BIOLOGICAL LIMITS OF FECUNDITY IN SOWS – DO THEY EXIST?

Klaus-Peter Brüssow¹, Martin Wähner², Jędrzej M. Jaśkowski^3
1 Centre of Veterinary Sciences, Nicolaus Copernicus University, Toruń, Poland
² Anhalt University of Applied Sciences in Bernburg
³ University of Life Sciences in Poznań

ABSTRACT

The profitability of pig production considerably depends on the number of born alive and fostered piglets. Therefore, it could be of interest to estimate the reproductive potential of sows. The progress regarding the number of piglets born for German Landrace, Large White and Pietrain in Germany during 1970–2006 is still moderate (GL: 10,4–11,0; LW: 11,0–11,0; PI: 10,2–10,0). Comparing actual international data, variations between 11.3 to 14.2 piglets born alive are observed. The reproductive performance of sows is mainly determined by (i) the number of ovulated follicles and fertilized oocytes, (ii) the percentage of surviving embryos and fetuses, and (iii) the morphological and functional performance of the uterus to support fetal development until birth. Therefore, the question whether the ovary and/or uterus are limiting factors will be discussed. Although only less than 0.1% of oocytes present in the ovary are ovulated during the sows' lifetime, the pool of ovarian follicles is not the limiting one. Selection for ovulation rate increases the number of ovulating follicles, but not of piglets born alive. Limited uterine capacity and function seems to exert greater influence. Although a relationship exists between uterine dimension and the number of fetuses/piglets, uterine length alone is not a prerequisite of higher uterine capacity. Placental efficiency and the degree of placental blood supply appear to be essentially for litter size. At present, the (realistic) presumed potential of fecundity is 16.0 piglets born alive, 2.4 litters/year, <10% losses and 34.0 piglets per sow/year (compared to current data of 12.2, 2.33, 14.8 and 24.2, respectively).

Key words: Pig, fecundity, ovulation, uterus.

INTRODUCTION

The objective of modern pig breeding is to take advantage of the genetic potential in reproduction performance of sows regarding to litter size and number of weaned piglets per litter, and finally to realize an economic output of pigs for breeding and slaughter. As the profitability of piglet production depends on litter size [22], it is therefore necessary to get a high number of vital born piglets and uniform bodyweights of newborn and weaned piglets. Long living high performance sows are absolutely necessary, too. In comparison to other Suidae (e.g. Pekari, Babirusa, European Wild Pig), modern pig breeds have a much higher reproductive performance. The litter size of these wild pigs is limited between 1 to 5 offspring. This situation demonstrates the high potential of reproduction performance of modern breeds. On the other hand, during the last 100 years in the USA, the litter size of sows increased from 7 to 8 piglets [15] to 8 to 9 in the all registered pig farms, only. In dam lines the litter size was increased to 10 to 11 piglets [34]. This progress is not very advanced. Also the development in litter size of Landrace, Large White and Pietrain sows in German from 1970 to 2009 is moderate (Table 1).

The comparison of international data reveals considerable differences in reproduction performance between countries, where the number of piglets born alive (NBA) varies between 14.2 (Denmark), 12.1 (Germany) and 11.3 (Italy). When comparing the results between Germany and Denmark with regard to the lower 25% of farms (11.4 vs. 13.6 NBA) and the upper 25 % (12.8 vs. 14.5 NBA) the differences to results of Danish farms are between 1.7 to 2.2 piglets. Taking such differences into account, it is of interest to know what the biological potential of fecundity of sows is.

ARE OVARIES AND UTERUS LIMITED FACTORS?

The reproduction performance of sows is predicted by three main concerns: (i) the number of ovulated follicles and the number of fertilized oocytes, (ii) the number of surviving embryos and fetuses, and (iii) the functional efficiency of the uterus to ensure fetal development until birth.

Hence, the following question requires an answer: Are the ovary and /or the uterus limited factors? Generally, porcine ovaries have a large pool of about 500,000 follicles and oocytes, [14]. However, only less than 0.1 % of the existing "reserve of oocytes" matures into fertilizable oocytes during sow's lifetime. Further, only 60–70% of ovulated oocytes develop into a piglet born alive [24]. Nevertheless, the ovary appears not to be a limited factor. The number of ovulation can be increased in some degree by exogenous hormone application and by breeding selection.

Follicle growth and a superovulation response can be induced by exogenous gonadotrophins, e.g. with equine chorion gonadotropin (eCG) and increase the number of follicles for ovulation to 30–60%. However, the number of intact embryos and following the potential litter size will increase only to a smaller degree (0 to 40%) [1, 4]. Though, the individual variations are rather high and not predictable. Therefore it is not really practicable to use hormonal induced superovulation for higher piglet production.

Breeding selection for a higher number of ovulation is possible inasmuch the heritability is about 0.10 to 0.15 [8, 21, 16, 30]. Breeding experiment selecting for the number of corpora lutea (CL) during 11 generations revealed an increase from 14.0 to 20.5 CL (+ 6.5). The number of fetuses at day 50 of gestation was increased from 10.8 to 13.6 (+ 2.8), but the NBA was increased only from 9.9 to 10.7 (+ 0.8) [18]. Even though a breeding progress of 10 to 28 % concerning the number of ovulations could be achieved, the litter size increased moderately to only 8–10 % [8, 17, 19, 21, 31].
Ergo, selection for the number of ovulating follicles has shown that the ovary is not a limiting factor, but such breeding strategy is also not a key to increase the NBA.

A relation between an increased number of ovulations and the number of embryos/fetuses can be observed only during the first days of pregnancy [11]. This was confirmed by our experiments (Figure 1) [6]. Together, these results suggest that the uterine capacity is a limiting factor. Uterine capacity is the ability of the uterus to support only a limited number of embryos/fetuses and to maintain the development to term [10, 7]. Hereby, several physical, biochemical and morphological characteristics of the uterus (e.g. space, nutrients, gas exchange and surface of the placenta) may influence uterine capacity.  

Figure 1. Relationship between the number of ovulations and the number of intact embryos in Landrace sows on day of pregnancy (DP) 30 and 80

Uterus capacity depends on genotype of sow, as it was shown in experiments where 13 to 15 embryos were additionally transferred into inseminated sows [27]. A higher number of embryos (24.2) was found in the uterus of crossbred sows (Duroc x Yorkshire) compared to purebred animals (19.6 and 19.5). Davis et al. [9] reported on longer uterus in Duroc compared to Yorkshire sows (411 versus 375 cm) although the number of embryos was lower (9.9 versus 10.5). Differences were confirmed in uterine length and weight during early pregnancy between Hungarian Mangalica and German Landrace sows [5]. In Mangalica sows, which have a substantial lower fecundity, the length of uterus was shorter (124±5 versus 188±8 cm; P<0.01). Furthermore, there was no measurable uterine growth during the first 24 days of gestation and uterine weight commenced later compared to Landrace sows.

Although a relation between uterine length (space) and the number of fetuses/piglets exists, the length of the uterus is not the only one prerequisite of higher uterine capacity [33]. It has been shown that sows with low uterine length and weight revealed higher uterine capacity [12]. Nevertheless a minimum of space for embryos is necessary and it was suggested that each fetus requires until day 50 about 36 cm [36]. In our study [6] the mean length of uterus/fetus was 25.8±7.0 and 44.5±13.7 cm on days 30 and 80 of gestation, respectively. Proper developed placentas are essential for litter size and respective birth weight. An underdeveloped placenta in early pregnancy (day 20 to 30) influences growth and survival of fetuses. Furthermore, uterine capacity predicts prenatal losses until birth [20, 18]. Breeding selection for uterine capacity and higher placental weight increased the number of fetuses per uterine horn of about 0.8; whereas selection for the number of ovulations decreased the uterine capacity of 1.1 fetuses/horn [11].

What is characteristic for sows with a high number of piglets? For example, sows of the Meishan breed have 3 to 5 more NBA per litter compared to European breeds. Their piglets are smaller and lighter and they require less uterine space. Meishan sows have smaller placentas and a higher placental efficiency [3, 35]. Hereby, placental efficiency (PE) is defined as the quotient of fetal and placental weights (g). The PE can be considered as the "degree of efficacy" of placenta, i.e. it shows how much gram of a fetus is supported by one gram placenta. Of importance in Meishan is also the placental blood supply. Here, the number of blood vessels is greater and their diameter is larger; the density of blood vessels in Meishan sows is more than two times higher compared to Yorkshire sows and is continuously increasing during pregnancy [3]. A high density of blood vessels supports the transfer of nutrients to the fetus [28], i.e. blood supply of fetuses (blood vessel density) determines fetal viability under conditions of high fecundity. This is, particularly, advantageous for small fetuses to support their survival and as a result to increase the reproduction performance of sows. It should be considered that breeding selection for relatively smaller but more efficient placentas effects increased litter sizes, but also smaller bodyweight of newborn piglets [35].

Considering the aforementioned, hypothetically two breeding goals are possible: (i) increasing the placental efficiency and "maximizing" the number of piglets per litter; hereby, birth weights are reduced and the rearing losses can be increased; or (ii) select for a fewer number of piglets but of more uniform birth weights and of high vitality.

However, it is rational to consider both breeding strategies with regard to their advantages and/or disadvantages. This is because biological (e.g. different muscle growth and meat quality, health status, behavior) [25, 13, 29] and technological parameters (housing system, climate, feed input) [2] have a considerable influence on profit under specific production conditions. Based on average market conditions, prices and costs in Canada it was shown, that the economic weights for litter size will be reduced when the average litter size becomes large [26]. More than 28 piglets per sow and year decrease at present the economic output in German pig production [23].

In future, research is challenged to support the best choice. Thereby, the metabolic performance of the uterus in different trimesters of pregnancy and the supply of blood and nutrients to the fetuses in relation to fetal growth could be of interest. Additionally, it is necessary to find genetic markers for uterine metabolic performance, for placental efficiency and for uterine/fetal supply, which are usable for breeding selection.

ESTIMATION OF THE POTENTIAL OF FECUNDITY IN SOWS

Which potential of fecundity we can predict for modern swine breeds at present? To answer this question we should consider that breeding progress is a dynamic process and limits are shifting. Though, based on realistic physiological parameters a prediction of reproductive performance was done (Table 2). Our estimates base on relevant and realizable parameters as 25 ovulations, an oocyte fertilization rate of at least 80%, embryonic/fetal losses of 20%, and the number of stillborns should be lower than 1.0 piglet. Appropriate reproductive management enables on average 2.4 litters per year and 34 weaned piglets per sow and year.

The main biological basis to achieve such results is to increase the number of piglets born alive reducing embryonic/fetal mortality and to decrease rearing losses increasing piglets' vitality. Both biological processes presuppose optimal uterine environment and feto-maternal interactions.

Regarding this, it is interesting to consider breeding selection experiments for fecundity, litter size and litter weight in mice with more than 130 generations [32]. Animals of the experimental population realized nearly 100% higher litter size (18 vs. 11) together with increased litter weight (29.9 vs. 19.9 g) in comparison to controls. It would be of interest and hopefully to confirm similar tendencies in pigs.

* The manuscript base on a lecture presented at the University of Life Sciences Poznan, Poland, April 2011

REFERENCES

Accepted for print: 30.09.2011

Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.