Gilt development

Gilt development, management and parity control

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A proper gilt development program should be followed to maximise their genetic potential. For a good gilt development program, producers must be willing to accept that this phase of production is considered an essential step that many times will set the lifetime productivity of the sow. An appropriately matured reproductive tract, along with an appropriate weight for the age of the gilts together influence litter size. An increase in the non-productive days (days when the female is neither pregnant nor lactating) is often a consequence of the high number of gilts not entering the breeding herd at a desired age due to the lack of a well-designed gilt development program. The program needs to include isolation and acclimatisation as well as management and environmental factors for optimising puberty and target age for mating.

The goal must be to have gilts acclimatised prior to target breeding age. Regardless of the system and the age at herd introduction a defined gilt development plan must be implemented to optimise the breeding herd efficiency. This should include a 30-day all-in/all-out isolation and 30-day acclimatisation period to prevent disease introduction into the herd. This allows development of a common health status between incoming gilts and the herd. It also allows time to detect health concerns in both the source herd and the isolation.

Acclimatization should include the use of herd sentinel animals or feedback. The use of young, cull boars (10-12 months) or sows many enhance immunity and may also influence puberty.
Ideally, isolation is from 120-150 days of age, and acclimatization is from 150-180 days of age.

Disease and injury are two of the greatest challenges that producers face when attempting to introduce new genetics into the breeding herd. Disease agents may impact the successful introduction and mating of new gilts. Many gilts challenged with a health disturbance reach
puberty at a later age and lower weights compared with gilts that are not challenged. Health challenges that occur near the onset of puberty are most detrimental with respect to the age at puberty. Attaining an appropriate health status and immunity in the gilt developer, and in the breeding facility, is a challenge but necessary for achieving the highest reproductive efficiencies.

Puberty in females is defined as the first heat, or oestrus, and naturally occurs in gilts at about 200-210 days of age with no stimulation. However, there are many management and environmental factors that can decrease age at puberty. The female reproductive system reaches a state of readiness, but if not properly stimulated by external factors, the gilts will begin cycling at a later age leading to increased cost due to more non-productive days. Gilts make up a significant percentage of the breeding herd. Therefore, optimising non-productive days clearly defines an economic advantage. Also, there is evidence that gilts that cycle early will be more reproductively successful (with larger litter sizes and increased longevity), and age at puberty may be an indicator factor of subsequent performance. The external factors, discussed below, are the stimuli that will influence the reproductive system starting the reproductive cycle of the gilt. There are many practices that influence the onset of puberty, but the strongest and most important is stimulation by a mature boar. Other stimuli, used by many producers, include adding new pen mates (mixing), movement to a new building or location and new animal handlers.

It is well known that management practices have a large impact on alerting the gilt development process. Boar exposure, nutrition, housing conditions and moving/mixing gilts are excellent management tools used to stimulate the onset of puberty and enhance the signs of standing heat.

A. Housing of the Developing Gilts
Management factors during the gilt-rearing phase include pen spacing and the environment within each pen. Prepubertal gilts should be housed in pens rather than individual stalls. Interaction with other animals is an important factor for sexual maturity. Providing ample room for boars to move around in the pen and providing the proper physical stimuli are essential. This will provide and ideal exposure of boar pheromones, which are present in the saliva. Plenty of space ensures the pheromones will be distributed throughout the pen, properly stimulating the gilts. Spacing recommendations for proper boar exposure are 10-12 sq ft/gilt (0.93 –1.11 sq m) for gilts over 150 days of age. These space recommendations are often overlooked in many gilt pools because many producers follow recommendations for grower/finisher pigs, which are unsuitable in a gilt development program. If only fence line exposure is given, ideal facility design would provide pens that are wider (>12 feet or 3.66 m) along the alley but short in depth. This will enable more gilts to be at the fence next to the boar for the sight and smell stimuli when the boar is moved through the passageways. So, not only is pen size important, but pen “layout” can influence the effectiveness of boar exposure.

Pen flooring can affect the gilt development and should be of a type that drains correctly promoting a dry surface that is smooth and non-abrasive. Poor quality of such flooring compromises bone, hoof and pad integrity. Additionally, when using boars to enhance heat in gilt pens, non-slip floors will prevent leg injuries for both the gilts and the boars.

B. Nutrition
Gilts must be in good condition to maintain pregnancy and endure future lactational stress. A strong nutritional program should begin at weaning, because early bone and cartilage growth is crucial to ultimate integrity. Data indicates that nutrient shortages in early growth lower the potential for maximum bone mineralisation and strength at breeding. Additionally, gilts must be fed properly to maintain structural soundness throughout their lifetime.

Gilts on full-feed prior to puberty will be younger at the time of sexual development and have higher ovulation rates compared to those limit-fed prior to puberty. It is generally seen that in herds with improper nutrition, puberty is delayed increasing non-productive days with serious economic consequences.

Visual or backfat scoring with ultrasound of gilts is a common practice. At mating should have a target backfat of 16-18 mm, and if gilts are bred at 210 days of age, the target weight should be 127 kg. During pregnancy, gilts should put on a net weight of 34 kg so they arrive at parturition with the best body condition score and backfat level that should 18-20 mm.

To optimise gilt feeding, producers must have a functional gilt pool facility. In general, these facilities, known as developer barns, use similar feeders and water delivery systems to those used for grower/finisher pigs. Feeding on the floor should be avoided because it is difficult to full-feed gilts using this method. If gilts are limit fed to slow growth, they must be returned to full-fed (flushing) for two weeks before breeding to insure maximum ovulation rate. The feeding systems should optimise feed intake and provide adequate water consumption in an environment without stress.

C. Boar Exposure
The most important factor in stimulating early puberty in developing gilts is boar exposure.
Producers are aware of the importance of boar exposure but some do not optimize this technique for the following reasons:

  • Lack of people to expose gilts daily (competing labour priorities)
  • Lack of functional facilities, which don’t allow easy boar access to gilts in the pens
  • Poor penning arrangements that make fence line exposure difficult
  • Lack of a boar program to produce the necessary boars

Boar exposure will increase the number of gilts coming into oestrus and strengthen the signs of heat. Seventy-five percent of gilts should come into heat within 3 weeks after implementing boar exposure. Timely and adequate boar exposure will assure appropriate age and weight on the second or third cycle. Mating at this time results in a higher farrowing rate and pigs born alive.

The mechanisms of boar exposure that influence gilts coming into heat are smell (pheromones) and physical contact with the gilt. Older (at least 10 months of age), experienced boars with high libido, frequent grunting and strong smell are best for stimulating a strong heat response. Nose-to-nose contact is important as well as having the proper ratio of boars to gilts. One recommendation is a ration of boars to gilts of 1:200 for boars much older than 10 months and 1:50 for boars closer to 10 months. These boars must become part of the normal replacement program of the herd. It is also advantageous to rotate boars among gilts. Boars should not work more than 5 pens of gilts per day.

Physical boar exposure (direct contact of boar to gilt) is the best way to enhance puberty and good heat expression. However, fence line boar exposure (boar and gilt are separated by a fence/gate) can influence puberty compared to no boar exposure. It is important that nose-to-nose contact be made even with fence line exposure. Boars used for fence line boar exposure will maintain interest longer if allowed to have nose-to-nose contact. The boar stimulus may be enhanced by providing gilt backpressure and side nudging during boar exposure. This also provides an opportunity to detect the pubertal oestrus. Recording this data is a must for fitting gilts into breeding schedules. It is also advantageous from a management standpoint to regroup cycling gilts by week of heat dates.

The most effective procedure for physical boar exposure and heat detection is to move gilts and boars to a different pen. Combine the boar with the group of gilts (no more than 20 gilts per group) for at least 10 minutes. There should be ample space in the pen for the boar to stimulate all of the gilts. Unless carefully managed, gilts low in the dominance order, may not receive adequate stimulation. Once a day, boar exposure can be effective, but if time allows, boar exposure two times per day will increase sexual stimulation of the gilts. The importance of stimulation from the use of a mature boar cannot be over emphasized.

When using boars to stimulate puberty or enhance signs of heat, it should be remembered that gilts respond differently from sows in the presence of a boar. It is important for the stockperson, who is heat checking, to understand that gilts are less likely to show heat signs. The majority of gilts in oestrus will exhibit signs of heat in the presence of a boar. The standing oestrus response generally lasts for 5 to 15 minutes and gilts will become refractory for 45 to 55 minutes. Because identification of puberty is a goal, it is important that farm staff continually be aware of the behaviour of the gilts and make note of the heat date.

If the boar is housed near gilts, or the smell of the boar is present in the barn, the standing estrus response can occur at times when farm personnel are not present. Boars should be housed away from gilts in a separate room, building, or at least down wind, near an exhaust fan.

Boar exposure for very young gilts (less than 150 days) will not have a detrimental effect. It can, however, result in non-synchronous puberty, in a group of gilts. Delaying boar contact until around 150 days can result in a more synchronized first oestrous. Synchronous oestrus cycles may be advantageous when targeting weekly breeding.

It is recognised that some gilts will express heat when re-mixed or after being transported after 150 days of age. In general, producers should skip one or two heats so they can acclimatise the gilts to the new environment. The re-mixing or transport heat is not a substitute for the use of mature boars. If gilts are not detected in heat three weeks after re-mixing, repeat and continue heat checking. Along with moving/mixing gilts, boar exposure must be implemented.

Many producers use hormone treatments such as PG600 to bring prepubertal gilts into heat.
PG600 contains two hormones that mimic the naturally occurring hormones that are responsible for initiation and regulation of the oestrus cycle. PG600 can be used in conjunction with previously mentioned gilt movement and boar exposure but not in place of them. Use of this tool will enable producers to synchronise gilts resulting in ease of predicting gilt breeding targets. Use should be according to anufacturer’s suggestions.

Environmental factors such as season, day-length (light), temperature and air quality affect the developing gilt. These factors must be appropriately controlled within production systems to minimise deleterious effects.

a) Season
It is well known that season has an effect on reproduction due to temperature and day-length. In general, there are fewer gilts reaching puberty during the summer months (high temperature) than those in the winter months. Seasonal effects may be due, in part, to changes in day length, but there may be other unidentified factors. There is controversy surrounding the amount of light required during the period of gilt rearing, but eight to ten hours of light are minimal. Currently, few producers use additional artificial light while rearing gilts. It is unclear whether increasing day-length beyond the normal photoperiod for the season is useful in correcting reproductive problems.

b) Temperature
Thermoneutral zone, or the comfort zone temperature for gilts, will usually range between 18° to 24° C for developing gilts. Producers should make the effort to keep gilts in the thermoneutral zone. Temperature, whether too high or too low, has been shown to delay puberty and affect reproduction. Temperatures above 30° C for several days, especially if occurring during the night, can increase the time to reach puberty. This means that producers may need to add additional gilts during hot weather to achieve breeding targets. Producers in the US generally follow this practice.
The effects of temperature are also associated with decreases in feed intake. Cold temperatures increase the amount of energy needed for maintenance and limits growth.

c) Air Quality
Rearing developing gilts indoors imposes the necessity for systems that provide proper ventilation and air quality. Fresh air must be provided while maintaining thermoneutral temperatures and relative humidity. When ventilation is inadequate, there will be a build-up of ammonia and other air pollutants that will be detrimental to the developing gilt. In general, as buildings get older, equipment tends to fail making it more difficult to achieve optimum performance. Routine maintenance must be a part of any management system.

Age at first mating is dependent upon the age at puberty and how effectively the gilt development program has been implemented. The reason to target the desired age at mating (on the second or third heat cycle) is because of the required planning for induction of puberty at an early age for optimization of reproductive performance. Producers must weigh economic factors and breeding herd targets to estimate the appropriate time when gilts will be first mated. To optimize the genetic program, one must consider the introduction of a fixed number of gilts annually. The ability to meet gilt breeding targets is essential, and the failure to meet herd breeding targets is the
most important factor affecting pigs/sow/year. Producers must have 90% of their gilts bred within 30 days after being available for service. Inefficient management of gilts can contribute to 30% or more to non-productive days in many herds. When determining economic factors associated with non-productive days, feed labour costs along with inefficient facility usage, must be evaluated.

Ideally, gilts must be mated at the second or third oestrus. This allows the reproductive tract to mature. Several studies have indicated that mating on the second or third oestrus can result in a larger litter size, thereby increasing the number of pigs produced over the sow’s lifetime. A good program starts with the replacement gilt as it leaves the nursery stage. With gilts out of the nursery, the efficiency of how producers manage their gilt program results in different outputs of pigs produced in parity one gilts. Poorly managed gilts result in higher deaths, lower farrowing rates and fewer pigs born alive. Producers should analyse the productivity of parity one gilts and their influence on lifetime performance.

Step 1: Evaluate past records for:

  • Replacement rate and breeding targets
  • Cull and death rates of the replacement gilts

Step 2: Prevent disease introduction and develop common immune status

  • Introduce gilts into isolation and acclimatization barns
  • Use proper immunization procedures by veterinarian
  • Provide sentinels and feedback when necessary

Step 3: Feed gilts to encourage growth and sexual development

  • Provide gilts full-feed, if restricted, flush prior to breeding (2 weeks)
  • Routinely monitor watering system.

Step 4: House gilts to encourage:

  • House gilts in pens under desired temperature of 18° C to 24° C
  • After 150 days of age, raise gilts in pens of 16-20 with 10-12 sq ft/gilt (0.93-1.11 sq m)
  • Assure adequate ventilation and flooring

Step 5: Daily boar exposure (heat checking)

  • Begin boar exposure at age 150 days
  • Check heat with mature boar (10-24 months old, smells, grunts and salivates) for at least 10 minutes per pen
  • Provide back pressure and side nudging
  • Allow nose-to-nose contact
  • Boar exposure 1-2 times per day
  • Record date of heat for each individual gilt

Step 6: Target optimum age at first breeding

  • Target optimum age at first breeding. Target second or third oestrus for first mating (210 days of age). Target weight of 125 kg and back fat 16-18 mm.
  • Target second or third oestrus for first mating
  • Target weight of 125 kg and back fat 16-18 mm
  • Target farrowing rate of >88% and born alive > 10.8 pigs

Step 7: Use specially trained labour


The relationship between sow age and performance ability is well documented. However, it is not always so clear what effect the change in sow performance has on the overall profitability of the business. It is a subject that has come under discussion in the US industry recently where much of the explosive growth of the last few years is reaching maturity. The need to control more keenly the parity distribution of the herd (defined as the number of sows in each parity at any one time) is now becoming more of a priority.

Data show us that in some key areas the sow has a performance pattern through the course of her life. This particularly evident in Born alive, Still born, Mortality and Non-productive days. In all these traits sows will tend to start lower, peak somewhere between second and fifth parity and then start to decline from the sixth onward.

These factors can change significantly from situation to situation and it is only by knowing our own circumstances that we can set up a plan that is suitable for us.

It is clear that if a particular herd has a parity performance profile that drops off dramatically after the sixth or seventh litter then it will be detrimental to that herd’s performance to have many animals over the seventh parity, as they will be dragging down the average output. Equally, just as the older sows are pulling down the farm by their poor performance so, too, are the gilts.
Making better management of the gilt herd a priority will have a beneficial effect on overall performance and give us the opportunity to be more selective in culling the older sows. Better-managed gilts produce larger litters in their first parity, maintain superiority throughout their life and have greater longevity.

The challenge is to ensure that it is the older sows who are selectively culled by minimising the loss of younger sows from physical causes.

If we agree that a sow is at her most productive in second to fifth parity then it would be logical to want to have the whole herd in that range all the time. Unfortunately, that ideal is not a reality as it implies that sows do not age and that gilts enter at second parity. Broadly speaking there are two options open to us as we decide how we want a herd to look in the future.

A) Start the herd out at a low replacement rate and raise it later, this approach produces the familiar wave of sows getting older creating the “honeymoon” in years 2-3 with a large percentage of sows in second to fifth parity. The downside is that the herd experiences a performance drop as that group of sows goes above parity 7. Often an increased replacement rate is ntroduced in year 3 creating a second wave.

B) Start maintaining a constant replacement rate. This approach implies a heavier culling policy in the early stages of the herd and leads to a reduced “honeymoon” in years two and three. However, it leads to a structure that is sustainable over time and may have some interesting downstream benefits when we look at a constant flow of weaners through the growing system and a constant demand on the multiplication system for replacement gilts.

The first of these two options is certainly the more traditional but I think that we need to ask some more in-depth questions about what is the real value (or cost) of that “honeymoon period”

  • Which system will create the highest margin over the long term?
  • What is the value of having a constant throughput through our grower system?
  • Is there a value, in large systems, to operating a simpler management system?

Typically when we compare these approaches we see that the first produces more piglets in the first two years but the second then overtakes and in the long-term produces more pigs because it avoids the peaks and troughs created by the herd ageing in waves.

Every herd is shaped by variations in various key areas such as genotype, nutrition, gilt management, housing, etc. therefore each situation needs to be managed individually.

Broadly speaking, we could say that the replacement rate is directly proportional to the maximum parity we want to allow in the herd. A constant 50% will insure that sows leave the herd earlier that a 40% constant.

That decision is key to our parity control management and rests on some basic information about herd-specific circumstances.

  • Productivity by Parity (dependent on genotype and the management system)
  • The marginal value of producing an extra pig to the farm
  • First and second parity performance
  • Gilt development
  • Cost of replacement gilts
  • Salvage value of cull sows
  • Supply of replacement gilts

There are clearly huge benefits to be gained from establishing and maintaining the herd structure that will give the maximum profitability for the business in the long term. In order to achieve this it is essential to have a good understanding of each individual farm and to set up the strategy most applicable to the specific situation.

If the herd manager can work to this plan and can concentrate his energy on gilt development and selective culling, he will have the opportunity to take control of herd structure rather than allowing the herd to dictate the pace.

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Jon Bergstrom Jon Bergstrom
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