PHENOTYPIC AND GENOTYPIC VARIATIONS OF REPRODUCTIVE PERFORMANCE INDICATORS OF SHEEP IN ACCELERATED LAMBING SYSTEM

Hipoteze istraživanja bile su da unutarpasminske varijacije reprodukcijskih pokazatelja u visokoplodnih pasmina ovaca ovise o genetskim i paragenetskim čimbenicima te da reprodukcijski pokazatelji utvrđeni u početku rasplodne aktivnosti mogu biti prediktori za ukupan broj janjadi koji će ovca proizvesti u životu. Ukupno 134 ovce romanovske pasmine, uzgajane u sustavu učestalih janjenja s kontinuiranim razmnožavanjem, analizirane su s obzirom na 742 janjenja. Kao rani reprodukcijski pokazatelji analizirani su dob pri 1. janjenju, veličina 1. legla i trajanje 1. međujanjidbenog razdoblja, a životna proizvodnja ovce iskazana je kao ukupan broj ojanjene janjadi do navršene 4. godine života. Linearni modeli za tumačenje varijabilnosti reprodukcijskih pokazatelja uključivali su različite kombinacije paragenetskih i genetskih čimbenika: mjesec rođenja, pripusnu skupinu, ovna, prvi pripust, mjesec prvog janjenja, tip prvog legla, rodbinski odnos majka – kći, gen za sintezu čimbenika rasta i diferencijacije 9 (GDF9) i gen za sintezu inzulinu sličnog čimbenika rasta 1 (IGF1). Polimorfizam GDF9 i IGF1 gena utvrđen je lančanom reakcijom polimerazom uz uporabu Hha1 i HaeII restrikcijskih endonukleaza. Povezanosti između reprodukcijskih pokazatelja analizirane su fenotipskim i genotipskim korelacijama, a heritabiliteti su procijenjeni regresijom majka – kći. Za odabir optimalnog modela predikcije životne proizvodnje ovaca korišten je Akaikeov informacijski kriterij (AIC). Ovce obuhvaćene istraživanjem do navršene četvrte godine života janjile su se 4 do 6 puta i proizvele prosječno 11,87 janjadi. Prosječna vrijednost utvrđene dobi pri 1. janjenju bila je 373,2 dana, veličine 1. legla 1,98 janjadi, a trajanja 1. međujanjidbenog razdoblja 257,8 dana. Varijacije u veličini 1. legla zadanim su modelom relativno malo objašnjene (R2 = 7,64 %). Za varijacije u dobi pri 1. janjenju utvrđen je najveći doprinos čimbenika mjeseca rođenja ovce i prvog pripusta, a za varijacije u 1. međujanjidbenom razdoblju čimbenika pripusne skupine. Veličina 1. legla i trajanje 1. međujanjidbenog razdoblja najvažniji su izvori varijacija za životnu proizvodnju. Iste se varijable ističu i kao najpouzdaniji prediktori životne proizvodnje, pri čemu za 1. međujanjidbeno razdoblje kao važan izvor varijacija treba uzeti u obzir utjecaj pripusne skupine.INTRODUCTION Sheep reproduction is specific compared to the reproduction of other animal species, which creates unique opportunities and challenges in the production of sheep meat, milk and wool. Different breeds of sheep can manifest a wide range of reproductive performances depending on their genetic background and ecological/production system. High reproductive efficiency of prolific breeds of sheep can be achieved with a large number of lambs per litter and/or short interlambing periods. Reproductive efficiency is most often expressed by multiple indicators that can be registered at different stages of life and production. Within breed variations of reproductive performance indicators are caused by genetic and non-genetic factors. Understanding the sources of variation in reproductive performance indicators contributes to the effective reproduction and population improvement. Given the complex interactions of different factors that to a greater or lesser extent can contribute to the variability of a particular reproductive performance indicator, researchers are looking for optimal models that can explain the variability of a indicator in the best possible way. This research is based on the assumptions that genetic and non-genetic factors contribute to the variability of reproductive performance indicators in prolific breeds of sheep and that early reproductive performance indicators can be used as predictors of lifetime lamb production. MATERIAL AND METHODS The study included 134 ewes of the Romanov breed. The data were derived from the total of 742 lambings. All sheep originated from one farm in mountain part of Croatia. Ewes were kept at the semi-intensive production system. Reproductive management was based on the accelerated lambing with continuous mating. Ewe lambs were first joined with rams at the age of four to eight months. After joining, rams and ewe lambs were kept together permanently. The breeding group consisted of one ram and approximately 35 to 40 ewe lambs. Reproductive performance indicators included in the analysis were divided into early indicators (age at 1st lambing, 1st litter size and 1st interlambing period) and lifetime production expressed as the total number of lambs born within the first four years of ewes’ life. Linear models for the analysis of variability of reproductive performance indicators included following non-genetic and genetic factors: month of birth, mating group, type of first mating, ram, month of first lambing, type of the first litter, mother – daughter relationship, growth differentiation factor 9 gene (GDF9) and insulin-like growth factor 1 (IGF1) gene. Polymorphisms of GDF9 and IGF1 genes were determined by polymerase chain reaction (PCR) followed by restriction with Hha1 and HaeII restriction enzymes. For the analysis of lifetime production, ewes were divided into several groups in regard to the early reproductive performance indicators and birth weight of lambs at 1st lambing. Thus, the model included age at 1st lambing, 1st litter size, 1st interlambing period, GDF9 and IGF1 genotypes, birth weight of lambs at 1st lambing and mother – daughter relationship. All variables included in the model were categorical. The lowest value of Akaike information criterion (AIC) was used to determine the optimal model for the prediction of the lifetime production. Phenotypic and genotypic correlations were calculated, depending on the type of the variable, with Pearson and Spearman rank correlations. Heritability was estimated by mother – daughter regression. RESULTS By the age of four, the examined sheep lambed between 4 and 6 times. The mean age at first lambing was 373.2 ± 34.5 days, with the range from 304.0 to 469.0 days. Non-genetic factors with the greatest influence on this trait were type of first mating (F(2,123)=130.25; P>0.001) and month of birth (F(4,123)=45.72; P>0.001). Significant differences (P0.05) were observed between frequencies of singletons, twins and triplets in ewes divided in groups according to the ram, type of the first mating, month of birth, mother – daughter relationship, GDF9 genotypes and IGF1 genotypes. Model which included previously listed non-genetic and genetic factors explained relatively small proportion of the variance of this indicator (R2=0.0764; F(14,119)=0.703; P=0.768). Phenotypic and genotypic correlation of 1st litter size and 1st interlambing period were 0.19 and 0.04, respectively. Heritability of the 1st litter size was 0.20 (P=0.694). The 1st interlambing period lasted an average of 257.8 ± days, with the range from 196.0 to 393.0 days. The largest contribution to the variations in this indicator had the factor of the breeding group (F(3,121)=19.26; P0.398). The average lifetime production of the investigated sheep was 11.87 ± 2.33 lambs, with the range from 7 to 18 lambs. The model explained 25.49 % of the variations in lifetime production (R2=0.2549; F(16,117)=2.43; P<0.001). The greatest contribution to the individual variations in this indicator had size of the 1st litter (F(2,117)=2.77; P=0.047) and 1st interlambing period (F(2,117)=5.12; P=0.007). Significant differences (P