Consequences of Birthweight Differences on Carcass Traits, As Well As Muscle and Adipose Tissue Cellularity in Crossbred Bulls

Birthweight differences have been shown to affect body composition in pigs and other species. Data on effects of birthweight differences on morphological and cellular traits of carcass in bulls with a longer fattening period are scarce, however. Consequently, we have analyzed two groups of bulls from an experimental F₂ cross (Charolais x Holstein) with low birthweight (LBW: 35.6 ± 0.7 kg, n = 20) and high birthweight (HBW: 59.4 ± 0.9 kg, n = 20). All animals were kept in a tight stall barn from day 121, were fed a standardized, concentrate based diet ad libitum and slaughtered at an age of 547 days. The feed intake was higher in the HBW group but feed conversion ratio was not different. Live weights were not different between 4^th and 15^th month of age but then diverged resulting in significantly higher weights at slaughter with 18 months in the HBW group (717 ± 13 kg vs. 661 ± 10 kg; P < 0.05). Bulls of the HBW group accreted more protein (14.9 ± 0.15 % vs. 14.4 ± 0.17 %, P = 0.017) and LBW bulls more fat (17.2 ± 0.81 % vs. 14.3 ± 0.79 %, P = 0.016) in the carcass. The absolute and relative weights of omental, perirenal and visceral adipose depots were significantly higher in LBW bulls. Weight of M. longissimus (MLD) was higher in the HBW bulls (8.6 ± 0.15 vs. 7.7 ± 0.13 kg, P < 0.001) compared to LBW animals. However, there were no significant differences in muscle fiber traits in MLD. The intramuscular fat (IMF) content of MLD from LBW bulls was significantly increased compared to HBW animals (3.9 ± 0.5 % vs. 2.5 ± 0.3 %, P = 0.012). Analysis of cellular parameters of the IMF revealed increased diameters (94.4 ± 1.7 vs. 81.4 ± 2.4 µm, P < 0.01) of adipocytes and larger but not more marbling flecks as cause for higher IMF in LBW bulls. Our results demonstrate long-lasting effects of large differences in birthweight on carcass traits. Crossbred bulls could not compensate low birthweights even under ad libitum feeding regimen within a period of 18 months in this experiment. Our data indicate that metabolic differences resulted in different partition of nutrients with a shift to lipid accumulation in bulls with lower birthweight in this crossbred population.