ABSTRACT This study reports on the results of a selection experiment using Japanese quail (Coturnix coturnix japonica) with the aim of assessing the impact of a number of selection regimes on tolerance to heat stress as measured by growth rate. Four lines were selected for six generations for increased weight gain from 14-28 days of age under either high (W, 32ºC) or normal (C, 25ºC) temperature conditions on either a high (H, 250 g/kg) or low (L, 150 g/kg) protein diet. Both diets contained 13.0 MJ ME/kg. The four selected lines were thus designated WH, CH, WL and CL. Fortnightly hatches were produced each generation from matings between 12 males and 36 females per line, with the parents of the next generation selected from approximately 100 birds per sex per line. A randomly selected control line (line C) of similar size was also maintained and reared in all cages with the selected line birds. Each generation, a third hatch of birds was reared and approximately 20 birds per line per environment were placed in single-bird cages to measure correlated responses in weight gain and feed efficiency in all four temperature/ dietary protein environments. In addition, correlated responses were measured in body composition and an assessment of the relative responses of the lines to early post-hatch epigenetic heat conditioning and to dietary addition of betaine (an osmolyte) were undertaken. The final study involved measurement of correlated response in the lines in the components of reproductive performance. On the high protein diet, response in growth rate relative to the control line in all selected lines was positive under both temperature environments, and by generations 5 and 6 growth rate was higher in the WH than in the CH line birds in both temperature environments. As weight gain was the selection criterion, these results suggest that selection for increased growth rate under high temperature conditions may not only be beneficial for progeny reared under high temperature conditions, but might also be at least as effective as selection under normal temperatures for progeny reared under normal temperature conditions. Whilst there was no equivalent advantage in feed efficiency in the WH line birds in either temperature environment, and the results arise from a comparison between un-replicated lines, they certainly indicate that such a selection approach is unlikely to have deleterious consequences. There was no benefit in either growth rate or feed efficiency under either temperature environments achieved through selection of birds on a low protein diet, irrespective of the selection temperature conditions. This indicates that irrespective of environmental temperature either during selection or subsequently, any possible advantage obtained through a reduction in the protein breakdown rate conferred through selection on a diet limiting in protein is outweighed by other factors contributing to a lower heritability and poorer response in these lines. In contrast to the WH line, the CH line birds in the normal temperature conditions were significantly (P<0.01) leaner under high than normal temperature conditions, although these differences were not reflected in differences in feed efficiency. On the low protein diet, females were considerably fatter than males and the CL line birds had a considerably higher proportion of body fat than either their WL or C line counterparts. Responses in body composition emphasise the inadvisability of selecting birds for growth rate under protein deficient diets, particularly if they are expected to perform under high temperature conditions on either high or low protein diets. In a study of the effect of early post-hatch epigenetic heat conditioning on subsequent performance in the single-bird cages, half of the chicks from each line were exposed at 2 days of age to 38OC for 24 hours. The results of the study showed that there was a positive effect of heat conditioning, as measured by subsequent growth rate, in birds selected for increased growth rate under normal temperatures. However, selection for increased heat tolerance appeared to render the birds refractory to the beneficial effects of epigenetic conditioning. This suggests that the physiological pathways exploited by epigenetic heat conditioning are also utilised in selection for heat tolerance. Inclusion of betaine in the diet at 0.5 g/kg had a beneficial effect upon growth performance from 14-28 days of age in birds kept under high temperature conditions, but the degree of the effect was influenced by other factors possibly associated with the nutrient (and particularly the amino acid) composition of the diet. There was a lack of consistency in the relative responses of the lines to betaine supplementation indicating that it is unlikely that osmoprotection contributes in any meaningful way to heat tolerance expressed by the lines selected under high temperature conditions. A study of reproductive performance of the birds at generation six lent support to previous published findings showing deleterious effects upon the components of reproductive performance from selection for increased growth rate. There were, however, significant differences between the selection lines which suggested beneficial outcomes from selection for growth rate under moderately high temperature conditions, particularly in breeders housed under these same conditions. Relative to selection under normal temperatures, these benefits included: early onset of lay, increased egg production, increased egg weight and improved fertility.
| Identifer | oai:union.ndltd.org:ADTP/279222 |
| Creators | Abdullaziz Al-abdullatif |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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