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Journal of Animal Science - Animal Genetics

Selection for increased number of piglets at d 5 after farrowing has increased litter size and reduced piglet mortality1

 

This article in JAS

  1. Vol. 91 No. 6, p. 2575-2582
     
    Received: Oct 15, 2012
    Accepted: Feb 19, 2013
    Published: November 25, 2014


    2 Corresponding author(s): bni@lf.dk
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doi:10.2527/jas.2012-5990
  1. B. Nielsen 2,
  2. G. Su,
  3. M. S. Lund and
  4. P. Madsen
  1. Pig Research Centre, Danish Agriculture and Food Council, DK-1609 Axeltorv, Denmark
    Department of Molecular Biology and Genetics, Aarhus University, DK-8830 Tjele, Denmark

Abstract

Selection for litter size at d 5 after farrowing (LS5) was introduced in 2004 to increase the number of piglets weaned and to reduce piglet mortality in Danish Landrace and Yorkshire. The objective of this study was to investigate selection responses for LS5, total number born (TNB), and mortality [MORT, defined as (TNB − LS5)/TNB] when selection for increasing LS5 was a part of the breeding goal. Data were collected from nucleus herds recorded from 2004 to 2010, including first litters of 42,807 Landrace sows and 33,225 Yorkshire sows. The data were analyzed using a 3-trait animal model of TNB, MORT, and LS5. Significant (co) variances were estimated between the 3 traits in both populations. The heritabilities of TNB, MORT, and LS5 were 0.10, 0.09, and 0.09 in Landrace and 0.12, 0.10, and 0.10 in Yorkshire. The genetic correlations were 0.28 and 0.22 between TNB and MORT, 0.74 and 0.68 between TNB and LS5, and −0.43 and −0.57 between MORT and LS5 in Landrace and Yorkshire, respectively. The results show that the genetic improvement of LS5 was a combination of increased TNB and reduced MORT. During the observation period, the genetic improvement was 1.7 piglets per litter for LS5, 1.3 piglets per litter for TNB, and 4.7% for MORT in Landrace and 2.2 piglets per litter, 1.9 piglets per litter, and 5.9% in Yorkshire. Phenotypic improvement was 1.4 piglets per litter for LS5, 0.3 piglets per litter for TNB, and 7.9% for MORT in Landrace and 2.1 piglets per litter, 1.3 piglets per litter, and 7.6% in Yorkshire. In addition, genetic gain was evaluated in 3 phenotypic groups of TNB, representing the 25% smallest litters, the 50% medium litters, and the 25% largest litters. In all 3 groups, the genetic and phenotypic gains of TNB and LS5 increased, whereas MORT reduced in both populations.



INTRODUCTION

In commercial pig production, the number of weaned pigs is a key factor to increases productivity, and litter size has been one of the most important traits in pig production. Therefore, selection for large litter size has been a part of the Danish pig breeding program since 1992, and the selection strategy has resulted in a considerable increase in total number born (TNB; Sorensen et al., 2000; Su et al., 2007). It has been reported that the genetic correlation between TNB and mortality is positive but unfavorable (Lund et al., 2002; Damgaard et al., 2003; Su et al., 2007). Consequently, selection for increasing TNB will increase the number of stillborn as well as the preweaning mortality. Previous studies have showed that there is a substantial potential for genetic improvement of piglet survival, especially when piglet mortality is high as under outdoor production systems (Roehe et al., 2010). It was observed that most cases of death occurred at farrowing and during the first 5 d after farrowing in Danish Landrace and Yorkshire (Su et al., 2007, 2008). To avoid an additional increase in the number of stillborn and to reduce the mortality of the piglets in the early nursing period, the breeding goal in the Danish breeding program was changed in 2004 to focus on the litter size at d 5 after farrowing (LS5).

The trait LS5 is a composite trait as it combines TNB and mortality until d 5 after farrowing. It is expected that selection for LS5 will lead to an increase of TNB and a reduction of piglet mortality.

The objective of this study was to investigate the realized genetic improvement of TNB and piglet mortality (recorded as the ratio between the sum of stillborn and dead piglets up to d 5 after farrowing and TNB) after selection for increasing LS5 was introduced as a part of the breeding goal in Danish Landrace and Yorkshire populations.


MATERIALS AND METHODS

Animal Care and Use Committee approval was not obtained for this study because the data were obtained from an existing database of performance records.

Data

All data were supplied by the Danish Agriculture and Food Council, Pig Research Centre, Axeltorv, Denmark. The data were collected from Danish Landrace and Yorkshire nucleus herds during the period from January 2004 to December 2010. During the recording period, the populations were selected on the basis of a selection index in which LS5 was a main component trait. Sows were kept under commercial conditions, and all matings were by AI. There were no protocols for assisting the sows during the birthing process. At farrowing, TNB was recorded as the total number of fully formed piglets born, including the number of stillbirths. The data recording system does not account for the piglets that died in utero. At birth, the piglets were earmarked, and cross-fostering was permitted during the whole suckling period to ensure animal welfare. During the assignment of cross-fostering, piglets might be exchanged between litters. Different herds might have different strategies for cross-fostering to use the nursing capacity of sows efficiently. During the first 5 d after farrowing, the dead piglets in each litter were recorded and were assigned to the biological litters according to their earmarks. Litter size at d 5 after farrowing in each litter was calculated as the TNB minus the number of stillborn and dead piglets up to d 5 after farrowing. The mortality (MORT) was calculated as (TNB − LS5)/TNB in each litter. Survival status of the piglets that because of cross-fostering were transferred to another litter was recorded to the litter of the biological mother.

To avoid possible bias in estimation of genetic parameters and prediction of breeding values due to phenotypic selection of sows after first litter, only the first litter records of each sow were included in the data. Thus, the data in the analysis comprised records of first parity from 42,807 Landrace sows (gilts) and 33,225 Yorkshire sows (gilts; Table 1).


View Full Table | Close Full ViewTable 1.

Number of farms, year-week, and sows (litters)1

 
Breed Farms Year-week Sows (Litters)
Landrace 19 84 42,807
Yorkshire 20 84 33,225
1Only first parity was considered.

Statistical Analysis

Litter size and mortality up to d 5 after farrowing were analyzed using a 3-trait animal model. Su et al. (2007) claimed that the nursing sow had a small effect on piglet survival rate during the first 5 d after birth. Knol et al. (2002) reported that including the nurse sow effect in a model for piglet survival gave erratic results. In the current study all 3 traits were considered as the traits of biological sows, ignoring the cross-fostering effect. The basic model to describe the observations waswhere y is the vector of observations for the 3 traits (TNB, LS5 and MORT); b is the vector of fixed effects, including effects of year-month and litter genotype (purebred or crossbred), as well as effects of age at first parity (regressions on the first and second orders of age covariables); u is a vector of random herd-year-month effects; d is a vector of random sow genetic effects; e is a vector of random residuals; and X, Zu, and Zd are incidence matrices associating b, u, and d with y. Assumptions for random effects werewhere , , and are covariance matrixes for herd-year-month effects, additive genetic effects of sow, and residuals, respectively; Iu and Ie are the identity matrices of dimension equal to the number of herd-year-month classes and the number of observations, respectively; and A is the matrix of additive genetic relationships among animals in the pedigree. The pedigree was traced back 4 generations and included 49,800 Landraces and 39,674 Yorkshires. The numbers of base animals were 441 and 422 in the Landrace and Yorkshire populations, respectively. The parameters in the models were estimated by REML using the software package DMU (Madsen and Jensen, 2010).

Phenotypic variance () was defined as for TNB and LS5 as well as MORT, where is the variance of herd-year-month effects, is the variance of sow additive genetic effects, and is the residual variance. All 3 traits, TNB, LS5 and MORT, were considered as traits of the sow. Therefore, heritability was calculated as the ratio of sow additive genetic variance to phenotypic variance, i.e., .

The asymptotic (co)variances for the estimates of (co)variance components were obtained from the approximated observed information matrix, and approximated SE for functions of estimated (co)variances were estimated using Taylor series expansion (Su et al., 2007).

Genetic trends were calculated as the year mean of EBV, and phenotypic trend was calculated as year mean of observations of sows on the basis of the birth year of the sows. Chen et al. (2010) reported that there was a nonlinear relationship between the additive genetic effect of the number of stillbirths and the environmental deviation of litter size and suggested that selection for increasing survival at birth for a large litter might not simultaneously reduce mortality for a small litter. It can be argued that the reduction in MORT might be different in large litters compared with small litters in the present populations. Therefore, it could be important to investigate whether the expected reductions of piglet mortality due to selection for LS5 were realized in different sizes of litters. For this purpose, the sows were divided into 3 phenotypic groups based on the phenotypic litter size (TNB) within each year, representing the 25% smallest litters, the 50% medium litters, and the 25% largest litters. The genetic and phenotypic gains of LS5, TNB, and MORT were calculated within each group.


RESULTS

Landrace sows had slightly larger litter size and slightly lower mortality rate than Yorkshire sows (Table 2). The mean TNB and the mean LS5 were 13.7 and 11.1 in Landrace and 13.5 and 10.8 in Yorkshire, respectively. The mean mortality was 0.18 in Landrace and 0.20 in Yorkshire. The mean mortality was calculated as the mean of the mortality obtained in each litter. The within-year SD of TNB and LS5 ranged from 3.6 to 3.9 in the 2 populations, and the within-year SD of MORT was 0.18 in Landrace and 0.20 in Yorkshire.


View Full Table | Close Full ViewTable 2.

Total number of litters, mean and SD within years for total number born (TNB), piglet mortality (including stillborn and number of dead piglets until d 5 after farrowing), and litter size at d 5 after farrowing (LS5)

 
Item No. of litters Mean SD
Landrace
    TNB 42,807 13.7 3.9
    Mortality 42,807 0.18 0.18
    LS5 42,807 11.1 3.6
Yorkshire
    TNB 33,225 13.5 3.7
    Mortality 33,225 0.20 0.20
    LS5 33,225 10.8 3.7

The sow genetic (co)variances between TNB, MORT, and LS5 were all significantly different from 0 in the 2 populations (Table 3). The genetic variances of TNB, MORT, and LS5 were 1.48, 0.003, and 1.07 in Landrace and 1.65, 0.004, and 1.34 in Yorkshire.


View Full Table | Close Full ViewTable 3.

Covariance matrices of herd-year-week effect, maternal genetic effect, and residuals related to the total number born (TNB), piglet mortality including stillborn (MORT), and litter size at d 5 (LS5) in Landrace and Yorkshire1

 
(Co)variances and SE
Variance component Trait TNB MORT LS5
Landrace
    Herd-year-week effect TNB 0.17 (0.03) 0.003 (0.001) 0.09 (0.02)
MORT 0.001 (0.0001) −0.01 (0.001)
LS5 0.15 (0.02)
    Genetic, sow TNB 1.48 (0.13) 0.017 (0.004) 0.93 (0.10)
MORT 0.003 (0.0002) −0.02 (0.004)
LS5 1.07 (0.10)
    Residual TNB 13.19 (0.13) 0.072 (0.004) 9.41 (0.10)
MORT 0.027 (0.0002) −0.26 (0.004)
LS5 11.11 (0.10)
Yorkshire
    Herd-year-week effect TNB 0.25 (0.03) 0.002 (0.001) 0.15 (0.03)
MORT 0.001 (0.0001) −0.01 (0.002)
LS5 0.25 (0.03)
    Genetic, sow TNB 1.65 (0.15) 0.018 (0.006) 1.01 (0.12)
MORT 0.004 (0.0004) −0.04 (0.006)
LS5 1.34 (0.14)
    Residual TNB 11.86 (0.14) 0.050 (0.005) 8.48 (0.12)
MORT 0.035 (0.0004) −0.38 (0.006)
LS5 11.89 (0.13)
1Asymptotic SE are shown in brackets.

The environmental variances in the model were described by herd-year-month effects and residual effects. The residual variance described a significant part of total variation in Landrace as well as in Yorkshire (Table 3). Residual covariances between TNB, MORT, and LS5 were all significantly different from 0 in the 2 populations. The variances of herd-year-month effects were small and less than the genetic variances for all 3 traits in both populations because the model already included year-month as fixed effect.

The heritabilities of TNB, MORT, and LS5 were 0.10, 0.09, and 0.09 in Landrace and 0.12, 0.10, and 0.10 in Yorkshire (Table 4). The genetic correlation between TNB and MORT were 0.28 and 0.22 for the Landrace and Yorkshire, respectively. The genetic correlations between TNB and LS5 were 0.74 and 0.68, and the genetic correlations between MORT and LS5 were −0.43 and −0.57 for Landrace and Yorkshire, respectively. The phenotypic correlations between TNB and MORT were 0.14 and 0.09 for Landrace and Yorkshire, respectively. The phenotypic correlations between TNB and LS5 were 0.77 and 0.71 and between MORT and LS5 were −0.47 and −0.59 for Landrace and Yorkshire, respectively. The phenotypic correlations between the 3 traits were on the same level as the genetic correlations except for the phenotypic correlations between TNB and MORT, which were less than the genetic correlations. The genetic correlations indicated that selection for large TNB would increase both LS5 and MORT, whereas selection for large LS5 would increase TNB but decrease MORT.


View Full Table | Close Full ViewTable 4.

Heritability (on the diagonal), phenotypic correlation (below the diagonal), and genetic correlation (above the diagonal) for total number born (TNB), piglet mortality including stillborn (MORT), and litter size at d 5 (LS5) in Landrace and Yorkshire1

 
Landrace Yorkshire
Item TNB MORT LS5 TNB MORT LS5
TNB 0.10 (0.008) 0.28 (0.06) 0.74 (0.03) 0.12 (0.01) 0.22 (0.07) 0.68 (0.04)
MORT 0.14 (0.005) 0.09 (0.008) −0.43 (0.05) 0.09 (0.06) 0.10 (0.01) −0.57 (0.05)
LS5 0.77 (0.002) −0.47 (0.004) 0.09 (0.008) 0.71 (0.003) −0.59 (0.004) 0.10 (0.01)
1Approximated SE are given in parentheses.

As shown in Figs. 1 and 2, there was a clear phenotypic trend and a clear genetic trend that LS5 and TNB increased, whereas MORT decreased over the years in both populations. During the period from 2003 to 2009, the genetic improvement was 1.7 piglets for LS5, 1.3 piglets for TNB, and 4.7% for MORT in Landrace and 2.2 piglets, 1.9 piglets, and 5.9% in Yorkshire. The phenotypic improvement was 1.4 piglets for LS5, 0.3 piglets for TNB, and 7.9% for MORT in Landrace and 2.1 piglets, 1.3 piglets, and 7.6% in Yorkshire. These improvements confirmed that selection for LS5 had resulted in a considerable direct response for LS5 itself and a considerable correlated response for TNB and MORT.

Figure 1.
Figure 1.

Phenotypic means by sow year of birth for total number born (TNB), mortality (MORT), and litter size at d 5 after farrowing (LS5) in the first parity (gilt results) of Landrace (line with circles) and Yorkshire (line with triangles) populations.

 
Figure 2.
Figure 2.

Mean EBV by sow year of birth for total number born (TNB), mortality (MORT), and litter size at d 5 after farrowing (LS5) in Landrace (line with circles) and Yorkshire (line with triangles) populations. The EBV were adjusted so that sows with birth year at 2003 have a mean EBV equal to 0.

 

Figure 3 shows the changes of year means of EBV for 3 groups of litters with small, medium, and large TNB, and Fig. 4 shows the year means of phenotypes for the 3 groups. It was observed that MORT was slightly greater in the group with large TNB compared with the groups with small and medium TNB. However, the trends for both phenotypic and genetic changes over the years were almost the same for each trait in the 3 groups. Clearly, selection for LS5 reduced piglet mortality not only for small and medium litters but also for large litters.

Figure 3.
Figure 3.

Mean EBV by sow year of birth for total number born (TNB), mortality (MORT), and litter size at d 5 after farrowing (LS5) in 3 phenotypic levels of total number born: the 25% small litters (line with circles), the 50% medium litters (line with triangles), and the 25% largest litters (line with crosses) in first litter of Landrace (L-sows) and Yorkshire (Y-sows). The EBV were adjusted so that sows with birth year at 2003 have a mean EBV equal to 0 in the 50% medium litters.

 
Figure 4.
Figure 4.

Phenotypic means by sow year of birth for total number born (TNB), mortality (MORT), and litter size at d 5 after farrowing (LS5) in 3 phenotypic levels of total number born: the 25% small litters (line with circles), the 50% medium litters (line with triangles), and the 25% largest litters (line with pluses) in first litter (gilt results) of Landrace (L-sows) and Yorkshire (Y-sows).

 

The expected response to selection for LS5 was calculated approximately. In both breeds, the selection proportion was about 1:150 for males and 1:3 for females. This gave a theoretical selection intensity i = 0.5(2.8 + 1.1) = 2. However, some animals were not available for selection because of various reasons (e.g., poor health, fertility problems, and restriction on inbreeding), and thereby, the real selection intensity was assumed to be i = 1.5. The breeding values of LS5 were predicted using a multitrait BLUP model based on the data consisting of records from both nucleus herds and multiplier herds. The standard deviation in EBV of LS5 was σEBV = 0.35 for the animals without LS5 records (most animals did not have their own LS5 record at the time of selection), and the correlation between EBV of LS5 and selection index was r = 0.65. Assuming 1 generation per year, the expected genetic gain of LS5 was about i × r × σEBV = 1.5 × 0.65 × 0.35 = 0.34 piglets per litter per year, which was consistent with the observed gain of LS5.


DISCUSSION

This study showed that selection for LS5 has led to genetic and phenotypic increases in TNB and LS5 and reduction in MORT. The results were consistent with the estimated genetic parameters of the 3 female traits for which the estimates of heritabilities for the 3 traits ranged from 0.09 to 0.12, and LS5 had a strong positive genetic correlation with TNB and a strong negative correlation with piglet mortality.

The heritabilities of TNB and LS5 ranged from 0.09 to 0.12 in Landrace and Yorkshire, which is similar to the average of 0.10 reported by Haley et al. (1988). However, the estimates were greater than those previously reported for the same breeds (Su et al., 2007). There were 2 possible reasons: 1) the study by Su et al. (2007) was based on a smaller number of litters (9,310 in Landrace and 6,861 in Yorkshire) for a period of 2 yr and therefore had larger SE of the estimates, and 2) the data in their study included records from various parities, and the mixture of various parities together with phenotypic selection of sows (mainly for litter size) after first parity in practice could have an influence on estimates of heritabilities.

The heritability of mortality was found to be 0.09 and 0.10 in Landrace and Yorkshire, which for Landrace was in accord with the results of Su et al. (2007); however, for Yorkshire it was a bit greater. In the literature there is rather large variation among heritabilities of mortality. In a review by Rothschild and Bidanel (1998), the mean heritability was found to be 0.05, covering a large variation, whereas Lamberson and Johnson (1984) reported a heritability for preweaning survival at 0.03. Ferguson et al. (1985) reported a value of 0.14 in Yorkshire and 0.18 in Duroc. Damgaard et al. (2003) reported a heritability of 0.13 for the proportion of stillbirths in Swedish Yorkshire.

The positive but unfavorable genetic correlations at 0.28 and 0.22 between TNB and MORT in Landrace and Yorkshire, respectively, could explain why in the years between 1992 and 2004 breeding for increased TNB increased the piglet mortality. Lund et al. (2002) also found an unfavorable genetic correlation between TNB and piglet mortality. In Landrace they found a negative correlation of −0.39 between maternal genetic effects on total number born and proportion surviving from birth until 3 wk. Furthermore, an unfavorable positive genetic correlation between the number of live-born piglets and the proportion of dead piglets during suckling was also found by Damgaard et al. (2003). Similarly, Su et al. (2007) reported a negative genetic correlation of TNB with piglet survival at birth and survival during suckling. Other studies had reported that selection based on ovulation rate and embryonic survival had an unfavorable effect on number of stillborn piglets (Johnson et al., 1999; Petry and Johnson, 2004). The results of the present study as well as previous reports in the literature indicate that breeding only for TNB will increase piglet mortality.

The negative but favorable genetic correlations of −0.43 and −0.57 between LS5 and MORT in Landrace and Yorkshire show that breeding for LS5 can reduce the mortality at the same time as it increases the number of live piglets at d 5 after farrowing. Su et al. (2007) reported that genetic correlations between LS5 and number of piglets born alive were 0.84 and 0.82 and genetic correlations of LS5 with piglet survival at birth and survival from birth to d 5 ranged from 0.38 to 0.58 in Landrace and Yorkshire, respectively. Therefore, selection for high LS5 is expected to reduce both mortality at farrowing and mortality during the early suckling period.

The genetic correlations between LS5 and TNB at 0.74 and 0.68 in Landrace and Yorkshire were greater than those reported by Su et al. (2007). The positive and high genetic correlation shows that even if TNB is not part of the breeding goal, selection for LS5 will increase TNB per litter and the favorable correlation to mortality will decreases the mortality rate in litters. However, if selection is based on TNB, LS5 will still increase, but the unfavorable genetic correlation between TNB and mortality will lead to increased mortality in litters.

The favorable genetic correlation between LS5 and mortality and between LS5 and TNB was well demonstrated by the genetic gains due to selection for high LS5 from 2004. The genetic gain increase in TNB and LS5, and at the same time a genetic decrease in mortality, was found regardless of whether the dam was categorized as having a low, medium, or high litter size. For the phenotypic group of the 25% largest litters, which had the greatest mortality, the decrease in piglet mortality was about 6% during the period from 2003 to 2010 (birth years of sows). Similarly, decreases in piglet mortality were also found in the groups of small and medium litters. Similar patterns were observed in the change of phenotypic means.

The simultaneous reduction of mortality in small, medium, and large litters is in contrast to the results of Chen et al. (2010), who found that selection for increasing survival at birth for large litter size might not simultaneously reduce mortality for small litter size. Thus, these authors suggested that the genetic sensitivity of the number of stillbirths to litter size should be considered as a selection criterion to improve piglet survival. On the basis of our results this does not seem necessary, possibly because the mean litter size is relatively high for Danish Landrace and Yorkshire.

The trait of LS5 can be considered a composite of 2 traits related to fertility (TNB) and mortality (MORT), and response to selection for LS5 will be a combination of the 2 traits. Let us assume a mean litter has a total of 13 piglets born and the genetic reduction in mortality is 6%. Then over the period from 2003 to 2009 the mean number of live piglets at d 5 after farrowing was increased by 0.78 piglets because of the reduction of mortality. However, at the same time the genetic gains of LS5 for Landrace and Yorkshire were 1.7 and 2.2 piglets, respectively, indicating that 35% to 45% of the selection response for LS5 was allocated to piglet welfare by reduced mortality, whereas the remaining selection response was allocated to increased fertility described by TNB. The results show that breeding can be a useful tool to increase animal welfare (Kanis et al., 2004). Breeding against mortality to reduce the number of dead piglets in the litter is just one example. The same tools that are used successfully to increase production can also be used to improve animal welfare. However, translation of welfare aspects into a clear breeding goal is not always straightforward. The presented results of the MORT trait provide an explanation of what has occurred, but the selection for TNB and LS5 seems to be the driving factor. Traits such as LS5 evaluated in this paper will help to efficiently breed pigs for welfare-friendly husbandry.

A further selection for reduction in piglet mortality might be achieved in the future by using more sophisticated models that allow for selection for the maternal and the direct genetic components jointly (Knol et al., 2002; Ibáñez-Escriche et al., 2009; Roehe et al., 2009, 2010). In those models the observations are binary responses at the piglet level scored as a live or dead piglet. However, in the present study, a preliminary analysis was conducted for mortality using a generalized linear mixed model considering mortality as a binary distributed trait. The inferences from the models clearly indicated an overdispersion, as the observed number of litters with zero mortality was greater than that predicted by the model. Varona and Sorensen (2010) suggest a hierarchical zero-inflated negative binomial model for the trait of stillbirth. To allow for predicting maternal and direct components jointly and using information from correlated traits, the hierarchical 0-inflated negative binomial model calls for an extension to a generalized linear multitrait model that can handle the covariance structure of direct additive and maternal additive genetic effects for mortality as well as additive genetic effects for other traits. On the other hand, many studies (e.g., Gunsett, 1984; Mather et al., 1988) have reported that a ratio trait is not a good selection criterion because the distribution of data for a ratio trait is undefined and have suggested improving a ratio trait by selection for a linear index that maximizes the correlation between the index and the breeding value of the ratio trait.

Conclusions

The results of this study show that LS5 is a combined trait that has favorable genetic correlations with TNB and piglet survival. Selection for LS5 since 2004 has led to an increase of TNB and a reduction in mortality rate at farrowing and the first 5 d after farrowing in Danish Landrace and Yorkshire nucleus herds.

 

References

Footnotes


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