Performance of meat-type chickens test-positive and -negative for lymphoid leukosis virus infection

Four "sire-line" and seven "dam-line" breeding stocks of meat-type chickens were purchased as 1-day-old chicks from commercial breeders. One egg from each of 61 to 81 hens per stock was tested at 245-280 days of age for presence of group specific antigen of the lymphoid leukosis virus (LLV). Subsequently, cloacal swabs collected at 42 days of age from 93 male and 188 female progeny of particular "dam-line" stocks were tested for LLV. The percentage of LLV shedders in the individual stocks varied from 0 to 15%. Egg production to 385 days was lower in shedders than in non-shedders by 10 eggs per hen housed and by eight eggs per surviving hen. In the crossbred progeny of the "dam-line" stocks tested for LLV at broiler age, males had a significantly lower percentage of positives (2%) than females (11%). The broiler-age body weight of the test-positive females was 5% lower and, to 315 days of age, their mortality was 29% higher and egg production per hen housed 28 eggs lower than in the negative birds.