Abstract. The objective of this study was to determine the effect of the body condition of late pregnant sows on fat reserves at farrowing and weaning, including the scale of the relevant changes in that period, and on the reproductive performance of sows and the results of piglet rearing.

The experiment involved 97 hybrid sows (Polish Large White × Polish Landrace [PLW×PL]) divided into two groups, according to backfat thickness: I – (P₂+P₄)/2>20 mm (10 primiparous, 36 multiparous) and II – (P₂+P₄)/2≤20 mm (19 primiparous, 32 multiparous). It was carried out between pregnancy day 104 and weaning after 21 days of lactation. During late pregnancy, at weaning and at the end of lactation, the body weight of sows, fatness (points P₁, P₂, P₃, P₄) and longissimus muscle depth at point P4M were determined. Feed consumption during lactation, reproductive performance traits and the results of piglet rearing were also analyzed.

Higher backfat thickness percentages and body weight noted in group I sows vs. group II sows (P≤0.001) during late pregnancy were maintained at farrowing and weaning. At weaning, the values of longissimus muscle depth were significantly (P≤0.001) higher in group II than in group I. The level of changes in fat reserves differed between late pregnancy and farrowing for P₄ (P≤0.01) and (P₂+P₄)/2 (P≤0.001); fat reserve loss was noted in group I, while an increase in adipose tissue was observed in group II. Body weight losses (%) during late pregnancy and at weaning were non-significant in sows of both groups. Based on feed consumption levels, group I sows were characterized by a lower appetite during lactation than group II sows. A regular trend in feed intake was noted in lactating sows of both groups: feed consumption increased in week 2, compared with week 1, while a decrease was noted in week 3 in comparison with week 2. Significant differences were reported in favour of group I vs. II (P≤0.05) as regards the piglets stillborn, litter weight at weaning, the average piglet weight at weaning and placental weight. Multiparous sows as compared to primiparous sows were characterized by significantly (P≤0.05) larger total number of piglets born and the number of piglets born alive. The difference in the litter weight at birth was found to be highly significant (P≤0.01).