Abstract
Two breeds, White Leghorn (WL) and Barred Plymouth Rock (BP), inbred to some extent, were mated within and between the breeds, using 2 males and 24 females from WL and 2 males and 29 females from BP. The number of birds produced from different matings is shown in Table 1. The experimental birds were maintained without any culling up to 18 weeks of age, and every dead bird during this period was autopsied.
The total mortality of experimental female chicks one day to 18 weeks of ago, was considered as growing mortality. Mortality in the pure breeds and in the reciprocal F1 hybrids between them is given in Table 2. The growing mortality of the reciprocal F1 hybrid chicks was lower than that of the pure breed chicks.
Although the difference of 7.2% mortality in thefirst experiment is statistically non-significant, the difference of 30.2% growing mortality in the second experiment is highly signficant at the 1% level.
Important causes of death in the second experiment are summarized in Table 3. It is evident that the main cause of the difference in mortality between parental breeds and the F1 hybrids was the loss by blackhead. Mortality from this disease in the pure breeds was as high as 38.3%(WL =43.0%, BP=31.5%) at 18 weeks of age, while it was only 9.5%(WLρ×BPσ=8.7%, BPρ×WLσ=10.6%) in the F1 hybrid chicks. The difference of 28.8% mortality at 18 weeks of age is statistically highly significant at the 1% level.
