Inbreeding consequences

Consequences of long-term inbreeding accumulation on preweaning traits in a closed nucleus Angus herd¹

J. A. Carrillo and F. Siewerdt²
Department of Animal and Avian Sciences, University of Maryland, College Park 20742
² Corresponding author: siewerdt@umd.edu


The effect of individual and dam (maternal) inbreeding was quantifiedfor preweaning traits in an Angus nucleus herd that has beenclosed to outside breeding for 70 yr. The effectiveness of 5models (linear, quadratic, exponential, Michaelis-Menten, andRumford-Newton) was evaluated for describing the effect of inbreedingon growth traits, hock length (HL), and scrotal circumference(SC). Pedigree information and production records were retrievedfor 10,938 animals and analyzed in an animal model that includedthe fixed effects of year of birth, age of the dam, sex, andage at weaning (for traits measured at weaning). Average individualand maternal inbreeding in the herd were 0.068 and 0.066, respectively,for all animals; in the last calf crop these values were 0.120for the calves and 0.121 for their dams. Inbreeding depressionwas observed for BW at birth (WB), weaning weight (WW), BW adjustedto 205 d of age (W205), ADG, HL, and SC. The effect of maternalinbreeding was smaller than for individual inbreeding for WB,WW, W205, and ADG. Nonlinear prediction was done more effectivelyby the exponential and Michaelis-Menten models. Quadratic polynomialswere an inadequate descriptor of inbreeding effects. Geneticgain from selection at an intensity equivalent to 0.25 can benullified by an inbreeding accumulation of 0.187 (WB), 0.056(WW), 0.068 (W205), 0.065 (ADG), or 0.092 (SC). Inbreeding accumulationof 0.018 is required to nullify genetic gain for HL; this particularprediction is valid for non-inbred cows due to an observed interactionbetween individual and maternal inbreeding. At current inbreedingaccumulation levels in this herd, 7 generations of inbreedingaccumulation will be necessary to nullify the genetic progressfrom 1 generation of selection in growth traits.

Seven generation of mild? inbreeding to overcome one performance selection. Mike, what else struck you as important take-home info?