Abstract
In turkey hens, the egg production rate is relatively high early during a reproductive period, but declines as the period progresses. Among lines with different egg production potential, the interval between preovulatory surges of LH is the primary determinant of the egg production rate. The main objective of this study was to determine whether the decline in egg production rate late during an egg production period is also associated with a difference in the interval between LH preovulatory surges. A group of photosensitive turkey hens (Early) were photostimulated with continuous light (24L:0D) at 40 wk of age to induce egg laying, and serial blood samples were collected after about 3 wk of egg production. A second group of hens (Late) were housed in floor pens and photostimulated with 14L:10D at 40 wk of age for a normal 36-wk reproduction period and were then switched to 24L:0D lighting for 2 wk before collection of serial blood samples. Continuous light photostimulation was used for at least 2 wk before and during serial blood sampling to avoid potential masking effects of diurnal lighting on the interval between LH surges. The Early (n = 12) and Late (n = 16) hens were cannulated 3 days before being serially bled hourly for 10 days. The mean interval between preovulatory surges of LH was shorter in the Early hens than in the Late hens (26.1 ± 2.5 h and 34.7 ± 3.9 h, respectively). The intra-hen LH surge interval coefficient of variation was lower in the Early hens than in the Late hens (7.2% and 18.6%, respectively). The inter-hen LH surge interval coefficient of variation was similar in the Early and Late hens (9.5% and 11.2%, respectively). The incidence of blind surges of LH (those not retrospectively associated with ovipositions) was not different between Early and Late laying hens (8.4% ± 15.2% and 7.3% ± 14.6%, respectively). In conclusion, in turkey hens, longer intervals and greater intra-hen variation between LH surges were associated with a poorer rate of egg production late in the reproductive period relative to early in the reproductive period.
Introduction
The egg production rate of turkey and chicken hen flocks is characterized by a period of relatively constant high production followed by a period of gradually declining production with advancing duration of the reproductive period (turkey [1]; chicken [2–6]). The duration of the period of relatively constant high (about 85%) production lasts about 15 wk in a line of turkey hens (Egg line, [7]) previously used in studies of the relationship of LH surge interval and egg production rate. In Egg line hens, the rate of egg production then slowly declines after the period of constant production to about 65% by 36 wk of production [1].
In birds, as in mammals, ovulation is hormonally controlled by the hypothalamus-pituitary-gonadal axis [8, 9]. Before ovulation of a mature follicle, LH is released from the anterior pituitary in a surge, which is probably stimulated by a surge of GnRH-I secreted into the portal vessels supplying the anterior pituitary. The preovulatory surges of LH are associated with surges of progesterone (P4). The association of surges of LH and P4 with ovulations and ovipositions in turkey hens has been reported in detail for single [10] or multiple [1, 11, 12] ovulation cycles. Preovulatory LH surges are characterized by increasing concentrations over 2–3 h to a peak. The LH concentration then gradually declines to a basal level over 3–5 h. Pulses of LH between surges, infrequently observed, are of low amplitude and short duration and are not coincident with P4 surges when observed [10]. Preovulatory surges of P4 begin slightly earlier than or at the same time as LH surges and have a slightly longer duration [10]. In comparison to the relatively sharp peak of plasma LH, the peak of plasma P4 is broad and typically lasts 4–6 h before declining to baseline levels by ovulation [10, 12]. Ovulation of the largest mature hierarchical follicle(s) (F1) occurs 6–8 h after a preovulatory surge of LH [13, 14] and about 15–30 min after a preceding oviposition in a laying sequence [15]. Oviposition of completely formed eggs normally occurs about 25 h after ovulation [15]. Direct evidence for LH surge secretion controlling ovulation in birds is given by in vivo studies in which injection of exogenous LH in acutely hypophysectomized laying chicken hens induced single or multiple ovulations of hierarchical follicles [16, 17]. Examination of several hundred LH and P4 preovulatory surges [1, 11, 18] has shown that they are, without exception, coincident in laying turkey hens.
The interval between LH and P4 surges has been shown to be associated with the egg production rate in turkey hens at the peak of production. The interval between preovulatory surges of LH was longer in a line of hens (RBC3 line) with a poor rate of egg production than in a line of hens (Egg line) with excellent egg production [1]. The RBC3 line hens also had a higher incidence of blind LH surges (an LH surge not coupled with an oviposition about 30 h later) and more intervals between surges of >33 h, probably associated with pauses between sequences [19], than the Egg line hens. In comparison to the RBC3 line hens, the Egg line hens had a decrease in intrasequence LH surge intervals, a decrease in the incidence of blind LH surges, and a decrease in the incidence of LH surge intervals of >33 h (intersequence intervals), resulting in a higher rate of egg production in the Egg line hens.
Several factors may contribute to poorer egg production of laying hens late in the reproductive period, including 1) an increase in intrasequence LH surge intervals, resulting in longer intervals between intrasequence ovipositions [20]; 2) an increase in the incidence of intersequence intervals of >33 h between clutches of eggs [2], resulting in a decrease in clutch size [21]; 3) an increase in follicular atresia, resulting in a lower rate of follicular maturation and number of follicles available for ovulation [22, 23]; 4) a loss of synchronization of ovulation and oviposition [24–26], resulting in an increase in the number of defective eggs late in the reproductive period, and 5) abnormal secretion of P4, associated with a blockage of LH surge secretion and ovipositions [12].
The objective of this study was to determine if the decline in egg production rate late during an egg production period is associated with a difference in the interval between LH preovulatory surges, the incidence of blind LH surges, or changes in the characteristics of LH surges or of P4 baseline and surge amplitude concentrations. Measurements of overall mean plasma estradiol-17β (E2) concentrations and ovarian and oviductal morphologies were also made.
Materials and Methods
Animals
Egg line turkey hens, selected for increased egg production rate for over 40 generations [1, 7], were used in this study. Hens were studied either early (Early) or late (Late) in their reproductive period. Both Early and Late groups of hens were exposed to 24L:0D from hatch to 8 wk of age; 12L:12D from 8 to 16 wk of age, and then 6L:18D at 16 wk of age. Feed and fresh water were supplied ad libitum throughout the study. The Early hens were housed in individual cages (60 × 60 × 80 cm) bedded with wood shavings in light-tight rooms with two overhead 100-W incandescent bulbs, with light intensity varying from 20 to 100 lux from the bottom tier inside the cages to the top tier outside the cages. The Early hens (n = 12) were photostimulated with 24L:0D lighting at 40 wk of age to induce sexual maturity. Egg production by the Early hens started 2–3 wk after photostimulation. The Late hens were housed in floor pens [7] and photostimulated with 14L:10D lighting at 40 wk of age for a normal 36-wk production period. Then 16 laying hens were selected randomly, housed in individual cages as described for the Early hens, and switched to continuous light (24L:0D). The hens were given continuous light for 2 wk to allow the circadian rhythm of LH surges to free run [18, 27] before and during collection of serial blood samples. Egg production was recorded daily before cannulation and then 3 times a day during the 10 days of serial bleeding. During serial bleeding, the hens were also checked for the presence of an egg in the shell gland by digital palpation three times a day.
The Early and Late hens were weighed and cannulated 3 days before the start of serial bleeding. The procedures of cannulation and serial bleeding have been described previously in detail [1, 28]. Blood samples (1.5 ml) were collected hourly for 10 days. After the samples were centrifuged and plasma was removed for storage at −20°C, the red blood cells were resuspended to original volume with sterile saline and returned to the individual hen of origin every 3–4 h to avoid hemodilution. One of the 16 Late hens stopped laying after cannulation but before the start of serial bleeding, and in 1 of the 12 Early laying hens, cannula patency failed during serial bleeding; both of these hens were removed from the study. All hens were killed by an overdose of pentobarbital sodium (Socumb; The Butler Co., Columbus, OH) and autopsied immediately after completion of serial bleeding. The following measurements were taken at autopsy: 1) body weight, 2) oviductal and ovarian weights (without yolky follicles), 3) number and weight of each hierarchical yolky follicle (follicles >1.0 g), and 4) number of ruptured and of atretic follicles. An animal use protocol for these procedures was approved by the Institutional Laboratory Animal Care and Use Committee (protocol 99 AG008).
LH, P4, and E2 Assays
Concentrations of LH were measured by RIA as previously described [29] using 100 μl plasma for each duplicate. The intraassay and interassay coefficients of variation (CV) determined with a pool plasma (mean, 3.33 ng/ml) were 5.9% and 6.3%, respectively.
Samples for P4 analysis were selected retrospectively in relation to LH surges as previously described [1]. Samples collected at 5 and 6 h before the LH surge peak concentration were assayed to estimate baseline concentrations of P4. Samples collected at 2 and 3 h after the LH surge peak concentration were assayed to estimate preovulatory surge concentrations of P4. Concentrations of plasma P4 were measured by RIA as previously described [10] using 15 μl plasma for each duplicate. The mean recovery of labeled P4 after extraction was 74.8%. All samples were corrected for percentage recovery. The intraassay and interassay CV determined with a pool plasma (mean, 8.53 ng/ml) were 11.5% and 10.4%, respectively.
Samples for E2 analysis were those taken every 8 h, independently of LH surges. Concentrations of plasma E2 were measured by RIA as previously described [30], using 100 μl plasma for each duplicate. The mean recovery of labeled E2 after extraction was 75.24%. All samples were corrected for percentage recovery. The intraassay and interassay CV determined with a pool plasma (mean, 0.57 ng/ml) were 9.5% and 5.8%, respectively.
Statistical Analyses
The plasma LH data were evaluated by PC PULSAR algorithm [31] to identify surges and to calculate surge duration and baseline and peak amplitude concentrations. The interval between LH surges was calculated based on the period between consecutive LH surge peaks. The G values used in the PC PULSAR algorithm for the LH analyses were G(1) = 50, G(2) = 2.6, G(3) = 1.9, G(4) = 1.5, and G(5) = 1.2. The G values specify the number of assay SDs above baseline concentration to which 1, 2, 3, 4, or 5 consecutive datum points must be elevated to qualify as a peak. The SD of the LH assay calculated and used in the PULSAR analyses was (9.63x)/100, where x is the sample mean of plasma LH concentration.
The following measurements were analyzed by one-way ANOVA with the main effect the duration of the laying period (Early vs. Late) and hen as the error term: 1) egg production rate, 2) body weight, 3) ovarian weight, 4) oviductal weight, 5) weight of the largest (F1) yolky follicle, 6) number of yolky hierarchical follicles (follicles >1.0 g), 7) number of atretic follicles, 8) incidence of double hierarchical follicles, 9) interval between LH surges, 10) intra-hen CV of LH surge interval, 11) rate of blind LH surges, 12) LH baseline and surge amplitude concentrations of LH, 13) LH surge duration, 14) P4 baseline and surge amplitude concentrations, and 15) E2 mean concentration. Each preovulatory surge of P4 was coincident with an LH surge. Not all surges of LH and P4 were retrospectively associated with a specific oviposition, identified by digital palpation. The unassociated surges were called blind surges [1, 11]. The percentage of blind LH surges was transformed by Arcsin transformation and analyzed by one-way ANOVA with duration of laying as an effect and hen as the error term.
The distributions of all intervals between sequential surges of LH were examined using the approach of Yoo et al. [27]. First, the distributions of intervals were classified into 2-h windows, and the distributions then fitted with six degree polynomial equations for the Early and Late hens. We then arbitrarily set boundaries for short-period and long-period groups of intervals, based on the first derivative estimation of constants for the first and second perigees for the short-period group of intervals, and the second and third perigees for the long-period group of intervals. Descriptive statistics (means, SDs, and modes) and relative distribution of intervals in the short-period and long-period interval groups were then calculated for both the Early and Late hens.
Results
Egg production during the 10 days before cannulation was not different from egg production during the 10 days of serial bleeding for both groups of hens (P > 0.05, data not shown). Egg production was 86.4% ± 10.2% and 65.4% ± 12.0% (P < 0.001) for the Early and Late groups of hens, respectively.
Body weight was greater for the Early hens than for the Late hens (Table 1). Body weight of both groups was not affected by 10 days of serial bleeding (P > 0.05, data not shown). Ovarian weight, oviductal weight, and weight of F1 follicles were all greater for the Late hens (Table 1). The number of hierarchical yolky follicles, number of atretic follicles, and incidence of double hierarchal follicles were not different between the Early and Late hens. Only one atretic follicle was found in an individual Early hen (hen 44) and an individual Late hen (hen 105). The ovary of 1 Late hen (hen 100) contained 12 gray solid masses ranging in weight from 1 to 50 g. This hen had a total of 6 surges of LH (with one blind LH surge) over the 10-day period of serial blood sampling, near the mean number of LH surges (6.47) for this group of hens. Data from this hen were included in the study.
Body weight and ovarian and oviductal measurements of Egg line turkey hens either early or late in the reproductive period.
| Stage of production | Body weight (kg) | Ovarian weight (g) | Oviductal weight (g) | F1 follicle weight (g) | No. of hierarchical follicles per hen | No. of atretic follicles per hen | No. of double follicle hierarchy per hena |
|---|---|---|---|---|---|---|---|
| Earlyb | 5.8 ± 0.3 | 9.6 ± 1.3 | 79.6 ± 4.8 | 18.6 ± 1.3 | 7.5 ± 1.2 | 0.10 ± 0.30 | 0.73 ± 0.90 |
| Lateb | 5.4 ± 0.4 | 15.0 ± 2.9 | 93.1 ± 10.2 | 21.0 ± 2.3 | 7.8 ± 1.1 | 0.07 ± 0.26 | 0.20 ± 0.41 |
| Pc | <0.01 | <0.005 | <0.005 | <0.05 | >0.05 | >.05 | 0.057 |
| Stage of production | Body weight (kg) | Ovarian weight (g) | Oviductal weight (g) | F1 follicle weight (g) | No. of hierarchical follicles per hen | No. of atretic follicles per hen | No. of double follicle hierarchy per hena |
|---|---|---|---|---|---|---|---|
| Earlyb | 5.8 ± 0.3 | 9.6 ± 1.3 | 79.6 ± 4.8 | 18.6 ± 1.3 | 7.5 ± 1.2 | 0.10 ± 0.30 | 0.73 ± 0.90 |
| Lateb | 5.4 ± 0.4 | 15.0 ± 2.9 | 93.1 ± 10.2 | 21.0 ± 2.3 | 7.8 ± 1.1 | 0.07 ± 0.26 | 0.20 ± 0.41 |
| Pc | <0.01 | <0.005 | <0.005 | <0.05 | >0.05 | >.05 | 0.057 |
Weight of two yolky follicles within 1.0 g of each other.
Data from the Early hens were collected after 3-5 wk of egg production, and data from the Late hens were collected after 38-40 wk of egg production.
Probability of a difference between Early and Late means.
Body weight and ovarian and oviductal measurements of Egg line turkey hens either early or late in the reproductive period.
| Stage of production | Body weight (kg) | Ovarian weight (g) | Oviductal weight (g) | F1 follicle weight (g) | No. of hierarchical follicles per hen | No. of atretic follicles per hen | No. of double follicle hierarchy per hena |
|---|---|---|---|---|---|---|---|
| Earlyb | 5.8 ± 0.3 | 9.6 ± 1.3 | 79.6 ± 4.8 | 18.6 ± 1.3 | 7.5 ± 1.2 | 0.10 ± 0.30 | 0.73 ± 0.90 |
| Lateb | 5.4 ± 0.4 | 15.0 ± 2.9 | 93.1 ± 10.2 | 21.0 ± 2.3 | 7.8 ± 1.1 | 0.07 ± 0.26 | 0.20 ± 0.41 |
| Pc | <0.01 | <0.005 | <0.005 | <0.05 | >0.05 | >.05 | 0.057 |
| Stage of production | Body weight (kg) | Ovarian weight (g) | Oviductal weight (g) | F1 follicle weight (g) | No. of hierarchical follicles per hen | No. of atretic follicles per hen | No. of double follicle hierarchy per hena |
|---|---|---|---|---|---|---|---|
| Earlyb | 5.8 ± 0.3 | 9.6 ± 1.3 | 79.6 ± 4.8 | 18.6 ± 1.3 | 7.5 ± 1.2 | 0.10 ± 0.30 | 0.73 ± 0.90 |
| Lateb | 5.4 ± 0.4 | 15.0 ± 2.9 | 93.1 ± 10.2 | 21.0 ± 2.3 | 7.8 ± 1.1 | 0.07 ± 0.26 | 0.20 ± 0.41 |
| Pc | <0.01 | <0.005 | <0.005 | <0.05 | >0.05 | >.05 | 0.057 |
Weight of two yolky follicles within 1.0 g of each other.
Data from the Early hens were collected after 3-5 wk of egg production, and data from the Late hens were collected after 38-40 wk of egg production.
Probability of a difference between Early and Late means.
The patterns of change in concentrations of LH, P4, and E2 over 10 days of serial blood sampling are given in Figure 1 for 4 Early hens and 4 Late hens. The hens presented in Figure 1 were chosen based on the following criteria. Hens 31 and 105 had the shortest mean LH surge interval for the Early and Late hens, respectively. Hens 43 and 89 had relatively high incidences of blind LH surges for the Early and Late hens, respectively. Hens 48 and 87 had LH surge intervals near the mean for the Early and Late hens, respectively. Hens 57 and 97 had relatively long intervals between LH surges for the Early and Late hens, respectively.
Changes in concentrations of plasma hormones over 240 h. LH (solid line) concentrations were measured in each hourly sample. P4 (▴) concentrations were measured only in samples collected 5 and 6 h before LH surge peak samples to determine P4 baseline concentrations and 2 and 3 h after LH surge peak samples to determine P4 surge concentrations. E2 (♦) concentrations were measured in every eighth hourly sample regardless of preovulatory LH surges. The four hens on the left side were sampled for 10 days early in the reproductive period, starting after 3–5 wk of egg production. The four hens on the right side were sampled late in the reproductive period, starting after 38–40 wk of egg production. Closed circles (•) at the top of each panel represent approximate (±4 h) times of ovipositions based on digital palpation and egg records of the hens. Open circles (○) at the top right hand corner of some panels represent a soft-shelled or hard-shelled (hen 48 only) egg in the reproductive tract at autopsy. Arrows in the panels give the assigned association of LH surges to ovipositions. Elevated P4 concentrations were coincident with each LH surge, but not all LH surges were associated with the subsequent oviposition of an egg. Surges of LH not associated with ovipositions are blind LH surges and are indicated by question marks (?) in some of the panels
The number of LH surge intervals for the Early and Late hens was 90 and 82, respectively. The distributions of mean LH surge interval per hen are given in Figure 2a. The interval between LH surges was shorter (P < 0.001) for the Early hens than for the Late hens. The inter-hen CVs of LH surge intervals were similar for the Early and Late hens (9.4% and 11.1%, respectively). The distribution of LH surge intervals for the Early hens is given in Figure 2b and for the Late hens in Figure 2c. The intra-hen LH surge interval CV was lower (P < 0.001) for the Early hens than for the Late hens (7.2% and 18.6%, respectively). Also graphed in Figures 2b and 2c are lines estimated from fitting the data to six degree polynomial equations.
a) Distributions of mean LH surge interval per hen for the Early (n = 11) and Late (n = 15) hens. The hen mean LH surge interval was different between the groups (P < 0.001). b) Distribution of all LH surge intervals of the Early hens from 3 to 5 wk of egg production (n = 90 intervals). c) Distribution of all LH surge intervals of the Late hens from 38 to 40 wk of egg production (n = 82 intervals). Also graphed in (b) and (c) are lines estimated from fitting the data to six degree polynomial equations. d) Distribution of LH surges throughout the 24-h solar day. The hens were given 24-h lighting for at least 2 wk of egg laying before initiation of the 10-day hourly serial blood collection. A total of 101 and 97 surges were detected in the Early and Late groups of hens, respectively
The distributions of intervals were arbitrarily separated into short-duration and long-duration groups for the Early and Late hens as described previously. The incidence of short-duration intervals was higher, and the interval of the short-duration intervals was shorter for the Early hens than for the Late hens (Fig. 2b and Table 2). The incidence of long-duration intervals was lower for the Early hens than for the Late hens (Fig. 2c and Table 2), but there was no difference in the interval of the long-duration intervals between the Early and Late hens (Table 2). The distributions of LH surges over the 24-h solar day are shown in Figure 2d. Surges of LH occurred throughout the 24-h solar day for both the Early and Late groups of hens.
Descriptive statistics for intervals between LH surges of relatively short and long intervals of turkey hens either early or late in the reproductive period.
| Observation | Early | Late | ||
|---|---|---|---|---|
| Short interval | Long interval | Short interval | Long interval | |
| Intervals (n) | 85 | 5 | 49 | 33 |
| Distribution (%) | 94 | 6 | 60 | 40 |
| Interval mean ± SD (h) | 25.3a ± 2.2 | 35.4c ± 2.1 | 29.0b ± 3.0 | 40.8c ± 7.1 |
| Interval mode (h) | 25.0 | 35.0 | 28.0 | 41.0 |
| Observation | Early | Late | ||
|---|---|---|---|---|
| Short interval | Long interval | Short interval | Long interval | |
| Intervals (n) | 85 | 5 | 49 | 33 |
| Distribution (%) | 94 | 6 | 60 | 40 |
| Interval mean ± SD (h) | 25.3a ± 2.2 | 35.4c ± 2.1 | 29.0b ± 3.0 | 40.8c ± 7.1 |
| Interval mode (h) | 25.0 | 35.0 | 28.0 | 41.0 |
Means with different letters within the short interval classification are different (P < 0.001).
Means with identical letters within the long interval classification are not different (P > 0.05).
Descriptive statistics for intervals between LH surges of relatively short and long intervals of turkey hens either early or late in the reproductive period.
| Observation | Early | Late | ||
|---|---|---|---|---|
| Short interval | Long interval | Short interval | Long interval | |
| Intervals (n) | 85 | 5 | 49 | 33 |
| Distribution (%) | 94 | 6 | 60 | 40 |
| Interval mean ± SD (h) | 25.3a ± 2.2 | 35.4c ± 2.1 | 29.0b ± 3.0 | 40.8c ± 7.1 |
| Interval mode (h) | 25.0 | 35.0 | 28.0 | 41.0 |
| Observation | Early | Late | ||
|---|---|---|---|---|
| Short interval | Long interval | Short interval | Long interval | |
| Intervals (n) | 85 | 5 | 49 | 33 |
| Distribution (%) | 94 | 6 | 60 | 40 |
| Interval mean ± SD (h) | 25.3a ± 2.2 | 35.4c ± 2.1 | 29.0b ± 3.0 | 40.8c ± 7.1 |
| Interval mode (h) | 25.0 | 35.0 | 28.0 | 41.0 |
Means with different letters within the short interval classification are different (P < 0.001).
Means with identical letters within the long interval classification are not different (P > 0.05).
Blind surges of LH were detected in 4 of the 11 Early hens and 4 of the 15 Late hens. Two of the Early hens (hens 55 and 43) and 1 of the Late hens (hen 89) had high incidences of blind surges of LH during the 240-h of serial blood sampling (4/10, 3/9, and 3/6, respectively), whereas other hens had 0, 1, or 2 blind surges. The rates of blind surges of LH in the Early and Late hens were 8.4% ± 15.2% and 7.3% ± 14.6%, respectively (P > 0.05).
Baseline and surge amplitude concentrations of LH were greater for the Early hens (Table 3). The duration of LH surges was not different between the Early and Late hens.
Plasma LH concentrations at baseline (between surges) and during surges and duration of LH surges.a
| Stage of productionb | LH baseline concentration (ng/ml) | LH surge amplitude concentration (ng/ml) | LH surge duration (h) |
|---|---|---|---|
| Early | 2.68 ± 0.34 | 3.38 ± 0.63 | 7.69 ± 1.31 |
| Late | 1.90 ± 0.31 | 2.70 ± 0.56 | 7.06 ± 1.18 |
| Pc | <0.005 | <0.05 | >0.05 |
| Stage of productionb | LH baseline concentration (ng/ml) | LH surge amplitude concentration (ng/ml) | LH surge duration (h) |
|---|---|---|---|
| Early | 2.68 ± 0.34 | 3.38 ± 0.63 | 7.69 ± 1.31 |
| Late | 1.90 ± 0.31 | 2.70 ± 0.56 | 7.06 ± 1.18 |
| Pc | <0.005 | <0.05 | >0.05 |
Values are expressed as the mean ± SD.
Data from Early hens were collected after 3-5 wk of egg production, and data from the Late hens were collected after 38-40 wk of egg production.
Probability of a difference between Early and Late means.
Plasma LH concentrations at baseline (between surges) and during surges and duration of LH surges.a
| Stage of productionb | LH baseline concentration (ng/ml) | LH surge amplitude concentration (ng/ml) | LH surge duration (h) |
|---|---|---|---|
| Early | 2.68 ± 0.34 | 3.38 ± 0.63 | 7.69 ± 1.31 |
| Late | 1.90 ± 0.31 | 2.70 ± 0.56 | 7.06 ± 1.18 |
| Pc | <0.005 | <0.05 | >0.05 |
| Stage of productionb | LH baseline concentration (ng/ml) | LH surge amplitude concentration (ng/ml) | LH surge duration (h) |
|---|---|---|---|
| Early | 2.68 ± 0.34 | 3.38 ± 0.63 | 7.69 ± 1.31 |
| Late | 1.90 ± 0.31 | 2.70 ± 0.56 | 7.06 ± 1.18 |
| Pc | <0.005 | <0.05 | >0.05 |
Values are expressed as the mean ± SD.
Data from Early hens were collected after 3-5 wk of egg production, and data from the Late hens were collected after 38-40 wk of egg production.
Probability of a difference between Early and Late means.
Baseline concentrations of P4 were not different between the Early and Late hens (Table 4), but the surge amplitude concentration of P4 was lower in the Early hens than in the Late hens. The mean concentration of E2 was higher in the Early hens than in the Late hens.
Plasma progesterone (P4) baseline and surge concentrations and plasma estradiol-17β (E2) concentrations.
| Stage of productionb | P4 baseline concentration (ng/ml) | P4 surge amplitude concentration (ng/ml) | E2 overall mean concentration (ng/ml) |
|---|---|---|---|
| Early | 1.53 ± 0.33 | 5.95 ± 1.04 | 0.359 ± 0.047 |
| Late | 1.53 ± 0.21 | 6.86 ± 1.13 | 0.309 ± 0.050 |
| Pc | >0.05 | <0.05 | <0.05 |
| Stage of productionb | P4 baseline concentration (ng/ml) | P4 surge amplitude concentration (ng/ml) | E2 overall mean concentration (ng/ml) |
|---|---|---|---|
| Early | 1.53 ± 0.33 | 5.95 ± 1.04 | 0.359 ± 0.047 |
| Late | 1.53 ± 0.21 | 6.86 ± 1.13 | 0.309 ± 0.050 |
| Pc | >0.05 | <0.05 | <0.05 |
Values are expressed as the mean ± SD.
Data from Early hens were collected after 3-5 wk of egg production, and data from the Late hens were collected after 38-40 wk of egg production.
Probability of a difference between Early and Late means.
Plasma progesterone (P4) baseline and surge concentrations and plasma estradiol-17β (E2) concentrations.
| Stage of productionb | P4 baseline concentration (ng/ml) | P4 surge amplitude concentration (ng/ml) | E2 overall mean concentration (ng/ml) |
|---|---|---|---|
| Early | 1.53 ± 0.33 | 5.95 ± 1.04 | 0.359 ± 0.047 |
| Late | 1.53 ± 0.21 | 6.86 ± 1.13 | 0.309 ± 0.050 |
| Pc | >0.05 | <0.05 | <0.05 |
| Stage of productionb | P4 baseline concentration (ng/ml) | P4 surge amplitude concentration (ng/ml) | E2 overall mean concentration (ng/ml) |
|---|---|---|---|
| Early | 1.53 ± 0.33 | 5.95 ± 1.04 | 0.359 ± 0.047 |
| Late | 1.53 ± 0.21 | 6.86 ± 1.13 | 0.309 ± 0.050 |
| Pc | >0.05 | <0.05 | <0.05 |
Values are expressed as the mean ± SD.
Data from Early hens were collected after 3-5 wk of egg production, and data from the Late hens were collected after 38-40 wk of egg production.
Probability of a difference between Early and Late means.
Discussion
Changes in egg production rate during a reproductive cycle for the Egg line turkey hens used in this study have recently been reported [1]. The egg production rate (number of eggs produced by a hen per day) can be estimated by the relationship: [24 h ÷ mean interval (h) between LH surges] × [100% − percentage blind surges]. Egg production rates estimated from the LH surge intervals of the Early and Late hens were 84.2% [(24 h/26.1 h) × (100% − 8.4%)] and 64.1% [(24 h/34.7 h) × (100% − 7.3%)], respectively. The estimated and observed egg production rates are near the previously reported [1] rates of 85% at peak of production between 3 and 16 wk of lay and 65% at 36 wk of egg production for this line of turkey hens under diurnal lighting. The observation that egg production rate is primarily determined by frequency of LH surges is in agreement with the findings of our earlier study [1]. In this earlier study, a much longer interval between preovulatory surges of LH at peak of production was observed in a line of turkey hens (RBC3) with relatively poor egg production in comparison to the Egg line hens used in the present study.
Blind surges of LH are probably associated with internal ovulations [1]. If the frequency of LH surges remained constant, then a higher incidence of blind surges would result in a decreased rate of egg production. The decline in egg production in the present study was not due to an increased incidence of blind LH surges, however, since no difference in incidence of blind surges was detected between the Early and Late hens. This is in comparison to the findings of our earlier study [1], in which a higher incidence of blind surges was observed in a line of turkey hens (RBC3) with poorer egg production and greater mature body weight than the Egg line hens (23.3% and 8.9%, respectively).
The intervals between LH surges not only became longer but also more variable in duration late in the reproductive period. In the chicken, the growth rate of ovarian follicles has been reported to decrease [22] and the time for ovarian follicles to reach maturity (become F1 follicles) to be longer late in the reproductive period [32]. In the current study, the weight of the F1 follicles was greater in the Late hens, but the number of hierarchical follicles was not different between Early and Late hens. Because there was no difference in the incidence of atretic follicles between the Early and Late hens, these observations suggest a slower rate of follicles entering the hierarchical phase of follicular development and a longer duration of this phase of development in the Late hens. Prolongation of this phase could result in longer intervals between maturation of F1 follicles in the Late hens. The duration of hierarchical follicular growth was not measured in the present study, however.
In laying chickens, a decrease in follicular maturation rate and egg production rate late in an egg production period may be caused by a higher incidence of follicular atresia [32–34]. However, the present results showed that higher incidences of follicular atresia were not detected late in the reproductive period, suggesting that this phenomenon does not contribute to the decrease in egg production late in the egg production period in the Egg line of turkey hens.
Continuous lighting was used in the current study to allow free running of the circadian rhythm of LH and P4 preovulatory surges [18]. In chicken hens, ovulation and oviposition times start to free run within a day or two of when hens are treated with continuous light, and become distributed over the 24-h solar day within 7–10 days [19, 27, 35]. But on return to diurnal lighting, within 1 day, ovipositions become restricted to a period of approximately 8 h duration [35]. Thus, in chicken hens, LH surges are presumed to be mostly restricted to a certain region (the “open period”) of the photoperiod under diurnal lighting [8, 36, 37], but to free run under continuous lighting. In comparison to chicken hens, in some turkey hens, the open period has been reported to be less restricted (of longer duration) or to be essentially absent under diurnal lighting [38]. In chickens, the period of the circadian rhythm of oviposition time has been reported to be line dependent [27]. In turkeys, the circadian rhythm of LH surges has been reported to free run under continuous lighting [18], with the interval between surges being line dependent [1].
In chicken hens, intervals between ovipositions of longer than 33 h are associated with pause days between sequences [19] and are observed under both diurnal and continuous lighting treatments. Intervals between LH surges of >33 h were observed in both the Early and Late groups of hens in the current study when the hens were given continuous light for at least 2 wk before and during serial bleeding. More intervals of >33 h, potentially associated with pause days, were detected in the Late hens than in the Early hens, suggesting an increased incidence of pauses between sequences in the Late hens. These results showed that the interval and increased variation between LH surges are major factors associated with decreasing egg production during its seasonal decline. Liu et al. [11] recently reported that hen age is not a major factor affecting egg production rate or LH surge interval in Egg line hens during peak egg production at a relatively old age. Thus, duration of the egg laying period, not hen age alone, appears to be associated with an increase in the interval between LH surges late in the reproductive period in turkey hens.
The duration of LH surges was not associated with the poorer egg production rate late in the reproductive period. This observation is in agreement with the results of an earlier study [1] in which the duration of LH surges at the peak of egg production did not differ between Egg line turkey hens and turkey hens from a line with poor egg production. Thus, the duration of LH surges does not appear to be associated with the egg production rate in turkey hens.
The possible effects of the baseline concentration of LH on egg production remains unknown, but a high baseline concentration of LH has previously been shown to be associated with initiation of egg production. Baseline levels of LH increase after switching turkey hens from short-day lighting to long-day lighting and reach a maximum about 2 wk before the start of egg production [39]. The baseline level then declines gradually until it reaches a relatively low but constant level after a few weeks of laying [10, 18, 28, 40]. Plasma concentrations of LH also declined late in the laying period in chicken hens, coincident with advancing age, decreasing egg production, and increasing egg size [41]. Sharp et al. [42] reported that old laying broiler breeder hens had lower plasma concentrations of LH and reduced pituitary responsiveness to chicken LH releasing hormone-I and -II (cLHRH-I and -II) in vivo in comparison to young laying hens and suggested that young laying hens might be more sensitive to preovulatory surges of LHRH. Direct measurements of LHRH secretion have not been made in turkeys or other avian species, however.
The higher LH surge amplitude observed in the Early hens and the lower baseline concentration of LH in the Late hens were not associated with the incidence of blind LH surges. This suggests that the incidence of blind LH surges is not associated with LH surge amplitude or baseline LH concentration, which change with progression of the reproductive period.
Secretion of P4 in laying hens is mainly by the granulosa cells of mature F1 follicles (chicken [43, 44]; turkey [45]). The slightly higher P4 surge concentration and lower LH surge concentrations of the Late hens suggest that surge secretion of P4 might be more sensitive to LH late in the reproductive period.
Most E2 is secreted by stroma tissue and theca cells of small ovarian follicles in domestic hens [34, 46, 47] and turkeys [48]. Although ovarian weight was greater in the Late hens, this increase in ovarian weight was not associated with increased plasma concentrations of E2. Similar results were found in an earlier study [1] in which ovarian weight (without yolky follicles) was greater in a line with a greater mature body weight but lower plasma concentrations of E2. Injection of FSH into hens late in a reproductive period [23] caused a dose-dependent increase in E2 concentrations. The authors suggested that the decreased rate of ovarian follicular development late in the reproductive period may be due to an inadequate stimulation by FSH. Since the number of hierarchical yolky follicles was not different between Late and Early hens in the present study, there is no evidence to indicate that E2 is associated with the number of hierarchical follicles in the ovary of turkey hens.
In conclusion, longer intervals between preovulatory surges of LH late in the egg production period are associated with the decline in egg production. Late in the reproductive period, there was also a higher intra-hen CV of LH surge intervals. The percentage of blind LH surges and the incidence of atretic yolky follicles were not important factors in determining the seasonal decrease of egg production rate in the turkey hens.
Acknowledgments
The authors thank Dr. Karl E. Nestor for kindly providing the Egg line turkey hens used in this study and Mr. John W. Anderson and Ms. Ruthi A. Patterson for helping with serial bleeding of the turkey hens. The LH reagents were kindly provided by Dr. John Proudman, U.S.D.A., Beltsville, MD.
References
Author notes
Salaries and research support provided by state and federal funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University.
