Y. Eitan

Selection Induced Genetic Variation

Selection of broiler chickens for juvenile growth rate and proportion of breast weight is an ongoing instance of intense long-term directional selection. The response was accompanied by correlated effects on reproductive performance and livability. Thus, the modern broiler is a unique resource for studies of the genetics of response to long term selection. Two remarkable aspects characterize this response. (1) After more than sixty generations of selection, genetic progress continues with no indication of a plateau, and (2) Correlated effects appeared in a punctuated and coordinated manner; that is, they appeared sequentially in time and simultaneously in the stocks of all mainline breeders. We present two models to explain these aspects: (1) A combination of Island Model and group selection together with internal changes caused by selection (endo-environmental effects) and external changes due to changed management practices introduced to ameliorate the correlated effects of selection (exo-environmental effects). (2) A new model: Selection Induced Genetic Variation, which explains the genetic variation required for the long continued response as resulting from sequential changes in genetic background by selection bringing new cohorts of genes into play through epistatic interactions, in a programmed manner.

Body composition and reproductive performance at entry into lay of anno 1980 versus anno 2000 broiler breeder females under fast and slow release from feed restriction

During the 1990s, various disturbances arose affecting broiler breeder females at entry into lay. These disturbances were associated with even slight overfeeding during release of feed restriction in this critical maturation period. The present experiment was carried out to gain some insight into the causes of these disturbances by comparing the effect of fast (FF) and slow (SF) release from feed restriction at entry into lay in 2 broiler breeder populations: B1980, representing the genetic level of 1980, and B2000, the genetic level of 2000. Under the FF treatment, B1980 entered lay 19.2 d earlier than B2000; this increased to 37.4 d earlier under SF. The B1980 population entered lay at virtually the same mean age for SF and FF, whereas B2000 entered lay 15.7 d earlier under the FF. Body weight at first egg were 2,621 g for the B1980 and 3,591 g for B2000. Differences in BW at first egg between feeding treatments within lines were minor. As a percentage of BW, ovary, oviduct, and follicle weights were the same for B1980 and B2000; breast weight was 14.9% for B1980 and 21.2% for B2000; abdominal fat pad weight was 5.37% for B1980 and 2.67% for B2000. Follicle weight and absolute difference in weight between successive follicles was greater in B2000 than in B1980. It is concluded that body fat content does not limit entry into lay, and that threshold BW for onset of sexual maturity of broiler breeder hens increased by about 1,000 g between 1980 and 2000, indicating a tight association between juvenile growth rate and threshold BW for onset of sexual maturity. It is also concluded that disturbances at entry into lay due to overfeeding are not due to smaller differences between successive follicles in B2000 compared with B1980. There are hints, however, that overfeeding may contribute to these disturbances by decreasing differences between successive follicles.