Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Discourses mapped by Q-method show governance constraints motivate landscape approaches in Indonesia
    Langston, James Douglas ; McIntyre, Rowan ; Falconer, Keith ; Sunderland, Terry ; Noordwijk, Meine Van; Boedhihartono, Agni Klintuni - \ 2019
    PLoS ONE 14 (2019)1. - ISSN 1932-6203

    Interpreting discourses among implementers of what is termed a “landscape approach” enables us to learn from their experience to improve conservation and development outcomes. We use Q-methodology to explore the perspectives of a group of experts in the landscape approach, both from academic and implementation fields, on what hinderances are in place to the realisation of achieving sustainable landscape management in Indonesia. The results show that, at a generic level, “corruption” and “lack of transparency and accountability” rank as the greatest constraints on landscape functionality. Biophysical factors, such as topography and climate change, rank as the least constraining factors. When participants considered a landscape with which they were most familiar, the results changed: the rapid change of regulations, limited local human capacity and inaccessible data on economic risks increased, while the inadequacy of democratic institutions, “overlapping laws” and “corruption” decreased. The difference indicates some fine-tuning of generic perceptions to the local context and may also reflect different views on what is achievable for landscape approach practitioners. Overall, approximately 55% of variance is accounted for by five discourse factors for each trial. Four overlapped and two discourses were discrete enough to merit different discourse labels. We labelled the discourses (1) social exclusionists, (2) state view, (3) community view, (4) integrationists, (5) democrats, and (6) neoliberals. Each discourse contains elements actionable at the landscape scale, as well as exogenous issues that originate at national and global scales. Actionable elements that could contribute to improving governance included trust building, clarified resource rights and responsibilities, and inclusive representation in management. The landscape sustainability discourses studied here suggests that landscape approach “learners” must focus on ways to remedy poor governance if they are to achieve sustainability and multi-functionality.

    Use of models for the environmental risk assessment of veterinary medicines in European aquaculture: Current situation and future perspectives
    Rico, Andreu ; Vighi, Marco ; Brink, Paul J. van den; Horst, Mechteld ter; Macken, Ailbhe ; Lillicrap, Adam ; Falconer, Lynne ; Telfer, Trevor C. - \ 2019
    Reviews in Aquaculture 11 (2019)4. - ISSN 1753-5123 - p. 969 - 988.
    Antimicrobials - Antiparasitics - Aquaculture - Environmental models - Environmental risk assessment

    Veterinary Medicinal Products (VMPs) are used in intensive aquaculture production to treat a wide range of bacterial and parasitic infestations. Their release into the environment poses concerns regarding their potential ecotoxicological risks to aquatic ecosystems, which need to be evaluated making use of appropriate Environmental Risk Assessment (ERA) schemes and models. This study presents an overview of the major aquaculture production systems in Europe, the VMPs most commonly used, and the environmental quality standards and regulatory procedures available for their ERA. Furthermore, it describes the state-of-the-art on the development of environmental models capable of assessing the fate, exposure, ecotoxicological effects and risks of VMPs in aquaculture production systems, and discusses their level of development and implementation within European aquaculture. This study shows that the use of environmental models in regulatory ERA is somewhat limited in many European countries. Major efforts have been dedicated to assess the fate and exposure of antiparasitic compounds in salmonid cage systems, particularly in Scotland, while models and scenarios for assessing dispersal of antimicrobials, in general, and antiparasitic compounds in the Mediterranean as well as in Scandinavian regions are less available. On the other hand, the use of ecological models for assessing the effects and risks of VMPs is almost absent. Recommendations are provided to improve the chemical exposure and effect assessments and the ecological realism of the modelling outcomes, paying special attention to the protection goals set for the regulatory ERA of VMPs in Europe.

    Genetic analysis of production, immunity and behaviour in laying hens
    Biscarini, F. - \ 2010
    Wageningen University. Promotor(en): Johan van Arendonk, co-promotor(en): Jan van der Poel; Henk Bovenhuis. - [S.l. : S.n. - ISBN 9789085857860 - 132
    genetische analyse - hennen - eierproductie - immuniteit - diergedrag - dierveredeling - fokwaarde - heritability - immuniteitsreactie - single nucleotide polymorphism - genetic analysis - hens - egg production - immunity - animal behaviour - animal breeding - breeding value - heritability - immune response - single nucleotide polymorphism
    The new regulations about the husbandry of laying hens and the so-called genomic revolution offer both opportunities and challenges for the breeding of layers. Hens are currently housed mainly in battery cages of 4 individuals each. Following recent developments of the communitarian legislation, many countries will soon adopt furnished cages or non-cage systems, which will lead to larger groups of hens. Also, beak-trimming will be prohibited in EU countries in the near future. Advancements in sequencing technology are making an always greater number of genetic markers available at increasingly cheaper prices, making genome-wide studies possible and helping geneticists to start unraveling the mystery of the genetic make-up of animals, which until a few years ago was considered a black-box. This thesis touches upon the impact of such innovations on the breeding of laying hens.

    Use of pooled data in the genetic evaluation of laying hens
    Hens are usually housed in cages and therefore pooled instead of individual egg records are often available: a pooled egg record is the total production of a cage, when the egg production of the individual hens is unknown. Current selection schemes are carried out in nucleus herds where hens are housed individually, so that egg production of individual birds can be recorded and used for genetic evaluations. Based on this information sires and dams are selected. Such a selection scheme based on individually housed hens introduces a discrepancy between the environment where hens are selected and the environment in which hens are kept for commercial egg production (group housing). Selecting animals in one environment and using them in a different environment might lead to genotype x environment interaction (Besbes and Ducroq, 2003), thereby reducing the realized response to selection. Future husbandry conditions, with larger groups of hens or hens housed in furnished cages might make this problem even worse. A method to use pooled data in the genetic evaluation of laying hens would therefore be of interest. In Chapters 2 and 3 of this thesis it is described how to use pooled records for the estimation of heritability and breeding values. In chapter 2 the use of individual and pooled observations is compared. Individual body weights of hens at different ages were available: these were then pooled by cage in order to create pooled records. Heritabilities estimated from pooled and individual data correlated well: the standard error of estimates based on pooled records was however about twice that of estimates based on individual records. The accuracy of EBVs from pooled data is lower than the accuracy of EBVs from individual data; in the case of sires with at least 10 offspring the reduction in accuracy was about 23%. This loss of precision in estimating genetic parameters and breeding values is understandable considering that pooled records are a less detailed of information. However, this lower accuracy should be interpreted in the context of direct vs indirect selection. The breeding goal is the trait under commercial conditions (group housing), and if testing is under individual housing, the genetic correlation between group and individual housing is relevant. The ratio of the selection response for direct and indirect selection is a function of the accuracies for both situations, the standard deviations of the traits and the genetic correlation between the traits (Falconer, 1989). Similarly, the ratio between accuracies based on pooled and individual data provides a threshold for the genetic correlation between individual and group housing below which pooled data would result in a greater selection response. In practical breeding also the costs of individual housing relative to the costs of group housing are relevant. Since group housing is cheaper than individual housing, more selection candidates could be tested for the same level of costs. This would in turn result in higher selection intensity and larger response to selection.
    In chapter 3 the method of analyzing pooled data developed in chapter 2 was compared with an approximation consisting in assigning cage means to each hen in a cage, then treating them as individual observations. Cross-validation was used to compare the two methods: the method developed in Chapter 2 performed consistently better than the approximate method in terms of predicting ability.
    In the general discussion, finally, it was described how to estimate genetic and phenotypic correlations from pooled data.

    Across-line association studies for immune response and feather pecking behaviour
    The great number of genetic markers available at increasingly lower prices has been fostering developments in genomic research. Association studies between genetic markers and phenotypes are typically conducted within populations (breeds, or lines): the amount of LD conserved in a population is exploited using high marker density, such as SNP chips, and markers relatively close to QTLs are expected to show significant effects in association studies. In this thesis we propose to take it one step further and perform association studies across lines. This requires higher marker density but increases the resolution. The amount of LD conserved across lines is expected to be lower than within lines and the phase of the marker-phenotype association might be different in the different lines. On the other hand markers that happen to show significant effects in an across-line association study are likely to be close to the QTL. These issues in conducting marker-phenotype association studies across populations were addressed in Chapters 4 and 5 of this thesis, where it was shown how to deal with multiple populations when analyzing hens from 9 different genetic lines of White Leghorn and Rhode Island Red origin genotyped for a panel of 1536 SNP (Single Nucleotide Polymorphism) markers.
    The traits analysed were immunological parameters and plumage damage due to feather pecking behaviour, two classes of traits for which, given that they have relatively low heritability and are difficult and expensive to measure, genomic information may be particularly valuable. Immunological parameters might be used in selection programmes aimed at improving disease resistance of laying hens, while information on the genetic background of feather pecking behaviour can be useful in reducing problems due to this behavioural disorder of layers. Under future husbandry conditions susceptibility to infectious diseases and feather pecking are expected to become more serious problems: both aspects of layer production are in fact related to the number of individuals that interact with each other, which will increase as a result of the application of the EU directive 1999/74/EC. In addition, the ban of beak-trimming will make it more difficult to control the consequences of feather pecking (plumage damage, cannibalism, mortality). Genetic selection might represent an appealing addition to the current control measures. The association studies identified several regions of interest. The gene for interleukin 17 (IL17), on chromosome 3, was found to be associated with natural and acquired antibody titres, and with the classical and alternative pathways of complement activation. The major histocompatibility complex (MHC) genes on chromosome 16 showed significant association with natural and acquired antibody titres and classical complement activity. The interleukin 12B gene (IL12B) on chromosome 13 was associated with natural antibody titres. As for feather pecking behaviour, a role of the gene for the serotonin receptor 2C (HTR2C) on chromosome 4 was found. This supports existing evidence of a prominent involvement of the serotonergic system in the modulation of this behavioural disorder in laying hens. The genes for IL9, IL4, CCL4 and NFKB were found to be associated to plumage condition, revealing relationships between the immune system and behaviour.

    Guidelines for spatial distribution of ecotopes, based on ecological networks and geomorphological boundary conditions
    Rooij, S.A.M. van; Liefveld, W.M. ; Maas, G.J. - \ 2001
    In: River basin management. Southampton (UK), WIT, 2001. Progr. Water Resourc. 15 / Falconer, R.A., Blain, W.R., - p. 261 - 269.
    geomorfologie - landschapsecologie - rivierkunde - Maas
    The arable crops regime and the use of pesticides
    Falconer, K. ; Oskam, A.J. - \ 2000
    In: CAP regimes and the European countryside / Brouwer, F.M., Low, P., - p. 87 - 102.
    Induction and characterization of micronuclei in plant cells : perspectives for micronucleus-mediated chromosome transfer = Inductie en karakterisering van microkernen in plantecellen : perspectieven voor chromosoom overdracht via microkernen
    Verhoeven, H.A. - \ 1989
    Agricultural University. Promotor(en): A. van Kammen; B. de Groot. - S.l. : Verhoeven - 117
    cytologie - plantenfysiologie - genetische modificatie - recombinant dna - celfysiologie - cytology - plant physiology - genetic engineering - recombinant dna - cell physiology

    In this thesis, micronucleation in plant cells has been investigated and systems for isolation and transfer of organelles have been established.
    The discovery, described in chapter two, that the phosphoric amide herbicide amiprophos-methyl induced micronuclei at a high frequency in cell suspensions of N.plumbaginifolia, has opened the possibility to develop a microcell-mediated chromosome transfer system analogous to that in mammalian cell lines. In mammalian cells, micronuclei are induced by prolonged exposure of cells to spindle toxins (colchicine, Colcemid), resulting in up to 60% micronucleated cells (Matsui et al., 1982). Micronucleated cells are isolated by the "shake-off' method, and subjected to high speed centrifugation, which results in fractionation of the cells into microcells, containing micronulei with one or a few chromosomes. Subsequently, microcells are fused to the recipient cells. The transferred chromosomes were found to remain intact and mitotically stable (Fournier, 1982). This technique has hitherto not been available for plant cells or protoplasts, due to the lack of efficient procedures to induce micronuclei. Gamma-irradiation is now often used in the construction of monochromosomal addition lines by somatic hybridization (Bates et al., 1987), to induce chromosome damage which promotes chromosome elimination from one of the fusion partners. As has been pointed out in the introduction (chapter one), ionizing radiation induces chromosome rearrage ments, deletions and insertions (Menczel et al., 1982). From research on mammalian cells, it is known that these phenomena occur with a lower frequency after microcell-mediated chromosome transfer (Fournier, 1981). If microprotoplasts would become available for plant genetic manipulation, transfer of a limited number of chromosomes by microprotoplast fusion would offer an alternative to the use of gamma-irradiation. With the finding that APM induces micronuclei at high frequency in plants, transfer of low numbers of chromosomes after micronucleation can now be tested for use in plant genetic manipulation. The APM treatment was found to be reversible, as was demonstrated by washing the cell suspension cultures free from APM. After washing, normal growth and cell division were soon resumed, with some abnormal, multipolar spindles in the first division after washing. This observation is in good agreement with the the reversible inhibition of microtubule polymerization by APM (Falconer and Seagull, 1987). This low cytotoxicity makes APM a useful tool in the induction of micronuclei in plants.

    The flow cytometric analysis of the nuclear DNA content of APM-treated cel suspension cultures of N.plumbaginifolia, revealed the presence of many micronuclei with a DNA content equivalent to one metaphase chromosome (which consists of both sister chromatids). Similar observations have been made in micronucleated rat kangaroo cells after treatment with Colcemid (Sekiguchi et al., 1978). Sorting of the micronuclei on the basis of the fluorescence of ethidium-bromide, followed by analysis of the DNA content by Feulgen staining (chapter three), shows that it is possible to separate micronuclei on the basis of their DNA content by flowcytometry, like it has been shown for isolated plant metaphase chromosomes. Chromosome identification is sometimes possible with isolated metaphase chromosomes (de Laat and Blaas, 1984; Conia et al., 1987a; 1987b). Identification of chromosomes present in a particular micronucleus is not possible. This is due to different degrees of chromosome decondensation in the micronuclei (which influences the fluorescence signal of the fluorochrome -DNA complex by quenching), and due to the various combinations of chromosomes in micronuclei containing more than one metaphase chromosome. This is illustrated by the DNA histograms of isolated micronuclei in chapter two, which lack the specific chromosome peaks, present in metaphase chromosome preparations (chapter four). When micronuclei are present in large numbers, the overall DNA histogram will show no appreciable contribution of a particular type of chromosome combination in micronuclei, since chromosome grouping appears to be a random process, as was shown by the analysis of the number of micronuclei per cell in chapter two, and by cytological data in chapter two and three. Furthermore, the reduction of the number of micronuclei per micronucleated cell, which appears to be the consequence of fusion of micronuclei into a lobed restitution nucleus, gives rise to even more combinations of chromosomes.

    The processes, involved in the formation of micronuclei, are studied in chapter three and four. The effects of the anti-microtubular herbicides APM, oryzalin and the alkaloid colchicine, used for metaphase arrest and induction of micronuclei in mammalian cells, on the mitotic index and micronueleus formation are compared. The disruption of the spindle by direct inhibition of microtubule assemble is responsible for the accumulation of cells at metaphase. The concentrations of the inhibitors required for complete metaphase arrest, vary from 3 μM for APM and oryzalin to 500 μM for colchicine, as a consequence of differences in binding specificity (Hertel et al., 1980; Dustin 1984). The differences in the percentage of ball metaphases indicate specific effects of the above mentioned inhibitors on chromosome scattering. Apart from the disruption of the microtubules, APM and oryzalin have been shown to influence the accumulation of calcium in the mitochondria (Hertel et al., 1981). Moreover, oryzalin disturbs the active excretion of calcium by the plasma membrane. These combined effects result in an increased cytoplasmic calcium concentration (Hertel et al., 1980), which will be higher after oryzalin treatment than after APM treatment, due to the reduction of active calcium excretion by oryzalin. Our 'data suggest that the APM or oryzalin induced increase of the cytoplasmic calcium concentration is involved in both formation and fusion of micronuclei. Colchicine, which does not influence the cytoplasmic calcium concentration, is not effective in the induction of micronuclei. The higher cytoplasmic calcium levels after oryzalin treatment, would increase the fusogenic properties of the nuclear membranes, which would explain why micronuclei exist for a shorter time after oryzalin treatment as compared to APM treatment. This hypothesis will be tested in future experiments by treatments with the calcium ionophore A23187 in combination with the calcium-specific chelator ethyleneglycolbis- (2-aminoethylether)-N,N'-tetra acetic acid (EGTA), with simultaneous measurements of the cytoplasmic calcium concentrations with the new calcium specific fluorochromes Fluo-3 and Rhod-2 (Haugland, 1989).

    In order to obtain both large numbers of micronucleated cells, and large numbers of micronuclei per micronucleated cell, the
    effect of DNA synthesis inhibitors was investigated. The results in chapter five show, that a considerable increase in the number of micronucleated cells can be achieved by HU or APH treatments, and that the time at which micronuclei appear can be controlled. The results further indicate that metaphases have to be exposed to APM for at least 12h, before micronucleation occurs, and that their lifetime is in the same order. These data demonstrate that it is possible to manipulate the conditions of the treatments in order to obtain either a high yield of metaphase chromosomes, or a high yield of micronuclei, with little contamination by micronuclei or chromosomes, respectively. In this way, it becomes possible to determine the moment at which the number of micronuclei per cell is at its maximal value.

    The isolation and characterization of microprotoplasts from micronucleated suspension cells is described in chapter six. Data obtained from DNA content measurements and flow cytometry demonstrate the presence of up to 40% of subprotoplasts with a DNA content less than the G1-level of the APM treated suspension cells. This indicates that genome fractionation has occurred, and the data on the FDA-staining show that most of the subprotoplasts still possess an intact plasma membrane, since FDA can not be retained by vacuolar membranes only (Lesney, 1986). The viability of the microprotoplasts and other types of subprotoplasts is indicated by the successful culture after gradient fractionation. As it is impossible to measure the DNA-content of microprotoplasts in a non-destructive way, no preselection could be performed to use only microprotoplasts for fusion. In a mass fusion system, the smallest microcells will be the least likely to fuse when electrofusion is used, because their small diameter will prevent alignment and membrane breakdown, which are both related to particle diameter (Zimmermann et al., 1982). Individual selection and fusion could overcome this problem (Koop et al., 1983). This control is essential for the efficient application of microprotoplasts, since the DNA content per microprotoplast will depend upon the DNA content per micronucleus in the cell suspension. Microprotoplast fusion will result in transfer of a part of the total number of chromosomes, directly followed by spontaneous chromosome elimination when two distantly related species are fused, since chromosome elimination seems to be directed by genome dose effects (Graves, 1984; Gilissen et al., 1989). Sofar no successful fusion experiments have been performed, which makes it impossible at the moment to comment on the usefulness of microprotoplasts in chromosome transfer. However, fusion experiments with karyoplasts indicate that it is possible to perform fusions in a controlled way (Spangenberg et al., 1987).

    In addition to the microprotoplast fusion, microinjection was developed for transfer of organelles and micronuclei. Glass needles with a large orifice (5pM) were prepared, along with a pressure system, based on the application of mercury. With the injection system, described in chapter seven, it is possible to suck donor material from a donor protoplast, and inject this directly into the recipient. The data on the complementation of the albino tobacco by injection of mature green chloroplasts or etiolated plastids, indicate that protoplasts can survive the injection treatment, and that the injected plastids can be replicated by the recipient. In this way, the organelles to be transferred are not subjected to damaging isolation procedures and they can be preselected visually. Selective transfer of organelles offers a number of advantages when compared to fusion techniques, or transfer of isolated genes. One of the advantages is the protective nature of the membranes associated with chloroplasts, mitochondria and nuclei. Although structural integrity and functionality has been demonstrated for isolated chloroplasts and mitochondria, it is not known whether isolated organelles are still physiologically intact. The isolation of intact nuclei from plant cells has also been described, with data indicating their structural integrity, as well as their ability to transfer genes into recipient protoplasts (Saxena et al., 1986). Transfer of marker genes does not necessarily implicate the functional integrity of isolated nuclei, since transfer of marker genes can be achieved by uptake of isolated genomic DNA. Preliminary results obtained from experiments with microinjection of micronuclei, indicate that it is possible to remove micronuclei from the donor by suction. Sofar, transfer into a recipient has not been achieved. The kanamycine- resistance, which was introduced into N.plumbaginifolia by transformation with Agrobacterium tumefaciens , will be used as selectable marker after transfer of micronuclei. The transfer of chromosomes will be tested with species specific repetitive DNA probes, which are able to discriminate between the donor genome N.plumbaginifolia and the recipient (either Lycopersicon esculentum or Solanum tuberosum ) . Several probes with the required specificity have already been characterized, from a series of highly repetitive sequences, isolated from N.plumbaginifolia (data not shown).

    With the methods, described in this thesis, the transfer of chromosomes via micronuclei has come within reach. Future work will focus on achieving transfer, and study the fate of the introduced micronuclei. This should provide an answer whether micronuclei can be used as chromosome carriers in plants, as has already been shown in mammalian somatic cell genetics.

    Maternal and genetic influences on production and reproduction traits in pigs
    Steen, H.A.M. van der - \ 1983
    Landbouwhogeschool Wageningen. Promotor(en): R.D. Politiek. - Wageningen : Van der Steen - 112
    dierhouderij - meerlinggeboorten - varkens - polyembryologie - productiviteit - rentabiliteit - selectie - zeugen - superovulatie - animal husbandry - multiple births - pigs - polyembryony - productivity - profitability - selection - sows - superovulation
    The profitability of pig production may be expressed as a function of reproductivity and productivity. The optimal selection pressure on reproductivity relative to productivity depends on the response to selection and the economic value of the response. Reproductive performance is primarily a function of the dam and involves age at first oestrus, conception rate, litter size and the interval between weaning and oestrus. An increase in the litter size would improve the reproductive performance. Mean litter size has been rather constant in most countries over the last decades. This means that there has been no response to selection or no selection pressure on litter size. It could also imply a relatively deterioration in the environment or negative effects of selection for production characteristics. It has not been established whether selection for litter size is worthwhile. The heritability of the trait, possible selection differential and the economic value of litter size are important components determining the response to selection. Of these, heritability seems to be the major limiting factor. Estimates of this factor have been consistently low (~0.10). A negative correlation between direct genetic and maternal effect might reduce the effective heritability or response to selection. A dam may influence her offspring through the environment she provides as well as through the genes transmitted to the offspring. This environmental effect of the dam on her offspring is referred to as maternal influence. The present study was focussed on the effect of the post-natal maternal conditions on the fertility of the daughter.

    Maternal influences are partly due to the size of the litter in which a gilt is raised. Gilts raised in small compared with large litters might produce larger litters. This maternal influence affects h 2estimated by daughter-dam regression but not h 2estimated by paternal half sib analysis.

    It is not clear how important the maternal influence on litter size is. Systematic differences in h 2estimated by daughter-dam or paternal half sib analysis, have not been found. A negative effect of being raised in large litters on the size of the first litter from gilts raised in large litters was reported (table 2.6). So maternal effects might counterbalance the response to selection for litter size.

    The experiments were performed to estimate the effect of standardization level (litter size during the suckling period) on the development of the gilt, age at puberty, size of the first litter and number of corpora lutea after first oestrus of the weaned primiparous sow. The effect of maternal influences on the response to selection was studied by simulation.

    Three generations of Dutch Landrace gilts were reared. The first generation (192 gilts) was purchased and the litters produced were standardized at 8 piglets within 24 hours of birth. Litters produced by the gilts of generation two were standardized at 6 (low level) or 12 piglets (high level). Thus the effect of the standardization level on production and reproduction traits was measured in gilts of generation three. Gilts were inseminated at a fixed age of approximately 255 days.

    Growth of litters from birth to weaning at five weeks of age, expressed per piglet, at the high standardization level was 24% lower than those at the low level. From weaning to 56 days of age the difference was 8%. This resulted in a weight at 56 days of 15.8 and 18.7 kg for gilts raised at the high and low levels respectively. Milk energy intake was ca. 26% lower for piglets raised in large compared with small litters. The intake of milk and creep feed (ME) from 21 days to weaning was reduced by ca. 19% (table 5.1). Relations between intake of milk and creep feed, and growth were stronger at the high than at the low standardization level. Milk production is a limiting factor for growth especially in large litters.

    The significant weight difference between both standardization level groups at 56 and 74 days of age had disappeared at insemination. At a fixed weight (~100 kg) high standardization level gilts deposited more backfat than those standardized at the low level.

    At the high standardization level 20% of the gilts that produced a litter did so as a result of insemination at the first oestrus while at the low level this per centage was 49. This result was obtained although the age at first oestrus and conception rate were not affected by the standardization level, and gilts were inseminated at a fixed age.

    From a total of 111 high and low standardization level gilts which produced a litter, 96 were halothane negative. The estimate of standardization level effect on litter size obtained from halothane negative gilts, after correction for oestrus number at insemination, was -0.48 piglets (high-low; table 5.5). The realized difference in standardization level was 5.4 piglets which resulted in an estimate of the "m-value", defined by Falconer (1965), of -0.09. The coefficient m is the partial regression coefficient of daughters' phenotypic value on mothers' phenotypic value for litter size in the absence of genetic variation among the mothers. This m value can be split into a pre- and a post-natal component m 1 and m 2 (m = m 1 + m 2 ). Uterus weight was higher in low compared with high standardization level sows. The former also tended to have longer uterus horns. A tendency to a higher number of corpora lutea in low than in high standardization level gilts was observed in sows in which oestrus after weaning of the first litter was induced. Results obtained suggest that a low standardization level positively affects weaning weight and development of the uterus. This, in combination with a larger pool of primordial follicles might explain the positive effect on litter size.

    From a simulation study and derived formulae it was concluded that daughter- dam regression coefficients (trait: first litter size) are seriously biased by pre-and post-natal maternal effects while granddaughter-granddam estimates are biased to a much smaller extent (fig. 5.5). Standardization of litters will eliminate the post-natal maternal effect as determined by litter size during the suckling period. Maternal influences affect the response to selection in two ways. Firstly the relation between additive genetic value and phenotypic value will be changed. A value of -0.1 or -0.2 for m reduced the regression coefficient by 5 and 11% respectively. Secondly, maternal effects result in an important permanent negative effect on litter size as, in a selection programme for litter size, the next generation of gilts will be born and raised in large litters. Selection of boars only from the largest litters does not result in a permanent negative effect.

    The reduction of the regression coefficient of additive genetic value to phenotypic value by maternal effects can be eliminated by correction of the data. For this purpose accurate estimates of m 1 and m 2 are needed. It is doubtful whether this correction is worthwhile as the effect on the regression coefficient and thus on the response to selection is small. Standardization of litters will be more efficient as it also removes the permanent negative effect on litter size as far as is determined by post- natal maternal effects. It cannot be concluded from a negative daughter-dam correlation for number born that selection of replacement gilts born and raised in large litters would not bring about desirable genetic changes in litter size. The value of the regression coefficient of additive to phenotypic value is of significance. Over a range of plausible values for m this regression coefficient, and thus genetic change, decreased by 5 to 10% due to maternal effects. So the genetic implications of maternal effects on litter size are limited. Selection will result in an additive genetic response. The permanent negative environmental effect on litter size, if replacement gilts are born and raised in large litters, does reduce the phenotypic value. To a large extent, standardization of litters will remove this, from an economic point of view, unfavourable effect. The response to several generations of selection for litter size might be zero because of this permanent negative environmental effect and/or the limited size of the experiment. Selection for fertility is possible but is not always evident because too few animals have been used. Large scale experiments or lines in which litters are standardized are needed to improve litter size or overall reproductive performance by selection. Points of interest for the future are - assessing the economically important components of overall reproductive performance - achieving accurate estimates of genetic and phenotypic correlations and heritabilities involved - combining the sources of information in an index and - assessing an optimal selection scheme. It may be postulated that the rather constant mean litter size over the last decades is probably caused by a low realized selection pressure. A higher selection pressure in combination with more efficient methods (larger scale experiments, standardization of litters, accurate data collection, combining sources of information) practized over a period of 10 years or more will result in a response to selection which is of economic interest.

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