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Genetic parameters for reproduction and production traits of Landrace sows in Thailand¹

N. Imboonta^*, L. Rydhmer and S. Tumwasorn^*,2

^* Department of Animal Science, Faculty of Agriculture, Kasetsart University, Bangkok 10900, Thailand; and and Department of Animal Breeding and Genetics, Swedish University of Agricultural Sciences, Box 7023, S- 750 07, Sweden

	Abstract

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION IMPLICATIONS LITERATURE CITED

 
Data from Thai Landrace sows were used to estimate genetic parameters for reproduction and production traits in first and later parities. The reproduction traits investigated were total number of piglets born per litter (TB), number of stillborn piglets (SB), and number of piglets born alive but dead within 24 h (BAD). The reproduction data pertained to 12,603 litters born between 1993 and 2005. The production measures were ADG and backfat thickness (BF); these were recorded in 4,163 boars and 15,171 gilts. Analyses were carried out with a multivariate animal model using average information REML procedures. Heritability estimates of reproduction traits for first parity were 0.03 ± 0.02 for TB, 0.04 ± 0.02 for SB, and 0.06 ± 0.02 for BAD. For later parities, they were 0.07 ± 0.01 for TB, 0.03 ± 0.04 for SB, and 0.02 ± 0.01 for BAD. Heritability estimates for production traits were 0.38 ± 0.02 for ADG and 0.61 ± 0.02 for BF. Genetic correlations between ADG and TB tended to be favorable, and genetic correlations between BF and TB tended to be unfavorable in all parities. However, BF was genetically correlated unfavorably with SB in later parities, and the genetic correlations between TB and BAD tended to be unfavorable in all parities. The genetic correlations of TB, SB, and BAD between first and later parities were 0.85 ± 0.13, 0.79 ± 0.16, and 0.71 ± 0.24, respectively. Selection for high growth rate will probably increase TB, and selection for low BF will decrease TB and increase SB. The results obtained also indicated that BAD will increase if there is selection pressure for high TB.

Key Words: average daily gain • backfat thickness • genetic correlation • Landrace sow • litter size • piglet mortality

	INTRODUCTION

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION IMPLICATIONS LITERATURE CITED

 
Sow productivity is recognized as a key factor affecting the efficiency and economic viability of the pig industry and is a leading concern of commercial producers and breeders (Kim, 2001). In current pig breeding programs, great emphasis is placed on improving reproduction traits in dam lines (Hanenberg et al., 2001). The breeding goal is generally to increase the number of piglets weaned per sow per year.

Lean growth rate and litter traits are economically important traits in swine production, and thus, both should be emphasized in a swine selection program. Selection practices have improved the ADG, backfat thickness (BF), and lean meat percentage of the pig carcass. However, a correlation between selection for production traits and decreased reproductive performance has been reported (Zhang et al., 2000; Holm et al., 2004a). To combine production and reproduction traits optimally in selection programs, accurate estimates of variance and covariance components for all traits are necessary.

The swine genetic material used in Thailand mostly originates from temperate areas, such as Western Europe and North America, where the climate is different from that in Thailand. Thailand has greater average temperatures and humidity and an almost constant day length. Genetic parameters may vary between populations and environments; it is therefore necessary to estimate genetic parameters specifically for tropical areas. To date, few genetic studies based on data from sows in tropical areas are available.

Hence, the purpose of this study was to estimate genetic parameters for reproduction and production traits in sows raised under tropical conditions.

	MATERIALS AND METHODS

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All animals in the study were cared for according to the guidelines of the Department of Livestock Development, Ministry of Agriculture and Cooperatives, Thailand.

Data

Purebred Landrace pigs, which came from 1 nucleus herd located in eastern Thailand, were studied. Litter records were available, including the total number of piglets born in first and later parities (TB1 and TB2+), the number of stillborn piglets in first and later parities (SB1 and SB2+), and the number of piglets born alive but dead within 24 h of birth in first and later parities (BAD1 and BAD2+), for sows farrowing between 1993 and 2005. Performance test data were also available for gilts and boars tested between 1993 and 2005. In this population, selection has been based on an index composed of lean percentage, growth rate, and total number of piglets born. Until 1996, individual animals were selected on the basis of their phenotypic results (growth rate and backfat). Thereafter, single-trait BLUP values of lean percentage (estimated from back-fat and loin eye area records), ADG, and litter size were used in the index.

The sows were kept in individual stalls during mating and gestation and in individual farrowing pens during lactation. The boars were kept in individual pens. Since the establishment of the farm, sows and boars had been housed in open buildings with cooling systems such as water dripping, sprinkling, and fans. After 1996, sows and boars were housed in evaporative cooling (EVAP)-system buildings. Evaporative cooling reduces the air temperature by humidification. Water is sprayed on a cooling pad in one end of the closed building. Hot outdoor air passes through the pads, using the exhaust fan at the other end of the building, and the air temperature is reduced when the water evaporates. This process reduces the temperature with a complementary increase in relative humidity and water vapor content in the air (Simmons and Lott, 1996). The temperature inside an EVAP-system building can be reduced by approximately 7°C below the outside temperature (Saengsukeeluck, 2001). The side wall along the length of an EVAP building is made, in part, of translucent plastic sheets; therefore, an EVAP building has ambient photo-period.

All boars, gilts, and sows received feed of the same composition at all stages of the reproductive cycle. The feed contained approximately 17% CP and 13 MJ of DE per kilogram, as-fed basis. The boars were fed 2.5 kg/d. Gilts were fed 2.5 kg/d from the performance test until the first successful mating. Gilts and sows were fed 1.8 kg/d from mating to 12 wk of gestation and, thereafter, 3 kg/d until 7 d before expected farrowing, when the feed was reduced to 2 kg/d. Lactating sows were fed 2.5, 4.5, and 6 kg/d during wk 1, 2, and 3 of lactation, respectively. After weaning, the sows were moved to the mating-gestation area, where they were fed 2 kg/d until mating. All animals had free access to water via nipples.

Replacement gilts were penned in groups of 3 to 5 animals. Before the expected second estrus, they were moved into the mating area for boar contact, where they were kept in individual stalls. Gilts were mated on the second observed estrus or later at a minimum BW of 130 kg. Sows were inseminated artificially 3 times per estrus, every 12 h. All sows produced pure-bred piglets. Sows were moved to the farrowing house 1 wk before the expected farrowing date. Farrowing was supervised over a 24-h period. Newborn piglets were monitored and handled once a day, between 1000 and 1200. Numbers of piglets born in total (TB), stillborn piglets (SB), and piglets born alive but dead within 24 h of birth (BAD; animals being, for example, weak, crushed, or malformed) were recorded at this handling. No crossfostering was performed before the piglets were handled on the first day. Light BW male piglets and male piglets with abnormal characteristics (e.g., hernia, cryptorchidism, abnormal feet and legs) or from parents with undesirable breeding values were castrated in the farrowing house. The lactation period was approximately 18 d.

All gilts and intact young boars were performance-tested from 9 to 22 wk of age. During the performance test, these animals were fed ad libitum and penned in groups of 18 to 20 animals of the same sex. Backfat thickness was measured ultrasonically at the 10th rib and 6.5 cm from the midline and adjusted to 100 kg by using the following equation:

[1]

Growth was expressed as ADG from 9 to 22 wk of age. After the performance test, 10% of the selected young boars and 50% of the selected gilts were used for replacement on this nucleus farm, and the rest were sold as breeding animals to other farms.

After editing, the final data contained 19,334 production records of 4,163 young boars and 15,171 gilts, and reproduction records of 3,074 sows, with 12,603 litters. Mummified piglets were excluded from TB. Only litters with at least 1 TB were included. Parities greater than 9 were excluded. The minimum number of observations per year-month was 14 for first parity and 36 for later parities.

Statistical Analyses

Initially, several fixed effects and covariates were examined for their significance (P < 0.05) in univariate models using ordinary least squares. For GLM, only fixed effects with a significant influence were included in the final models. For the genetic analyses, ages at first parity conception were grouped into 3 classes (less than 32, 32 to 40, and more than 40 wk). Parities were grouped into 7 classes (1, 2, 3, 4, 5, 6, and 7 to 9). Two-month farrowing periods (January/February, March/April, May/June, July/August, September/October, and November/December) were constructed and used for reproduction traits and 2-mo birth periods were used for production traits. Multivariate analyses were performed for all traits for (1) first parity and (2) second to ninth parity (repeatability model). Because of computational limitations, it was not possible to fit all of the traits into the same analysis. Furthermore, SB and BAD are autocorrelated because stillborn piglets cannot die after birth. Therefore, parameters (ADG, BF, TB, and SB or BAD) were estimated with 4-trait analyses of data from first parity or later parities.

The statistical model, in matrix notation, was y = Xb + Za + Wp + e, where y is the vector of observations of the traits studied (4 traits simultaneously); X, Z, and W are the known incidence matrices for fixed and random effects; b is the vector of fixed effects, a is the vector of additive genetic effects; p is the vector of permanent environmental effects; and e is the vector of residuals. The matrix W and the vector p were only included in the repeatability model. The (co)variance matrices of random effect factors in a, p, and e were assumed to be

where A is the additive genetic relationship matrix between animals; I is the identity matrix; and G, P, and R are the (co)variance matrices for the vectors a, p, and e, respectively. Models for TB, SB, and BAD included nongenetic effects of contemporary group (farrowing year-month), age at conception for first parity, or parity for later parities. For ADG and BF, a combination of birth year and month, sire line (the fathers of gilts and young boars belonged to 3 Landrace lines used in the nucleus herd), and sex were included in the model.

The (co)variance components were estimated with REML (Johnson and Thompson, 1995), using the DMU package (Madsen and Jensen, 2000). The heritabilities and correlations presented were averages of the estimates of the multivariate analyses, including SB or BAD.

	RESULTS

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION IMPLICATIONS LITERATURE CITED

 
Descriptive statistics of the traits analyzed in this study are given in Table 1. In 84% of litters, no piglets were stillborn (SB = 0), and in 72% of litters, no live born piglets died (BAD = 0). The distribution of reproduction records over parities is shown in Table 2.

Only 9% of gilts conceived at an age less than 32 wk, 78% conceived at 32 to 40 wk, and 13% conceived at greater than 40 wk. Gilts in the first, second, and third age group had 9.85 ± 0.17, 10.04 ± 0.06, and 9.6 ± 0.14 piglets/litter, respectively. The second age group gilts had more TB (P < 0.05) than the older age group. Age at conception had no effect on SB or BAD of primiparous sows.

The effect of parity number on TB, SB, and BAD is shown in Figure 1. The TB decreased in the second parity and thereafter increased with the number of parities, reaching a plateau in parities 4 and 5. Stillborn piglets were low in parities 2 and 3 but high both in parity 1 and after parity 4. The number of BAD in parities 2 and 3 was significantly lower than BAD in other parities. The effect of parity on SB and BAD was also significant when TB was included in the model.

View larger version (8K): [in this window] [in a new window]   Figure 1. Effect of number of parities on variation in total number of piglets born (TB), stillborn piglets (SB), and born alive but dead within 24 h (BAD). a–eLeast-squares means without a letter in common are significantly different (P < 0.05).

View larger version (13K): [in this window] [in a new window]   Figure 2. Seasonal effect on total number of piglets born (TB), stillborn piglets (SB), and born alive but dead within 24 h (BAD) in first parity (1) and later parity (2+) sows. a–c, A–CWithin a line, least-squares means without a letter in common are significantly different (P < 0.05).

View larger version (9K): [in this window] [in a new window]   Figure 3. Effect of birth month on ADG and backfat thickness (BF). a–dWithin a line, least-squares means without a letter in common are significantly different (P < 0.05).

Heritabilities and the proportion of permanent environmental variances to total variances of reproduction and production traits are presented in Table 3. Heritability for TB1 was relatively low. Heritabilities for BAD and SB were somewhat greater in the first than in later parities. There was no permanent environmental variance for SB in later parities.

Table 6 shows genetic and phenotypic correlations between reproductive traits measured in different parities. Genetic correlations were high, but phenotypic correlations were low.

	DISCUSSION

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION IMPLICATIONS LITERATURE CITED

Litter size increased as the age at first conception increased from the first age group (<32 wk) to the second age group (32 to 40 wk). This confirms previous studies (Clark et al., 1988; Dewey et al., 1995). Gilts in the third age group produced smaller litters than gilts in younger age groups. In this study, 1, 5, and 33% of gilts in the first, second, and third age groups, respectively, had to be mated twice to conceive (data not shown). Schukken et al. (1994) showed in a retrospective study that sows conceiving at an advanced age are culled for reproductive failure more often than sows conceiving when younger. It might be hypothesized that gilts with low fertility became pregnant at later ages (>40 wk) and subsequently had smaller litters.

Litter size increased with parity number, reaching a plateau at parities 4 to 5, and then declining. This finding is in accordance with earlier studies (Roehe and Kennedy, 1995; Tantasuparuk et al., 2000). Our finding that second parity sows had small litter size disagrees, however, with studies by Dewey et al. (1995) and Roehe and Kennedy (1995).

There was no seasonal effect on TB in the older sows, but the seasonal effect on gilt litters in this study was strong. Tummaruk et al. (2004) also showed that season has a more pronounced effect on gilts than it does on older sows in Thailand. It appears, then, that sows adapt to climatic changes with increasing age. This study showed a reduction in the litter size of first parity sows farrowing during the rainy season. These sows had been mated during the hot season. This finding confirms the results of other studies, which indicate that sows bred during the hot season produce smaller litters (Tantasuparuk et al., 2000; Tummaruk et al., 2004). It has been reported that high ambient temperature alters reproduction by acting directly on ovarian function or via the hypothalamic pituitary axis by affecting estrus, ovulation, gametes, and embryo survival (Wettemann and Bazer, 1985; Armstrong et al., 1986). The small litters born in the rainy season in this study might be explained by a poor fertilization rate, reduced levels of embryonic survival, or both. There was no seasonal variation in SB, when corrected for TB. This finding is in accordance with a previous study conducted in Thailand (Tantasuparuk et al., 2000).

Genetic Parameters

The estimates of heritability for TB in the current study are lower than the average value of 0.11 presented in the literature review undertaken by Rothschild and Bidanel (1998). Heritability for TB was estimated at 0.10 in Gu et al. (1989). Roehe and Kennedy (1995) estimated heritabilities for TB from 0.09 to 0.16 in Landrace and Yorkshire sows. Estimates of permanent environmental effects for TB vary in the literature from 0.00 to 0.12 (Lamberson et al., 1991; Hanenberg et al., 2001), and the estimate from this study falls in between these values. Heritability estimates for mortality traits in the current study are low, but significantly greater than zero, except for SB in later parities. Knol (2001) estimated slightly greater heritabilities for SB (0.05) and litter mortality (0.08), calculated as the percentage of live piglets dying from birth to weaning.

Estimates of genetic correlations between the traits examined in the current study ranged from –0.23 to 0.40 and had high standard errors. In general, these estimates did not diverge from zero. The only favorable and significant genetic correlation was between ADG and TB in later parities. The genetic correlation between ADG and TB in first parity was also favorable, in keeping with previous studies (Kerr and Cameron, 1996; Serenius et al., 2004). Holm et al. (2004a), by contrast, found a positive (unfavorable) genetic correlation between age at 100 kg and number of piglets born alive. Tummaruk et al. (2001) reported that gilts with greater growth rate had larger litter size than those with lower growth rate. Perhaps gilts with high growth rate consume more feed, are healthier, and have a better nutrient base for subsequent reproductive performance than slow-growing gilts. Because age is the most important factor in gilts attaining puberty (Hughes, 1982), gilts with a high ADG tend to be heavier at puberty. King (1989) showed that live weight at 165 d of age, rather than BF, influenced ovulation rate in gilts. Thus, selection for high ADG may increase TB. The current study, however, indicates that such selection would increase piglet mortality in the first parity.

The estimated genetic correlation between BF and SB in later parities was unfavorable. This is in agreement with Knol (2001), who found that genetic correlations between piglet survival and BF were moderately positive, and Arango et al. (2005), who reported a negative genetic correlation between sows’ backfat and number of piglets born dead in their first parity. This may explain the unfavorable genetic correlation between BF and SB found in the current study.

The genetic correlations between TB and SB in first parity and between TB and BAD in all parities were unfavorable. This is in line with the finding of Johnson et al. (1999) that selection for large litters increases the number of stillborn piglets. Lund et al. (2002) also found unfavorable genetic correlations between total born and piglet survival. Increased litter size generally results in lower birth weights, with a decreased probability of survival (Roehe and Kalm, 2000). On the other hand, Grandinson et al. (2002) reported a positive genetic correlation between birth weight and stillbirth. Other factors, such as time from beginning to end of parturition, also determine the number of stillborn piglets (Holm et al., 2004b). According to Thornbury et al. (1993), delayed gut maturation was found in piglets with low birth weight. Moreover, the development of the central nervous system may be compromised by a critical endocrine component in piglets with low birth weight (Wise et al., 1997). Low birth weight is also genetically correlated with greater piglet mortality caused by crushing (Grandinson et al., 2002). Selection only on total litter size will, therefore, almost certainly increase piglet mortality. The inclusion of selection on BAD or SB, although these traits have low heritabilities, will likely increase the number of piglets produced per litter.

	IMPLICATIONS

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION IMPLICATIONS LITERATURE CITED

 
The results indicate that selection for high growth rate will result in more piglets born per litter, but selection for low backfat thickness will increase the incidence of stillbirth. Selection for litter size will result in greater mortality after birth. Therefore, piglet mortality should be included in the breeding evaluation, together with litter size and production traits.

	Footnotes

 
¹ Financial and data support for this project were provided by the Thailand Research Fund under the Royal Golden Jubilee Project and Charoen Pokphan Co., Ltd (Thailand); the study was performed at the Department of Animal Breeding and Genetics, Swedish University of Agricultural Sciences.

² Corresponding author: sornthep{at}ksc.th.com

Received for publication December 9, 2005. Accepted for publication July 31, 2006.

	LITERATURE CITED

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION IMPLICATIONS LITERATURE CITED

 


Arango, J., I. Misztal, S. Tsuruta, M. Culbertson, and W. Herring. 2005. Threshold-linear estimation of genetic parameters for farrowing mortality, litter size, and test performance of Large White sows. J. Anim. Sci. 83:499–506.[Abstract/Free Full Text]

Armstrong, J. D., J. H. Britt, and N. M. Cox. 1986. Seasonal differences in function of the hypothalamic-hypophysial-ovarian axis in weaned primiparous sows. J. Reprod. Fertil. 78:11–20.[Abstract/Free Full Text]

Clark, L. K., A. D. Leman, and R. Morris. 1988. Factors influencing litter size in swine: Parity-one females. J. Am. Vet. Med. Assoc. 192:187–194.[Medline]

Dewey, C. E., S. W. Martin, R. M. Friendship, B. W. Kennedy, and M. R. Wilson. 1995. Associations between litter size and specific sow-level management factors in Ontario swine. Prev. Vet. Med. 23:101–110.

Grandinson, K., M. S. Lund, L. Rydhmer, and E. Strandberg. 2002. Genetic parameters for the piglet mortality traits crushing, stillbirth and total mortality, and their relation to birth weight. Acta Agric. Scand. Sect. A. Anim. Sci. 52:167–173.

Gu, Y., C. S. Haley, and R. Thompson. 1989. Estimates of genetic and phenotypic parameters of litter traits from closed line of pigs. Anim. Prod. 49:477–482.

Hanenberg, E. H. A. T., E. F. Knol, and J. W. M. Merks. 2001. Estimates of genetic parameters for reproduction traits at different parities in Dutch Landrace pigs. Livest. Prod. Sci. 69:179–186.[CrossRef][Medline]

Holm, B., M. Bakken, G. Klemetsdal, and O. Vangen. 2004a. Genetic correlations between reproduction and production traits in swine. J. Anim. Sci. 82:3458–3464.[Abstract/Free Full Text]

Holm, B., M. Bakken, O. Vangen, and R. Rekaya. 2004b. Genetic analysis of litter size, parturition length, and birth assistance requirements in primiparous sows using a joint linear-threshold animal model. J. Anim. Sci. 82:2528–2533.[Abstract/Free Full Text]

Hughes, P. E. 1982. Factors affecting the natural attainment of puberty in the gilt. Pages 117–138 in Control of Pig Reproduction. D. J. A. Cole and G. R. Foxcroft, ed. Nottingham University Press, Nottingham, UK.

Johnson, R. K., M. K. Nielsen, and D. S. Casey. 1999. Responses in ovulation rate, embryonal survival, and litter traits in swine to 14 generations of selection to increase litter size. J. Anim. Sci. 77:541–557.[Abstract/Free Full Text]

Johnson, D. L., and R. Thompson. 1995. Restricted maximum likelihood estimation of variance components for univariate animal models using sparse matrix techniques and average information. J. Dairy Sci. 78:449–456.[Abstract]

Kerr, J. C., and N. D. Cameron. 1996. Genetic and phenotypic relationships between performance test and reproduction traits in Large White pigs. J. Anim. Sci. 62:531–540.

Kim, H. J. 2001. Genetic Parameters for Productive and Reproductive Traits of Sows in Multiplier Farms. Ph.D. Thesis, Georg-August-University of Gottingen, Göttingen, Germany.

King, R. H. 1989. Effect of live weight and body-composition of gilts at 24 weeks of age on subsequent reproductive efficiency. Anim. Prod. 49:109–115.

Knol, E. F. 2001. Genetic aspects of piglet survival. Ph.D. Diss. Wageningen University, Wageningen, the Netherlands.

Lamberson, W. R., R. K. Johnson, D. R. Zimmerman, and T. E. Long. 1991. Direct responses to selection for increased litter size, decreased age at puberty, or random selection following selection for ovulation rate in swine. J. Anim. Sci. 69:3129–3143.[Abstract]

Lund, M. S., M. Puonti, L. Rydhmer, and J. Jensen. 2002. Relationship between litter size and perinatal and pre-weaning survival in pigs. Anim. Sci. 74:217–222.

Madsen, P., and J. Jensen. 2000. A user’s guide to DMU. A package for analysing multivariate mixed models. Version 6, release 4. Danish Institute of Agricultural Sciences, Research Centre Foulum, Tjele, Denmark.

Roehe, R., and E. Kalm. 2000. Estimation of genetic and environmental risk factors associated with pre-weaning mortality in piglets using generalized linear mixed models. Anim. Sci. 70:227–240.

Roehe, R., and B. W. Kennedy. 1995. Estimation of genetic-parameters for litter size in Canadian Yorkshire and Landrace swine with each parity of farrowing treated as a different trait. J. Anim. Sci. 73:2959–2970.[Abstract]

Rothschild, M. F., and J. P. Bidanel. 1998. Biology and genetics of reproduction. Pages 313–344 in M. F. Rothschild and A. Ruvinsky, ed. The Genetics of the Pig. University Press, Cambridge, UK.

Saengsukeeluck, P. 2001. Comparative studies of semen quality and production cost between the boars raised under open and evaporative cooling system houses. M.S. Thesis, Kasetsart University, Bangkok, Thailand.

Schukken, Y. H., J. Buurman, R. B. M. Huirne, A. H. Willemse, J. C. M. Vernooy, J. Vandenbroek, and J. H. M. Verheijden. 1994. Evaluation of optimal age at first conception in gilts from data collected in commercial swine herds. J. Anim. Sci. 72:1387–1392.[Abstract]

Serenius, T., M. L. Sevon-Aimonen, A. Kause, E. A. Mantysaari, and A. Maki-Tanila. 2004. Genetic associations of prolificacy with performance, carcass, meat quality, and leg conformation traits in the Finnish Landrace and Large White pig populations. J. Anim. Sci. 82:2301–2306.[Abstract/Free Full Text]

Simmons, J. D., and B. D. Lott. 1996. Evaporative cooling performance resulting from changes in water temperature. Appl. Eng. Agric. 12:497–500.

Tantasuparuk, W., N. Lundeheim, A. M. Dalin, A. Kunavongkrit, and S. Einarsson. 2000. Reproductive performance of purebred Landrace and Yorkshire sows in Thailand with special reference to seasonal influence and parity number. Theriogenology 54:481–496.[CrossRef][Medline]

Thornbury, J. C., P. D. Sibbons, D. van Velzen, R. Trickey, and L. Spitz. 1993. Histological investigations into the relationship between low birth weight and spontaneous bowel damage in the neonatal piglet. Pediatr. Pathol. 13:59–69.[Medline]

Tummaruk, P., N. Lundeheim, S. Einarsson, and A. M. Dalin. 2001. Effect of birth litter size, birth parity number, growth rate, back-fat thickness and age at first mating of gilts on their reproductive performance as sows. Anim. Reprod. Sci. 66:225–237.[CrossRef][Medline]

Tummaruk, P., W. Tantasuparuk, M. Techakumphu, and A. Kunavongkrit. 2004. Effect of season and outdoor climate on litter size at birth in purebred Landrace and Yorkshire sows in Thailand. J. Vet. Med. Sci. 66:477–482.[CrossRef][Medline]

Wettemann, R. P., and F. W. Bazer. 1985. Influence of environmental temperature on prolificacy of pigs. J. Reprod. Fertil. Suppl. 33:199–208.[Medline]

Wise, T., A. J. Roberts, and R. K. Christenson. 1997. Relationships of light and heavy fetuses to uterine position, placental weight, gestational age, and fetal cholesterol concentrations. J. Anim. Sci. 75:2197–2207.[Abstract/Free Full Text]

Zhang, S., J.-P. Bidanel, T. Burlot, C. Legault, and J. Naveau. 2000. Genetic parameters and genetic trends in the Chinese x European Tiameslan composite pig line. I. Genetic parameters. Genet. Sel. Evol. 32:41–56.[CrossRef][Medline]

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