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Understanding Regular vs. Irregular Returns to Estrus

To improve open sow days, litters/female/year and farrow-to-farrow intervals, we will review a case study looking closer at the impact of

To improve open sow days, litters/female/year and farrow-to-farrow intervals, we will review a case study looking closer at the impact of returns to estrus.

The definition of a “regular return” to estrus is a female that did not conceive when mated but has a heat cycle every 18-24 days until rebred or culled. An “irregular return” to estrus is a female that does not have consistent, regular return to estrus every 18-24 days.

It is important to understand the difference between “regular” and “irregular” returns to estrus. For example, returns 17 days and less are irregular, 18-24 days are regular, 25-38 days are irregular, 39-45 days are regular, and, 46+ days can be either.

Irregular returns can be caused by several factors. Early irregular returns are probably females that were bred that were not in standing heat. Irregular returns at 25-38-day intervals are females that were bred, conceived, but the pregnancy failed because of body condition, disease, small litter size, injury, a single mating, moldy feed, hormonal problems, etc. These maladies caused the female to absorb the embryos and the female, consequently, returned to estrus.

In order to analyze the data, the breeding crew needs to do a good job of heat checking and recording information, such as the date of the returns and the action taken.

Regular returns to estrus 39-45 days and after weaning were open on Days 18-24, but were missed by the breeding crew. If heat checking is being done well, the crew should find over 60% of regular returns on Days 18-24. Finding these open females as soon as possible reduces open sow days, lowers the farrow-to-farrow interval and increases litters/female/year.

During heat checking, it is important to control the path and speed of the heat-check boar. Going too fast with the heat-check boar causes the heat-check crew to miss some of the regular returns that are in heat on Days 16-25. Boar carts, the Boar Bot, harnesses, gates and panels are often used to control the boar. It is important that all sows being checked get contact with the heat-check boar, especially if they are in heat or just coming into heat.

Also, remember, sows like variety in the boars being used for heat checking. We suggest rotating boars daily and replacing the heat-check boar after 1-2 hours of heat checking. The best heat-check boars are the older, more mature, stinky and ugly ones. Good heat-check boars are usually at least 12 months old, aggressive, and produce lots of pheromones during the heat-check process.

The farms that are the most successful at heat checking have two-person teams with one person controlling the boar, the other applying back pressure to each sow and looking for those females showing physical signs of heat.

Table 1 is from the Swine Management Services’ titled: “In-Depth Breeding Analysis Report – Returns to Estrus.” This report helps identify when the second or third-service females were found in heat and, if rebred, their subsequent farrowing rate. The data is broken out by parity and the time the females were found in heat.

The first area to look at is “% of Services” (right column), which shows only 29.7% of regular returns are found during Days 18-24. Unless this farm is dealing with a health challenge or feed quality issue, to review the Standard Operating Procedures (SOP’s) for the entire heat-checking process, including the age of heat-check boars and the time spent looking for returns.

This dataset also shows 42% of the returns are found after Day 38, which again means several open females are being missed by both heat checking and pregnancy checking. Looking at “average days” (bottom row), we find a return-to-estrus average of 40.1 days; the best job was done finding sixth-parity females at 26.5 days.

The top farms that are finding 60+% of regular returns at Days 18-25 will average less than 30 days to find a return to estrus. Some farms have pulled that average down to 22-25 days. The data also shows a lot of variation in farrowing rate by parity and when rebred.

To reduce open sow days, increase litters/female/year and lower farrow-to-farrow interval requires that you look into more detail, checking to see when regular and irregular returns to estrus are being found and the effect it is having on breeding performance. Remember to make appropriate changes to the standard operating procedures and training manuals.

Key Performance Indicators
Tables 2 and 3 (below) provide 52-week and 13-week rolling averages for key performance indicators (KPI) of breeding herd performance. These tables reflect the most current quarterly data available and are presented with each column. The KPI’s can be used as general guidelines to measure the productivity of your herd compared to the top 10% and top 25% of farms, the average performance for all farms and the bottom 25% of farms in the SMS database.

If you have questions or comments about these columns, or if you have a specific performance measurement that you would like to see benchmarked in our database, please address them to: or

Mark Rix and Ron Ketchem
Swine Management Services LLC

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