The aim of the study was to establish the influence of the month and the year of birth of gilts from the hybrid combination 'Y' ((Tai Zumu x Landrace) x Large White), as well as of the age of the first insemination on the age of the first mating, the first farrowing, the duration of pregnancy, and the total number of born piglets. The experimental animals were divided into three groups depending on the age of the first insemination (by the 249 th day (n=71), from the 250 th to the 271 st (n=42), and after the 272 nd one (n=20). The average age of the first insemination and the first mating was respectively 258.8±0.78 and 276.71±3.54 days, on average. The age of the first farrowing was 391.56±3.54 on average, and the duration of pregnancy-115.03±0.2 days. The total number of born piglets in a litter was 11.87±0.32 pigs on average. The age of the first insemination has a reliable influence on the age of the first mating and the first farrowing (p<0.001), as well as on the number of born piglets (p<0.05). The month (p<0.001) and the year of birth of the gilt pigs (p<0.05) have reliable influence on the age of the first insemination.

The purpose of this study was to establish the influence of the boar and the age category of sows as factors for the fertility of hybrid sows ((Tai Zumu x Landrace) x Large White). The mother is a reliable source of variation on the number of the alive pigs (p<0.05) and the total number of the delivered ones in a farrow (p<0.01). The difference between the gilts and the sows is significant in the number of the alive and the total number of pigs in a farrow (p<0.05). The total number of delivered pigs from the sows are within the range of 10.6±0.4 to 11.4±1.2, while with the gilts ones they are from 8.9±0.6 to 11±1.8. High positive correlation was established between the live-born pigs and the total number of pigs born by gilts (rp= 0.858 р<0.01), as well as with the next parity (rp= from 0.702 to 0.861, р<0.01). The connection between the stillborn and the total number of born pigs, we reported a moderate and reliable correlation, both with the gilts (rp= 0.381, р<0.01) and the sows (rp = from 0.408 to 0.584, р<0.01

An evaluation was made of the effectiveness elements of the pig breeding industry in Bulgaria in the period 2001-2016, through cluster analysis. The studied period was divided in 3 subperiods, each one with three similar groups (clusters). Through application of cluster analysis, the proximity of the different administration regions in the country was defined in accordance with certain indicators of the pig breeding effectiveness.

The aim of the study was to establish correlation and regression dependencies between some traits of sperm production of terminal boars (Large White x Pietrain). The analyzed material consisted of 347 ejaculates, received in the period from 2011 to 2014. The boars were divided in three groups according to the age the ejaculate was obtained at (up to 12 months, from 12 to 24 months, and above 36 months). A moderate and negative correlation was established (r p =-0.34, p<0.001) between the volume of the ejaculate and the concentration of the sperms in the second age group. With the youngest boars we established poor and positive correlation (r p =0.29, p<0.05) between the volume of the semen and the motility of the sperm. The correlations between the traits concentration and motility are poor in the third (r p =0.18, p<0.05), and moderate in the second age group (r p =0.49, p<0.001). Between the motility from one side and the agglutinated and dead sperms from the other, they are within the range of r p =-0.46 to r p =-0.87, p<0.001. The correlations between the agglutinated and dead sperms are positive, from moderate to high and reliable (p<0.001) in all three age classes. The regression trend between the traits of sperm production are almost the same in all three age groups with the exception of volume of the ejaculate to the concentration of sperms.