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The aim of the study was to determine the effect of birth type and sex on lambs survival parameters of polish heath and żelazneńska sheep. The research included data, which was used to determine the prolificacy index, the percentage of live-born lambs and the lambs rearing index up to 100 days of age. A higher prolificacy index (P≤0.01) and a higher percentage (P≤0.01) of reared lambs were determined in the polish heath compared to the żelazneńska sheep. In the herd of the first breed, a greater proportion of twin and more numerous litters than singleton births were noted. Single and twin litters were dominant in the żelazneńskie sheep. The rearing rate of lambs from single litters was at a similar level in both observed breeds. However, in the herd of polish heath sheep, lambs from multiple litters shoved also high rearing rate. Single ram lambs were characterized by higher (P≤0.01) body weight at birth and at 56 days of age compared to twin born and ewe lambs, both in polish heath sheep and żelazneńska sheep. Statistical differences (P≤0.01) in body weight at birth and on 56th day of age were also recorded between twins and triplets in polish heath sheep. The analysis of the distribution of lambs' mortality during the rearing period has shown that the most critical is the first week of life of the born offsprings.

The aim of the study was to compare two mating systems and their impact on reproductive parameters in sheep of the synthetic WROBER line and to evaluate the profitability of lamb meat production. Sheep were raised at the Agricultural Experimental Station in Żelazna (Warsaw University of Life Sciences) in a traditional system with mating once a year and at the Werbkowice Experimental Farm (Institute of Soil Science and Plant Cultivation – National Research Institute) in an intensified system with a shortened lambing interval. The study confirmed the suitability of the WROBER line for intensive production of slaughter lambs. In this system, 0.5 more lambs were obtained per ewe (in all seasons) than in the traditional system. Due to the increased number of lambs produced, the WROBER line can become more profitable.

The aim of the study was to investigate the distribution of prion protein PRNP alleles in flocks of Polish Merino and Suffolk sheep. The research was conducted in 2012-2017 on ewes and rams kept in the Golina Wielka sheepfold (Greater Poland Voivodeship, Poland). All animals (264 ♀ and 64 ♂ Polish Merino; 98 ♀ and 73 ♂ Suffolk) were up to one year old. PRNP gene polymorphism was identified in the sheep. The frequency of scrapie alleles and genotypes was found to be highly significantly influenced by the breed and significantly influenced by the year in the case of Polish Merino sheep. Five alleles (ALRR, ALRQ, ALHQ, AFRQ and VLRQ) were detected in Polish Merino sheep, leading to the identification of 10 PRNP genotypes. In the Suffolk breed, three alleles (ALRR, ALRQ and ALHQ) and three genotypes were identified. In the Polish Merino breed, the frequency of the ALRR/ALRQ genotype was high and was the highest among all genotypes, followed by ALRR/ALRR. The level of genotypes containing valine at codon 136 was very low. In the Suffolk breed, the frequency of the ALRR/ALRR genotype was very high, and there were no alleles with valine at codon 136. In addition, in Polish Merino sheep, phenylalanine at codon 141 was detected only in the AFRQ allele, which appeared in two genotypes (in combination with ALRR and ALRQ).

The short-tailed sheep, native of an area stretching from Russia to Iceland, are generally considered a primitive type. These robust northern sheep seem to have been spread by Norse vikings to several countries in this area from the late eighth century to the middle of the eleventh century ad. They have several common characteristics in addition to the fluke-shaped and tapered short tail, such as a wide range of colour patterns, dual-coated wool and the ability to thrive under harsh environmental conditions, often in isolated marginal areas. While 34 short-tailed breeds of North European origin can still be identified, it is clear that their population sizes have declined in most cases and several of them are now rare and endangered. Although these breeds have mainly been confined to certain localities, some of them have gained considerable distribution due to their genetic merits, such as prolificacy. Of these, the Finnsheep and the Romanov are best known being exported to several countries in the world where their genetic material has been utilized through crossbreeding with local sheep. This has resulted in the production of some new synthetic breeds. Meat is now generally the main product of the North European short-tailed breeds and their crossbreds, whereas wool, skins and milk are normally regarded as byproducts, yet of considerable economic importance in some cases. Such breeds have clearly a role to play in sustainable grassland-based production systems in the future.