An improved mathematical model of the ovulatory cycle of the laying hen

1. The mathematical model of the hen's ovulatory cycle proposed by Etches and Schoch (British Poultry Science, 25: 65-76, 1984) predicts ovulation times for sequences of 2 to 9 ovulations only. 2. Continuous functions have been produced, representing the changes required to the parameters lambda1, lambda2, S1, S2, b1, b2 and b3, such that the prediction of any sequence length is now possible. 3. This improved ovulation model is capable of predicting ovulation times and intra-sequence ovulation intervals for any ovulation rate between 0.5 and 1.0. 4. The improved ovulatory model lends itself to stochasticity. The rate of lay of a population of hens at a time may be modelled with the use of means and standard errors for each of the parameters in the model. 5. Age-related changes in the ovulation rate of the population may be predicted using a combination of three methods, which are consistent with published theories that account for the decline in performance with time.     

Posture and heart rate in the domestic hen

When unanaesthetised fowls were hung upside down on shackles the heart‐rate immediately rose by about 30 per cent; on re‐inverting them after 2, 4 or 6 min it returned rapidly to normal. It behaved similarly in anaesthetised birds save that the response to inversion was somewhat smaller and slower, depending on the depth of anaesthesia, and there was some over‐compensation before return to normal. It is concluded that the effect was probably a response to changes in the activity of baroreceptors in the cardiovascular system and that the data do not disclose evidence of serious distress.     

Exogenous leptin advances puberty in domestic hen

The present study was undertaken to examine the effect of recombinant chicken leptin administered to fed ad libitum and feed-restricted immature chickens of a layer strain on ovarian development and the timing of sexual maturity. In the first experiment 11-week-old pullets (77 days of age) fed ad libitum were injected daily with leptin at four dose levels (4, 16, 64 and 256 microg/kg body weight) until sexual maturity (lay of the first egg). Leptin treatment at the highest dose significantly (P<0.05) advanced the onset of puberty (day 116.3+/-1.0) in comparison to controls (day 121.3+/-1.2). The rises of luteinizing hormone, estradiol and progesterone in blood plasma were also advanced by leptin treatment. In the second experiment, both full-fed and feed-restricted pullets (79 days of age) were injected daily with leptin (256 microg/kg body weight). In birds fed ad libitum, exogenous leptin again significantly (P<0.05) advanced first ovipostion (day 118.4+/-1.4 versus day 124.4+/-1.7), while abolishing the significant (P<0.05) delay caused by feed restriction (day 131.5+/-1.6) and restoring the normal onset of sexual maturity (day 125.7+/-1.6). Analysis of the ovaries in 106-day-old pullets revealed that leptin injections advanced follicular development, particularly in birds fed ad libitum, and significantly (P<0.01) reduced follicular apoptosis both in full-fed and feed-restricted birds. In conclusion, we have shown that in female chickens exogenous leptin advances the onset of puberty by attenuation of ovarian apoptosis and enhancement of folliculogenesis.     

Physiological and behavioural responses of the domestic hen to hypoxia

Physiological and behavioural responses of adult hens and chicks were monitored in atmospheric and increasingly hypoxic conditions to investigate whether anoxic killing was a suitable alternative method for use in the slaughter industry. All birds were placed in a perspex box for easy observation and nitrogen gas was slowly administered when subjecting birds to anoxic conditions. When subjected to decreasing oxygen concentration adult birds slowly became unconscious, without showing any signs of distress, until respiratory failure supervened. Chicks showed similar results but loss of motor control was observed while still conscious which might cause some distress. Further investigations are suggested to alleviate this problem.     

A study of the progressive deposition of shell in the shell gland of the domestic hen

It is known that the organic matrix of the shell is not distributed evenly throughout the calcified portion and an experiment has been described in which incompletely shelled eggs were removed from a group of domestic hens after being in the shell gland for various known periods.

Rate of shell deposition and the timing of the deposition of the various layers in the shell has thus been determined, and it was found that shell deposition was initially slow for the first 3 h in the gland and thereafter more rapid, at a constant rate of 322 mg h^?1, until termination after 17 or 18 h. The narrow layer low in matrix at the top of the mammillary layer of the shell was laid down during the 7th to 9th hour in the gland and did not appear to be associated with any change in shell deposition rate. It was also found that the characteristic pole to pole thickness patterns of shells were present at the earliest measurable stage, that is after about 8 h in the gland.     

Breeding for late egg shell quality in the domestic hen

An experiment is reported, in which shell weight per unit surface area was measured for eggs laid by pedigreed pullets at 33, 54 and 65 weeks of age, using a Rhode Island Red strain and a White Leghorn strain. Specific gravity and albumen quality were also measured and records of individual egg production were available.

The heritability of “shell thickness “ (i.e. shell weight per unit surface area) in the Rhode Island Reds declined from 0.57 in the first period to 0.33 in the last period. The genetic correlation between thickness in the first period and thickness in the last period was 0.90. The relative selection efficiencies expected by basing selection on early shell thickness with the object of improving late shell thickness were greater than 1.0, indicating that early selection would be the best method of improving shell quality in this strain. In the Leghorn flock, estimates of the heritability of shell thickness were low (0.07 to 0.20) and variable. The genetic correlations between thickness measured in different periods were generally lower than for the Rhode Island Reds, and relative selection efficiencies varied widely. The Leghorns had much thicker shells than the Rhode Island Reds at all times of the year but showed little prospect of further improvement in shell thickness whichever time of measurement was used as a basis for selection.

Specific gravity was found to be highly genetically correlated with shell thickness and so would be a good estimator of shell thickness, although more eggs per pullet must be measured to give the same accuracy as direct measurement of shell weight per unit area. There were no adverse genetic correlations between thickness and the other traits measured, except perhaps in the case of albumen quality.

It is concluded that the lack of any general improvement in end‐of‐year shell strength is probably not due to low heritability, to adverse genetic correlations or to inaccurate methods of measuring shell thickness, but to a lack of selection pressure.     

Fine structure of the facial nerve nucleus of the domestic hen]

The facial nucleus of the domestic hen is composed of a dorsal, intermediate, and ventral portion. The dorsal and intermediate one consist of medium-sized and rare small polygonal and oval neurons. The ventral subnucleus shows medium-sized and large neurons. Electronmicroscopically, the latter exhibit numerous Nissl bodies with short cisterns and a lot of polysomes. The small neurons comprise few but large Nissl bodies with long cisterns of the rough endoplasmic reticulum. According to the shape of synaptic vesicles and the number of mitochondria three distinct types of synapses can be distinguished.     

Role of magnesium in egg shell formation in the domestic hen

1. The effects of feeding diets containing various amounts of magnesium on plasma concentration of calcium and magnesium in the domestic hen were investigated. 2. Plasma concentrations of calcium and magnesium decreased during shell formation in all birds. 3. Plasma magnesium content and egg shell thickness were severely reduced in birds given diets containing either 207 or 132 mg Mg++/kg. 4. Using electron microscopy, a precise correlation was observed between the normal distributions of magnesium and organic material across the egg shell of the domestic hen.     

Influence of various nesting materials on nest site selection of the domestic hen

Nesting behaviour of LSL hens from a deep litter house and from a battery system was compared. Every hen was tested in one of two trial chambers containing a wire mesh floor and a nest unit with 5 nesting materials: wire mesh, perforated plastic, synthetic grass, wheat straw and oat husk. Wheat straw and oat husk were preferred as nesting materials to perforated plastic, synthetic grass and wire mesh. The results from these trials were confirmed in a deep litter house. After the nesting material, oat husk, was changed in two of 10 communal nests the hens did not accept those two nests for the trial period of two weeks and laid elsewhere. Hens of the same breed and age reared together on deep litter showed no differences in nest site selection and nesting behaviour regardless of whether they had previously been housed in a deep litter house or in cages.     

Effects of an advance of darkness on the ovulatory cycle of the hen

Hens maintained on 16 h light: 8 h darkness were subjected to an abrupt advance of darkness by between 4 and 13 h. When darkness was advanced so that it occurred less than 6 to 8 h after ovulation, the next expected preovulatory release of luteinising hormone (LH) and ovulation was usually delayed or blocked. This was associated with a reduction in the ability of injected ovine LH to stimulate the secretion of progesterone. When the advance meant that darkness occurred at more than 6 to 8 h after ovulation subsequent ovulations were advanced by as much as 5 h within a 3 d period. The hastening effect of an advance of darkness on follicular maturation was associated with an increased secretion of LH in response to injected progesterone. An advance of darkness by 4 or 7 h was associated with an earlier onset of the circadian increase in the secretion of corticosterone and changes in the tonic secretion of LH, notably a tendency for plasma concentrations of LH to increase during the first 4 to 5 h of advanced darkness, followed by a steep decline.     

Plasma progesterone concentration in the hen in relation to the ovulatory cycle

The concentration of peripheral plasma progesterone was measured by a double isotope derivative assay in hens slaughtered at various stages during the ovulatory cycle. A peak value was obtained 2 to 6 h before ovulation and basal levels were reached at ovulation. No secondary peaks were observed during the cycle. Similar results were obtained using a protein‐binding assay on plasma from individual birds taken serially during the cycle. Birds which did not ovulate on a particular day showed no peak of progesterone. In two birds sampled at the beginning of a laying cycle, the progesterone peak was of longer duration.     

Occurrence of waxy-textured yolk in the egg of the domestic hen (Gallus domesticus)

An unusual yolk material (waxy yolk) was identified in some hard boiled yolks of one strain of domestic hen. Its structure was consistent with it being formed from normal yolk through disruption of the yolk spheres. These are normally stable during even long periods of storage. Formation occurs mainly after ovulation, during passage of the egg down the oviduct; only rarely could it be found in the larger follicles of the ovarian hierarchy. In the flock of birds examined the incidence of eggs with waxy yolk was 75% of all eggs laid over a 7 d period. Of the 173 birds tested, the eggs of only 17 birds were totally unaffected; all eggs of 82 birds contained this material. The results of a hatchability trial indicated this material had no apparent influence on the embryo or hatchability.     

Calcitonin receptor binding in the hen anterior pituitary during an oviposition cycle

The equilibrium dissociation constant (K_d) and the maximum binding capacity (B_max) of calcitonin (CT) receptor in the plasma membrane of the anterior pituitary in hens were examined by Scatchard analysis of specific binding of ¹²⁵I‐labeled chicken CT. Values of K_d and B_max of CT receptor were smaller in laying hens than in non‐laying hens. A decrease in the K_d and B_max value of CT receptor was observed in the anterior pituitary after the injection of estradiol‐17β and progesterone into nonlaying hens, but not changed after the injection of 5α‐dihydrotestosterone. During an oviposition cycle, the K_d and the B_max value decreased 3 h before oviposition. In non‐laying hens, neither the K_d nor the B_max value changed during a full day period. The present study suggests that the CT action on the anterior pituitary may increase 3 h before oviposition by the effect of estradiol‐17β and progesterone in laying hens.     

Relationship between plasma concentration of luteinising hormone and intensity of lay in the domestic hen

1. The concentration of plasma luteinising hormone (LH) in samples taken at frequent intervals from 6 weeks of age until the onset of lay, and also at 9 months after the onset of lay, was significantly greater in hens with a high rate of egg production than in comparatively poor layers.

2. The difference was most marked during the period 7 to 9 weeks of age when there was a transient increase in plasma LH in hens that subsequently had a high rate of egg production, while LH concentration remained stable in comparatively poor layers.

3. The concentration of LH in plasma, particularly at 7 to 9 weeks of age, before gonadal growth, or during the prepubertal peak of LH secretion, at about 2 to 4 weeks before the onset of lay, may be a useful criterion in the selection of laying strains.     

Influence of food restriction and of the presence of chicks on the reproductive system of the domestic hen

An experiment was conducted with 4 groups of hens: a control group of laying hens, a group subjected to food deprivation for 7 d, a group subjected to food deprivation for 7 d, then re-fed for 10 d and a group subjected to food deprivation for 7 d then re-fed for 10 d with two chicks per hen introduced during the last 7 d of refeeding. Food deprivation provoked the involution of the oviduct and ovarian regression, as well as a decrease in the plasma concentrations of luteinising hormone (LH) and sex steroids (progesterone, testosterone and oestradiol). After 3 d of re-feeding, there was a significant increase in the plasma concentrations of LH and steroid hormones. This phenomenon was even more marked after 10 d of re-feeding; most of the hens of the third group which were not given chicks, were at the point of lay. The presence of chicks resulted in the expression of maternal behaviour and suppressed a rapid return to laying. This was especially marked in hens showing typical maternal behaviour traits, for which the morphological and endocrine measurements indicated a decrease in the activity of the hypothalamo-hypophyseal-gonadal axis.     

Unimportance of yolk loss in the egg as a factor limiting egg production in the domestic hen

The injection of yolk material into the body cavity to replace that lost in the egg did not increase the rate of oviposition. During the first few weeks of this procedure a drop of 18 per cent in the rate of laying was observed.

It was concluded that the loss of material in the formation of yolk is not a limitation to subsequent ovarian activity.     

Histological studies of changes in the magnum of the domestic hen associated with the production of 'watery white' eggs

Abnormalities in the magnum of hens producing eggs with 'watery whites' are described. The histological staining characteristics of the epithelium were altered and there was a decrease in the number of ciliated cells. The structure of the tubular gland cells was also changed, with an increase in the number of type-B cells.