Difficult calvings and stillbirths are problems in Swedish dairy production that result in large expenses for the farmers and welfare issues concerning the animals. A common opinion is that the live weight of SRB and SLB is increasing, and whether increased weights have caused the increase in calving problems has been discussed. The aim of this study was to investigate the relationship between the relative birth weight (calf/cow) and calving difficulty and stillbirth, respectively. The importance of breed and parity for the relative weight has been investigated, and comparisons with previous studies have been made to evaluate whether the relative weight has changed over the last decades or not.
The study was based on 807 calvings, registered during the period 1993-2003 in the experimental dairy herd of the Swedish University of Agricultural Sciences (Jälla). During this period a selection experiment was in progress, consisting of two selection lines of SRB with low or high milk fat content at equal and high milk energy production. Because of this, three breeds/selection lines were investigated; SRB-low fat, SRB-high fat and SLB. Cow and calf were individually weighed after the calving and calving performance was assessed based on the extent of assistance required; easy calving (unobserved successful calving or unassisted calving), normal calving (assistance of one or two persons required), difficult calving (assistance of two or more persons required), and very difficult calving (veterinary assistance required). Calves dead at birth or within 24 hours of birth were considered stillborn.
The frequencies of difficult calvings and stillbirths were highest in SLB, 11,2 % and 8,1 % respectively. The proportion of difficult calvings was comparable between the two SRB lines, 3,3 % in the high fat line and 3,0 % in the low fat line. The frequency of stillbirths on the other hand, was twice as high in the high fat line (4,5 %) compared to the low fat line (2,2 %). In general, heifers had more problems at calving than cows.
The results indicate an increase in the live weight of SRB and SLB during the last decades. The weight of the mother at calving has increased more (in per cent) than the birth weight of the calf, resulting in decreased relative weight. The low fat line had slightly lower relative weight (6,5 %) than the high fat line and SLB, and no difference could be shown between the latter two (6,7 %). Bull calves had higher uncorrected relative weight compared to heifer calves, and the difference ranged between 0,46-0,60 percentage points among the three breeds/selectionlines. The relative birth weight had an influence on the viability of the calf, but no effect on calving performance was found.
The cow weight at calving, expressed as a percentage, increased with age most in the low fat line of SRB, followed by SLB and SRB high fat line. Heifers showed the greatest change in weight, and the change decreased gradually in later parities. Higher dam weight appeared to result in increased birth weight of the calf. The increase in birth weights did not, howewer, show the same trend as the dams weights, but were comparable between the breeds/selection lines. The birth weight of calves born by heifers seemed to have increased more than the birth weight of calves born by cows, and the largest increase was found in calves of SRB heifers. The uncorrected mean birth weight was slightly less than 46 kg in the calves of SLB heifers, and in the calves of SRB heifers the corresponding weight was just over 41 kg. The calf birth weight ...
