Reproduction in Domestic Animals

Genetic Improvement of Dairy Cow Reproductive Performance 93based measures of fertility have higher heritability thanthe traditional measures of fertility (Royal et al. 2002;Petersson et al. 2007). Veerkamp et al. (1998) suggestedthat selection on days to first luteal activity (CLA) basedon monthly analysis of progesterone in milk may addfurther accuracy to the genetic evaluation for fertility.Van der Lende et al. (2004) suggested to select siresbased on a measure when 50% of the daughters of a sirehad an active corpus luteum (CLA 50%) based on 3- to6-week intervals of progesterone sampling. Peterssonet al. (2007) found that direct selection on progesteronebased measures of fertility may increase the accuracy inthe genetic evaluation for an early start of cyclic ovarianactivity after calving compared to the commonly usedmeasure CFI, even with an infrequent sampling such asin the regular milk recording system. Progesteroneanalysis in the first monthly collected milk samplescould also be used as a management tool. Peterssonet al. (2008) showed that four out of five cows withdelayed cyclicity could be predicted within 60 days aftercalving enabling an earlier treatment. The cost forprogesterone analysis of milk samples could thereforebenefit from management returns as well as fromimprovements in genetic gain for fertility.Automization of recordingsAutomatic milking systems, e.g. robotic milking systemsmay allow automization of recordings. Commercialsystems for on-line recordings allowing herd-profiles offertility (e.g. based on progesterone analysis) and healthparameters are underway (Friggens and Løvendahl2007). These may offer recordings with a high accuracythat could be built into selection programmes. Løvendahland Chagunda (2006) used activity meters forautomatic heat controls and estimated a heritability of0.17 for this trait.Juvenile predictorsThe genes controlling fertility are present and potentiallyexpressed early in life, but it takes at least four yearsbefore a bull has milking daughters and can be progenytested for fertility. Thus early fertility predictors would bevery valuable. These could be reproductive hormones ormetabolic traits. Hayhurst et al. (2007b) suggested apossibility of using the correlation between the prepubertalresponse to gonadotrophin releasing hormone inbull calves and the fertility of their daughters as a possibleselection tool. In another study by Hayhurst et al.(2007c), it was suggested that selection for bull calveswith lower concentrations of glucose and FFA couldresult in female offspring with genetically better fertility.Gene mapping studiesThe identification of quantitative trait loci (QTL) is afirst step towards novel selection methods based on bothphenotypic and molecular information. Using QTL inselection is most beneficial for low heritability traits, sexlimited traits and traits expressed late in life such asdaughter fertility. Holmberg and Andersson-Eklund(2006) found regions with several QTLs on chromosome9 and 11 for both reproduction and health traits. A QTLfor non-return rate was fine mapped to an interval ofless than 3 cM on chromosome 9 (Holmberg et al.2007). Marker assisted selection (MAS) can be used forpre-selection among full-sibs before progeny testing byaccounting for the Mendelian sampling and also toavoid genetic defects for which there are availablemarkers. For quantitative traits the benefit from MAS islimited by the proportion of the genetic varianceexplained by known QTL. The marker density in theQTL region can be increased by use of single nucleotidepolymorphism (SNP) markers. Single nucleotide polymorphismsare single base-pair differences betweenindividuals within a species and where the differentvariants (most often only two allelic forms) are relativelycommon in a population.Expression profiles of genes regulating reproductionGene expression profiling is a relatively recent toolcontributing to our knowledge about the various processesunderlying reproduction such as the function ofgenes and their products determining the phenotype forreproduction. Beerda and Veerkamp (2006) summarizedgene expression studies related to reproduction in cattle,sheep and swine in a paper at the World Congress onGenetics Applied to Livestock Production in Brazil.They concluded that a vast amount of information hasalready been achieved in this area and as informationwill grow exponentially over the next few years theyunderlined the importance of a major effort in bringingall information together in a ‘broadly accessible ontology’.The need for compiling and analysing largeamounts of molecular data has created a new field ofscience referred to as bioinformatics. An EU integratedproject SABRE was started in 2006, which aims toprovide fundamental knowledge on the genomics andepigenetics on, e.g. reproduction traits in dairy cattle tobe used in selection for improved reproduction efficiency.An update on studies on expression profiles ofgenes regulating dairy cow fertility was given at theInternational conference on fertility in dairy cows inLiverpool, 2007 (Beerda and Veerkamp 2007).Genomic selection and integration of molecular data intogenetic evaluation programmesGenomics is the study of variation of base pairs in thenucleic acids and genomic selection means using thisinformation in genetic selection programmes. The availabilityof large arrays of SNPs is changing the approachof predicting breeding values from molecular information.Genomic selection (GMAS) uses all markers, orrather haplotypes consisting of a pair of contiguousSNPs, spanning the genome for prediction of breedingvalues. Thus, by summing the effects of all markerhaplotypes in the genome of a bull calf, a breeding valueis obtained directly at birth whereby the generationinterval can be considerably shortened. This informationcan be used directly, hence theoretically no progenytesting is needed. Muir (2007) showed that by usingGMAS for traits of high (0.5) or low heritability (0.1) theaccuracy of selection increased between 10% and 30%.Ó 2008 The Author. Journal compilation Ó 2008 Blackwell Verlag