GILT DEVELOPMENT

 

Monte B. McCaw

North Carolina State University
College of Veterinary Medicine

Pig Health and Management Group

 

Gilts are often viewed as a necessary evil.  They are hard to breed, farrow fewer piglets, require special housing, feeding, handling, close observation, may introduce disease, and count against DENR maximum animal inventories even though they are considered “nonproductive”.  Therefore they are often neglected to one degree or another, but with serious consequences for the overall productivity of the herd.  Since gilts make up at least25% of the herd, whatever their performance is will significantly affect the herd’s performance.  Therefore, it makes sense to work hard to maximize their productivity and ensure their long life within the herd.

                For this investment in effort and resources we expect the gilt to be productive, that is to have piglets, successfully raise them, rebreed and become pregnant again, thus remaining in the herd, all without initiating any production losses from clinical disease.  She must be fertile, come into heat at a predictable time (earlier rather than later), and enable elective replacement of an older sow.  She should farrow a maximal number of piglets, lactate her piglets, and likely some from an older sow with too many liveborn and too few functioning teats.  The gilt should be robust, have sufficient body weight and capacity to eat enough feed to not loose excessive weight during lactation to enable her to recycle and rebreed quickly and thus remain in the herd.  Finally, she should be healthy, not carrying in any new diseases nor susceptible to any diseases that are already in your herd.

 

            Optimizing gilt fertility requires coordination of feeding management, housing, properly timed boar exposure and mating.  Gilts apparently do not require a special ration during the pre-selection / finishing stage.  If gilts are full-fed standard finishing rations during their growth it appears that there is no long-term detrimental effects upon her fertility or longevity.  Once gilts are selected (approximately 210 pounds) they should be placed upon a lactation ration.  Lactation ration provides the gilt the necessary elevated levels of protein, lysine, calcium, phosphorus, and micronutrients.  This will minimize any risk of insufficient bone mineralization and fractures during later matings or pregnancies.  The level of feeding leading up to mating also can impact the gilt’s performance.  Flushing (increasing the gilt’s feed allowance by 50 to 100% for 10 to 14 days prebreeding) has been shown to increase ovulation rate and subsequently increase the number of piglets born as much as 1.0 pig per litter when gilts were being limit fed during development.  This effect may be herd specific, but often most pronounced in herds with low average gilt total born.  Flushing, to be effective, must be followed by limit feeding immediately after mating (full feeding following mating may increase embryo mortality).  Modern fast-growing gilts should be fed to maximize their body weight by mating to maximize their chances of remaining in the herd after their first litter.   Feeding modern gilts at or near ad libitum after final selection through puberty up until the time of mating will maintain them in the positive energy status (“flushed”) necessary to maximize the number of ova shed during estrus.  Gilts may have reduced litter size if they are bred on their first heat after transportation and entry into the herd because they were in a negative energy state resulting from reduced daily feed intake during transportation and adjustment to the new herd.  Also, pen feeding of new gilts may cause the least dominant animals to not get full access to feed and therefore have small litter size or not cycle as early.  Full feeding gilts up to the time of mating ensures their rapid growth, sufficient size, young age at first estrus, and maximal number of ova shed.  Finally, the age at which gilts first start to cycle is dependent upon their breed. Crossbred animals may start to cycle 20 days earlier than purebreds.  Certain breeds like the Landrace and Large White begin cycling younger than Yorkshire or Duroc gilts.

 

Housing

 

Housing of gilts also has a major impact upon their age at puberty, fertility, and fecundity.   It is well known that transporting gilts from one facility to another will stimulate most mature gilts to cycle within days of arrival.  However, how the gilts were raised before that transport will affect how many will be ready to cycle and breed successfully after arrival.  A greater percent of gilts reared in outdoor lots will attain puberty and at an earlier average than confinement reared gilts.  Outdoor rearing of gilts is not practiced widely today, mainly for biosecurity reasons.  None-the-less we must keep in mind the negative impact confinement rearing has upon their sexual development.  Seasonal effects on gilt maturation are more marked in outdoor reared vs confined gilts.  However, even the negative effects of seasonal on puberty still do not negate the positive effects outdoor rearing has upon early maturation of gilts.    Gilts apparently should be reared in groups rather than individually to optimize their reproductive development.  When gilts were crated individually or penned in groups of 3 they reach puberty at a much older age and at a lower rate than gilts housed in groups of 10 to 27.  Gilts housed individually in crates were also observed to have nearly double the incidence (28%) of silent heats and irregular cycles as pen reared gilts.  Also, teathered gilts have been found to have a significantly greater incidence of immature reproductive tracts at 10 to 12 months of age (13/87 vs. 0 / 87) versus pen reared controls.  Conversely, gilts raised in groups of 50 or more also had a lower conception rate than smaller groups.  Therefore, it appears gilts are best reared in groups of approximately 10 to 30 animals.  Density of gilts in the developer pens appears to have no effect on the age of puberty.  However, PIC recommends 12 square feet per gilt space, possibly to minimize the risk of injury from fighting among animals.  Gilts can be raised with barrows with no bad effect on their sexual development.  However, gilts should not be exposed to mature boars until they are approaching puberty (see below).  The gilts’ environment can also affect her development and fertility.  Dr. Flowers found environmental temperatures of 90oF reduced the proportion of gilts reaching puberty by 230 days from 90% to > 30%.  Approximately half of the gilts reared at the higher temperatures had cystic ovaries.  Also, excessive environmental temperature increases embryo mortality if the gilt is exposed between mating and day 16 gestation, thus decreasing the number of piglets born live.  Conversely, gilts housed individually in wet conditions or with a lot of draft will experience greatly reduced effective environmental temperatures.  These gilts will need more feed to maintain their expected rate of growth and body condition and thus a positive energy status for optimal fecundity at mating.  Finally, environmental gases may also play a role in the gilt’s reproductive development as 20ppm ammonia was found to delay puberty by 7 to 10 days in 15% of gilts tested.

 

Boar Exposure

 

Boar exposure is essential to stimulating gilts to mature earlier and express visible estrus.  Properly timed boar exposure can stimulate gilts to cycle as much as 40 days earlier than non-stimulated controls.  This is critical to minimize feed and housing costs of gilt development.  Also, early puberty enables producers to wait and breed gilts on their second or third heat.  One pig or more liveborn per litter can be gained by waiting to mate gilts until their second or even third estrus.  Given that gilts generally farrow 25% of the herd’s litters, this may be a very significant gain for herds with low liveborn problems. 

 

To achieve early puberty and optimal gilt reproductive performance four things must be accomplished: 1) do not start boar exposure at too young an age (<160 days), this makes gilts much less responsive to his effect when they approach puberty 2) use mature (>10 months old) boars for gilt exposure 3) take gilts daily to a strange pen to be exposed to the boar (either the boar pen or a breeding pen)  4) direct physical contact with the boar  by the gilt is critical, fence line contact not good enough.  Only 5 to 15 minutes of boar exposure is needed per day to get the desired effect.  The rest of the time, house gilts away from boar contact and scent.

 

Once the gilts approach puberty their estrus cycles can be synchronized by transporting them to a new facility.  If the final stages of gilt development are performed “on farm” or if boar exposure in the gilt developer is not possible then maximizing the number of sexually mature gilts AND synchronization of gilt estrus may be difficult to achieve simultaneously.  Rotating boars used to expose gilts with may improve the proportion of gilts that will begin cycling and therefore mate.  Realize, that there appears to be an association between how quick gilts come into heat following boar stimulation and their subsequent lifetime fertility.  Usually gilts quick to show estrus are more fertile (return to heat quickly, high conception rates) than slower to cycle animals.  If gilts are to be naturally mated, mate them immediately upon observing standing heat.  Vasectomized boars can be used to stimulate heat in young gilts and still prevent unwanted / recorded matings.  Allowing the vasectomized boar to mount and “breed” the gilts on their first estrus may have positive effects upon subsequent litter size.  However, be certain to wait 6 weeks after surgery to ensure no viable sperm is in his ejaculate.  Also make sure the vasectomized boar is mature (old) enough to stimulate the gilts with his odor and mating activity.  Finally, abortion of bred gilts early in pregnancy (< 42 days of pregnancy) and rebreeding them upon their first estrus (96 hours later) can be a very reliable method of timing their entry into specific breeding groups.

 

We expect gilts and sows to lactate or feed their piglets.  That we can’t do it easily or economically is evident by the reduction in swine units attempting to use the Segregated Early (7 to 14 days) Weaning management system.  However, how frequently do we cull based upon the number of pigs weaned?  At what level of priority is lactation ability ranked among factors used to decide whether a sow is kept in the herd or not?  Certainly it’s ranked below ability to walk and become pregnant, and farrow a few liveborn piglets.  But how much lower should it be ranked, if at all?

 

Lactation is half of the sow’s purpose in the herd. 

 

Sows must be able to feed the piglets that are born alive; otherwise the piglets die. Piglets commit to and suckle from only one teat. Therefore, the number of functional teats controls the number of piglets that are weaned.  Functional teat number is vastly overlooked!  Functional teat number is the most limiting factor to the number of pigs weaned per sow or per week in most of out herds.

 

Today, with our aggressive breeding programs and sows selected for large litter size / liveborn, it is rare to find herds where the number of pigs born alive doesn’t exceed the number of functional teats available to feed them.  Also, with many states implementing waste management control plans that also limit the number of sows per site, we are now unable to “over farrow” each week to meet our production goals in less efficient herds.  Therefore, to maximize our sow herd’s productive efficiency we must include the number of pigs each sow weans in our culling decisions and select gilts with a large number of well spaced and formed teats in their underlines.  The following calculations illustrate the impact of number of functional teats available (number of pigs that can be weaned) upon preweaning mortality and obviously the number of pigs weaned per litter.

 

                                     10  litters

Ave LB            10.5    105  born

Ave Weaned    9.2       92  weaned

13    died  =  12.4% PreWeaning Mortality

 

 

Ave LB            11.0    110  born

Ave Weaned    9.2       92  weaned

18    died  =  16.4% PreWeaning Mortality

 

After increasing number of functional teats in the group by 5 to handle the increased number of pigs born alive.

 

Ave LB            11.0    110  born

Ave Weaned    9.7       97  weaned                       

 13  died  =  11.8%  PreWeaning Mortality

 

How can we change the average number of functional teats per sow (per crate)?  

 

By increasing the average number of functional teats per sow we will increase the average number of pigs weaned per crate, therefore per room and ultimately per week by the herd!   Simply put, the average number of functional teats per sow can be increased by: 1) carefully selecting gilts with a large number of evenly spaced well formed teats and 2) culling sows with poor underlines (damaged teats, rear udders that swing freely, mastitis, or are too big for piglets to reach the teats easily) and / or wean low numbers of pigs.  Obviously we need to have 1) enough gilts to replace these voluntarily culled sows and 2) be certain that the gilts will definitely wean more pigs than the sow she replaces.  I feel that, barring obvious disease of the litter or sow, that low number of pigs weaned by middle to older aged sows will very likely be repeated in the next litter.  What we may lose in number of pigs born alive by replacing that sow with a gilt I believe will be made up for by the increased number of pigs weaned by the gilt.  Remember, we currently are farrowing more pigs than most of our sows can wean, therefore fostering (at birth) some of the extras onto gilts will help increase the number of pigs weaned and decrease preweaning mortality at the same time!  In short we must maximize the number of functional teats in each room to maximize the farrowing herd’s weaning performance.  Therefore, it is critical that we manage the gilt pool so we have enough gilts available to replace older poor milking (as well as non-breeding or lame) sows that need to be replaced.  This allows for voluntary sow culling rather than the involuntary and therefore reactionary or emergency type replacement programs we’re often forced to use when gilt supplies are tight.

 

We need to ensure gilts enter the breeding herd in robust body condition.  We must maintain that condition through gestation and especially their first lactation to maximize the chances of their rebreeding successfully and remaining in the herd long-term.  Therefore active attention must be focused on both pre-pubertal as well as lactation gilt feeding practices and rations.   If the gilt is not robust upon entry into the breeding herd, and not maintained in that condition, she is very likely to fail to cycle or conceive and be culled by second parity.  This is particularly true of gilts that produce a lot of milk / feed a large first litter.  Particular attention must be focused upon the amino acid levels as well as energy and calcium / phosphorus levels of the gilt rations.  Gilt growth targets should be an 100 pound gain from breeding to farrowing, sows 75 pounds.  Particular care needs to be made in ensuring all gilts in pen gestation situations get their needed  amounts of feed for growth and condition maintanance.  The least dominant animals are at a high risk of not gaining the necessary amount of weight during their first pregnancy and therefore are likely candidates for culling after weaning only one litter.  In lactation for every 1% loss in weight (as fat) by parity 1 sows there was a decrease of 0.12 pigs born, 0.14 pigs weaned, and 2.2 pounds less litter weaning weight in the second parity.  Sows should gain 25 to 35 pounds between farrowings.  For some of our hybrid sows we need to also pay attention to their size and body condition for entering lactation.  If they are too fat, they may not eat as much in lactation, lose a higher percentage of body weight, enter a negative energy state, and ultimately not rebreed.  To minimize gilt weight loss and maximize their lactation feed intake we may need to consider feeding them three to four times a day after first week of lactation.   It appears that nitrogen (protein) loss during pregnancy may actually be more important than energy / fat losses to subsequent reproductive performance and retention in the herd.  This appears to be particularly true during the first lactation.  We may need to consider a gilt weighing and ultrasonic probing to measure back fat and loin eye size before and after lactation to access the effectiveness of our feeding management.

 

Finally, we expect gilts to be Healthy additions to the herd.  Health in this setting means that the gilts neither bring in new disease nor are susceptible to diseases already present in the herd.  In the second situation, commonly seen in PRRSV infected herds, as the proportion of non-immune animals in the breeding herd increases, so to does the risk of clinical disease reoccurring in the herd.  To prevent introduction of unwanted new diseases we must test animals both before delivery and while in isolation to ensure they are not infected before being added into the herd.  Closely observe new gilts and boars daily in isolation for any abnormal signs including cough, diarrhea, lack of appetite, depression, reluctance to move, rough hair coat, bad colored feces, pale color, rapid breathing, etc.  The cull animals from your herd can be used as both clinical signs sentinels an should be tested at the same time for sero-conversion to unwanted diseases following exposure to the new gilts.  Also, blunt discussions by your veterinarian with the selling producer and their veterinarian (should include diagnostic test charts of regular repeated testing in the source herd) should be had before sale as to whether the disease has been observed.  Also ask what are the measures taken to identify whether the infection may be in the herd and what is done to keep it out.   Gilts must be made immune to diseases already present in your herd.  This can be accomplished by vaccination with commercial vaccines, exposure to clinically affected animals, or feed-back of infected tissues or feces (TGE, rotavirus, PRRSV?) while the animals are in isolation.  Each herd needs its own set of immunization procedures since they each have different disease profiles.  The gilt feed-back or exposure procedure must be limited to use in isolation only because the gilts may shed enough bacteria or viruses to actually overcome the immunity of older sows in the herd causing clinical repeated disease even in these animals.  Feed-back can be accomplished with feces from piglets in the farrowing house, infected pig intestines, mummies, or with housing gilts with cull animals from the main herd.  The best cull animals to use are un-bred cull gilts or possibly very old and thin sows.  Successfully infecting gilts in isolation is actually very difficult to do consistently.  To improve the rate of infection, trade pens of gilts with the cull animals daily (keep the same animal groups, move the groups to different pens).  Also, don’t wash out the isolation room between groups of gilts to improve chances of successfully infecting them.  However, if an unwanted disease is detected in gilts during isolation, thoroughly clean and disinfect the isolation unit after selling the infected gilts and before bringing in a new group.  Success or failure of infection must be monitored by testing gilts at entry and again prior to release into the breeding herd.  We have to assume they are immune following developing an antibody response.  All testing, exposure, and observation procedures must be repeated religiously for each set of gilts entering the herd.  One single slip or certainly repeated systematic lazy omissions in gilt vaccination or acclimatization (exposure to home-herd infections in isolation) will result in exceptionally costly losses (for example PRV, TGE, PRRSV, Influenza, disentary, etc).  To maximize the usefulness of the isolation time expose gilts to a boar, observe and record estrus dates in isolation to aid efficient and effective gilt insertion into the herd.

Take-Home Messages:

 

·        Make sure gilts are fed for rapid growth

·        Make sure gilts get fed heavily before breeding and for growth during gestation (maintain a 3.5 body condition score)

·        Make sure gilts are exposed to boars to stimulate early estrus

·        Breed gilts only after their first estrus

·        Select gilts also by the number and spacing of their teats

·        Cull sows that don’t wean many pigs

·        Maintain enough gilts in the gilt pool to allow for voluntary (non-death non-reproductive) culling.

·        Make sure gilts and sows are fed as much as they will eat in lactation to ensure they can come into fertile heat again and be rebred.  Feed gilts 3 – 4 times per day in the second and third weeks of lactation.

·        ISOLATE AND TEST ALL HERD ADDITIONS!  REJECT groups found to be infected with unwanted diseases.

·        Determine which diseases you have and which ones you don’t have in your herd.  Design (with your veterinarian) and follow religiously a vaccination, exposure, or feedback program in isolation.  Test new gilts negative for new diseases before allowing them to enter the sow herd.  Test gilts to cross-check that they have been successfully exposed (acclimatized) to the diseases in your herd that they were not immune to upon arrival.