Ovarian dysfunction in broiler breeder hens
Commercial breeds of the domestic chicken include egg-layers (L) that produce table eggs, and meat strains. The former are lightweight hens that lay more than 250 eggs/bird at 60 weeks old. Meat strains are heavier birds bred for good carcass conformation and have a high rate of growth. Broiler breeders (BB), the parental and grand-parental strains, share these characteristics but the female lineage produces fewer eggs than egg-laying strains (175 eggs/bird at 60 weeks old). The lower rate of lay is the result of several phenomena. The average sequence length is shorter in BB, possibly due to a lower frequency of follicular maturation. Secondly, they lay irregularly. Thirdly, there is a much higher incidence of double-yolked eggs, which may be linked to an irregular development of the follicular hierarchy of the ovary. Some observers suggest the presence of pairs of follicles of similar size in BB, but others point to atresia in the smaller follicles. This irregular ovarian development is strongly dependent on the nutrition of the growing pullet. BB females normally receive a restricted ration equivalent to approximately 40% of the ad libitum feed intake to achieve the rates of lay cited above. If they are fed ad libitum, the rate of lay is less and there is a greater incidence of abnormalities in the ovary. Additionally, mortality and morbidities (the ascites syndrome, and leg problems) are higher in ad libitum fed hens. The pre-pubertal period from approximately 14 to 18 weeks of age seems to be particularly critical for feeding management. Attempts to simplify this management using fibre dilution of the diet have not been successful. Behavioural studies on L and BB strains reveal that the latter do not display the full inventory of behaviours. Exploratory behaviours (preening, scratching and perching) are hardly expressed, and restricted feeding necessarily limits feeding behaviour to < 15 minutes per day in BB, whereas this is spread throughout the day in L and interspersed with other behaviours. BB peck at the emptied feeder indicating that the bird is not satiated and may experience undue hunger. There is a clear ethical problem in the use of BB females in the meat production chain. Without feed rationing, mortality and morbidity are high, egg production is lower; the system is clearly unsustainable. Severe rationing is economically effective, but the acceptability of this technique is open to question if the animals experience chronic hunger. An alternative is a mid-weight strain (used in organic farming systems) but the end product costs more.
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