Effect of Season and Parity on Reproduction Performance of Iberian Sows Bred with Duroc Semen

1. Introduction

The Iberian pig is a highly valued breed from the Iberian Peninsula, recognized for its organoleptic and nutritional characteristics [1,2]. Whereas traditional rearing is extensive or semi-extensive, demand has led to intensive management of the animals [1]. These are typical of Iberian × Duroc crossings, adding the meat characteristics of the Iberian breed to the advantages of Duroc genetics (precocity, leanness, and higher performance). The use of artificial insemination (AI) with liquid semen doses from Duroc boars enables increased efficiency [3] and allows for the planning of crosses according to the desired product by modulating the proportion of Iberian genetics [1]. Despite the increasing importance of this production system for the breed, there are little data on the reproductive efficiency of Iberian sows or their crosses after AI with semen from Duroc boars. Current data are limited to reports from producers, which lack systematic research on this topic, and we are not aware of publications on the reproductive performance of these crosses.The reproductive performance of pigs depends on many factors. Due to its importance for swine production, it has been extensively analyzed for many breeds and situations [4,5,6,7,8]. Parity is a relevant factor, with the number of farrowings influencing sows’ fertility [9,10]. There are important differences between gilts (nulliparous) and multiparous sows [4]. Gilts present physiological features that result in overall lower fertility and prolificacy [11,12,13]. Moreover, low-parity sows could be more sensitive to environmental stress [14,15].The effect of parity is complicated by the influence of management and breeding systems [8], and whereas gilts can show a good prolificacy, their performance can vary greatly depending on the breed and management [16]. Similarly, older sows, with many farrowings, are more prone to reduced reproductive performance. Sows in parities between two and five are considered the most productive, showing an optimal prolificacy [5,17].Another critical factor for pig reproduction is the environment, mainly influenced by the season and local climatology. Whereas modern farms can efficiently control the temperature (especially in dry climates [18]) and photoperiod, these factors still significantly impact pig production. However, the seasonal effect is more important when using breeds with a lower level of genetic selection or in latitudes with a higher thermal or humidity variation [11,19,20]. Indeed, we detected a seasonal effect in studies using commercial breeds (Landrace, Large White, and their crosses) [21,22,23]. However, when studying the sperm cryopreservation of the autochthonous breed, Gochu Asturcelta (Northern Spain), the seasonal effect was small and, interestingly, it reflected on a lower freezability in winter [19]. In this regard, the Iberian pig is peculiar in many aspects [1,2], and this could affect parity and season effects on its AI performance differently compared with other breeds. Iberian boar semen could be more sensitive to specific conditions during extension and storage [24], and a role for environmental factors can be speculated. Therefore, a characterization of Iberian × Duroc production farms is relevant for improving reproductive efficiency.

In this study, we analyzed records from an Iberian sow farm using Duroc semen for AI, with the objective of evaluating the impact of both sow parity and season on their fertility and prolificacy. Due to the interest and peculiarities of the Iberian breed, the study aimed to find the similarities and differences with more common commercial breeds in order to optimize the management of the Iberian × Duroc breeding system.

4. Discussion

The effect of season on sow farms’ productivity and reproductive performance is still a challenge for modern industry. Seasonal infertility remains a problem for pig breeding, and studying it is difficult due to many confounding factors. Whereas most problems are due to heat stress affecting the sows during ovulation or pregnancy [33,34], other factors are involved. Seasonal effects are more evident with extensively or semi-extensively reared animals, tropical climates, farms with limited control of the environment, or local breeds with low genetic selection [35]. Other environmental effects such as the photoperiod or an effect on semen quality could be highly relevant, at least in some cases [19,36,37].A seasonal effect was not detected in previous studies in which AI was performed using the semen supplement Suinfort® in commercial breeds (Landrace and Large White boars, and Landrace × Large White sows) [23]. However, another study on Gochu Asturcelta (a local breed extensively reared) [19] and on Iberian × Duroc [38] reported a significant influence of the period of the year on semen freezability and fertility performance, respectively. The variability among studies highlights the need for considering individual situations. Breeds, environment (latitude and climate), and farming procedures could cause variability between large-scale studies performed with commercial breeds and modern facilities. The present study focused on the Iberian pig [1], a breed raised for its organoleptic and nutritional qualities. Iberian sows have not been subjected to genetic selection, are well adapted to the hot and dry Iberian climate (Southern/inland), and might show some physiological differences from typical commercial breeds.Whereas the Iberian × Duroc crossing is increasingly popular due to its possibilities for increasing productivity and the quality of the offspring, the bibliography is still scarce [2,38,39]. Here, we demonstrate a critical role of season and parity in the productive performance of Iberian sows. These results are not surprising since parity is known to impact sow fertility [11,12,13]. First, gilts show reduced reproductive performance, as clearly evidenced in our study for fertility and prolificacy. Moreover, consecutive pregnancies affect the sow’s physiology, leading to decreasing reproductive performance, although this depends on individual, breed, and rearing conditions [13,40,41]. Whereas the physiological mechanisms behind the effects of the age of the sow and parity on fertility and prolificacy are still little known [11], it is undeniable that both lower-parity and old sows present suboptimal performance [17,42], and the results on the Iberian × Duroc crossing in our study support those previous findings, with the most consistent and highest fertility and prolificacy results in the parity 2–4 group.Nevertheless, from the fifth farrowing (parity 5 and at up to 10), sows showed acceptable reproductive performances at the expense of higher variability. The case for primiparous sows was mixed, with fertility comparable with the reference group but having lower prolificacy. These sows could be at a higher risk of miscarriage [28], resulting in fewer farrowed piglets.In a first approach for analyzing the seasonal variability of the reproductive performance of the Iberian × Duroc breeding, we analyzed the reproductive data within each natural season. Seasonal effects are apparent in tropical or subtropical climates, especially when high humidity combines with heat [11,20,28,43]. However, the climate also affects farm performance in temperate climates, with a decrease mainly observed in the summer and early autumn, and both with liquid-stored [13,22,27,28,44,45,46] and cryopreserved semen [47]. In a recent study on Spanish herds [48], the miscarriage occurrence was higher in the summer and early autumn. These effects are related to insufficient thermoregulation in this species [27], impacting ovarian function [33,49]. However, other effects such as the photoperiod could also be relevant, although potentially minor in farms having photoperiod-controlled facilities [7]. Our results agree with our preliminary report on Duroc × Iberian [38] and differ from previous studies in swine (with a decrease in fertility in summer and the early autumn) by presenting a clear distinction between the spring–summer and autumn–winter periods.Interestingly, neither farrowing rate nor total/live piglets at farrowing showed evidence of interactions between parity and season and, therefore, our results could not be attributed to a skewed distribution due to parity groups. A hypothesis is that early miscarriages could have a role in skewing our data towards lower results (both fertility and piglets born) earlier in the year. It is important to consider that the sows were Iberian, a breed still little researched regarding the reproductive parameters and physiological peculiarities [50], and possibly with lower reproductive performance.Moreover, the use of Duroc semen could influence our results, as these boars show lower fertility than other commercial breeds [51]. Interestingly, we obtained an inverse distribution when studying the post-thawing semen quality of another autochthonous Spanish breed, the Gochu Asturcelta [19], with better post-thawing quality when the semen had been collected in spring and decreasing towards the winter. While the experimental conditions are not comparable (liquid vs. cryopreserved storage), these observations support the need for further research on the reproductive characteristics of local breeds for better management. However, the possible environmental influence on sperm fertility must be cautiously considered since we could not study sperm quality in detail. This aspect remains to be further investigated in future studies.Since the interpretation of our results is complex due to the interaction of many factors in seasonal infertility (breed, local climate, and farm management), we further explored the data to assess the importance of the environmental variables on the reproductive performance of Iberian sows. Cosinor models, to our knowledge, have not previously been used to analyze reproductive performance in farm animals. The periodograms produced from the cosinor models reflected the trend found by the seasonal analysis, with the reproductive parameters for farrowing rate and farrowed piglets dropping by spring and reaching a minimum by summer but recovering between summer and autumn. Moreover, the cosinor analysis allowed for the detection of some striking differences between our reference group (parities 2–4) and the gilts. The fertility and prolificacy of different parity groups were the lowest by summer, but this effect was shown earlier for gilts, especially for pregnancy. Moreover, the variation in stillborn piglets was higher than in other groups and presented the minimum and maximum incidences earlier in the year (end of winter and summer, respectively). Gilts are more sensitive to heat stress or photoperiod variations regarding their reproductive readiness [52], partly explaining our results.A limitation of this study is that some data, such as sow age or pre-farrowing removal, was not available in the farm’s dataset. The age at AI could be a relevant factor for reproductive success, especially for gilts, and could affect many levels. For instance, age at first AI has been related to gilt fertility and the lifetime performance of sows [8]. Moreover, gilts could be more sensitive to heat stress, including peripartum death [53]. In this study, the maximum temperature and the photoperiod change were the environmental parameters of choice, following previous reports [54], and breed could modulate their impact on reproductive performance [13,41]. Both parameters were indeed significant in our analyses.Although animals housed in modern farms are partially insulated from the natural environment, hot weather and changing photoperiod still affect them. Here, we confirmed a role for both the ambient temperature and the photoperiod change, helping to explain our findings when comparing seasons and the yearly rhythms. Pigs are sensitive to heat stress [53], affecting spermatogenesis and the capacity of the sows to prepare for and maintain a pregnancy [27,33,55]. Interestingly, whereas heat influenced fertility and the number of piglets farrowed in the three periods studied, the sows seemed not to be affected by the photoperiod change during the period following AI. MDT could impact the optimal 2–4 parity group more, considering the interactions from our models. Therefore, although Iberian pigs could be more adapted to the warm–hot climate of the farm’s environment, farms might improve sow performance if animals remain in their thermoneutral zone [7]. This should be a priority in the face of more frequent heat waves due to climate change [56]. However, the increasing photoperiod could play a critical role during the spring and even summer [54,57,58].


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