Inbreeding and Relationship to Performance in a Closed Herd of Small-Framed Angus Cattle

Abstract

The objective of this study was to estimate level of inbreeding and relate it to performance in a closed herd of small Angus cattle. This herd of Angus cattle was created in 1973 when the shift from small-framed, compact cattle to the largeframed, more upstanding type of cattle was advocated by the breed association. From 1974 to 1991, 396 cows were exposed to bulls, resulting in 318 live births. Data included percentage of inbreeding for all animals, calf crop percentage for each year, and birth, weaning, and yearling weights of progeny.Regression procedures were used to examine increase in inbreeding over time. Correlation analyses were used to examine relationships between calf crop percentage and average inbreeding. Relationships of inbreeding to birth and weaning weights of progeny were examined by analysis of variance using a model that included terms for year, sex of calf, year by sex of calf interaction, and percentage of inbreeding of calf and(or) dam as covariates. The model for yearling weights included only terms for year and inbreeding. Average inbreeding of progeny ranged from 3.25% in 1975 to 18.37% in 1990, and average inbreeding of cows exposed for breeding ranged from .89% in 1977 to 11.76% in 1989. Average inbreeding of cows actually calving was comparable, ranging from .89% in 1977 to 10.58% in 1988. A linear relationship (P<.01) was observed between inbreeding and time for all three groups. Inbreeding of progeny = -51.17 + .76 (year), inbreeding of cows exposed = -48.13 + .66 (year), and inbreeding of cows calving = -45.05 + .62 (year). Calf crop percentage was not related to average inbreeding of progeny, cows exposed, or cows calving (r = .069, -.005, and .070, respectively; P>.10). Inbreeding both of calf and of dam influenced (P<.05) birth and weaning weights but not yearling weight. Inbreeding of calf was more important than inbreeding of the dam for birth weight, and inbreeding of the dam had a greater effect on weaning weight than inbreeding of the calf.