Effect of Single-Sire and Multiple-Sire Natural Mating on Pregnancy Rate of Beef Cattle

Although artificial insemination (AI) is widely used in dairy cattle, AI has found only limited application in beef cattle. Use of bulls in natural mating programs accounts for more than 95 percent of the pregnancies achieved each year in the U.S. beef cattle industry. Success of such natural mating programs depends on the reproductive capacity and fertility of the individual herd sires used, but very little research on the natural mating fertility of beef bulls has been conducted. Because of the general lack of information on effective techniques for identifying sires with superior fertility, procedures currently used for selecting herd sires for natural mating are based on factors other than reproductive potential and include factors (such as body wt, growth rate, appearance, etc.) that have little relationship to bull fertility. It is not surprising that a large range in pregnancy rate of beef bulls used in single-sire natural mating has been reported, and commercial cattle producers have resorted to using multiple sires in pasture breeding, assuming that more fertile bulls compensate for less fertile bulls within multiple-sire groups. Conflicting reports exist in the available literature, with some results indicating that pregnancy rate may be higher during multiple-sire breeding. Studies using AI indicate that increasing the number of inseminations per estrous female increases pregnancy rate, and that inseminating mixtures of semen from two or more bulls generally increases pregnancy rate. However, it is unknown if increasing the number of services per female or increasing the number of sires servicing each female results in increased pregnancy rate in natural mating programs. This report presents results of a study conducted to determine the effect of number of services on natural-mating pregnancy rate and to investigate the effect of multiple-sire natural mating on pregnancy rate in beef cattle