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. (
http://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.