Abstract for CRU-seminar February 12^th 2003, by Susanne Eriksson, Dept. of animal breeding and genetics, Swedish University of Agricultural Sciences, Box 7023, 750 07 UPPSALA.

Calving performance and viability of the calf are of great economical importance to beef producers, as well as an important ethical issue. These traits are subjectively scored categorical traits, influenced by both maternal (genes of the cow) and direct (genes of the calf) effects and with low heritabilities, i.e. they are largely influenced by environmental factors. Swedish calving records of beef breeds have until now not been used in the genetic evaluation. Instead, birth weight has been used as an indirect trait to avoid calving difficulties.

The incidences of dystocia and stillbirth are considerably higher in first compared with later parities. There are several possible causes of difficulties at calving, and these have been suggested to be of different importance in heifers and older cows. Feto-pelvic incompatibility, a large calf in relation to the inlet pelvic dimensions of the cow, is the most common cause of dystocia and more important in heifers than in cows. Posterior or abnormal presentations of the calf, weak labor, uterine torsion and insufficient dilatation of the cervix have been suggested to be of importance especially in older cows.

The aim of our study was to estimate genetic parameters for, and genetic relationships between, calving difficulty, stillbirth and birth weight in first and later parities of Charolais and Hereford. This information is needed to evaluate whether more direct measures of calving traits could be considered in the genetic evaluation. The data included 74,538 Charolais and 37,077 Hereford calves born 1980-2001. Calving difficulty was scored as 1 (unassisted), 2 (easy pull by one person), and 3 (difficult, i.e. strong pull by more than one person, caesarean section or hormone injection). The frequency of caesareans and induced calvings was very low (<0.5 %). The viability of the calf was scored as 0 (live born) or 1 (stillborn or dead within 24 hours).

The frequency of difficult calvings was more than 6 % at first parity and 1 % at later parities for both breeds. Stillbirth was also more common at first (close to 6 %) compared with later parities (< 2 %). Bull calves were heavier than heifer calves at birth, and stillbirths and calving difficulty were almost twice as common for bull calves compared with heifer calves. Of all first-parity calvings where the calf was stillborn, 42 % for Charolais and 45 % for Hereford were difficult. At later parities 21 % and 25 % of stillborn calves had difficult calvings.

The statistical model for estimating variance components included the effects of herd-year, season and combination of sex of the calf and age group of the dam. For birth weight the age at weighing was included, as was a permanent environmental effect of the dam for later parity traits. The genetic effects of the calf and dam and a residual effect were also included.

Moderate direct heritabilities and low maternal heritabilities were estimated for birth weight. Direct heritabilities on the visible scale of calving difficulty at first parity were low (0.11-0.16 on a scale from 0 to 1) and the maternal heritabilities were lower than the direct, indicating that the calves own genes are more important for the calving traits. Heritabilities were lower at later than at first parities. Heritability estimates for stillbirth were generally very low and strong genetic correlations were found between direct effects of stillbirth and birth weight and calving difficulty, respectively. The genetic correlations between direct effects of calving difficulty and birth weight were strong and positive, but birth weight did not explain the whole genetic variance in calving difficulty.

Calving difficulty, at least at first parity, should be possible to include in the genetic evaluation of beef breeds in Sweden, while stillbirth probably will have to be considered indirectly by inclusion of birth weight and calving difficulty. Strong genetic correlations were generally found between calving traits in first and later parities. However, partly different sets of genes control calving traits at first and later parities.

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