The endocrine control of egg production in poultry
Williams, John Boyden
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(i) Yellow yolky ovarian follicles were shown to be an important, though not the sole source of progesterone in laying birds. Levels of progesterone and LH in the plasma were directly related to the extent of ovarian follicular development. However, during the brooding period an increase in plasma LH levels is not associated with an immediate increase in plasma progesterone levels.(ii) The largest, mature, yellow yolky follicle seems to produce more progesterone than smaller follicles and progesterone from this source may be largely responsible for the pre-ovulatory increase in plasma progesterone levels.(iii) A combination of a diurnal increase in plasma LH levels and other changes in plasma concentrations of other hormones (possibly androgens or prolactin) was suggested to initiate progesterone secretion by the mature, pre-ovulatory ovarian follicle. Increased progesterone secretion triggers the pre-ovulatory increase Of LH acting via a positive feedback system which has an important neural component.(iv) Hens were observed to lay shorter sequences of eggs as they become older, but as the times of lay of first and last eggs of a sequence did not vary with age, shorter sequences were not due to a shortened 'open period' of the ovulatory cycle. The higher rate of lay of modern egg-type strains compared to strains used over twenty years ago cannot be explained by differences in the times of lay or lag within a sequence. Times of lay and lag were identical in both types of hen.(v) An age-dependent shortening of sequence length was observed in three different strains of hen (Ross I broiler breeder; Ross Ranger mid-weight egg layer; Babcock B300 lightweight egg layer) with markedly different egg production rates. It was suggested that this may result from an increase in the time taken for successive follicles to reach an ovulable condition.The ovulable condition, or the maturity of the follicle was thought to be related to its capacity to secrete, and sustain secretion of large amounts of progesterone sufficient to activate the positive feedback mechanism controlling LH release. This maturity was independent of the size of the follicle and the amount of yolk it contained.(vi) A further cause of declining egg production with age in all three strains studied was the occurrence of short breaks in laying of 2-7 days. These breaks may have been due to undetected soft-shelled eggs, internal ovulation or occasional follicular atresia. Longer breaks in laying, of the order of 28 days, were an important source of lost production in the broiler breeder and mid-weight egg laying strains. The relatively early incidence of long breaks in broiler breeders may account for the poor overall egg production of these hens.(vii) The pattern of LH levels in growing hens was different in all three strains. All strains showed a pre-pubertal peak of LH associated with the development of the progesterone positive feedback mechanism, but this occurred at a different time in each strain relative to the onset of lay. The mean plasma LH levels.were higher in developing Ranger than Ross I or Babcock hens. Rising plasma progesterone titres were associated with the onset of lay, but they were not associated with the decline in plasma LH titres from peak pre-pubertal values. It was suggested that the pattern of pre-pubertal LH secretion, by affecting early ovarian follicular development, may have a bearing on subsequent egg laying performance.(viii) The basal progesterone and LH plasma levels during the first laying year differed in the three strains. Furthermore, the fluctuating basal hormone levels did not correlate with each other, nor could basal hormone levels be correlated with egg production in any way. It was suggested that basal levels of gonadotrophins may regulate the incidence of follicular atresia.(ix) The morfhology of the ovaries of broiler breeder and egg laying type hens are similar and age-related changes in the ovarian follicular hierarchy are seen in both strains. Follicles grow to a larger size in old birds before they ovulate, but this may only have a minor effect on egg production rates. Abnormal, irregular laying patterns seen in old hens were not the result of abnormal, irregular follicular hierarchies.