Managing Sows for Optimum Retention

By Zach Rambo, Dr. Jerry Torrison, Mark E. Wilson

Sow removal and replacement is a necessary part of optimizing herd performance. It is a balance between keeping the herd young enough, while maintaining a significant number of sows between parities three and five in the system. Sow longevity is a complex phenomenon, influenced by many factors, including genetics, gilt and sow price, nutritional programs, gilt development, reproductive performance and health status. Recent analysis of herd records from more than 700,000 sows across 355 U.S. swine farms revealed an average replacement rate of 53.55 percent, calculated as number of animals culled plus number of sows that died (PigCHAMP, 2013). While this may have underestimated the true replacement rate, many herds see the largest percentage of breeding female losses in gilts and first parity sows.


Setting targets to reduce the loss of gilts and young sows from the herd may pay greater dividends than focusing on overall herd culling rates. Models suggest that pork production systems with the lowest commercial replacement rates were the most profitable. Greater economic value was captured on maintaining sows in the herd from zero through fifth parity and additional returns were minimal when a sow was kept beyond fifth parity.
Similarly, an improvement in longevity of an additional parity in the sow herd was calculated to be equivalent to improving lean meat content by 0.5 percent (Sehested, 1996). More recently, John Deen (2003) suggested that parity of sows at culling and removal rates may not be the best measure for characterizing sow retention problems. He recommended measuring the proportion of animals that make it through early parities as a more important management metric.

A high sow turnover rate means that more gilts must enter the herd to maintain inventories, increasing the number of progeny born to first parity sows. This may result in several consequences: lower gilt litter size compared to multiparous sows (Gatford et al., 2010); lower birth and weaning weights than older parity sows; and more importantly, a 12 and 17 percent disparity in pre-weaning growth rates (Smith and Collins, 2009; Gatford et al., 2010). These differences in weight at weaning have been shown to increase in magnitude by the end of finishing.

Gilt progeny are also more susceptible to disease than pigs born to sows, with increased rates of mortality and higher medication costs (Holyoake, 2006). The performance differences in the gilt progeny may be due to lower intakes of colostrum, poor transfer of immunoglobulins in colostrum and reduced immunoglobulin concentrations in milk (Cabrera et al., 2012). The overall impact of these indirect effects of high culling rates is that feed efficiency and growth performance is impacted negatively with increased proportions of gilt progeny in the grow-finish groups (Smits, 2011).

The other major advantage to lower the percent replacements below the current industry averages is to help stabilize herd health. If replacement rates are more consistent all year, there is not a push on the system to bring gilts into the breeding herd too early without enough time for acclimatization to the herd and causing a potential decline in herd health status.

“The overall impact of these indirect effects of high culling rates is that feed efficiency and growth performance is impacted negatively”


If we assume the average replacement rate in the U.S. is approximately 53 percent, moving this target to a range of 42-45 percent will have a small impact on sow herd feed efficiency. However, improved performance in growing and finishing will make a major change in whole herd feed efficiency. There is a mathematical limit to how low the replacement rate can be maintained in a sow herd. Replacement rates below 40 percent lead to excessive aging of sows in the herd, causing the need for an eventual higher influx of gilts and subsequent bimodal distribution of parities, with many young and old sows and not enough “prime” parity sows in the herd. Setting a targeted goal close to 60 percent of the gilts entering the reproductive pool farrow at fourth parity will optimize retention and profitability of the sow herd.


The two largest reasons for removal or culling of young sows are reproductive failure and lameness. The age and weight of a gilt are important determinants for achieving sexual and physical maturity. An important goal is to develop a female that will lose less body mass and condition through first parity, increasing the potential for greater reproductive performance in subsequent parities.

Different genetics have different feeding recommendations for gilts to optimize their litter size and reproductive performance. Some groups target an age range of 220-235 days with a body weight of 136 kg (300 lb) for first mating, while others target older age ranges and heavier body weights prior to first mating. An excellent paper on gilt development, by Dan Bussieres, reported production records on lifetime productivity showed that optimal age was 229-243 days for total born lifetime (PigCHAMP, 2013). Also, this paper suggested that total born lifetime was highest for gilts ranging from 130 kg to 160 kg body weight. Additional data generated in this area is necessary to find the optimum performance of these females so that approximately 60 percent of gilts remain in the herd for four parities.

Growth rate of gilts has been shown to be a factor in the incidence of osteochondrosis and restricting feed after 10 weeks of age may reduce the incidence of this joint disease (de Koning, 2013). One must be careful to understand the interaction between specific genetics and feeding regimens on growth and lactation performance. For example, restricted feeding has been shown to reduce milk producing cells in the mammary glands (Farmer, 2013)


Lactation is one of the most energetically expensive and challenging activities a female can undertake. Therefore, feed consumption is critical during lactation. Anil et al., (2006) reported that sows consuming ≤ 3.5 kg (7.7 lb) of feed per day during the first two weeks of lactation were more likely to be removed from the herd before their next parity. The odds of removal were highest for sows that did not consume any feed during any single day for the first 14 days of lactation. The bottom line from this study is “if the sows don’t eat in lactation, they are going to leave the farm.”

Younger first litter gilts were more sensitive to negative effects of reduced feed intake during lactation compared with older gilts and multiparous sows. Voluntary feed intake of hyperprolific sows may be insufficient, especially for young sows, to meet the high nutrient demands of milk production, maintenance and growth. Insufficient feed consumption in lactation is a greater risk to the first parity sow nursing a large litter because most of the nutrients are prioritized for milk production and lean body growth. Subsequently, nutrients for reproduction are reduced. Fat and protein mass are mobilized to help meet nutrient needs, which increases the risk of failing to return to estrus after weaning. Body protein mass loss greater than 9 to 12 percent rapidly reduced ovarian function (Clowes et al., 2003). A higher daily lactation feed intake reduced body weight loss, improved litter weight gain and reduced the probability of a prolonged wean to estrus interval by 42 percent for each kg increase in average daily feed intake (ADFI) (Eissen et al., 2003). Young sows with poor daily feed consumption in lactation are at increased risk for a prolonged wean to estrus interval. A practical outcome of this is to cull for presumed reproductive failure.

A low feed intake during lactation involves mobilization of body tissues and can lead to excessive loss of body weight, decreasing sow longevity (Gaughan et al., 1995) and reproductive performance (Quesnel 2005). Limited follicular development and incomplete recovery of the reproductive axis at weaning seem to be the most likely causes of decreased embryonic survival in second parity sows with earlier weaning age (Willis et al., 2003).

There are two major ways to impact nutrition utilization by a sow. First, inadequate nutrient intake causes the body to prioritize the partitioning of nutrients for survival over production needs. Second, stressors or inflammatory reactions can change the prioritization o fenergy and even the utilization of specific amino acids needed for the immune system. This inflammation again causes immune function to take precedence over production needs for nutrients. Alternatively, over-conditioning gilts and sows in late gestation has a large, negative impact on feed intake in early lactation. Once you have met the nutrient requirement for growth of the piglet in utero in late lactation, feeding extra feed does not improve piglet birth weight.


Lameness is one of the major determinants of premature removal of sows (Anil et al., 2006). Several factors that cause inflammation, such as lameness and health problems, along with lactation feed intake, were predictive of removal of the sow from the breeding herd (Anil et al., 2008). Inflammation affects the partitioning of nutrients toward the immune system and away from production. Thus, it is better to prevent inflammatory conditions, such as lameness and disease, because they impact nutrient utilization and decrease reproductive productivity.


  • Increase lactation length
  • Encourage lactation feed intake through:
    • Unlimited, high quality water
    • Reduced ambient
    • temperature for the sow
    • Fresh feed at least once a day
    • Ad libitum feed intake
    • Synthetic amino acids
  • Selection of skeletally correct gilts
  • Prevention of stressors (i.e., disease and lameness)
  • After weaning, skip the next heat for first parity females with low feed intake during lactation
  • Avoid overweight gilts and sows in late gestation


  • Larger litters
  • Heavier pigs at birth and weaning
  • Decreased non-productive days
  • Acquired immunity to herd diseases
  • Improved herd health stability
  • Higher sow salvage values
  • Improved reproductive performance<
  • Decreased mortality of progeny production
  • Improved whole herd feed efficiency
  • Decreased fixed production costs
  • Reduced involuntary culling vs. voluntary culling

Zach Rambo earned his doctoral degree in swine nutrition from Purdue University in 2013. There his research focused on the effect of zinc and skeletal muscle growth and metabolism. He also completed work evaluating the impacts of dietary fat source on carcass composition and quality. Before attending Purdue he earned his master’s degree in beef cattle nutrition and his bachelor’s degree in animal science from Texas A&M University. Zach joined Zinpro in June of 2013.

Dr. Torrison received his DVM degree from the University of Minnesota. He worked in a mixed practice for two years before returning to the University of Minnesota to pursue MS and PhD degrees, studying the epidemiology of pseudorabies virus in pigs. In 1993 he was awarded a one-year Fogarty International Fellowship to conduct research on PRRS virus epidemiology at the Station de Pathologie Porcine in Ploufragan, France. He was the Health Assurance Manager for the Pig Improvement Company for five years and a consulting swine veterinarian with the Swine Vet Center in St. Peter, Minnesota prior to returning to the University of Minnesota in 2004 as a diagnostician at the Veterinary Diagnostic Laboratory. He joined Zinpro Corporation in 2011 as a swine veterinarian.

Mark received his PhD at the University of Kentucky in reproductive physiology. He was Professor at the University of Minnesota, Waseca for 11 years where he won several National awards for undergraduate education and coordinated the swine research at the Southern Experiment Station. He spent 9 years as director of technical service at United Feeds while overseeing the boar and sow research. He served as vice president of research and technology transfer at Minitube of America. Mark is an adjunct professor for the University of Wisconsin and the University of Minnesota. He is one of the swine scientists of the Research and Nutritional Services Team for Zinpro Corporation.