Selection practices and genetic and phenotypic trends estimated from Nebraska SPF swine field records.

Records collected during 1971 through 1979 from 101,606 pigs raised in 18 herds that were members of the Nebraska SPF Swine Accrediting Association were evaluated for phenotypic trends and predicted and observed genetic trends. Traits considered were backfat at 100 kg (BF) and weight at 140 d of age (WT). Phenotypic change on average was beneficial for BF (-.05 mm/yr) but undesirable for WT (-.2 kg/yr). However, the average observed genetic trend was nil for BF and .6 kg/yr for WT. An average, predicted response based on observed selection differentials and estimates of within herd-sex genetic parameters was in good agreement with observed response for BF, but was three times higher for WT.