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Effects of Inbreeding and Environment on Fertility of Beef Cattle Matings^1,2,

Virginia Polytechnic Institute and U. S. Department of Agriculture

Abstract

The data for this study consisted of breeding and calving records of 1,504 purebred Angus matings of 395 cows and 48 service sires, and 1,606 purebred Shorthorn matings of 435 cows and 49 service sires, during 13 calf crops produced from 1950 through 1962. The ages of the breeding animals ranged from 2 through 12 yr. for the cows and 2 through 11 yr. for the service sires. Seven breeding lines were developed within each breed to compare breeding systems, i.e., four inbred lines, two single trait selection and one outbred line were established. Inbreeding of individual matings ranged to 40 percent in the Angus herd and to 50% in the Shorthorn herd, also average per line ranged to 19.9 in the Angus and 30.0 in the Shorthorn herd. Each mating was scored by a "0" if it did not produce a calf or a "1" if a calf was born. Factors having a highly significant effect upon fertility were: in the Angus herd breeding system, inbred lines within breeding system, year of calving, age of service sire; and in the Shorthorn herd inbred lines within breeding system, year of calving and age of cow. Other important factors were selection lines within breeding system and age of cow in the Angus herd; also breeding system, selection lines within breeding system and age of service sire in the Shorthorn herd. The fertility means of all inbred lines was not significantly different from all non-inbred lines. None of the partial linear regressions of fertility on inbreeding of mating, cow and service sire were significant. The weighted linear regressions on years were negative for all lines.

¹ Published with the approval of the Director, Agricultural and Life Sciences, Research Division, Virginia Polytechnic Institute.

² Research conducted in cooperation with the Beef Cattle Research Branch, A.H.R.D., A.R.S., U.S.D.A. and the Virginia Agricultural Experiment Station as part of the Southern Regional Research Project, S-10, Improvement of Beef Cattle Through Breeding Methods.

³ Present address: A.H.R.D., A.R.S., U.S.D.A., Lincoln, Nebraska.

⁴ Department of Animal Science, Blacksburg, Virginia.

⁵ Beef Cattle Research Station, Front Royal, Virginia.