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Aim. To determine the factors, affecting compensatory growth and performance of the Ukrainian meat (UM) pig breed. To analyze the impact of selection traits on the live weight of pigs during different age periods, using several growth and development indices. To determine compensatory growth by two groups of piglets (based on their individual weight, which was above the average mean piglet weight in the litter (M+) or with a weight which was below average mean piglet weight in the litter (M–), in relation to the average daily gain, ADG), in order to use these factors in a specific breeding program. Methods. Uniform microclimate conditions to rear experimental piglets were maintained using Eletor SC-12 (Poland) equipment. When selecting animals for research groups, physiological conditions were determined (by direct observation), age (according to primary zootechnical records), and live weight weighing on electronic scales (Axis (Ukraine) with a measurement accuracy of 0.02 Kg. The basis of our research was the live weight of pigs of Ukrainian meat breed, n = 381 animals. First two groups of piglets were formed (M+; M–) n = 143(М+); n = 158(М–); based on their individual weight, which was above the average mean piglet weight in the litter (M+) or with a weight which was below average mean piglet weight in the litter (M–). Furthermore, two groups were later formed based on presence or absence of compensatory growth (M++, M+–, M–+ and M– –) n = 66(M++), n = 77(M+–), n = 68(M–+) and n = 90 (M– –). Results. The group M++ at the stage of rearing, at the age of 2–6 months, exhibited superior average daily gains by 22.2 % (P < 0.001) during the period from 2 to 4 months and by 8.8 % (P < 0.01) during the period from 4 to 6 months as compared to the other groups. An ANOVA analysis showed that the changes in weight gain of compensatory growth from 60 to 120 days affects the live weight of pigs at the age of 3–8 months (P < 0.001). The variability of piglet mean live weight in a litter at 60 days influenced the live weight of pigs at the age of 3–7 months (P < 0.001) and at the age of 8 months (P < 0.05), while the interaction between these two factors affects the live weight of piglets at the age of 3–5 months (P < 0.001) and 6 months (P < 0.05). Conclusions. New data have been obtained regarding the impact of piglet weight above or below the average mean piglet weight in the litter and the degree of compensatory growth in Ukrainian meat breed pigs on their average daily gains. Animals from group M–+ at 60 days of age, in the presence of compensatory growth, still outperformed their counterparts from group M+– at 60 days of age by 22.2 % (P < 0.001) during the period from 2 to 4 months and by 8.8 % (P < 0.01) during the period from 4 to 6 months, when not exhibiting compensatory growth. The influence of the aforementioned factors was also determined on the growth rate from 2 to 6 months, with the growth rate index in the M++ group being 1.81 times higher than in the M+– group and 1.54 times higher than in the M–+ group. The highest impact of litter composition on the average daily gain (ADG) in weight was observed at the age of 2–4 months (20.5 %; P = 4.2*10–12). Group compositions towards weight above piglet average weight in the litter and compensatory growth (M++) have been shown to be useful as selection and breeding criteria for the Ukrainian meat pig breed and are possibly so for other pig breeds, which will be investigated in future.

An increase in the productivity of animals is associated with a general improvement of the population, which largely depends on the accuracy of the genotype assessment. The most accurate estimates can be obtained with the help of modern indices: evaluation and selection. At the same time, it is important to calculate the weight coefficients of the traits included in the selection index structure, separately for each herd (breed). Evaluation by indices allows selecting the best animals in the herd for further use. According to the results of the evaluation of the reproductive qualities of sows in crossbreeding using evaluation and selection indices, the superiority of large white breed sows covered with boars of the landrace breed was established, and the lowest among pigs of the combinations ♀P×♂D and ♀D×♂P. At the stage of studying the growth dynamics of pigs, we studied the patterns of individual development of animals in ontogenesis according to criteria that would characterize growth indicators and allow us to determine the type of their formation. Animals of the ♀D×♂P group outperformed their counterparts and other experimental groups. Animals of the ♀P×♂D and ♀D×♂P groups had the highest index of growth uniformity. At the same time, the control group and the combination ♀VB×♂L were inferior to them. The superiority of pigs in terms of growth intensity of the ♀D×♂P group shows the high energy of the formation of animals, they reached slaughter conditions faster under the same conditions of keeping and feeding compared to other groups. Animals of the combination ♀P×♂D were characterized by the most uniform growth at this stage. During the fattening period, the superiority of the offspring of the combination ♀D×♂P in terms of the index of fattening qualities was preserved. In order to select highly productive individuals, for further use in crossbreeding, we calculated selection indices for evaluating gilts by reproductive capacity, for evaluating the fattening and fattening and meat qualities of the offspring. Keywords: selection index, evaluation index, selection, crossbreeding, Landrace, Duroc, Pietren, Large White breed, pigs.