Efficiency of sheep production is conditioned by fertility. According to some authors number of offspring obtained per lambing is more important than gain of weight. Genetic relationships involving reproductive traits were seldom studied. Reproductive traits have low heritabilities, a discrete phenotypic expression, and are expressed only in sexually mature ewes leading to low selection intensities and long generation intervals. Documentation of realized selection response is also often complicated by the low heritabilities of fertility traits. Existence of a major gene affecting prolificacy had been suggested and at that time there were many sceptics who strongly doubted that a trait as complex as reproduction could be profoundly influenced by a single gene. Major genes affecting prolificacy in sheep was founded. A mutation in the bone morphogenetic protein 15 gene (BMP15, also known as GDF9B) responsible for high prolificacy in Inverdale sheep had been discovered and evidence of segregating major genes was being reported from flocks around the world. Development of gene mapping techniques, and locating alleles that are responsible for the fertility of sheep began a new chapter in predicting and controlling the fertility of sheep. The beneficial effects of nutrition on reproduction in sheep are well known. This procedure is known as flushing. The effect of Body Condition Score (BCS), before mating, during mating and after mating period, on reproductive efficiency of different breeds of sheep in the different rearing systems were studied. The farm manager has the ability to control or at least to manipulate the factors that have an impact on fertility.

The aim of the study was to determine effect of environmental and paragenetic factors on body mass of lambs at birth. Investigation was realised on experimental sheep farm of Institute for Animal Husbandry, Belgrade-Zemun. Animal included in this research were representatives of Mis sheep population. The results of the research confirm that values of the birth mass range from 4.43 kg to 4,58 kg observed by years and 4.48 kg to 4.55 kg depending on the lambing season. Statistical analysis showed that the existing differences still significant on the level (P <0.05). Variations of body mass in lambs depending on the mother's age range in the interval from 4.29 kg to 4.52 kg kg and statistically very significant(P<0.01). Type of birth has also expressed a significant effect on the body mass of lambs at birth and variations range from 4.31 kg (twins) to 4.59 kg (single). Existing differences was statistically very significant (P<0.01). Average body mass in lambs depending on sex of lamb were almost the same, 4.54 kg male and 4.52 kg female, and the difference is not statistically significant (P> 0.05).

The aim of this study was to determine the influence of environmental factors affecting body weight variability of lambs in two crossbreed groups: Pirot x Württemberg and Sjenica x Württemberg. Both populations were managed under the same farm conditions. The data were analyzed to determine the effect of age of the dam, weight of dam, birth type, sex, year and season, on the birth weight and weaning weight of crossbreed lambs. Statistical analysis was performed by GLM procedure using the SPSS statistical package program. The average birth weight of Pirot x Württemberg lambs was 3.56 kg while Sjenica x Württemberg lambs was slightly higher at 3.69 kg. The difference on birth weight between the two crosses was not statistically significant (P>0.05). The average weaning weight of Pirot x Württemberg lambs was 23.54 kg while Sjenica x Württemberg lambs had higher weight at 24.37 kg. The difference of 0.83 kg on weaning weight was statistically significant (P <0.05). Body weight, depending on the environmental factors, ranged from 3.17 to 3.96 kg at birth and from 22.12 to 24.18 kg at weaning in Pirot x Württemberg lambs. Body weight of Sjenica x Württemberg lambs ranged from 3.39 to 3.99 kg at birth and from 22.69 to 25.44 kg at weaning. Statistical analysis showed that the differences were statistically significant (P<0.05) and highly significant (P<0.01).

The Effects of Crossing Balkan and Saanen Goat Breeds on Carcass Traits and Certain Quality Parameters of Kid MeatThe possibility for improvement of carcass traits and quality of kid meat of the autochthonous Balkan goat breed by crossing with Saanen breed was investigated in this study. The trial was carried out on one group of Balkan goat kids and three groups of kid crosses of Balkan and Saanen goats with different proportion of Saanen genes: 25, 50 and 75%. Each group had 16 male kids, which were slaughtered at the average body weight of 18 kg. With the increase in the proportion of Saanen genes, the age of kids that reached preslaughter weight decreased, the chilling loss increased and the proportion of fat tissue (kidney and pelvic fat) in the carcass side decreased (P≤0.05). The crossing also increased the proportion of carcass parts of the first category (leg and loin section), as well as muscle tissue in those parts. The highest proportion of muscle tissue in the thigh (74.91%) and loin section (75.66%) was determined in kids from the group with 75% of Saanen genes, and kids from this group also had the highest proportion of intramuscular fat (2.48%) in samples of m. longissimus dorsi. Slight differences between kid groups were established in indicators of technological meat properties, such as water binding capacity and tenderness, with the increase in the proportion of Saanen genes in the genotype. Sensory score for roasted meat was high, and scores for tenderness and juiciness were slightly higher in kid crosses with 50% and 75% of Saanen genes (P≤0.05). Results presented in this study confirm the positive effect of crossing the Saanen breed with the Balkan breed on carcass traits and for obtaining meat of more desirable quality.

In this paper are described the most important stressors in intensive sheep production on farms and pastures. Stressors on farms and pastures mostly originate from adverse housing conditions, improper zootechnical procedures, veterinary procedures (treatment, vaccinations, blood tests, surgery), animal husbandry procedures (labeling, weaning, grouping, shearing, shortening the tail, hoof care), unsuitable weather conditions (extreme heat and cold) and insufficient nutrition. The paper puts out special emphasis on the consideration of period of life in which sheep were subjected to additional loads and susceptible to effects of these stressors, such as birth, juvenile period, puberty, oestrus, advanced pregnancy, parturition and the puerperal period. [Projekat Ministarstva nauke Republike Srbije, br. TR 31086 i br. TR 31053]