Value of Selecting Bulls for High Potential Fertility
By David W. Forrest, professor, Texas A&M University

Among bulls of satisfactory breeding potential, there are other reproductive factors to consider that may give you a higher percentage of females bred early in the breeding season.

The first test, which has been used by veterinarians for decades to classify satisfactory or unsatisfactory potential breeders, is the breeding soundness exam (BSE). This exam assesses the external characteristics of the bull, the internal organs of the reproductive tract and the semen quality.

The external exam ensures structural soundness, vision and physical health are adequate for the bull to efficiently detect females in heat.

Scrotal circumference is an important indication of capacity for sperm production. The recommended minimum acceptable scrotal circumference is 34 centimeters for bulls older than 24 months.

Reproductive system soundness can be assessed by internal (accessory sex glands and inguinal rings) and external (testicular symmetry and consistency, penis, prepuce and epididymides) evaluation.

Ejaculate is evaluated to determine percentage of progressively motile sperm; percentage of morphologically normal sperm; and whether cells other than sperm (such as leukocytes or red blood cells) are present.

At least 30 percent of sperm must exhibit progressive forward motility and at least 70 percent of sperm must exhibit normal morphology (shape of sperm head and tail) for the bull to be classified as a satisfactory potential breeder.

Have your veterinarian conduct BSEs each year, at least 60 days prior to the breeding season. This will allow time to re-test any bulls or to get a replacement bull before the start of the breeding season.

More than satisfactory
Serving capacity is another test for satisfactory potential breeders. Bulls with moderate to high serving capacity can impregnate more cows at first heat and achieve higher overall pregnancy rates than bulls of low serving capacity.

Bulls classified as satisfactory potential breeders were evaluated for serving capacity (number of services achieved when exposed to females in heat during a defined time period) in South Texas.

High (HSC) or low serving capacity (LSC) bulls were placed in single-sire pastures with 50 cycling heifers for 60 days. HSC bulls achieved higher average pregnancy rates during the first 25 days of breeding (62 percent) than did the LSC bulls (45 percent). At the end of the breeding season, HSC bulls produced 83 percent pregnancy rates. The low serving capacity bulls produced 67 percent pregnancy rates at the end of the breeding season.

The HSC bulls also generated more pounds of calf weaned per female because there were more older and heavier calves at weaning. There was a projected economic advantage of more than $50 per female exposed to a HSC bull.

Social rank
Studies conducted in multi-sire herds under range conditions in Australia identified a positive relationship between social dominance rank of bulls and number of calves sired. The number of calves sired by individual bulls was highly variable among bulls, but was repeatable between years within individual bulls.

Their results also indicated that stretching the ratio of bulls to females from 1:27 up to 1:36 did not delay conception or affect overall pregnancy rate. The decreased ratio of bulls to females to an average of 1:36 can increase the potential value of each bull by approximately $400 and increase the longevity of bulls due to fewer injuries.

In 2002, a southwest Texas herd was studied to measure the influence of physical and behavioral traits on the number of calves sired per bull.

Twelve bulls of two breeds, aged 18 to 24 months, were placed with 305 crossbred cows under range conditions for a 90-day breeding season.

The final pregnancy rate was 91.5 percent. Sires of 94.4 percent of the weaned calves were determined by DNA typing.

The number of calves sired by each of the 12 bulls ranged from none to 38, with an average of 21 calves per bull.

The percentages of motile and normal sperm, social dominance and breed of bull accounted for 75 percent of the variation in the number of calves sired per bull. More calves were born during the first 40 days of the calving season when cows were exposed to bulls that produced ejaculates with greater than 80 percent normal sperm than for bulls whose ejaculates contained 80 percent or less normal sperm.

Helpful proteins
Several proteins have been identified in bovine semen that are associated with fertility. Heparin-binding proteins (HBP) are produced by the accessory sex glands and can promote capacitation of bovine sperm in the female reproductive tract (necessary for sperm to acquire the ability to fertilize the egg).

Bulls with high fertility have been shown to have a specific form of HBP attached to their sperm that is not present in bulls with lower fertility. Average pregnancy rates were 17 percent higher for cows exposed to bulls that possessed the desirable HBP fraction on sperm than for cows exposed to bulls which lacked the HBP fraction.

This HBP fraction is described as the fertility-associated antigen (FAA). Bulls that were classified as both FAA-positive and high serving capacity stock impregnated more cows in multi-sire herds during the first 20 days and by day 60 of the breeding season than bulls that were either FAA-negative or low serving capacity.

A chute-side cassette has been commercially available since 2004 to classify an ejaculate as either FAA-positive or FAA-negative.

An annual BSE of each bull is fundamental to enhance pregnancy rate of the herd. Selecting bulls that produce FAA-positive semen, containing more than 80 percent normal sperm and are high serving capacity, should increase the number of females that conceive early in the breeding season. In multi-sire pastures, grouping bulls of similar age and dominance rank may reduce bull attrition due to injury.