Thanasis Sougias, th.sougias@nuevo-group.com
Seasonal infertility continues to represent one of the key areas requiring further improvement in modern sow production systems. Despite more than four decades of continuous research into this phenomenon, we are still unable to define with absolute certainty the exact mechanisms responsible for its occurrence.
Seasonal subfertility remains a complex and multifactorial challenge, influenced by the interaction of environmental conditions, physiology, genetics, nutrition, and management practices. Interestingly, its severity may vary considerably from one production year to another, sometimes appearing intensely and other times remaining barely noticeable -even within the very same farm under seemingly similar production conditions.
Table 1. Pregnancy losses in the same farm across consecutive years
Seasonal infertility is defined as a reduction in reproductive performance in breeding animals, affecting both gilts and multiparous sows. It is predominantly observed during specific periods of the year, most commonly throughout the summer months and early autumn.
The condition is typically characterized by a decline in key reproductive performance indicators, particularly farrowing rate, which represents one of the principal parameters used to monitor reproductive efficiency at herd level.
Under conditions of seasonal subfertility, farrowing rate may decrease by approximately 3% to 10% compared with the herd’s annual average performance (Auvigne et al., 2010). Nevertheless, in recent years, cases associated with substantially higher levels of reproductive failure have also been reported under commercial production conditions.
Reduced reproductive performance at herd level is typically expressed through three major patterns of reproductive dysfunction, the first of which is an increased proportion of animals exhibiting impaired or delayed estrus expression.
This commonly includes:
- Delayed onset of puberty and estrus in gilts
- Prolonged weaning-to-estrus interval in multiparous sows
- Increased incidence of anestrus in sows
These reproductive disturbances are among the most consistent clinical and productive indicators associated with seasonal subfertility in modern swine production systems.
Table 2. Typical example of seasonal subfertility associated with pregnancy losses
The second pattern is characterized by an increased incidence of pregnancy failure. In practical terms, this is most commonly reflected by a higher proportion of irregular returns to estrus, typically occurring between 25 and 35 days after artificial insemination.
In addition, several herds may also exhibit a reduction in total piglets born per litter, affecting both gilts and multiparous sows, which may indicate a compromised reproductive efficiency and an underperformance of modern hyper-prolific sow genotypes under seasonal stress conditions.
The third and final pattern focuses on the negative effects of seasonal conditions on boar reproductive performance, particularly semen quality and male libido. This represents an equally critical component of reproductive efficiency, as impaired sperm quality, reduced semen fertility, and decreased sexual behavior in boars can significantly contribute to lower conception and farrowing rates at herd level.
Consequently, the reproductive performance of both females and males should be considered together when evaluating and managing seasonal subfertility in modern swine production systems.
A substantial body of scientific research has demonstrated the pivotal role of melatonin in the pathogenesis of seasonal infertility in swine. Melatonin secretion is closely associated with changes in day length (photoperiod) across different seasons, and these physiological fluctuations are known to influence ovarian activity, endocrine function, and early embryonic development. As a result, alterations in photoperiod can directly affect reproductive performance in both gilts and sows.
Ôhe occurrence of seasonal subfertility is inevitably linked to the extended daylight periods observed from summer through early autumn. However, photoperiod alone does not fully explain the phenomenon. Elevated environmental temperatures during this period also play a major role, as heat stress can negatively affect feed intake, hormonal balance, oocyte quality, embryo survival, and overall reproductive efficiency.
In addition, management-related factors appear to substantially influence both the severity and the expression of the problem between farms. Differences in housing conditions, cooling systems, nutritional strategies, boar management, insemination practices, and overall herd management protocols may either exacerbate or mitigate the impact of seasonal infertility under commercial production conditions.
Modern production facilities and upgraded technologies -including advanced cooling systems, controlled lighting programs, and environmental air management systems- are increasingly common in contemporary swine operations. Nevertheless, in many cases, these improvements alone are still insufficient to fully control or eliminate the occurrence of seasonal subfertility.
Having said this, a critical question arises for the swine industry: Is seasonal infertility alone responsible for the reproductive failures commonly observed during the summer period?
A more detailed and in-depth evaluation of reproductive performance suggests that the answer is likely more complex. Studies involving endocrine profiling and hormonal monitoring of sows -allowing the identification of whether pregnancy was successfully established and the stage at which reproductive failure occurred- have demonstrated that a large proportion of non-pregnant (“empty”) sows are actually associated either with conception failure, expressed as regular 21-day returns to estrus, or with early pregnancy loss, typically manifested as irregular returns occurring between 25 and 35 days after insemination.
The first category, namely regular 21-day returns, is in many cases associated with inadequate or ineffective estrus detection and breeding management.
In contrast, the second category -irregular returns between 25 and 35 days- is more commonly linked to insufficient pregnancy monitoring or the absence of effective early pregnancy diagnosis protocols.
These findings highlight that, beyond the biological effects of seasonal subfertility itself, management efficiency and reproductive monitoring practices play a fundamental role in determining the overall reproductive outcome during the high-risk summer period.
Table 3. The Reality of Summer Pregnancies in Modern Sow Herds
Risk Factors Associated with Seasonal Subfertility
Several sow-related factors have been consistently associated with an increased risk of seasonal subfertility, including:
- Sows in their 6th parity or older
- Sows exhibiting a weaning-to-estrus interval longer than 5 days
- Sows weaned at an excessively early stage of lactation
- Sows weaning fewer than 8 piglets
Interestingly, these same risk factors are also commonly associated with reduced reproductive performance throughout the entire year, and not exclusively during the seasonal infertility period.
Consequently, sows with a documented history of impaired fertility or those exposed to inadequate management practices are considerably more susceptible to developing seasonal subfertility and should therefore be regarded as a high-risk reproductive group within the herd.
Management strategies for seasonal subfertility in modern sow herds
The management of seasonal subfertility and its negative consequences continue to be an area of extensive investigation, with new scientific evidence and practical tools constantly being incorporated into modern reproductive management programs.
According to recent research findings, three key physiological aspects should be taken into consideration when evaluating seasonal infertility in swine production systems:
- Sows may ovulate earlier during estrus throughout the summer months and into early autumn
- Hormonal support of pregnancy appears to decline during the 3rd to 4th week following insemination
- Ovulations occurring during the warm season are often of lower biological quality compared with those occurring during the rest of the year
These alterations may result in reduced fertilization efficiency, impaired embryonic development during the early stages of gestation, and suboptimal corpus luteum formation. Consequently, progesterone secretion may become insufficient to adequately support pregnancy establishment and maintenance, ultimately increasing the risk of reproductive failure.
Practical Herd-Level Interventions to Reduce the Impact of Seasonal Subfertility
An effective management strategy should initially focus on the previously described risk factors, with proper sow evaluation and selection aimed at reducing the proportion of animals classified within the high-risk reproductive group. The most important management measures should include the following:
1. Estrus Detection and Breeding Management
- Estrus detection should be performed twice daily
- Estrus detection and inseminations should be carried out during the coolest hours of the day
- Artificial insemination should be initiated at the first clear signs of estrus
Key Practical Point: Accurate estrus detection becomes particularly critical during periods of seasonal infertility, as altered ovulation timing may significantly reduce insemination efficiency if breeding protocols remain unchanged.
2. Artificial Insemination Practices
- Increased availability of females for insemination during high-risk periods
- Adequate preparation and acclimatization of replacement gilts
- Strict monitoring of semen quality
- Appropriate semen storage and handling procedures
- Implementation of effective cooling systems in boar housing facilities
Key Practical Point: Given the sensitivity of boar fertility to heat stress, maintaining semen quality during summer months is essential for preserving overall reproductive performance.
3. Early Identification of Non-Pregnant Females
- Frequent heat detection using boar exposure between 18 and 23 days after insemination in both gilts and sows
- Pregnancy diagnosis between 18 and 25 days post-insemination using appropriate ultrasonographic equipment
- Re-examination of pregnancy status between 40 and 50 days after breeding
Key Practical Point: Early identification of reproductive failure allows for faster intervention and minimizes the number of non-productive days within the herd.
4. Maximization of Feed Intake
- Increasing the number of daily meals during hot periods
- Optimization of dietary energy and protein levels, with particular emphasis on young sows and gilts during summer months
Key Practical Point: Maintaining adequate nutrient intake during periods of heat stress is fundamental for supporting ovarian function, embryo survival, and overall reproductive efficiency.
5. Sow Housing and Environmental Management
- Maintenance of a stable photoperiod of 16–18 hours of light per day for breeding animals
- Grouping animals according to body size and weight while ensuring adequate floor space per sow
- Precise feeding management during the first 4–5 weeks following insemination
- Ensuring adequate availability and quality of drinking water
Key Practical Point: Special attention should be given to water supply during summer, as water requirements increase dramatically under high environmental temperatures.
6. Staff Organization and Ventilation Control
Proper staff organization, routine monitoring of ventilation systems, and continuous evaluation of environmental conditions remain essential components of any successful seasonal infertility control program. Even small deficiencies in environmental management during high-risk periods may significantly compromise reproductive performance.
Sources:
Prof. Hughes P (2010), Seasonal Infertility in Pigs, Australia Pork CRC
Van Wettere, W. (2010). Determining the effects of season on timing of ovulation and luteal function.
Belstra, B.A., Flowers, W.L., See, M.T., 2004. Factors affecting temporal relationships between estrus and ovulation in commercial sow farms.
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