Artificial Insemination in Swine

SWINE REPRODUCTION

0749-0720/92 $0.00

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ARTIFICIAL INSEMINATION IN SWINE Bo G. Crabo, DVM, PhD, and Gary D. Dial, DVM, PhD

Artificial insemination (AI) is a management technique that has been available to American producers for several decades. While having very few disadvantages, AI offers producers real opportunities to improve their financial competitiveness. Increased intensification of swine production in the United States coupled with increased consumer demand for higher quality pork have resulted in AI being a long-proven technique that has finally come to age in this country. This article is intended to demonstrate the applicability of AI in the commercial swine industry. CURRENT USE IN THE UNITED STATES

In 1989, AI centers in the United States sold approximately 190,000 doses of boar semen (Table 1). Purebred organizations registered 4.3% of litters as a product of AI.l1 This relatively low rate of AI indicates that the American swine industry falls far short of the extensive utilization of the technique observed in many European countries. Many parts of Europe have set a goal of having at least half of the breeding stock inseminated with genetically proven or tested boars. Holland, which used 1,992,000 doses of semen from cooperative centers, and the Scandinavian countries are perhaps leading the trend to combine AI with progressive genetic programs. s Several AI centers provide American swine breeders with access to tested boars. In addition, seed stock producers sell semen to commercial and purebred producers. On commercial farms, the use of semen from terminal boars in AI programs (as opposed to AI with semen shipped from a center) is becoming an economically important management tool. On-farm AI requires conSiderably fewer boars and, in comparison to hand-mating, typically cuts labor by 50%2 (see Table 3). Com-

From the Department of Animal Science (BGC), the Department of Clinical and Population Sciences, College of Veterinary Medicine (GOD), and the Swine Center (BGC, GOD), University of Minnesota, St. Paul, Minnesota

VETERINARY CLINICS OF NORTH AMERICA: FOOD ANIMAL PRACTICE VOLUME 8· NUMBER 3. NOVEMBER 1992

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Table 1. USE OF FRESH SEMEN PURCHASED FROM AI CENTERS BY COUNTRY Country Canada Denmark Finland France Germany The Netherlands Norway Sweden United States

No. Females Inseminated/Year

% Females Inseminated

45,000 NA NA

280,000 1,115,000 1,992,000 NA

165,000 190,000

NA

25 37 9 23 56 71 >20 8

Data from Johnson LA, Rath D (eds): Boar Semen Preservation II. Reproduction in Domestic Animals (suppl 1). Berlin, Paul Parey, 1991.

mercial farms in Holland have used on-farm AI in 800,000 services. Approximately 7.5% of commercial swine producers in the United States reported having experience with AI,ll indicating that AI may be fairly widely used on commercial farms. ADVANTAGES OF AI Advantages of AI Centers

There are at least two advantages of using semen from AI centers. Genetics Despite what is generally accepted as a slow rate of genetic progress in the United States, there are many boars located in American AI centers that have test-station performance data. The location of these genetically superior boars in AI centers allows them to be available to all producers and potentially allows their genes to be dramatically amplified in America's swine herd. Disease In recent years, there has been growing intolerance of American producers toward diseases that compromise the performance of both the breeding and finishing herds. Although there is growing concern about other modes of disease entry, diseases still are most commonly introduced into herds via other swine, typically with purchased replacement stock. In herds for which replacements are purchased, boars provide a primary route of introduction into a naive herd. It is not uncommon for start-up and established herds to have epidemics of disease after the purchase of boars. The minimization of the chance that boars will bring new diseases into a herd is one of the most compelling reasons for implementing AI on both commercial and breeding-stock producing swine herds in the United States. Boar semen contains a variety of pathogens. 15 AI centers routinely test their boars for the more common diseases for which there are definitive diagnostic tests. Typically, these centers keep their boars isolated from pigs in other herds. This biosecurity has proven to be a reasonable safeguard for disease introduction. There

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are only a few tests commercially available to swine producers that can be conducted on a living animal. Few tests have sufficient sensitivity and specificity to be used on an individual animal. 3 The transmission of pathogens via semen appears to be relatively uncommon owing to an incompletely understood mechanism. Thus, AI appears to be a potentially useful tool for controlling the introduction of pathogens, including those diseases not having a definitive individual animal diagnostic test. Advantages of AI from On-Farm Boars

Although having similar genetics and disease advantages of AI centers, the use of AI with semen collected on-farm from herd boars has several additional advantages. Boar Power

The use of AI allows boar inventory to be reduced. The ramifications of reduced boar inventories are manifold. Boars take up space where sows could be housed, resulting in a reduced total number of pigs being weaned from the farm. Not only are extra pigs produced when boar inventories are reduced through AI programs, but all pigs produced have a reduced cost of production. That is, there are essentially no fixed costs in producing pigs from sows that are housed in places where boars were previously kept, only variable costs. Boars also have a per diem cost for maintenance that results in a higher amount of breeding herd feed per weaned pig. Lastly, boars typically are expensive to buy and therefore contribute significantly to the total breeding costs of all pigs marketed. In on-farm AI programs, fewer boars are purchased. This allows producers to purchase higher quality boars. Using fewer boars, but of higher genetic-merit, in an AI program allows the production of a more uniform group of pigs having better growth performance and higher quality carcass. AI also offers the producer an opportunity to evaluate the quality of semen at the time of collection. Subfertile or infertile boars may be more common than generally believed, judged from analysis of semen from slaughtered boars.1 Although infertile boars can be identified, subfertile boars are difficult, if not impossible, to detect using records under conventional breeding management schemes. This is due to two things: (1) the almost exclusive use of heterospermic services in commercial swine farms, which allows one boar to disguise the subfertility of another boar, and (2) the large number of homospermic services needed to identify variations in boar fertility with acceptable statistical accuracy. Labor

Although AI programs require females to be checked for estrus prior to insemination, the labor required for heat-checking is identical to that required for natural services. Heat-checking should be performed by a trained human observer in the presence of a boar and should be done much more rigorously than would be done with natural mating programs. Unlike natural services, in which only females in estrus will stand for mating, it is possible to inseminate a sow that is not in heat. When AI programs are set up with time efficiency in mind, semen is collected from the donor boar(s) while he is mounted on a dummy or while he is mounted on one of the females to be bred. Moving animals is minimized, because the inseminator carries the semen to sows previously found to be in estrus. Time for

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semen collection from each boar is identical to that for breeding one female and typically takes less than 5 minutes. Similarly, the insemination of the estrous female takes only a few minutes. DISADVANTAGES AND REASONS FOR FAILURE OF AI PROGRAMS

There is a widespread belief that fertility and litter size will suffer from the use of AI. If this was true, the use of AI would be significantly impaired in commercial swine production. Although fertility is lower when frozen boar semen is used in AI programs, extensive European experience and the data given in Tables 2 and 3 indicate that fertility after fresh-semen AI will be similar to that after hand-mating, when AI is used properly. Failure of AI programs is most often due to improper heat-checking. Typically, sows are bred asynchronous with the time of peak estrous intensity and, thus, semen is deposited too long before or after the time of ovulation. When producers switch from pen or pasture-breeding systems to hand-mating systems, they must learn proper heat-checking procedures to use AI optimally. Another reason for failure with AI is lack of commitment. It takes time and patience for producers to train boars for collection, to process semen, and to inseminate sows. Semen and animals must be handled, which may be aesthetically unpleasing to some producers. AI commonly fails when it is used only on an interrupted basis, such as when the number of available boars is insufficient for the number of females that must be mated. Although mishandling of semen and improper insemination technique are occasional causes of failure, AI more commonly fails because of the producer not having sufficient confidence that he or she can achieve the results of natural service. USE OF FRESH VERSUS FROZEN SEMEN

Cryopreservation allows semen to be collected independently of the time that it is going to be used. However, when cryopreservation is used, each collection yields fewer doses than with fresh semen, and the fertility generally is lower. Producers can expect a farrowing rate of approximately 55% and a loss of one pig per litter with frozen semen, regardless of whether the pellet or straw method of freezing is used. 6 The semen of some boars maintains its fertility after freezing, but that of others drops to virtually zero. This difference is not always reflected in sperm motility after thawing. The advent of overnight couriers has made transportation of fresh semen without loss of quality available to almost any location in the world. Overnight shipment makes the use of frozen semen obsolete except for instances when it is advantageous to save semen from a boar after this death. Whereas breeding stock producers often have justification in freezing semen, fertility losses and technical considerations make semen freezing impractical on most commercial farms. EXPECTED RESULTS USING FRESH SEMEN IN AN AI PROGRAM Natural versus Artificial Service

To date, few studies have compared the fertility achieved with artificial service with that occurring with natural mating using the same boars4 (Table 2) or

'I

~

U1

89.7 82.4 88.3

145 34 179

11.8 10.1 10.8

81.9 79.7 80.4

166 226 392

11.7 9.2 11.3

Litter Size

NA NA

81.8

38

Farrow Rate NA NA

No.

Natural +AI

Data from Hagen CD: Artificial insemination with fresh semen. In Minnesota Swine Day Reports, St. Paul, MN, University of Minnesota. 1986, pp 45-47.

SOWS GILTS TOTAL

Farrow Rate

No.

Litter Size

Farrow Rate

AI

No.

Natural

11.8

NA NA

Litter Size

Table 2. FARROWING RESULTS AFTER NATURAL SERVICE, ARTIFICIAL INSEMINATION, OR THE COMBINATION USING THE SAME BOARS IN ONE HERD

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Table 3. FARROWING RESULTS AFTER NATURAL SERVICE, ARTIFICIAL INSEMINATION, OR THE COMBINATION USING THE SAME BOARS IN ONE HERO*

No. sows Farrowing rate Litter size Labor180 sows (hr)

Natural Service

AI

Natural/+ AI

80 87.3 10.3t 60.8t

80 80.9* 9.4:t: 28.8§

80 93.2 10.2t 43.2:t:

*Time/farrowing rate interaction (P < 0.05)-farrowing rate increased from 60% to 90% in 8 weeks. t*§Means within a row with different superscripts differ (P < 0.05). Data from Flowers B, Stambaugh G, Hudson R, et al: Combinations of natural service and artificial insemination in swine breeding programs. J Anim Sci 68(suppl 1):483, 1990.

multiple herd boars in various treatments2 (Table 3). The results of these studies indicate that when sows were artificially inseminated twice during estrus, mated twice naturally, or mated once naturally followed by AI, they have very similar farrowing rates and litter sizes. Although the average farrowing rate after AI in the study of Flowers et aF was lower, this was shown to be caused by a timefarrowing rate interaction over the 8-week study. Farrowing rate increased from 60% during the first week to 90% during the 8th week, confirming an observation by Reed12 that farrowing rate improves with the experience of the inseminator. A British study14 comparing multiple herd boars to semen purchased from AI centers found that the farrowing rates for multiparous sows (91.1 % versus 90.5%) and gilts (85.9% versus 89.2%) was similar for herd boars used naturally on the farm and for their semen. However, the farrowing rate of parity-1 sows and the litter size across all parities was slightly less than those of the naturally mated sows. Influence of Breed

Factors that influence fertility in natural mating systems have similar effects with AI programs. Thus, insemination of crossbred sows likely will result in higher farrowing rates and greater litter sizes than insemination of purebred sows. There is a difference among pure breeds in both farrowing rate and litter size, which almost exclusively can be attributed to breed differences in the SOW.10 Influence of Number of Inseminations per Estrus As with natural service, the number of inseminations per estrus potentially influences reproductive performance. Two inseminations per estrus versus one has been shown to improve farrowing rate by 8 to 12%, and litter size by 0.2 pigs. An additional third insemination increased farrowing rate another 3%.5,13

Influence of Semen Extender and Time of Insemination

The most common extenders used worldwide are the Kiev and Beltsville thawing solution (BTS) extenders. Both are available commercially and have been found to give fertility levels at least as good as all other commercially available extenders. Recent trials 7,16 demonstrated that farrowing rates and litter sizes were similar for Kiev and BTS extenders for semen stored up to 4 days (Table 4). In a

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Table 4. FERTILITY OF DILUTED SEMEN OVER TIME IN SEMEN DILUTED IN BTS

No. sows Farrowing rate (%) Total born/litter

No. gilts Farrowing rate (%) Total born/litter

Day 1

Day 3

Day 4

271 79.2 11.2

297 78.6 11.4

135 80.3 11.4

43 84.4 10.2

42 58.5 9.5

18 77.8 8.6

Data from Johnson LA, Aalbers JG, Grooten HJG: Artificial insemination of swine: Fecundity of boar semen in Beltsville TS (BTS), Modified Modena (MM), or MR-A and inseminated one one, three, and four days after collection. Zuchthygiene 23:49 - 55, 1988

Norwegian study,S a slight but significant drop in litter size (0.23 pigs) following single inseminations was observed for storage times exceeding 27 hours. Extended semen should be stored at 15°C. In the absence of thermoregulated incubators, storage of semen at room temperature is preferred over storage in a refrigerator, which actually is detrimental to fertility. Extenders should contain antibiotics if the semen is not used immediately. Penicillin/streptomycin, neomycin, and lincomycin/spectinomycin/gentamycin are commonly used in commercial extenders. However, commercial extenders purchased in powder form do not always contain antibiotics. American AI centers usually collect semen on Mondays and Thursdays so as to ensure satisfactory fertility throughout the week. There usually is no need to store semen collected on the farm for more than overnight. METHODS FOR ON-FARM USE OF AI

Semen Collection

Semen normally is collected with the gloved hand" technique from a boar mounted on either an estrous sow or on a dummy. However, some types of latex gloves are spermicidal, so vinyl gloves are preferred. 9 A dummy may be useful (1) with large boars that may injure the sow or gilt during mounting, (2) if boars are overly aggressive to females or have undesirable mating behavior, (3) if a producer wants to avoid moving either the boar or the sow to a breeding pen, (4) if an estrous female is not available, and (5) if a producer wants to reduce collection time by using boars trained to mount a dummy. The procedure for collecting semen from a boar is simple. The boar will extend the penis and ejaculate when the correct pressure, which varies among boars, is applied on the cork-screw shaped glans, preferably with the last three fingers of the hand. Typically, the semen is collected into a wide-mouthed thermos covered with cheese cloth (gauze) that filters out the gel while allowing the liquid components of the semen to pass through. Either the sperm-rich fraction or the entire ejaculate may be collected, but the gel may plug insemination pipettes if the entire ejaculate is collected. /I

Evaluation of Semen and Dilution

After collection, semen volume is measured using a volumetric cylinder. In most cases, it is sufficient to estimate sperm concentration according to its opacity. Exact semen concentrations are rarely necessary, because the amount of semen

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usually exceeds the need. Opalescent, watery semen contains 50 to 200 million sperm/mL, milky semen 300 to 500 million/mL, and creamy semen approximately 1 billion sperm/mL. If greater accuracy is desired, the sperm concentration can be determined using a hemocytometer after dilution 1: 50 or 1: 100 with saline. Sperm concentration can also be determined using a spectrophotimeter. The saline should contain a drop of formaldehyde to inhibit sperm motility. The total number of sperm in the ejaculate(s) is calculated using the sperm concentration and volume. Based upon estimated concentration, sufficient volume of fresh or diluted semen is inseminated to ensure that each inseminate includes at least 2.5 billion spermatozoa. Extended semen may be saved at room temperature or stored in an incubator at 15°C for use on the same or following day. Once the number of insemination doses needed has been determined by heat-checking, a sufficient amount of extender should be added to the semen under isothermal conditions to provide insemination doses of 80 to 100 mL volume. If possible, extender should be added to semen, rather than semen to extender. It may be practical to dilute the semen 1: 1 with extender in the thermos and to make the final dilution at the time each dose is inseminated. Alternatively, the required number of sperm can be inseminated followed by the appropriate volume of extender. If sperm motility is to be evaluated microscopically, evaluations can be made immediately after collection. Stored semen commonly has poor motility even though fertility is normal. Producers usually find it most practical to buy extender in powder form and then dissolve it in distilled water as needed. As mentioned previously, antibiotics may not be included in commercial extenders. A simple alternative to commercial extenders is skim milk that has been heated to 92°C; however, such semen should be used on the day of collection. Expected Sperm Numbers per Ejaculate

The average ejaculate of a normal, mature boar contains 40 to 50 billion spermatozoa when the boar is collected two to three times per week. However, a large variation should be expected among boars. If the collection frequency increases to twice a day, the number of sperm per ejaculate will rapidly decease to 5 to 8 billion spermatozoa per ejaculate. 1 If collected less frequently than weekly, the total number of sperm in the ejaculate may increase to well over 100 billion. Young boars may give unexpectedly low sperm numbers owing to delayed puberty. If each inseminate requires a minimum of 2.5 billion sperm and the average boar produces 40 to 50 billion spermatozoa, a "normal" boar ejaculate should theoretically be sufficient for one insemination each of 20 to 50 sows. In practice, the number of doses will be lower, because it is safer to not dilute semen maximally. Also, if sperm concentrations are not determined, an excess of sperm should be inseminated. If boars are collected every second to third day, only four to six normal boars theoretically are needed to breed the 50 sows per week available for estrus in a 1000-sow herd. At a 20: 1 female: boar ratio (common to most commercial herds), a 1000-sow herd typically would have about 50 boars. When one considers that each boar potentially takes the place of one sow, the extra boars on a farm using natural service cost the producer a significant number of pigs, with the result that the cost of producing all pigs is more expensive. Producers commonly carry a larger boar inventory to make sure they have sufficient boars on hand in case one or more cannot be collected. The cost of carrying excess boar inventory, in terms of lost pigs, for a 1000-sow herd is

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40 displaced sows X 2.35 litters/female/year X 9.5 pigs weaned/sow farrowed =893 pigs Because of the frequency by which boars can be collected and the large number of inseminates from each collection, boars potentially can sire thousands of litters each year: three collections/week X 20 inseminates/collection X 52 week/year X 80% farrowing rate = 2496 litters/year The number of doses of semen collected from boars by AI averages approximately 1300 doses per year. Heat-Checking

Accurate and regular heat-checking is critical for the success of an AI program. A sow in heat will stand to manual back pressure by a human or will allow a boar to mount. Lordosis is the only valid indicator of estrus, although other signs such as unruliness and mounting behavior are useful aids for estrus detection. Although predictably associated with proestrus and estrus in the gilt, reddening and swelling of the vulva is difficult to recognize in sows. Depending upon parity and genotype, standing estrus will last 24 to 72 hours. Insemination

The preovulatory luteinizing hormone surge coincides with the onset of estrus. Ovulation occurs within approximately 40 hours of the initiation of the surge, or in the morning of the second day of estrus in many sows. If capacitation of semen takes approximately 12 hours, sows should be inseminated about 28 hours after they initially show heat. When heat checks are done only once or twice each day, accurate timing of inseminations in relation to time of ovulation is not possible. Thus, multiple timed inseminates are recommended to achieve optimal fertility. Considering that semen is abundant on the farm and there is a fertility advantage from multiple inseminations, it is probably wise to immediately inseminate sows when they are first found in estrus and then every day thereafter as long as the sow is standing. The equipment used for insemination consists of a catheter and a large syringe or a plastic, collapsible squeeze bottle of at least 100-mL capacity (Table 5). The Melrose spirette has long been used to make inseminations. It is made of rubber and intended for multiple use. The tip of the Melrose spirette is threaded counterclockwise, similar to the penis of the boar. Several types of disposable pipettes are commercially available. One of the more common types has a spiral tip, similar in design to the Melrose spirette. Disposal spiral-tipped pipettes are manufactured by Continental Plastics* and Minitiib.t The spiral-tipped pipettes have the advantage in that the inseminator can feel when the catheter is "locked" into the cervix of the female. A relatively new pipette, the "Golden Pig" manufactured by IMV:J:, has a ·P.O. Box C, Delavan, WI 53115 t214 Shiloh Drive, Madison, WI 53707 t6870 Shingle Creek Parkway, Minneapolis, MN 55430

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Table 5. AI EQUIPMENT Collection

Processing/Storage

Insemination

Insulated thermos Gauze/cheese cloth Disposable vinyl gloves

Graduated cylinders Incubator Extender & diluent Thermometer Microscope and slides Hemocytometer or spectrophotometer

Pipettes Collapsible bottles

cylindrical tip made of plastic foam. This latter pipette is preferred by some producers because of the ease with which inseminations are made and because of reduced retrograde flow of the inseminate from the vagina. A plastic pipette of the same type as used for the insemination of cows but with the forward 3 to 4 cm tip bent at a 30° angle has been available on the market for a long time. Although the fertility results with this pipette appears to be the same as with the other types of insemination pipette, many people feel less confident in using it because they have difficulty feeling when the pipette passes into the cervix. The insemination procedure is simple. The pipette normally is lubricated with extender, inserted into the vulva, directed dorsally along the roof of the vagina, and firmly pushed until resistance is met. The threaded pipettes are then turned counter-clockwise while pressure is maintained until the pipette locks in the cervix. The Golden Pig is pushed anteriorly in the vagina until it can be felt to pass over one or two cervical"rings." The semen is then injected for approximately 2 minutes. Back flow may occur but does not appear to be related to lower conception rate if the insemination is done slowly. Nearly everyone feels confident doing the procedure after having done only a few inseminations. Sources of Semen and Equipment

There are three major AI centers in the US: International Boar Semen· (IBS), Swine Genetics International, Ltdt (SGI), and Birchwood Genetics.+ In addition, many producers of seed-stock also sell semen to their customers. Semen typically costs between $8 and $30 per dose, dependent on the amount of semen and the boar. All firms carry Melrose's spirette. Minitube of America, Inc, and IMV International have complete lines of AI equipment, which are sold directly or through major AI centers. IBS and agricultural supply companies also sell the Continental Plastics' disposable " spirette." Most suppliers listed sell BTS and Kiev extender in powder form under various trade names. Thawing of Frozen Semen

Frozen semen is available as pellets or maxistraws. For thawing, the maxistraws are immersed for 55 seconds in water heated to 55°C. Pellets are thawed by dumping them into a styrofoam container, where they are held for 3 minutes. The ·P.O. Box 496, Eldora, IA 50627 tRoute I, Box 3, Cambridge, IA 50046 t465 Stephens Road, West Manchester, OH 45282

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pellets are then dumped in a beaker with 75 mL BTS heated to 42°C under vigorous swirling. Insemination should be done immediately after frozen semen is thawed. CONCLUSIONS

Swine AI offers many advantages for genetic improvement, disease control, and labor savings but has not been used as extensively in the United States and Canada as in many other countries. This may be because the difference in price paid to producers for high- and low-yielding hogs has been small. Furthermore, AI has been perceived to be labor-intensive and to result in slightly lower fertility. These perceptions not withstanding, AI requires less labor than hand-mating and, if done correctly, results in at least the same fertility as natural mating while requiring fewer boars. There is a place for AI in the continually changing American swine industry. Every producer will not use AI in the same way, and those now using pen- or pasture-breeding will first have to learn hand-mating. Some producers will demand fewer but better boars from breeding companies. Others will use semen from tested boars at AI centers to produce replacement stock for use on their own farms. It is likely that fewer and better boars will be sold at higher prices, and owners of top boars may increasingly choose to lease them to AI centers. The next decade may see a tremendous increase in swine AI led by performance-minded producers and packer pressure. A good genetic selection program combined with AI has the potential to reduce the spread of many of the horizontally and vertically transmitted infectious diseases (e.g., Actinobacillus pleuropneumoia, swine dysentery) and some noninfectious genetic diseases (e.g., porcine stress syndrome). Thus, producers who use AI will be less exposed to disease risks and will be more financially competitive. References 1. Crabo BG, Loseth KJ, Henry SC, et al: Evaluating fertility and evaluating semen. In Proceedings of the Am Assoc Swine Pract. Cincinnati, OH, April 17-19, 1983, pp 87-97 2. Flowers B, Stambaugh G, Hudson R, et al: Combinations of natural service and artificial insemination in swine breeding programs. J Anim Sci 68(suppl 1):483, 1990' 3. Gradil CM, Harding MJ, Molitor TW, et al: Boar semen: Transmission and detection of viruses. In Johnson LA, Rath D (eds): Boar Semen Preservation II. Berlin, Paul Parey, 1991, pp 273-285 4. Hagen CD: Artificial insemination with fresh semen. In Minnesota Swine Day Reports, St Paul, MN, University of Minnesota, 1986, pp 45-47 5. Hofmo PO: Commercial swine AI with liquid semen in Norway. In Johnson LA, Rath D (eds): Boar Semen Preservation II. Berlin, Paul Parey 1991, pp 317-320 6. Johnson LA: Freezing results using frozen boar spermatozoa: 1970-1985. In Johnson LA, Larsson K (eds): Deep Freezing of Boar Semen. Uppsala, Sweden, Swedish University of Agricultural Sciences, 1985, pp 199-222 7. Johnson LA, Aalbers JG, Grooten HJG: Artificial insemination of swine: Fecundity of boar semen stored in Beltsville TS (BTS), Modified Modena (MM), or MR-A and inseminated on one, three and four days after collection. Zuchthygiene 23:49-55, 1988 8. Johnson LA, Rath D (eds): Boar Semen Preservation II. Reproduction in Domestic Animals (suppl 1). Berlin, Paul Parey, 1991 9. Ko JCH, Evans LE, Althouse GC: Toxic effects of latex gloves on boar spermatozoa. Theriogenology 31:1159-1164, 1989 10. Koh TJ, Crabo BG, Tsou HL, et al: Fertility of liquid boar semen as influenced by breed and season. J Anim Sci 42:138-144, 1976

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11. Pond WG, Maner JH, Harris DL: Pork production systems: Efficient use of swine and feed resources. New York, Van Nostrand Reinhold, 1991 12. Reed HCB: Current use of frozen boar semen-future need of frozen boar semen. In Johnson LA, Larsson K (eds): Deep Freezing of Boar Semen. Uppsala, Sweden, Swedish University of Agricultural Sciences, 1985, pp 225-237 13. Reed HCB: Commercial requirements for an effective fresh semen diluent. In Johnson LA, Rath 0 (eds): Boar Semen Preservation II. Berlin, Paul Parey, 1991, pp 255-270 14. Reed HCB, Marchesi MG, Jones OW: Pig AI/natural service 'within herd' fertility investigation. In Proceedings of the 10th International Congress on Animal Production and AI. Urbana, IL, 1984, P 379 15. Thacker BJ, Larsen RE, Joo HS, et al: Swine diseases transmissible with artificial insemination. J Am Vet Med Assoc 185:511-516, 1984 16. Weitze KF: Long-term storage of extended boar semen. In Johnson LA, Rath 0 (eds): Boar Semen Preservation II. Reproduction in Domestic Animals (suppI1). Berlin, Paul Parey, 1991, pp 230-253

Address reprint requests to Bo G. Crabo, DVM, PhD 495 AnSc/VetMed University of Minnesota St. Paul, MN 55108