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Many factors contribute to variation in lamb weights. Factors such as age of lamb, sex, type of birth and rearing, and ewe age at lambing obscure genetic differences among lambs. Variation associated with these factors make selection for increased weight less effective. To improve selection response, selection must be based on genotypic rather than environmentally induced superiority. It is well-documented that corrections for classifiable sources of variation render selection and culling more accurate, thereby increasing rates of genetic improvement. The objective of this study were to develop within breed adjustment factors for post-natal lamb weights . Field records from Suffolk and Dorset lambs collected in the U.S . National Sheep Improvement Program (NSIP) from 1986 through 1989 were used to develop within-breed birth/rearing type, lamb sex, and dam age adjustment factors for birth weight (BW) and 30-d (W3o), 60-d (Woo), 90-d (W9o), and 120-4 (Wtm) weights. Data were analyzed using general linear models. Within breed and age group adjustment for lamb-age differences were suggested because growth rates vary among breeds and among the four postnatal lamb-age groups . Within-breed multiplicative adjustment of the five traits for the effects of birth/rearing type, lamb sex and dam age were suggested because variances of these factors were significantly (P |
