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ABSTRACT of the
STUDY OF THE HORMONAL FACTORS INVOLVED
IN THE MECHANISM OF FORCED MOLTING IN HENS
BRED IN INDUSTRIAL SYSTEM
by PHD student Cristina DASCĂLU (IONESCU)
This thesis aims to identify and describe the physiological and hormonal
mechanisms involved in the onset and development of the forced molt phenomenon in hens bred in industrial system, hypothesising differences by comparing to spontan (natural) molt.
The thesis is classicaly structured in Part I, aimed to the presentation of the
data from the literature regarding the current state of knowledge on the natural and forced molt processes, in terms of functional, metabolic and hormonal changes and the factors involved in these processes, and Part II, assigned to the personal researches.
Part I begins with an "Introduction" where the natural and induced molting
processes are briefly described in both wild and domestic birds.
Chapter 1.1 was reserved for the description of the particularities regarding
the physiology of the female reproductive system in birds.
In Chapter 1.2 it is presented the current stage of knowledge on natural
molting physiology, detailing the main changes in state of physiology: metabolism, reproduction, and particularly the hormonal mechanisms involved in the initiation, development and support of this process.
Chapter 1.3 is dedicated to the description of forced molting in terms of
involved hormonal mechanisms, the influence factors and the induction methods used.
Second part of the thesis consists in own researches and it is structured into
the following chapters: 1. Research objectives, 2. Material and methods, 3. Researches on the development of the physiological state during the process of spontaneous molting in hens, 4. Researches regarding the experimental involvement of thyroxine and progesterone in the mechanism of forced moulting in laying hens and 5. General conclusions.
Objectives set at the beginning of researches are presented in Chapter 2.1,
they are the following: 1. Evolution of physiological state during spontan molting process of hens in industrial breding conditions. 2. Experimental involvement of some hormones (thyroxine and progesterone) in the mechanism of forced molting in laying hens.
In Chapter 2.2. Material and methods, biological material is presented and
there are described in detail the methods of determination for the blood morphology and biochemical parameters, plasma metabolic enzyme activity and plasma levels of the hormones with relevance in the molt process: thyroxine, triiodothyronine, corticosterone, luteinizing hormone, prolactin, estrogen and progesterone.
Chapter 2.3 is entitled Researches regarding the evolution of the
physiological state during the spontaneous molting process in hen. In the frame of this chapter a broad analysis is performed on the physiological parameters with relevance in the development of the spontaneous molting process in hens bred in industrial system. Research has been conducted on an White Leghorn laying hen group, 63 weeks old, bred in the industrial system, characterized by spontan moulting phenomenon, which was not artificially induced or by the intervention of any stressful factors. These hens were monitored for the development of the physiological state and biochemical and hormonal status over a period of eight weeks, how long the process of molting was present, before returning to the normal rate of lay for their age. Research has been conducted in parallel with a control group of hens of the same age and breed, mentained under identical conditions. These groups were monitored primarly in terms of body weight development, feed consumption and rate of lay, observing the following: 1. body-weight of spontaneous molt group was 19.5% less, compared with control group in the first four weeks of observation; 2. food consumption in the hens with spontaneous molt was significantly lower vs. control group during the first four weeks of monitoring; 3. laying percentage of the molt group reached a zero level in the second week from the diagnosis of molt phenomenon, increasing significantly in the fifth week of monitoring, then seeming to be capped at a lawer level than control group.
Regarding the evolution of main morphological parameters of the blood
during the process of spontaneous molt, it was found that the erythrocyte number in molt hen group was higher vs. control group. The hematocrit of spontan molt hen group showed an increase, with a peak during profuse molting, after that presenting constant values over the control.
Analysis of the evolution of the derived red blood cell parameters (MCV,
MCH and MCHC) showed that changes in hematocrit is achieved at the expense of increasing the number of erythrocytes, increased erythropoiesis being real.
Total blood leukocyte number was increased in hen molt group, during
profuse molt. After exhausting of profuse molt period it was found that the number of leukocytes decreases, approaching to control group. Heterophile / lymphocyte ratio amounted to 0.43 in the midle process of moulting (first day of observations) and then decreased to 0.18 at the end of the molt process.
Regarding the evolution of the main blood biochemical parameters during the
process of spontaneous molting, blood glucose concentration in molt group increased directly proportional to the restart of feeding, reaching a peak in the eighth week of monitoring.
Total plasma proteins remained constant during the first four weeks of
monitoring, after which their levels showed an upward trend, differences between groups becoming no significant.
Total plasma triglycerides show values, significantly lower compared to the
control. Differences between groups decreased gradually becoming no significant from the sixth week of monitoring.
Evolution of plasma total cholesterol of molting group showed a high level
until the fifth week of monitoring, revealing an increase of its mobilization, then plasma cholesterol levels decreased.
In the experimental group, uric acid plasma levels remained relatively
constant low for about two weeks, revealing that the protein catabolism decreased. Then plasma uric acid level increased, reaching maximal values in the fourth week of monitoring.
Total serum calcium showed low levels during the first three weeks of
monitoring. Serum calcium level generally followed the curve of egg production, with significant increases after the fifth week and it was stationary after eighth week of monitoring.
Serum enzyme activity showed an evolution specific to the physiological
crossed state. Thus, in the molt group at moulting diagnosis moment, GOT activity was significantly lower than that of the control group (48.9 IU), remaining low during the next two weeks, after which there was a slow growth until the eighth week of monitoring, when molting process was considered complete. In parallel, the GPT activity in the molt group had a similar trend as GOT, amounting initially to 25.2 IU (lower), with an upward trend reaching at the end of the monitoring period the normal values of this age.
G-GT activity showed an initial value of 6.6 U/L compared with 12.6 U/L in
control group. Differences between groups were attenuated during the development process of molting.
LDH activity in molting group was initialy 244 IU (compared to the control
group with 212 IU) revealing an increased carbohydrate catabolism. Differences between groups became no significant after the first two weeks, remaining so until the end of the molt.
Plasma levels of thyroxine (T4) showed a higher peak compared with T3
(22.3 vs. 3.8 microg/dL) and subsequent decline was slower than those of T3. High levels of these two hormones during molting illustrate an intensified catabolism, materialized in lowering of the body weight.
Corticosterone levels showed average values over time amounting to 8.0
ng/mL in molt group, compared with 5.0 ng/mL in control group, revealing a relatively high alert and explaining the high value of the heterophile/lymphocyte ratio.
The LH plasma levels in laying molt hens amounted initialy to 1.4 ng/mL, the
value being about ¼ lower vs. control, maintaining relatively low in the next four weeks, then it presented a significant upward trend, parallel to the resumption of ovarian activity, reaching 3.8 ng/mL after the eight weeks of monitoring.
Plasma prolactin of molt group presented double values vs. control, with
significant differences, which where reduced in the second half of the monitoring period.
Estradiol showed minimum values in the initial period of molting,
corresponding to the absence of evolutive ovarian follicles, increasing slightly as this physiological period has been exceeded and the laying resumed.
Plasma levels of progesterone showed a mean value of 320.2 pg/mL, 221
pg/mL lower than in the control group, with a minimum in period of profuse molt, then it was increasing slowly as the molt condition was exceeded.
In Chapter 2.4, entitled Researches concerning the experimental involvement
of thyroxine and progesterone in the mechanism of forced molting in laying hens, an experimental model was done consisting of laying hens in the middle of laying period to whom thyroxine, respectively progesterone, experimental treatments were given, hipothesising an effect of molt induction.
Researches have been conducted on an experimental model represented by
three groups of Plymouth Rock hens aged 52 weeks, in full laying cycle, two experimental groups and one control group.
Experimental treatment with thyroxine lasted for 16 days and the one with
progesterone for 18 days, ending on installing molt phenomenon.
Plasma levels of thyroxine and progesterone in experimental groups during
treatment were significantly higher than control group revealing the efficiency of applied doses.
During experimental hormone treatments, it is noted that both hormones have
decreased the rate of lay. Thyroxine showed an effect of reducing the percentage of laying a few days late than progesterone, on one hand, and on the other hand, thyroxine induced a decrease in the percentage of lay lower than progesterone. Also, the return of the laying rate at technological values was faster in the group treated with progesterone than in the group treated with thyroxine.
Hormonal treatments induced a decrease in body weight of treated groups.
The lowest average body weight of hens that are part of the group treated with thyroxine was achieved in the fourth week of monitoring (age 56 weeks, respectively about a week later from the end of treatment). In the group treated with progesterone, the minimum weight was reached at the end of treatment.
Following that time, the body weight of both experimental groups began to
grow approaching to the technological weight values.
Based on the results concerning the gravimetric evolution of ovary and
oviduct mass in the two experimental groups, we can say that in case of thyroxine, weight loss was achieved predominantly due to the increasing of general catabolic processes in the body, while for the group treated with progesterone, predominantly by decreasing of reproductive system mass (ovary and oviduct).
Thyroxine administration triggered a massive collapse of the feathers after
seven days of treatment, leading to a complete molt while progesterone induced only a partial failure of feathers.
During forced molting induced by administration of thyroxine and
progesterone in hens subjected to experiment, some changes in blood plasma levels of hormones potentially involved in this process (corticosterone, luteinizing hormone, prolactin and estradiol) were revealed.
Blood levels of corticosterone in samples from hens treated with thyroxine
increased while maintaining relatively high during the experimental treatment, similar to the situation encountered in spontaneous molt. Corticosterone level maintained over 60% compared to baseline pre-treatment experimental values, returning suddenly to rest values over the 16 days of treatment, in the 56th week-of-age.
In case of the group of hens treated with progesterone, corticosterone
increases up to 36% compared to value prior to the establishment of experimental treatment, which proves that the experimental treatment with thise hormone did not exercise excessive stress influences.
Plasma LH level for the group treated with thyroxine remained constant
during Euthyrox administration. In group of hens treated with progesterone plasma levels of LH amended, identifying a decrease (compared to the value before administration of progesterone) during the 18 days of experimental treatment. In the week following the end of the experimental treatment with progesterone, plasma LH level was found higher than the control group.
For the group of laying hens treated with thyroxine, the level of plasma PRL,
presented a dynamic evolution, similar to the control group.
In the case of the group treated with progesterone, plasma PRL levels showed
a sharp drop. After interruption of progesterone treatment, plasma levels of PRL has already started to return to normal values, continuing to grow in the coming weeks to values which tend to exceed the control.
Treatment with progesterone caused a decrease in blood plasma levels of
estradiol, due to ovarian atrophy observed by gravimetric analysis and inspection of this organ. At the end of the 18 days of treatment with progesterone, estradiol blood levels are already raising, revealing resumption of ovarian activity.
Based on the results above related it can be concluded that in the molting
process the hormonal effects are complementary and concerted. Thyroxine acts faster and induces especially the feather drop and weight loss while progesterone effects are slower and can be specifically responsible for blocking the reproductive activity.
Chapter 2.5 contains general conclusions. The last chapter of the thesis was booked for references (numbered 261),
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