Economics of testing for fertility-associated antigen in bovine semen (Proceedings)

Fertility-associated antigen (FAA) is a non-glycosylated protein produced in all of the accessory sex glands of bulls; seminal vesicles, prostate and Cowper's glands. At ejaculation, as sperm traverse the male reproductive tract, FAA binds to sperm surfaces where it can be detected visually with specific antibodies produced against FAA.

The biological effects of FAA are to potentiate capacitation of bull sperm in a dose-dependent manner. Recombinant bovine FAA can be added to a freshly collected semen sample, improving post-thaw motility of semen samples as well as percent intact acrosomes 3 hours post-thaw, which is the physical trait with the highest correlation to fertility in the field.

At insemination, neutrophils are reunited across the lumen of the uterus where they bind to sperm and ultimately participate in phagocytosis. It was postulated that FAA, which possesses intrinsic DNase-1-like motifs embedded in its sequence, may serve to reduce sperm-neutrophil binding. In fact, addition of a recombinant bovine FAA to neutrophils in culture results in a dose-dependent reduction of sperm-neutrophil binding.

Presence of detectable FAA on bull sperm is a good thing. In every trial designed to compare fertility of herdmates categorized as positive or negative for sperm-associated seminal proteins, FAA-positive bulls are approximately 17% more fertile. This is true for natural mating multiple-sire pastures or utilizing artificial insemination.

Based upon screening thousands of semen samples, the proportion of bulls that will be diagnosed as FAA-negative is expected to approximate 25%.

Table 1 below illustrates how fertility within an individual herd might vary as a function of the percentage of sires used which are FAA-positive and FAA-negative.

Table 1. Theoretical herd fertility based upon varying frequencies of FAA-positive and FAA-negative bulls.

There is a "Chute-Side" Test for FAA

A recombinant FAA was used to immunize rabbits to produce polyclonal antisera. Midland BioProducts, Boone, IA incorporated the antisera onto a lateral-flow cassette which is marketed by ReproTec, Inc. (www.reprotec.us). The Figure below illustrates how a precipitin line on the cassette corresponds to a "postive" outcome for detection of FAA. The kit sells for $45 for a singlet test, or $30 if more than 100 tests are ordered. Table 2 illustrates the cost-benefit return of screening for FAA if a bull is used for 4 years and is exposed to 25 cows/heifers annually for breeding purposes. That table assumes that replacing a bull categorized as FAA-negative with one that is FAA-positive will yield 15% more calves in that exchange (based upon data cited earlier).

Figure 1. "Chute-side" lateral flow cassette to determine FAA status in bull semen.

Observations made from using the cassettes on over two dozen ranches encompassing over 1,000 bulls include the following: 1) About 25% of BSQ-qualified bulls tested will be categorized as FAA-negative; 2) very young bulls may be negative, but on a subsequent re-test will produce a positive result. Since FAA production is under testosterone control, a virgin bull early in puberty may provide a negative reading. In our opinion, if herd cohorts of the same age test positive, select those as breeding males because they obviously attained puberty sooner. 3) FAA status is heritable. In one herd of 22 negative bulls, every one had a common ancestor in the pedigree, mostly in the first or second generation back (n=18), but he also showed up back to five generations (n=4).

Table 2. Cost-benefit analysis of FAA testing of herd sires. Values are computed for two different cost scenarios.

As other commercially available kits for testing bulls for fertility-related proteins loom closer and closer for producers, the economic impact or potential gain in income should not go undiscussed. Table 3 represents theoretical calculations over a range of increased fertility when only those bulls are used that test positive for fertility-related proteins.

Table 3. Potential income gain based on fertility increasing when average weaned calf weight is 500 pounds in a 100 cow herd over three different market prices. Calculations include the cost of testing bulls, which was estimated at $1.00 per cow.

An 8 % increase in fertility would provide nearly $2,000 to $3,500 more in return, for the producer per 100 cows, depending on market prices. One must also consider the potentially more genetically valuable offspring resulting from fertility-related protein positive bulls for both commercial and seedstock producers.

Similar to OPN and lipocalin-type PGD synthase, the exact mechanisms of action on the sperm cell by FAA and TIMP-2 are not fully understood. Currently, studies are under design in an attempt to evaluate how they modify sperm membranes and/or trigger pathways to augment heparin-induced capacitation. A recombinant preparation of FAA has shown encouraging results because it stabilizes acrosomes, leading to a significant increase in percent intact acrosomes (PIA) post-thaw. The effects are dose-dependent. Since PIA is the semen trait most highly correlated with nonreturn rates, FAA may hold therapeutic value as a semen additive for semen being commercially processed for artificial insemination.

References

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