EVALUATION OF BREEDING ACTIVITIES OF A MINK FARM

Iwona Rozempolska-Rucińska, Grażyna Jeżewska-Witkowska, Grzegorz Zięba

ABSTRACT

Selection, one of the more important stages in fur animal breeding, is based on the phenotype of the animals only. Although a phenotype-based evaluation is the quickest and the simplest method that can be carried out in an on-farm environment, the results of such an evaluation, however, remain reliable exclusively in relation to highly heritable traits. The material comprised data collected from the breeding documentation of a standard mink reproduction farm and referred to 12 generations of animals. The covariance components of reproduction and conformation traits were estimated by means of the REML method based on a multitrait animal model, using the DMU software package. The components were used for a BLUP-based estimation of the breeding value of the animals. The genetic trends considerably differed from the phenotypic ones. Despite the fact that selection was carried out both for reproduction and conformation traits, only body size and conformation as well as fur quality were the trait

Key words: mink, breeding value, genetic trends, maternal effect.

INTRODUCTION

Application of present variability and its conversion to possibly greatest genetic progress is the goal of breeding and selection in all animals, obviously including the mink. Unfortunately, in fur animal breeding, the selection, one of the more important stages in breeding, is performed only basing on the phenotype of the animals, which – with the varied genetic variability, varied estimation repeatability, and accompanied by changing fashion – makes genetic progress difficult to achieve [6]. Thus, selection is performed within the herd, basing on litter size, pedigree, and conformation evaluation results, which an assumption that the order by phenotype reflects the order by genotype. Although phenotype-based evaluation is the quickest and the simplest method that can be carried out in an on-farm environment, the results of such an evaluation, however, remain reliable only in respect to highly heritable traits [1].

The aim of the study was to evaluate the effectiveness of breeding activities carried out on a mink farm.

MATERIALS AND METHODS

The material comprised the data on 12 generations of animals collected from the breeding documentation of a standard mink reproduction farm, managed during 1986-1997. The information on mink reproduction was collected from 7 376 breeding stock females (12 455 litters), whereas that on conformation from 10 163 animals (3 603 males and 6 560 females). During the analysed period, two different conformation standards were valid. Due to this fact, the evaluation results were standardised according to the present evaluation standard [7]. The scores were changed for each individual trait, converting a score from 30-point scale into the respective value on a 20-point scale. Length and density of hair, evaluated on a 30-point scale, was now termed “pelage quality”. The presented studies have not included one of the pelage traits, i.e. the colour type. In relation to standard mink, the animals achieve maximum score for this trait (no variability precluded any analyses).

The mean value of all the analysed traits, determined for subsequent years of the study, was used to estimate phenotypic trends, which represent an index of changes in the performance of the studies mink population. Reproduction and conformation traits covariance components were estimated by means of the REML method based on a multitrait animal model, using the DMU software package [3]. Table 1 presents the factors included in genetic analyses of individual traits. The components of variance and covariance were used for a BLUP-based estimation of the breeding value of the animals [3], in order to estimate:

1. Genetic trends of performance traits. The genetic progress was analysed for 16 generations of mink – the animals managed during 1980-1986 being the base generation – the breeding value of which was estimated based exclusively on the relationship matrix.

2. Genetic trends of additive maternal effect on individual traits.

Linear regression coefficients on birth year represented the measure of the trends. The coefficients, their significance, and standard errors where calculated using the SAS package [5].

3. Frequency of negative breeding value animals in relation to base animals.

4. Spearman's rank correlations between the performance and breeding value of the animals.

The following terms have been used in this report:

• Litter size at birth and at weaning – in the previous case, this term has been referred to the number of newborn pups found on the moment of nest post-whelping control (until 3 days after birth), the latter term means the number of pups on the moment of weaning (about 42 days of age).

• Total reproduction traits – the sum of the animal values for the reproduction parameters (number of born and raised offspring).

• Total conformation traits – the sum of the animal values for the conformation parameters (size and structure of the body, colour purity, pelage quality).

• Total value of reproduction and conformation traits – the sum of the animal values for the reproduction and conformation traits. No indices were applied in order to preserve the animal selection system applied on the farms, despite the fact that equal treatment of all the mentioned traits is certainly inadequate.

Depending on the breeding objective of the farm, individual traits should be assigned appropriate weighs.

A genetic total value of an animal was composed of the sum of the additive values for the given traits, whereas the phenotypic total value – analogical indices of performance value.

RESULTS AND DISCUSSION

The values of conformation traits of the animals remained at a similar level over the entire studied period (Fig. 1 and 2). Higher differences were found in the traits related to reproduction. In this case, the trend line was characterised by lower equality and showed considerable differences between the years. The level of these traits requires improvement. This fact is particularly clear in the picture of genetic trends (Fig. 3 and 4). Genetic trends differed considerably from the phenotypic trends. A disturbing effect was that the trend line for the reproduction-related traits in mink declined (Fig. 4). Despite variations in the breeding value, the general genetic trend for these traits shows a clear decline, confirmed with statistically highly significant negative regressions (Table 2). Particularly adverse changes have been recorded since 1989, when the breeding val ue of the animals was characterised by negative values compared to the base. This demonstrates low effectiveness of the selection, whereas increasing phenotypic trends may have resulted from improvement of the animals' environment.

Years

Years

Years

Years

As to the body size and conformation, as well as the pelage quality, distinct changes in the breeding value have been found which occurred over the studied period. In contrast to equalised lines of phenotypic trends, genetic trends are of a considerably “ragged” pattern (Fig. 3). The best genetic changes were those related to pelage colour purity, which may have resulted from the highest heritability of this trait in relation to the other traits [4]. The breeding value increased by 0.0013 points per year (Table 2). A slight increase in the genetic value of the animals in the case of the conformation traits and reduced reproduction-related breeding value demonstrate a necessity of changing the system of selecting parents for the next generation. In the analysed population if mink, selection was carried out according to the rules commonly observed on fur animal farms. The original litter size of an individual animal represented the first selection cri terion followed by its conformation evaluation score. The breeding stock was thus composed of animals derived from the largest litters and those of the best phenotypic traits. Despite the fact that the selection was carried out both for reproduction and conformation traits, only body size and conformation as well as fur quality were the traits that had been slightly improved. Improvement of animal performance trait basing on the phenotype proved difficult or impossible in this particular case. Low selection effectiveness based on the assumptions mentioned earlier has been confirmed by Socha [6], where the author also indicates to improper methods of evaluation and selection of animals to the breeding stock.

A genetic trends were also estimated in our studies for the additive maternal effect on reproduction and conformation traits of the animals (Fig. 5 and 6). Presented results have demonstrated that the breeding and selection led to improved maternal traits, particularly in relation to reproduction parameters (Fig. 5). Despite the fact that selection did not include directly the additive maternal effect, the way it was performed affected the parameter. Selecting parents of the next generation favours the animals derived from large litters, from dams characterised by well-developed maternal care instinct, and of high milk productivity. Such females are the only ones able to raise numerous offspring in good health. Such a selection system may underlie the pattern of trend lines for the analysed traits. Over the subsequent years, the additive maternal effect increased by 0.0074 per year in relation to born offspring and by 0.0107 in the case of the weaned offspring number (Table 2). Despite their low levels, the regressions proved statistically highly significant. Maternal trend lines for the conformation traits did not show such clear growth (Fig. 6). These trends can be found primarily for body size and conformation of the animals. Increase in the maternal effect over a year was in this case 0.0004 points (statistically significant) and may have resulted form the response to selection for increased body weight. Selecting animals of an appropriate size to feed the breeding stock, a selection was also carried out for improved additive maternal effect.

Years

Years

The results demonstrate that phenotype-based selection system led indirectly to improvement of additive maternal effect, particularly for the reproduction parameters, and (to a lesser extent) body size and conformation. This is a positive effect since the results of raising and condition of the pups to a large extent remain under maternal influence.

Despite the observed improvement of additive maternal effect on most of the analysed traits, the system of selection applied in fur animal breeding should be changed. This need may be confirmed by the values of rank correlations presented in Table 3. In relation to the reproduction traits, these correlations were close to zero and were -0.019 (litter size at birth) and -0.035 (litter size at weaning). This means that ordering the animals according to phenotypic value do not reflect such ordering by breeding value. The rank correlations for the remaining traits were equally low (from -0.016 to -0.394), which indicates that a breeder makes a major mistake using the performance of an animal as a selection criterion. Also, the negative value of these parameters may demonstrate a low precision of the conformation appraisal.

In the case of pelage quality, rank correlations were of the highest negative value (-0.394). When total value of reproduction and conformation was the selection criterion, the correlations were -0.203. It seems that the correlations were chiefly shaped by the conformation traits. Rank correlations for the total conformation score were -0.268, while for the total reproduction score only -0.099.

Phenotype-based selection led to feeding the breeding stock with a large percentage of animals of negative genetic value in relation to the base generation (Table 4). Amongst the animals considered phenotypic best individuals, as much as about 50% were characterised with a low breeding value, thus hampering genetic improvement of the population. Higher percentage of animals of negative breeding value in relation the base generation concerned the reproduction parameters (55.99%). This are reflected by the discussed earlier genetic trend lines. Lack of a clear genetic progress may have resulted from a large frequency of animals that reduced the population's breeding value. At the same time, selection of best animals based on phenotype is infeasible in the situation when no clear associations can be found between their performance and breeding values. Considerably different ordering of individuals according to phenotypic and breeding values may explain the high rate of th e animals, which – despite apparently low genetic merits – were included into the breeding stock.

The results demonstrate that the breeding practice applied on Polish farms needs to be changed. Low or very low rank correlations demonstrate that genetic improvement of the animals may be performed basing exclusively on the breeding value. This should also result on improved performance traits. It has been confirmed by observations on Scandinavian farms, where breeding-value-based selection has contributed to improved reproduction parameters of the populations evaluated this way [2].

CONCLUSIONS

1. Phenotype-based selection led to improvement of additive maternal effects, particularly in relation to reproduction parameters and body size and conformation.

2. Genetic improvement of mink, carried out basing on phenotype ranking, proved ineffective for reproduction and conformation traits, which is demonstrated by both direction of genetic trends and considerably different order of the animals according to their performance and breeding values.

3. A necessity has been proposed for introducing changes in the breeding and selection system presently applied on mink farm. Selection of animals to the breeding stock should be based on their breeding value, not only on their phenotype, which is the case today.

The study was carried out within a research project financed by the Ministry of Scientific Research and Information Technology, grant no. 3P06D 003 23.

REFERENCES

1. Falconer D.S., 1989. Introduction to quantitative genetics. Longman Scientific & Technical.

2. Lohi O., 1993. Reproduction results - Reproduction problems and future challenges for research with fur animals. Zesz. Nauk. Prz. Hod. 12, 19-25.

3. Madsen P., Jensen J. 2000. A users guide to DMU - a package for analysing multivariate mixed models. Version 6, release 4. Danish Institute of Agricultural Sciences.

4. Rozempolska-Rucińska I. 2003. Genetic factors of mink utility and functional traits. (w druku).

5. SAS institute INC. SAS Users Guide. Version 6.12 Edition, SPs Institute INC. Cary NC., 1996.

6. Socha S., 1995. Wyniki pracy hodowlanej nad lisami polarnymi na przykładzie fermy reprodukcyjnej [Results of breeding activities on polar foxes exemplified with reproduction farm]. Zesz. Nauk. Prz. Hod. 21, 27-53 [in Polish].

7. Wzorzec oceny pokroju norek. CSHZ. 1997. [Standard for mink performance evaluation]. Central Animal Breeding Office, Warszawa [in Polish].

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’ in each series and hyperlinked to the article.