Boar Stud


Gary C. Althouse
Department of Veterinary Clinical Medicine
University of Illinois
Urbana, IL 61802


Traditionally, boars have been chosen as AI studs based upon their genetic excellence in a particular trait(s). More often times than not, these genetic merits are related more to improved carcass quality and days to market than to reproductive performance. The determining factor for reproductive performance in these boars are that they are intact and have Aall the right equipment" for producing and transferring their genetic material (i.e., sperm). It is then up to the producer to implement management which will optimize health and sperm production from this AI boar. My talk today will discuss management strategies which can be used to optimize productivity of the AI boar. These strategies will extend from preparation and introduction of the boar into the herd, boar management, and quality control of the semen product.

A. Biosecurity and Isolation

The importance of trying to control the introduction of disease into a boar stud cannot be overemphasized. Not only is it important for the overall viability of the stud (i.e., health and productivity of the boars), but also to minimize the chance of transmitting disease(s) to other herd's through the extended semen product. A good biosecurity program should start in the stud development stage with the appropriate selection of stud location (if applicable) which can physically help to minimize disease transmission, along with mandatory restrictions in the movement of animals, personnel, equipment, supplies and sanitation protocols. The health of the boars standing at stud should be routinely monitored by providing cursory clinical exams during handling (i.e. movement, consistency/amount of manure, appetite, respiration, behavior, etc.) and through the use of various diagnostic tests. Additionally, preventative therapies should be carried out in an attempt to keep the resident herd stabilized for selected diseases.

As with any swine operation, boar studs have a consistent boar turnover. To maintain health and economic viability of the boar stud, isolation procedures should be utilized on all boars prior to their introduction. Preferably, isolation facilities should be off-site and away from other hogs. Animal movement into and out of isolation should be handled in an all-in-all-out fashion - this requires advanced planning on the part of the boar acquisition manager. Traditional 30-day isolation periods have been applied to boar studs; this time frame, however, has not controlled movement of certain diseases (e.g. PRRS) into some boar studs. Because of the disease breakthroughs in a 30-day isolation period, 45- to 60-day isolation periods are currently being implemented on many farms. Even with these extended isolation periods, however, isolated incidences have occurred where disease still got introduced into the boar stud. What can the producer do? First, the producer needs to accept that the swine industry currently does not have a 100% economically feasible and effective means of eliminating the chance of introducing disease through new herd additions into a resident herd. The producer does, however, have control over certain management strategies which would help to minimize the chance of introducing disease through new additions into their herd. These management strategies may include the following:

  1. Practice all-in-all-out isolation.
  2. Identifying similarities and differences in health between the genetic supplier and your farm, and obtaining boars from suppliers which at least match or are of better health status.
  3. Obtaining replacement boars from a single source.
  4. Strategic diagnostic profiling and monitoring of new additions in concert with a controlled stress.
  5. Application of preventative therapies (e.g., vaccinations, deworming, medications, etc.) at times which optimize their desired actions.
  6. Timed acclimatization of new boars while in isolation, with concurrent observation.

B. Boar Management

Optimizing reproductive capabilities of a boar is very much interrelated to the management of that boar. Management practices should be devised to aid in expressing a boar's natural reproductive potential. Economically, increases in sperm quality and sperm production mean more doses of semen per ejaculate and, thus, more females bred to a single boar. Good management practices should address such things as behavior, environment, nutrition, disease control and reproductive physiology.

1. Boar behavior - Certain boar behavior can be developed and/or modified by the boar's environment. Young, immature boars seem especially sensitive to behavioral conditioning through their environment. The pig is a social species, so tactile stimulation appears to be one of their most important influential senses in developing behavior. When young boars are curtailed of tactile stimulation through isolation in individual pens - a delay in puberty, poor sexual motivation, decreased mating performance and locomotion disturbances can occur, especially when compared to group-raised boars. Interactions of swine with humans are also very integral to conditioning either approach or avoidance responses as the pig matures. Once the boar is at stud, factors such as collection frequency, collection area and boar preparation for semen collection can have a big impact on semen collection and semen quality. Above all, you should allow sufficient time for collection of semen - pushing the boar can have negative repercussions on future semen collection sessions.

2. Feeding/Nutrition - Traditionally, nutritional needs for boars were thought to be similar to that of gestating sows. This approach has and still can work at maintaining a boar for a breeding program, especially when the boar is used under natural mating conditions. However, the reproductive potential (i.e. sperm production = number of doses) of the boar may never be reached if this same philosophical approach to nutrition is applied to the AI boar. Testicular development (e.g. size) occurs predominantly during boar adolescence. Recent work has suggested that if an adolescent boar is provided a diet which contains higher than the current NRC recommended levels of certain vitamins, minerals, and amino acids, testicular development may be more fully expressed. Consequently, with increased testicular development, the potential exists for increased sperm production. Once mature, nutritional requirements of the boar focus on maintenance of full expression of sperm production.

On some farms it is not feasible to have a separate diet and storage area for the small number of AI boars housed on the property. To address this problem, several feed companies have started to market products which contain high levels of nutrients so that they can be supplemented to the boar as a top dress on gestation or lactation diets. The availability of these products has provided the producer with a means of trying to capitalize on the testicular development phase and maintenance of full sperm production in the AI boar. Lastly, the amount of feed given to a boar should be dependent upon body condition and diet formulation.

3. Training - Allow for sufficient time for introducing the boar to the collection area. Patience is necessary when trying to train a boar to have semen collected off of an artificial dummy. Introduce the boar to the collection area and let him investigate. When the boar identifies the dummy, some will readily mount it. If the boar is interested in the dummy and/or has mounted the dummy, the boar should then be approached quietly from the rear without being touched or frightened. Gently reach around to the boar's prepuce and massage the prepuce in an attempt to stimulate pelvic thrusting. Once the boar exposes his penis, apply digital pressure to the curled tip of the penis. When applying pressure, be careful not to close the entire hand too far up the shaft of the penis, this will cause most boars to dismount. If properly stimulated, the boar should fully extend his penis and become very quiet when adequate pressure is applied. Once the boar has extended his penis, thrusting should cease, and the boar should start to ejaculate.

C. Semen Collection, Evaluation and Processing

1. Semen Collection - Semen is usually collected from boars off of some type of artificial mounting dummy. Adjustable mounting dummies can easily be made, or the producer can purchase a ready-to-use model from an AI supplier. Along with a sturdy mounting dummy, good footing around the dummy is essential to aid the boar in mounting, thrusting and the semen collection process. Most commercial studs prefer some type of rubber matting so that it can be cleaned between boar use.

A separate collection area should be used for collecting semen from boars. In the collection pen, at least two or three of the perimeter walls should be made up of 2-inch diameter pipe (plastic or metal), placed at 11-12 inch intervals on center, to allow the handler ease and availability of sites to enter or exit the collection area without opening the gate or scaling the wall. The collection dummy should be positioned in such a way to minimize boar movement around the dummy. Most studs will either attach the dummy in the corner of the pen or attach it to a wall so that the boar can not circle the dummy. This placement also helps the handler/collector when directing the boar to mount the dummy for the semen collection process.

As with other domestic species, boar semen is sensitive to a variety of external factors including various organic/inorganic chemicals, water, sun/UV light, temperature, and some types of rubber products. Because of the aforementioned, it is important that materials which come in contact with boar semen be assessed for toxicity to sperm prior to inclusion in the collection process. There are several businesses which market products for use in swine AI which carry products known to be non-toxic to sperm. Common sense dictates that for anything which may come into contact with boar semen use clean, dry equipment which is free of any residual agents (e.g. soap, alcohol, water, powder, etc.); single use, disposable products are preferred to minimize exposure to sperm-killing compounds along with controlling cross-contamination. Semen should come into contact only with things that are at the same temperature as that of the semen; differences of as little as 2oC or 4oF can be detrimental to sperm viability and/or longevity.

A pre-warmed (37oC) insulated bottle or cup is a convenient and economical semen collection vessel. The pre-sperm fraction which flushes out the urethra, usually consisting of less than 15 ml fluid, is usually ejaculated first and allowed to go on the ground. A small amount of gel is then usually ejaculated by the boar. This gel fraction is filtered out of the ejaculate by a filter or coarse gauze (placed over the mouth of the bottle/cup), because it coagulates into a semisolid mass that can interfere with evaluation and semen processing. Ejaculation of the milky-to-cream colored, sperm-rich fraction immediately follows the gel fraction. The post-sperm rich fraction is last and contains the largest volume of fluid and gel. Dictation as to what fractions and volume on the ejaculate which is collected is dependent upon the processing technique used in the laboratory.

2. Semen Evaluation - Good quality boar semen is essential to obtaining exceptional fertility rates. Standard tests currently used to evaluate boar semen include sperm motility, sperm morphology, sperm concentration and total sperm numbers. An effort to protect the raw ejaculate from temperature-, osmotic-, and/or pH-shock during handling and analysis should be made. All equipment/materials which come into contact with semen should be 35-39oC so as to avoid cold- or heat-shock. When used individually, these standard tests have limited usefulness in determining fertilizing potential of an ejaculate. These tests do, however, have the ability to identify ejaculates of overtly poor quality. Minimum semen quality values for fresh, unextended boar semen used for AI are indicated in Table 1.

Table 1. Minimum values of fresh, unextended boar semen for use in AI1

Semen Variable


1) Appearance

Milky to creamy consistency

2) Color

gray-white to white in color

3) Total Sperm Numbers

>15 x 109 sperm/ejaculate

4) Gross Motility (Unextended)

> 70 %

5) Abnormal Morphology
-cytoplasmic droplets#

< 20%*
< 15%

1Althouse, GC. Compend Contin Educ Pract Vet 19(3):400-404, 1997.
* the 20% maximum includes cytoplasmic droplets
# includes both proximal and distal cytoplasmic droplets

Most commercial studs evaluate all ejaculates processed through their facilities. For some on-farm AI laboratories, these same routine semen evaluations tend to be impractical because of limitations of equipment, skilled labor or time. At a minimum, it is recommended that initial ejaculates on all new herd AI boars be examined by a trained individual to critically assess the boar=s semen quality. Subsequently, a monthly screening of semen quality from all AI boars should be done during their use. Routine examination of AI boar semen quality is very important because their impact on herd reproductive efficiency is increased many fold when compared to natural mating. The producer needs to look at this examination as insurance against a reproductive catastrophe. The costs from using poor quality semen become quite high after considering its effect on herd farrowing rate, litter size, non-productive days, and inventory of sows and gilts.

a. Sperm motility - Visual assessment of the percentage of motile sperm in a gel-free ejaculate by light microscopy is still the preferred method. Examination of sample mass activity or swirl motion under low magnification from a drop of semen on a slide with or without a coverslip applied is of limited value when obtaining a true estimate of a sample=s motility. To correctly estimate motility, a small drop of semen is placed on a warmed (37EC) microscope slide and then overlaid with a coverslip. When viewed under a microscope, the sample should be thin enough to visualize the movement of individual sperm. If individual spermatozoa can not be seen, a small drop of extender (same temperature as the semen) can be mixed on the slide with a similar drop of semen before overlaying with a coverslip. Sperm motility is then estimated to the nearest 5% by viewing groups of sperm in at least 4 different fields on the slide at 200 or 400 magnification; these readings are then averaged. Only ejaculates showing at least 70% gross motility should be used for further processing. This is especially important because sperm motility and viability normally decrease during storage.

b. Sperm morphology - Visual assessment of boar sperm morphology gives us a means of assessing developmental and maturational aspects of the sperm cells and the ejaculate. When a large number of sperm in an ejaculate are morphologically abnormal, a disruption in spermatogenesis, sperm maturation or improper semen handling most likely occurred. Several stains are commercially available for examining boar sperm morphology using dry mounted slides. Stains provide the contrast needed to outline the sperm when using a light microscope, allowing for easier visualization by the observer. Higher resolution and more expensive phase contrast or other contrast-type microscopes have internal components that generate their own contrast, consequently, wet mount samples are usually used for morphological estimation.

To make a stained slide, equal volumes (e.g., 10 Fl) of stain and sample are adjacently applied to a microscope slide. The drops are mixed together using the edge of a second slide, with this second slide then used to draw the mixture across the flat slide to produce a thin layer. Allow the slide to air dry. Under oil immersion, a minimum of 100 sperm are then assessed and categorized into one of 3 categories : 1) normal sperm, 2) sperm with abnormal heads, and 3) sperm with abnormal tails (including cytoplasmic droplets). Ejaculates of the general boar population usually exhibit less than 20% abnormal sperm, therefore, ejaculates accepted for AI use usually contain greater than 80% normal sperm cells.

c. Sperm concentration - The most common way of determining sperm concentration in gel-free boar semen is by measuring the degree of sample opacity. Sample opacity is usually estimated using a photometer, an instrument that measures the percentage transmittance/absorbency of light through a sample. In boar semen, sample opacity is largely dependent upon the number of sperm cells and other ejaculate components which interfere with the movement of light through the sample. Boar ejaculates are normally too thick for light to pass readily through it, therefore, a small sample of boar semen is diluted into an isotonic solution before taking a measurement. The percentage transmittance/absorbency is then converted into sperm numbers/ml either internally by the photometer or by the producer using a conversion chart which accompanies the instrument. For this measurement to be relatively accurate, it is necessary that the instrument be calibrated specifically for boar semen. And because of inherent differences between instruments, photometric conversion charts are not interchangeable. Periodic re-calibration of the instrument is necessary to maintain accurate readings. Inaccuracies in photometric readings can occur if the measurements fall outside the optimum operating range of the equipment, human error (e.g., incorrect dilutions, improper warm-up time, solution mishandling), and innate differences between boar ejaculates. It is important that manufacturer recommendations be followed on the use of the instrument for determining sperm concentration.

Another method of directly determining sperm concentration in boar semen is by using a counting chamber (e.g., hemacytometer). The time and tediousness involved with hemacytometric counts make them impractical for most AI laboratories. Because of these impracticalities, photometric analysis is the most commonly used technique for determining sperm concentration per milliliter of gel-free ejaculate.

d. Total sperm numbers - Total sperm numbers are calculated by multiplying the total volume of the gel-free ejaculate (ml) times the sperm concentration per ml. Ejaculate volume can be determined by reading the volume off of a warm graduated measuring cylinder or by weighing the ejaculate, assuming that 1 gram (weight) is equal to 1 ml (volume).

3. Semen Processing and Storage - The majority of porcine semen extenders come packaged in a powdered form. When purchasing powdered extenders in bulk, they should initially be re-packaged into smaller units which will make the desired volume of liquid extender used in your production facility. If preservative antibiotics are not already present in the extender, they should be added on the day the powdered extender is reconstituted with water. Purchased extenders should have production dates, be kept in a frost-free refrigerator, and should generally be used within six months of purchase.

Powdered extender should be reconstituted with Type I or II quality water and incubated at 37EC in a water bath for at least 1 hour to allow for temperature and pH equilibration. To prevent contamination, it is best to prepare liquid extender in a single-use, disposable bag. A minimal concentration of 30 x 106 and no more than 100 x 106 sperm/ml is recommended by European and American boar studs when extending boar semen. Ejaculate quality, extender type and length of time sperm are to be stored should be used to dictate optimal dilution rate. When taking the aforementioned into consideration, total numbers of sperm per dose of semen have been found to range anywhere from 2 - 6 billion sperm per dose.

Pooling semen from different terminal boars is a popular technique for processing semen for AI. The benefits of pooling semen include 1) increasing processing efficiency in the laboratory by allowing for a large number of boar ejaculates to be processed simultaneously rather than individually, and 2) a means to reduce or even eliminate inherent differences in fertility found between sperm and between boars. For pooling semen, freshly collected ejaculates are examined for semen quality to eliminate overtly poor ejaculates. The ejaculate is then added to a set amount of extender (i.e., 500 ml) or diluted 1:1 with an extender, and then placed in a water bath (37oC). When pooling semen, all liquids (i.e., raw semen, extender) should be at similar temperatures. The number of boar ejaculates to be pooled should not exceed the volume capacity able to be processed at one time; 3-4 pooled boar ejaculates are sufficient in most production systems. After adding the final ejaculate to the pool, the entire sample is diluted to its final calculated volume with the remaining extender. The pooled, fully extended sample is then further processed using standard protocols.

Storage temperatures from 14-18oC are routinely used for extended boar semen storage. Temperatures above 18oC for short time periods (< 24 hrs) appear to not appreciably affect overt semen quality. These higher temperatures, however, increase utilization of the extender components and may even enhance bacterial growth. Conversely, temperatures at or below 10oC cause irreversible damage to both sperm motility and acrosome integrity and, thus, should be avoided if at all possible. Stored semen should be gently agitated at least twice a day to resuspend sperm cells in the extender.