Management of gilts prior to breeding is critical to the success of the breeding farm. Gilts represent the future production of the herd, and optimizing their performance requires proper introduction that includes phenotypic selection, proper isolation, immune management (acclimatization to the herd) and reproductive selection.

The methods for implementing phenotypic selection and acclimatization are described elsewhere in this Blueprint edition. This overview article will focus on farm management strategies to assist with proper gilt reproductive selection.

Reproductive Selection
While it is common for pork producers to select replacement gilts based on their phenotype (body type, feet and leg structure, number and placement of teats), less emphasis has been placed on reproductive selection.

Reproductive selection is the process of identifying the gilts that will have the highest reproductive potential for lifetime performance. While we can never know the future, we can make some educated guesses about what will happen based on previous performance. In the case of predicting gilt performance, we know that gilts that come into estrus soon after we begin estrous stimulation (boar exposure) have a higher lifetime performance than those that have a delayed response to boar stimulation.

Therefore, if we expose gilts from genetics line “X” to a boar at 150 days of age, and some come into heat at 160 days of age while others do so at 190 days of age, we know that the first gilts that come into heat will be more productive. This relationship is only true within a genetic line, and does not necessarily mean they will be more productive than another genetic line that expresses puberty at 180 days of age.

The real question is how to use this information to make a herd more productive.

Again, it is important to select gilts based on their response to estrus stimulation and be willing to cull those that are not early responders to boar exposure, regardless of what you paid for them. There is a temptation to think, “Gosh, I paid $100 over market for them, so I have to breed them.”

Well, you can and, trust me, I’ve been there, but if you want to make more money, you may want to pass them by. Here’s why: it costs money to feed a gilt; if she is not going to make pigs, her feed cost and your pigs/sow/year will both suffer.

A little math can help reinforce this approach, but first we have to make some simple assumptions:

First, we know that gilts that come into heat within 30 days of boar exposure have more pigs in their first litter and are more likely to stay in the herd.

Second, we expect about 70% of the gilts to be in heat within 23 days of boar exposure, 85% within 30 days. If we give PG 600 (Intervet/Schering-Plough Animal Health) at 23 days post entry to gilts without a recorded estrus, about 95% will have cycled by 30 days post entry.

Finally, it costs about $1/day to feed and house a gilt or sow.

Table 1 presents three options for handling gilts. With Option 1, all gilts that do not cycle within 30 days post entry are culled. In Option 2, all gilts are held until they cycle. With Option 3, all gilts that have not cycled by 23 days post entry receive a dose of PG 600; gilts that do not cycle within 30 days are culled.

Of the gilts that are bred, the highest pigs/sow/year (lifetime average) is achieved by culling all of the non-cyclers at 30 days post entry (42.3, 40.8, 42.4 for Options 1, 2 and 3, respectively). When we increase the percentage of gilts that come into heat by 30 days post entry with PG 600 and still cull non-cyclers at 30 days, we achieve the highest economic returns on the gilts entered into the herd.

Building a Successful System

Regardless of the layout of the barns and the skill level of the staff, these guidelines help accomplish optimal estrus stimulation and gilt management:
1. Approach gilt estrus stimulation and management in a systematic manner.

  • The entire farm and management team has to be engaged for it to work.
  • Have your best breeding herd management people work with the gilts. This is hard work, and only the best will be successful.
  •  Inspect what you expect. There is as much art as science to managing and breeding gilts, so make sure your program is on track all of the time.

2. Separate physical/phenotypic selection from reproductive selection.

  • Physical selection should come first. Do not put gilts into a system that will not hold up; these gilts should be sent to slaughter at the proper weight, not as culls.

3. Isolation of incoming stock is critical to the success of the herd, as it lowers the risk of introducing new diseases.

4. Allow ample time for proper acclimation to the sow herd prior to attempting estrous stimulation.

  • Even in porcine reproductive and respiratory syndrome (PRRS) naïve systems, make sure vaccines have enough time to work prior to placing gilts into the breeding herd.
  • Don’t forget about parvovirus and all of the other things that can wreck a sow farm. Feedback and exposure to cull gilts are invaluable.
  • Allow 45-60 days for acclimation before starting estrus stimulation.

5. Ensure that gilts have adequate space.

  • 15 sq. ft. of pen space per gilt is necessary for proper estrous stimulation.

6. Boars must have “the stench.”

  • Pheromones from the boar are critical to stimulating estrus in gilts.
  • The amount of “slobber” is highly correlated to the amount of pheromones the boar is secreting.
  • Older, mature boars that are still aggressive offer the best tools available for estrous stimulation.

7. Don’t give the boar too many gilts to choose from.

  • Groups of no more than 12 gilts are best.
  • If gilts are housed in large groups, use several boars for exposure. Again, this is not optimal.

8. Collect and compile detailed records of the gilt management activities.

  • These records are best kept on the farm by the herdsperson and manager to help them understand whether they are being successful, and in a manner that can be tracked over time. Owners, too, need these records to know whether the farm is performing up to expectations.
  • Most commercial software systems are very poor at tracking and managing gilt information.

Although estrous stimulation comes in various forms, a few key principles will help ensure success: daily boar contact with maiden gilts (without an estrous cycle) for at least 20 minutes; boars that are very “smelly” and aggressive; and, people who are focused on success.

Boar Contact is Essential

There are many ways to design a barn to help achieve the desired contact between boar and maiden gilt. Some are easier and more successful than others. With the right people, all can work.

The simplest procedure is to take a boar to the gilts’ pen. While this can work, the volume of pheromones may be limited, because it is difficult for one boar to have contact with all the gilts in the pen. Groups of boars can be used, but they also present safety risks to humans.

Two alternative systems have been very successful:
1. The Boar Exposure Area or BEAR system, developed by George Foxcroft at the University of Alberta.
2. The continuous stimulation (CS) method with and without Matrix, an estrous control product from Intervet/Schering-Plough Animal Health.

The strengths and weaknesses of each follow:

The BEAR System

This system features a series of individual boar stalls with pens on each side (Figure 1) where gilts are brought for a daily, 20-minute boar exposure period. Gilts are moved from their home pen, where they are allowed about 10 sq. ft./gilt, to the BEAR system with about 15 sq. ft./gilt, typically sized to hold 12-14 gilts per pen.

The boars are arranged so they face in both directions. As soon as the gilts enter the pen, those showing signs of estrus are removed. After 2-5 minutes of active estrous detection by the herdsperson, a boar is released from his stall and allowed to mingle with the gilts. During this period of boar exposure, the herdsperson will typically begin checking gilts in the other pen for estrus or move gilts to and from their home pen.

After 20 minutes of boar exposure, gilts found in heat are moved to the breeding area in the barn (pens or stalls), and their estrus dates recorded. Non-cycling gilts are returned to their home pen.

Gilts in the breeding area are bred on their second or third known estrus, depending on the service needs of the farm.

The advantages of the BEAR system include:

  • Provides optimal gilt-boar contact;
  • All estrus events are recorded;
  • Gilts can be bred on their known second or third estrus;
  • Allows greater control of the number of gilts bred;
  • Provides safe and efficient use of labor;
  • Lowers total floor space needed to house gilts;
  • Minimizes the risk of gilts being injured during estrous stimulation.

The disadvantages of this system are:

  • Specialized facilities are needed.
  • Requires higher level of staff skills to manage the system.

The advantages of the CS system include:

  • Low labor costs;
  • Estrous stimulation is done in isolation to optimize weight at breeding;
  • Allows farms that do not have the quantity or quality of labor resources to be successful.

The disadvantages of the CS system are:

  • Requires no control or knowledge of how many gilts are in heat at any time;
  • Many gilts might be bred on first estrus;
  • Many more vasectomized boars are required;
  • Risk of gilt injury is higher;
  • It is difficult to control the number of gilts bred per breeding group;
  • Requires more floor space than the BEAR system.

While this system can be successful, it is very difficult to meet a farm’s goal of breeding the same number of gilts each week. Adding the use of Matrix (altrinogest) to this program can help.

In this modification, we use the CS model, but three weeks after starting boar exposure we place part of the breeding group on Matrix for 14 days, beginning 19 days prior to the start of the breeding week. For example, if we have four week’s worth of gilts, we place 25% of the group on Matrix; with five week’s worth of gilts, 20% receive the altrinogest, and so on.

We have been successful in getting 85-95% of the gilts bred during the seven days following Matrix removal. Gilts that fail to cycle seven days after the product is removed are given PG 600. If they do not cycle within seven days of this treatment, they are culled.

The key advantage of this system is that it allows us to control the number of gilts bred each week with minimal labor.

We have conducted case studies where we have recorded gilt estrous cycles during the three weeks of boar exposure. It is interesting to note that regardless of the number of confirmed heats we observe prior to the Matrix treatment, we still see 85-95% of the gilts cycle within seven days following its removal. This tells me that there are a high number of gilts in many systems that have weak or silent estrous periods that are not recorded, yet we know they are cycling because they respond to Matrix administration and withdrawal.

The bottom line is this — stimulating estrus in maiden gilts is valuable because it improves the economic returns to the farm. Implementing an estrous stimulation process in a disciplined manner allows the farm to capture all of the benefits of early-cycling animals, while minimizing the number of select gilts that are culled and never bred. There are many ways to accomplish this goal. The BEAR and CS systems are two examples that have been very successful in our
veterinary practice.