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Natural moult
A natural moult was observed during the present study. As the phenomenon was not anticipated, provision for detailed recording was not made. However, as the moult interfered with the performance of some of the groups under evaluation, as referred in different sections of this thesis, a separate report of its confounding effects on growth and egg production will be made.
The plumage renewal started in early May each year, affecting growing pullets in the first experiment (1996), and laying hens simultaneously in both experiments (1997). The process was not synchronised in time and rather extended for about four months. At the onset of the moulting period, growing pullets were 13-wk old. The laying hens were 36-wk and 64-wk old and starting the 5th (mid-cycle) and 12th (late-cycle) month of production in the second and fIrst experiment, respectively. The average daylight length during the moulting period was 10.5 hr. Clearly visible de-feathering was observed in all birds with the exception of normal size normal feathered hens (nana ff Dw-), but the effects of moult on growth and production were considerably greater among the dwarfs irrespective of their feather coverage.
The effect of juvenile moult on the absolute body weight of Exp. 1 pullets was not clearly visible, as shown in Fig. 4.5. However, a deceleration in the rate of sexual maturing among the dwarfs, measured as body weight relative to the onset body weight, became evident after the 10th week of age (Fig. 4.7), with a resulting 7-day delay in age at first egg, in comparison with their normal size counterparts. As a consequence, early egg production (1-8 weeks) of dwarfs was diminished by 32 % in the fIrst experiment as farther presented in Table 4.17.
Among the Exp. 2 layers (mid-cycle moult), cessation of egg production was observed in 18 % of the dwarf population whereas in just 1 % of the normal birds. In the older group of hens (late-cycle moult), egg laying was interrupted in 12 % and 9 % of the existing dwarfs and non-dwarfs, respectively. At both ages, no normal feathered non-dwarf hen was represented. The mid-cycle moult had a higher depressing effect on the rate of lay of the dwarf hens than that occurred at the end of the productive year (Fig. 4.9), the difference to the normal counterparts being widened by 7.1 % (Exp.2) and 3.3 % (Exp. 1) during the moulting period, as compared with the laying intensity at the onset of the moult. All moulting hens in the second experiment resumed egg production, whereas 29 % of those that halted production in Exp. 1 never recovered. Mean body weight of the various groups of moulting layers was not reduced. However, in the post-moulting period, the weight gain of dwarfs was almost twofold that of normal size hens.
Laying rate
Fig. 4.11 illustrates the weekly laying rate (egg production/100 hen day) achieved by normal size and dwarf hens in each experiment, and averaged values by period are presented in Table 4.19. Appreciable differences in the shape of the curves were determined by the season of year, with special relevance to those occurred during the fIrst (1-8 wk) and the second (9-30 wk) parts of the fIrst cycle. Whereas laying intensity in the fIrst experiment increased gradually after the initial peak until the fIfth month, when climate was mild, it was seen to decline rather abruptly soon after the peak for equal period in the second experiment. Among the dwarfs, such decline was extended for a further eight weeks. At the end of the fast-growing phase, however, laying rate was higher in the second than in the fIrst experiment. Considerable fluctuations resulting from environmental changes in temperature and relative humidity were noticeable throughout both experiments. It is worth noting the higher fluctuation caused by abrupt rises in temperatures in the laying rate of normal birds as opposed to the more stable production observed in the dwarfs during the sununer ofExp. 1 (week 22 to 42). A specifIc gene interaction with environment determined another major difference between the curves and hence the production rates of the layers. A comparison between groups within experiments shows marked lowered production in the dwarfs in two particular periods. The fIrst elapsed approximately between weeks 18 and 33 and affected the summer layers, and the second occurred between weeks 44 and 59 involving the winter hens. Such reduced intensity of egg production among dwarfs accompanied an unforced moulting, described in a separate section within this thesis. Differences between body size groups were hence widened in the second and third periods of the fIrst cycle in the second experiment and in the second cycle in the fIrst experiment (Table 4.19). The values in this table are further showing that for the entirety of the fIrst 52- weeks of production, higher rates were observed in the fIrst experiment in both genotypes. Maximum production attained by non-dwarf hens was 97 and 94 eggs/1 00 hen day in the fIrst and second experiments, respectively, being the correspondent fIgures for dwarfs 84 % and 82.5 %.
1 INTRODUCTION
1.1 Poultry production in Mozambique
1.2 Strategies for poultry development in tropical countries
1.3 The scope of the study
2 LITERATURE REVIEW
2.1 Growth of pullets
2.2 Sexual maturity
2.3 Stress physiology in poultry
2.4 Productive penalties of heat stress
2.5 The efficiency of feed utilisation in hot environments
2.6 Major genes for improved tolerance to heat stress
3 MATERIALS AND METHODS
3.1 Experimental birds and management
3.2 Experimental design
3.3 Data collection and experimental procedures
3.4 Calculated traits
3.5 Statistical analysis
4 RESULTS
4.1 Climate and photoperiod within experiments
4.2 The path to sexual maturity
4.3 Natural moult
4.4 Productive performance
4.5 The penalties of heat stress on production and survival
4.6 Economic analysis
