ages were integrated to produce a model to predict age at 50% egg production following a single increase in photoperiod during rearing. 2. It was clear that the photosexual response in broiler breeders was strongly influenced by the feed allowance and hence the rate of prepubertal growth. Regressions for birds given either a constant photoperiod or a single increase indicated that mean age at 50% lay advances by 2 d for every 100-g increase in body weight at 20 weeks. 3. The general response of broiler breeders was similar to that previously reported for egg-type pullets, but with important changes in the ages at which the birds progressed from one physiological state to the next, depending on body weight. 4. Broiler breeders, unlike modern egg-type pullets, exhibit juvenile photorefractoriness and, depending on their body weight, require up to 20 weeks to dissipate this (faster growth allows quicker dissipation). As a consequence, a group of birds grown to a typical weight of 2.1 kg at 20 weeks do not start to be photoresponsive until about 10 weeks and are not uniformly responsive until 19 or 20 weeks. A transfer to a stimulatory photoperiod before a bird has dissipated photorefractoriness causes a delay of about 3 weeks in its sexual development, and this results in a bimodal distribution of ages at maturity when a flock is photostimulated between 10 and 20 weeks. 5. Once photosensitive, the response of broiler breeders to an increment in photoperiod is between 0.50 and 0.65 of that observed in ISA Brown egg-type pullets. However, a flock of broiler breeders with typical feed restriction starts to mature spontaneously under the influence of the initial photoperiod from about 25 weeks. 6. There is a difference of only 1 to 3 d in age at 50% egg production between a flock transferred to 11 or 12 h followed by further increases to 15 or 16 h and one increased abruptly to one of these photoperiods, and so this model can be used to predict maturity in a commercial flock of birds even though they are likely to be given a stepped, rather than a single, increase in photoperiod.

Broiler breeders do not respond positively to photoperiodic increments given during the laying period

1. Broiler breeders were given a 3-h increase in photoperiod to 11 h at 20 week and then a series of increases to reach 16 or 17 h either immediately after the initial increment or in 30-min, 1- or 2-h increments starting at various ages after peak rate of lay. Controls were maintained on 11 h from 20 weeks. Changes in plasma LH concentration (after 7 d) were measured in birds that had been transferred to 11 or 16 h at 20 weeks and given further increases in photoperiod at 41 or 61 weeks of age. 2. Birds that were transferred to 16- or 17-h photoperiods, irrespective of when and how the maximum photoperiod was reached, had inferior rates of lay between 52 and 60 weeks of age to birds maintained on 11 h from 20 weeks. However, the 11-h birds laid more eggs on the floor and produced a larger number of cracked and dirty eggs, resulting in similar numbers of settable eggs. 3. Although transferring birds from 11- to 16-h photoperiods at 41 weeks of age significantly increased plasma LH concentration, there was no effect on egg production during the ensuing 12 d. None of the other increases in photoperiod significantly increased plasma LH, whether given at 41 or 61 weeks. 4. It is concluded in broiler breeders, that increases in photoperiod applied during the laying period, from 11 or 16 h, have little or no effect on LH secretion, do not compensate for age-related declines in egg production, and adversely affect rate of lay.

New insights regarding pharmacologic approaches for ovarian cancer prevention

The pathogenesis of epithelial ovarian cancer is not completely understood, but it commonly is believed that the process of recurrent ovulation (incessant ovulation) causes genetic damage in ovarian epithelial cells and that sufficient genetic damage can lead to ovarian cancer in susceptible individuals. Under this model, it has been suggested that reproductive and hormonal factors, such as pregnancy and oral contraceptive use, decrease ovarian cancer risk mainly via their inhibitory effects on ovulation. There is mounting evidence that the ovarian epithelium is a hormonally responsive target organ whose biology can be impacted strongly by the local hormonal environment. Progestin-mediated apoptotic effects may be a major mechanism underlying the ovarian cancer protective effects of pregnancy (a high progestin state) and oral contraceptive pill use. Similarly, retinoids, vitamin D, and non-steroidal anti-inflammatory drugs may have biologic effects on the ovarian epithelium that are cancer preventive, whereas androgens may have stimulatory effects on the ovarian epithelium, leading to an increased ovarian cancer risk.

Effect of age of release from light or food restriction on age at sexual maturity and egg production of laying pullets

1. Lohmann Brown pullets, in one trial, and Hyline Brown pullets in another, were reared from day 2 on short daylengths, and from week 8 in trial 1 (week 16 in trial 2) on food restriction. These restrictions were lifted at various times during the rearing period as a means of determining the relative importance of the day length and food restriction stimuli on the attainment of sexual maturity and subsequent laying performance. 2. A total of 2304 pullets were used in each trial. The birds were reared in light proof rooms, and subjected to 8L:16D until they were moved to a laying facility where a light stimulus of 16L:8D was applied. In trial 1 the six ages at which light stimulation was applied were 115, 122, 129, 136, 143 and 171 d. Within each light treatment, food restriction of pullets, which consisted of feeding 72 g of food/bird d, was lifted at six different ages, namely, 115, 129, 143, 157, 171 and 185 d. In trial 2 both the light stimulation and the lifting of food restriction occured at 111, 125, 139, 153, 167 and 181 d of age, producing 6x6=36 treatments in both trials. 3. The first trial was terminated when the pullets were 28 weeks old, soon after all the birds had commenced laying, because of an outbreak of Egg Drop Syndrome. However, because age at maturity was the variable of major interest, data from this experiment could be used in the analysis. The second trial ended when the birds reached 40 weeks of age. Variables measured were age at maturity, food intake and body weight gain subsequent to the lifting of restrictions and, in the second experiment, rate of lay, peak rate of lay and egg weight at various ages. 4. The mean age at sexual maturity was influenced by the date of release from light restriction (P<0.001) and from food restriction (P<0.001) in both trials. In addition, the interaction between the age at release from light and from food restriction was significant. Regression equations were produced for each trial to describe the relationships between the age at sexual maturity and the age at release from light restriction and food restriction. 5. There was an effect of both light restriction (P<0.001) and of food restriction (P<0.001) on the increase in food intake (g/bird d) in the week following release from food restriction in both experiments. These effects were not independent: the effect of the interaction of light and food restriction on this increase in food intake was also highly significant. The longer the birds were subjected to light restriction, the less dramatic the increase in food intake when food restriction was lifted. The more sustained the period of food restriction, the higher the increase in food intake in the week following release from the restriction. 6. Mean egg weight was 4 g heavier at 22 weeks of age in birds released from food restriction at 16 and 18 weeks, than from those released at 24 and 26 weeks of age. However, by 30 weeks of age, birds restricted for longer produced heavier eggs than their earlier-maturing counterparts. This effect continued to the end of the trial at 40 weeks of age, at which time there was a 2.3 g difference in egg weight between these treatments. 7. Both light and food restriction have an effect on the age of maturity in laying hens. The length of time between the release from light or from food restriction to the onset of laying depended on the age of the pullets when the release occurred. Egg weight at a given age was significantly affected by the age at release from food restriction, but not from light restriction.

Effect of delayed step-up lighting on plasma luteinizing hormone and reproductive function in broiler breeders

The aim of the study was to establish, in contemporary broiler breeders, whether delayed photostimulation at 313 d results in a reproductive response similar to that after photostimulation at 134 d (standard practice). The standard lighting program was compared with a novel program in which daily hours of light were reduced to 3 h during rearing and kept at 3 h until photostimulation at 264 d (8 h) or at 313 d (16 h). This experiment was done with hens fed ad libitum or feed-restricted hens. In photostimulated and nonphotostimulated hens, feed restriction delayed the onset of egg production and enhanced the subsequent rate of laying. Standard photostimulation advanced the onset of lay and increased the subsequent rate of lay in hens fed ad libitum and feed-restricted hens. Delayed photostimulation of hens did not impair the photoinduced increase in the concentration of plasma luteinizing hormone (LH) or egg production. Delayed photostimulation in cockerels failed to stimulate LH secretion. Unexpectedly, for feed-restricted hens, transfer from 3 to 8 h light/d at 264 d resulted in an increased in plasma LH and increased egg production. A similar increase in plasma LH was observed for cockerels subjected to the same lighting treatment. We concluded that, in broiler breeder hens, the reproductive response to photostimulation is not impaired if photostimulation is delayed for up to 313 d. Cockerels may not respond well to delayed photostimulation.

Use of hatch date for broiler breeder production planning

Data on 109 batches of broiler breeders belonging to six strains and bred in conventional sheds were analyzed to determine the possible influence of natural day length cycle phase on age at start of laying, at sexual maturity, and at peak egg production. The dependence of these variables on date of hatching was characterized by polynomial regression analysis. The resulting equations constitute simple, novel empirical models that are in keeping with current theory on the effects of photoperiod on sexual maturation, and should facilitate production planning in broiler breeder farms.

Selection for high and low threshold body weight at first egg in broiler strain females. 7. Effect of photoperiod on body weight and age at onset of mature semen production, in males of the selection lines, and in commercial broiler and layer males

Effect of photoperiod on age and BW at attainment of mature semen production was examined in males of lines selected for high (HL) and low (LL) threshold BW at onset of lay, and in males of a commercial Cornish male line (MC), a commercial broiler breeder female line cross (BX), and a commercial layer cross (LX). Photoperiod treatments were: I, increasing natural spring light; and D, steadily decreasing artificial light provided in a light-controlled room. Under I, age at onset of mature semen production was 168.5, 133.6, 138.1, 124.0, and 118.0 d for HL, LL, MC, BX, and LX males, respectively. Under D, age at onset of mature semen production increased by 18.3, 8.0, 55.9, 60.9, and -2.0 d, respectively. The differential effect of Treatment D on the various genetic stocks is attributed to differences between stocks in photoperiodic drive. Therefore, the very great increase in age at onset of mature semen production in BX and MC males points to a severe reduction in photoperiodic drive in males of these stocks, and to future problems in male fertility in broiler breeder flocks entering lay in the summer and fall. Lines HL, LL, BX, and LX rank the same with respect to photoperiodic drive, whether measured in males as in this experiment, or in females, as in a previous experiment. This result suggests that genetic factors affecting photoperiodic drive come to similar expression in males and females. Thus, selection for increased photoperiodic drive in broiler line males may be able to increase photoperiodic drive in broiler line females, and in this way improve female reproductive performance.

Selection for high and low threshold body weight at first egg in broiler strain females. 6. Threshold weight for onset of photosensitivity in females of the selection lines

A two-way selection program, carried out for high and low threshold BW at onset of lay (TWL) resulted in a marked difference in TWL in the selected lines. Later studies showed that the selection procedure had decreased photoperiodic drive in the high line (HL) as compared to the low line (LL), in this way increasing the number of days required from onset of photosensitivity to onset of lay in the HL as compared to the LL. The present study was carried out to determine whether the difference in photoperiodic drive was sufficient to account for the entire difference in TWL between the HL and LL, or whether it is also necessary to postulate a difference in threshold weight for onset of photosensitivity (TWSM) as well. Birds of the two lines were raised under feed restriction to 22 wk reaching weights (1,881.8 g) for LL, 2,304.8 g for HL), well below anticipated TWSM. They were then provided gradually increasing amounts of feed, resulting in daily gains of 3 to 5 g for the LL and 8 to 9 g for the HL. Under this regimen, the HL entered lay an average of 64.4 d later and at a body weight 752.8 g greater than the LL. On the basis of previous studies of these lines, of the 64.4 d delay in onset of lay in the HL as compared to LL, 21 d can be attributed to the difference in photoperiodic drive. The excess of 43 d must thus represent a difference in TWSM. During this period the HL were gaining about 8.5 g/d, and at 22 wk HL body weight was already 153 g greater than LL body weight. In all then, TWSM of HL is estimated at 518 g greater than that of the LL. Thus, the selection procedure employed for TWL apparently affected TWSM as well as photoperiodic drive.

Effect of 5 hour increases in photoperiod and in feeding opportunity on age at first egg

1. ISA Brown pullets were given an 8-h photoperiod and fed ad libitum to 63 d of age. At 63 d the photoperiod was either kept at 8 h or increased to 13 h, and the photostimulated birds were subjected to 1 of 3 feeding systems: ad libitum, 8 h daily access to food or a daily individual allocation of food equal to that given to the 8 h control group. 2. Mean age at first egg (AFE) of the groups given the photoperiod increase was on average 33 d earlier than that of the 8 h controls. Within the photostimulated groups, limiting daily feeding opportunity to 8 h delayed maturity by 4 d compared with ad libitum feeding. The mean AFE of the birds which were given allocated quantities of food was intermediate and not significantly different from either of the other groups. 3. Light was the principal factor which determined AFE, but moderate food restriction had a small modifying influence, consistent with earlier evidence. 4. The 3 groups given a 5-h increase in photoperiod consumed similar quantities of food to first egg, which was laid around 15 weeks of age. The 8 h control group ate a similar amount of food to this age, but needed more than 40% extra food to reach their first egg at 20 weeks.

Two-way selection for threshold body weight at first egg in broiler strain females. 5. Replication of results in a two-generation selection experiment

Selection for high and low threshold weight for onset of lay (ThrWtLay), carried out in a broiler line over six generations, was previously found to increase reproductive performance in the low line (LL). Because of the potential importance of this effect, a two-generation replication of the original experiment was carried out using a modern broiler line. Selection was on the basis of ThrWtLay, measured as body weight at first egg, following gradual release from feed restriction. The total response to selection over two generations was 382 g, giving a realized heritability of .42. The difference in 6-wk body weight of the two lines was +19 g in favor (P > .05) of the high line (HL). Reproductive performance was measured in the S2 generation and in the first (R1) generation of relaxed selection. On the average, age at first egg of LL was 17.6 d earlier, and prepeak, postpeak, and total egg production of LL to a fixed age was 9.9, 1.1, and 11.1 eggs greater, respectively, than that of HL. The differences in age at first egg, prepeak, and total egg production were significant; those in postpeak production were not. There was a difference of 1.1 g in favor (P > .05) of HL for egg weight. Thus, the results of this short-term selection experiment are broadly consistent with those of the original, long-term selection. Namely, ThrWtLay responded strongly to selection, with a powerful accompanied response in age at onset of lay and in prepeak egg production, and only a small effect on 6-wk body weight. However, in contrast to the results obtained previously, LL in the present experiment showed only a small increase (P > .05) in postpeak production relative to HL. Nevertheless, although an effect on postpeak production was not obtained in the present experiment, the results do provide general support for the previously expressed view that selection for early onset of lay, on release from feed restriction, may provide a means of increasing photoperiodic drive and consequent reproductive performance of broiler breeders.

Selection for high and low threshold body weight at first egg in broiler strain females. 4. Photoperiodic drive in the selection lines and in commercial layers and broiler breeders

A two-way selection program, carried out for high and low threshold BW at onset of lay (ThrWtLay), decreased photoperiodic drive of the high line (HL) relative to the low line (LL). Photoperiodic drive in modern broiler breeder females was compared to layer females in experiments using the HL and LL lines, a commercial layer cross (LX), and a commercial broiler breeder female line cross (BX). Treatment groups were: NS, natural decreasing light to 20 wk, followed by natural light supplemented to 16 h daily with artificial light; NN, natural decreasing light to end of experiment; DD, dark room from hatch to end of experiment; ND, natural decreasing light to 20 wk, dark room thereafter. Under NS, mean age and BW at first egg was greater for the HL line than for the LL line. The difference increased by a further 8.9 d and 217.5 g under NN. Similarly, under NS, mean age and BW at first egg was greater for BX than for LX, and the difference increased by a further 22.6 d and 256.1 g under NN. These interactions indicate greater photoperiodic drive of LX and LL stocks relative to the HL and BX stocks. Prepeak and peak egg production (EP) was greatest for the LX, lower for the LL, and lower yet for HL and BX. Photoperiodic drive may be of major importance in determining prepeak and peak EP. Except for the DD treatment, age and BW of the LX cross at first egg were virtually identical across treatments, those of the LL line were similar, whereas those of the HL and BX stocks showed differences. Similarly, EP curves of the LX cross were virtually identical across treatments, those of the LL line were relatively uniform, but those of the HL and BX stocks again showed great differences. Standard deviation and coefficient of variation of peak EP were very low for the LX cross, still low, but significantly greater for the LL line, and significantly greater yet again for both HL and BX stocks. Thus, increased photoperiodic drive may also be associated with greater stability of reproductive performance. Selection for early onset of lay on release from feed restriction may provide a means of increasing photoperiodic drive and consequent reproductive performance of broiler breeders.

Photoperiodic control of reproduction in the domestic hen

Egg laying in domestic hens exposed to natural lighting begins shortly after the winter solstice, peaks in early spring, begins to decrease before the fall equinox, and is at its lowest during the late fall and early winter. The seasonal cycle of egg production phase-leads that of the changes in day length. This seeming anomaly can be explained if it is accepted that 1) short days are photoperiodically neutral and do not actively inhibit gonadotropin-releasing hormone (GnRH)-I neurons; and 2) long days are photoperiodically active, transducing both stimulatory and inhibitory inputs to GnRH-I neurons. The development of a long day-induced inhibitory input results in a form of photorefractoriness. Around the winter solstice, photorefractoriness is dissipated by prolonged exposure to short days, allowing GnRH-I neurons to express a photoperiodic-independent, genotype-dependent, level of activity. This is sufficient to stimulate egg laying before the minimum photoperiod for photoinduced gonadotropin release is reached in early spring. When day length begins to decrease after the summer solstice, the photoinduced stimulatory input to GnRH-I neurons is reduced, unmasking the photoinduced inhibitory input. As a consequence, the activity of GnRH-I neurons decreases rapidly and the intensity of egg laying decreases. The minimum and maximum day lengths required to stimulate reproductive function in short-day hens, calculated from the photoperiodic response curves (PRC) for luteinizing hormone release are about 10 and 13 h, respectively, depending on genotype.(ABSTRACT TRUNCATED AT 250 WORDS)