Livestock Research for Rural Development 25 (10) 2013 Guide for preparation of papers LRRD Newsletter

Citation of this paper

Risk factors associated with the second-litter syndrome under the tropical conditions of Mexico

J C Segura-Correa, R H Santos-Ricalde and J C Rodríguez-Buenfil

Facultad de Medicina Veterinaria y Zootecnia, Universidad Autónoma de Yucatán,
Km 15.5 carretera Mérida-Xmatkuil, AP 4-116, Mérida, Yucatán, México.


The objective of this study was to determine the effect of some risk factors on the second-litter syndrome (second parity sows with similar or lower litter size than first parity sows) in a commercial farm under the sub-humid tropical conditions of Mexico. Information of 7,193 sows was used and analyzed using a binomial logistic regression model. The model included the fixed effect of year of farrowing (2007-2012), season of farrowing (dry, rainy and windy), litter size (born alive) at first parity, lactation length (LL) and farrowing to service interval (WSI). Litter size, LL and WSI were included as continuous covariates.

Overall, 60.3% of the sows presented a reduced or similar litter-size at second parity. All factors studied had significant effect on the response variable. The risk of decreased litter size was higher in the dry and rainy seasons as compared to the windy season (ORs=1.31 and 1.24, respectively). Large litters at first parity increased the odds of the second-litter syndrome (OR=1.71), whereas longer LL (OR=0.965) and longer WSI (OR = 0.968), decreased it. Better care and management should be given to prolific primiparous sows in order to minimize the effect of the second-litter syndrome. While longer LL and WSI improve litter size of second parity sows, it is known to increase the number of non-productive days of sows.

Key words: second-litter size reduction, farrowings to service interval, lactation length, litter size, season


Pig production is an important activity in Yucatan, Mexico. However, it is highly dependent of grain and soya from abroad, to elaborate animal feed. In order to be more competitive, pig producers must be more efficient, improving reproductive performance and reducing mortality in the herds. Genetic improvement has increased prolificacy of sows in general and of first parity sows in particular. However, high prolificacy must be paired with better management, nutrition and care. First parity sows that produce large litters may have a poor body condition after lactation which in turn may affect the next litter size. Therefore, improving second parity reproductive performance might increase sow productivity and decrease replacement costs. In Mexico approximately 28% of the first and second parity sows are culled (Segura-Correa 20101

Morrow et al (1992) showed that 40% of the farms, in the United States of America, showed a similar or reduce size of the second litter of a sow as compared to the first one (second litter syndrome). This syndrome is often related to an excessive weight loss during first lactation (Thaker and Bilkei 2005; Schenkel et al 2010). Furthermore, short weaning to estrus interval might not be sufficient for sows to recover from lactation weight losses. Therefore, both the higher weight loss and short weaning to estrous interval can negatively influence follicle and oocyte development and/or embryonic survival and make sows more prompt for reduced litter sizes in parity 2. Factors such as herd, season, and litter size at first parity have been reported as risk factors for the second-litter syndrome (Boulot et al 2013).

A reduction of litter size from first to second parity has been reported in Japan (Saito et al 2010), United States (Morrow et al 1992) and Brazil (Kummer et al 2006) among other countries. In Mexico, results of Gomez-Medina et al (1999) show a reduction in litter size of sows from the first to the second litter. The knowledge of the frequency of sows with reduced litter size from first to second parity, within a farm, and factors associated with it is important to understand and take decisions, in order to make farms more profitable.

The objectives of this study were to estimate the frequency of sows showing the second-litter syndrome and to determine the effect of some risk factors in a commercial farm in the southeastern of Mexico.

Material and Methods

Data from a commercial farm locate in Yucatan, Mexico were used. Yucatan is localized at the southeastern of Mexico, characterized by a sub-humid tropical environment, with an average temperature of 26.6°C, 1,100 mm rainfall and 78% relative humidity (INEGI 2004). The farm had capacity for 3000 sows, which produced their own replacements. Breeding was carried out by artificial insemination and natural mating. Sows were fed commercial diets according to the stage of production. Young sows were given approximately 2.6 kg/day of a feed with 3,000 kcal EM/kg, 16% crude protein and 0.8% lysine.

Data from 2007 to 2012 from 7,193 sows kept in the Pigchamp® program were used. The information used was sow identification, date of farrowing, number of pigs born alive (litter size) at first and second parity, date of weaning, lactation length (LL) and weaning to service interval (WSI).

The second-litter syndrome was defined according to Morrow et al (1992), as those sows with the same or smaller litter size at the second vs first parity. Sows were categorized into two groups: 1 if the sow presented the second litter syndrome and 0 if not.

Data of the second litter syndrome were analyzed using a binomial logistic regression model. The risk factors evaluated were: year of farrowing (2007-2012), season of farrowing (Dry, rainy and windy), litter size (Pigs born alive), LL and WSI. Litter size, LL and WSI were included as continuous covariates. The statistical analyses were carried out with the SAS program (SAS 2008). The probability of a decrease in the size of the second litter for different LL, WSI, and litter size at first parity were calculated using the formula:

Probability (decrease=1)=1/(1+exp-(a+ΣbiXi))

Where: a is the intercept; bi the value of the regression coefficients for the ith risk factor and Xi is the matrix of incidences.

The probability of a decrease in the size of the second litter for effect of litter size at first parity (range 1 to 20 pigs) was calculated fixing the year 2012 and the windy season and using the means of LL and WSI. The probability of a decrease in the size of the second litter for the effect of LL (range 1 to 28 days) was similar to the previous model except that the means of LS and WSI were used. For predicting the effect of WSI (range 1 to 28 days) on the second litter syndrome, the means of LS and LL were used.

Graphs of the probability of a sow showing the second litter syndrome for litter size, LL and WSI were built using Excel.


The litter size means for parity 1 and parity 2 were 11.2 and 10.7 pigs respectively. The overall frequency of sows with the second-litter syndrome was 60.3%. The means for litter size at first parity, LL and WSI were 11.2 pigs, 21.6 days, and 8.69 days, respectively. The frequency distribution of sows showing an increase or decrease in the size of the second litter is shown in Figure 1. Approximately, 51% of the sows had a decrease or increase between -2 to 2 pigs, in their second litter.

Figure 1. Individual and cumulative frequency of sows with a decrease or increase in the size of the second litter

Year of farrowing was a significant risk factor for the second litter syndrome (Table 1). The odds of a drop in the size of the second litter were 1.25 and 1.20 times greater for a sow farrowing in the dry and rainy seasons, than for a sow farrowing in the winter season (Table 1).

Table 1. Factors associated with the second-litter syndrome in a commercial pig farms in Yucatan, Mexico, using a binomial logistic model.
Factor N Estimate Standard error Odds ratio 95%
Confidence limits
Year of farrowing
2007 1023 0.41 0.10 1.50 1.22, 1.84
2009 1202 0.52 0.10 1.69 1.39, 2.04
2010 1881 0.05 0.09 1.05 0.88, 1.25
2011 1789 0.161 0.090 1.18 0.98, 1.40
2012 1298 0 1
Season of farrowing
Dry 2697 0.221 0.072 1.25 1.08, 1.44
Rainy 2481 0.182 0.073 1.20 1.04, 1.38
Windy 2015 0   1  

A one pig increase in the number of pigs born alive increases the odds of a decrease in the size of the second litter of a sow, 1.71 times (b=0.54+0.02). Figure 2 shows that the probability of a decrease in the size of the second litter increased slowly up to 7 pigs to increase rapidly thereafter. However, a 1 day increase in LL or WSI decreases the odds of a decrease in the second litter 0.97 (b=-0.04+0.01) and 0.978 (b=-0.02+0.00) times, respectively. In other words, sows with large litter at first farrowing are more likely to have smaller litters at the second parity; whereas sows with longer LL and WSI are less likely of having a decrease in the size of their second litter (Figure 3). The intercept of the regression logistic model was -4.77.

Figure 2. Probability of a decrease in size of the second litter of sows kept under tropical conditions of Mexico


The overall percentage of sows showing the second-litter syndrome (60.3%), in this study, is at the superior limit of the range of values (40 to 60%) reported in the literature (Morrow et al 1992, Rathje and Himmelberg 2004; Kemp and Soede 2004; Vargas et al 2006; Saito et al 2010). Farm differences, in the frequency of sows showing the second-litter syndrome, were expected because of geographic and management differences.

Commonly, year of farrowing had significant effect on the second-litter syndrome; although, it is a factor hard to explain, because it involve management decisions with years, and farmer policies. Year differences in the percentage of sows showing the second-litter syndrome have been notified by Boulot et al (2013) in France.

Figure 3: Probability of a decrease in the size of the second litter of sows kept under tropical conditions of Mexico

The higher odds of the second-litter syndrome for sows farrowing in the dry and rainy seasons may be attributed to possible effect of the high temperatures in those seasons as compared with the cooler temperatures in the windy season (Table 1). Season effect on the second-litter syndrome has also been reported by Boulot et al (2013) in France.

Some authors (Morrow et al 1992; Boulot et al 2013) report that large litter-size at first parity is associated with smaller litter sizes at the subsequent parity. This can be the result of the high prolificacy at first farrowings of today sows, which may cause higher body weight loss during lactation and in consequence may cause low ovulation rate and high embryonic mortality (Whittemore 1996).

Morrows et al (1992) reported that first parity sows with longer LL produced larger litters at their second parity. Koketsu et al (1996) and Rathje and Himmelberg (2004) reported that one day increase in lactation increases in 0.1 pigs the second litter. Those results agree with those found in this study. Early weaned sows may represent a group of females with problems, probably associated with emergent weaning (disgalactia, locomotor problems, small litter size or reproductive problems). Also, early weaned sows may not be physiologically prepared for breeding, because short LL does not allow the reestablishment of the uterus and ovaries (Carregaro et al 2006). Therefore, it is important to use lactation periods that consider the minimum interval of recuperation of the reproductive tract. Studies show the need of at least two or three weeks for the uterine involution to occur and for the sow to get pregnant (Flowers 1998).

Longer WSI affected the subsequent litter size favorably. The decrease in the size of the second litter, found in this study, for sows with prolonged WSI, agrees with the results of Morrow et al (1992). In fact, a practice to allow the first parity sow to recover from the previous lactation is to inseminate her at the second estrous after weaning instead of the first one (skip an estrous). It have been shown that skipping the first estrous, improve pregnancy rate by 15% and next litter size by 1.3 to 2.5 pigs (Clowes et al 1994; Vesseur et al 1994). However, skipping an estrous will increase the non-productive days of the sows.



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Received 25 August 2013; Accepted 30 August 2013; Published 1 October 2013

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