Page 10 - Hooked on Herefords Feb 2020

Calving Ease Heritability of Mature Weight

Weight of dairy cattle has been reported to be a moderately
heritable trait; between 0.16 and 0.61. Overseas studies have
Courtesy of The AgriScience - September 2019 reported positive genetic relationships between calf birth weight
and mature cow weight, so selection on small mature weight
Genetics of Calving Difficulty would likely result in a decrease in birth weights as well.
We have had a good read of the studies around calving difficulty
and genetics, and a large amount were focused on beef cattle What does all this genetics mean?
rather than dairy cattle. We have found a few really good studies A heritability of 0 (or 0%) indicates that all of the variation in
on dairy cattle, unfortunately, not NZ based ones. calving difficulty was caused by environmental factors (e.g.
Birthweight and the length of gestation are determined by the nutrition, the weather etc.), whereas, a heritability of 1 (or 100%)
genetics (genotype) of the calf (direct genetic effect), genotype indicates that all of the variation in calving difficulty was due to
of the dam (maternal genetic effects) and environmental effects. genetics.
Although the majority of all the genetic effects are determined by In the case of calving difficulty here a heritability of 0.045 or
the genetics of the calf. 4.5% means that the majority of calving difficulty is caused by
The maternal effects contribute to the likelihood of a cow environmental factors and a small amount due to genetics.
experiencing dystocia when giving birth. The maternal ability of For birth weight and gestation length the moderate heritabilities
the cow to nurture the fetus influences the calf’s birthweight, of 40 - 50% mean that differences in birth weight or length
and the dam’s genetic potential for her own growth influences of gestation were fairly evenly split between genetics and the
the size of her pelvic area. environment.
For both beef and dairy cattle, the relationship between the
direct and maternal genetic effects is a negative one. This means Recording Calving Ease for Breedplan
that if bulls are selected for reduced calving difficulty then it is Calving Ease EBVs provide an estimate of genetic differences in
likely that their daughters will experience calving difficulty when the ability of calves to be born unassisted from 2 year old heifers
they calve. and are calculated from three main sources of information -
So if they have the genetics to not experience a difficult calving calving difficulty scores, birth weights and gestation length
when they are born they may have the genetics that increase records. By far the most important of these sources are calving
their chance of having a difficult calving when they calve difficulty scores.
themselves. As we mentioned earlier, we couldn’t find NZ data How do I record Calving Difficulty Scores? Calving difficulty
on dairy cattle for this section so it may not be the same for NZ- should be measured at birth by visually scoring females on
bred dairy cattle. the following scale of 1 - 6. Description 1. Unassisted: Cow
The use of a selection index that considered both direct and calved unassisted / No difficulty 2. Easy Pull: one person
maternal genetic effects is recommended by most studies on without mechanical assistance 3. Hard Pull: two people without
calving difficulty in beef and dairy cattle. More on selection and mechanical assistance: one person with mechanical assistance
breeding values is further on in this post. 4. Surgical Assistance: Veterinary intervention required 5. Mal-
presentation: eg. Breech Surgical. 6. Elective Surgical: surgical
Heritability of Calving Difficulty removal of calf before the cow has the opportunity to calve.
Calving difficulty as a trait of the calf has been reported as Note: For Calving Ease analysis, recorded data must have a
having a low direct heritability of 0.03 to 0.43. For the NZ genetic variation of scores within the same group.
evaluation of calving difficulty the heritability of calving difficulty
in calves born to heifers (first-calver) is 0.045 and in calves born
to cows (2nd and later calver) is 0.03.
NZHA Office where abouts…
Heritability of Birth Weight and Gestation Length In the hectic lead up to the Boehringer Ingelheim World
The heritability of both birth weight and gestation length were Hereford Conference, the NZHA office will strive to answer
similar at around 0.4–0.5 indicating that the calf’s genetics had your breeder enquiries as soon as we can,
a large influence on birth weight and gestation length. but we do request your patience over this time.
Birthweight increased with length of gestation by 0.15–0.38 Posy will be away from the office on WHC duties from
kg/day for different breed groups, and as a result, calves Monday 2nd March – Monday 23 March
with longer gestation periods tended to have an increased and Christine will be away
incidence of dystocia. Interestingly, the trend of longer gestation Wednesday 4th March – Monday 23 March.
periods being related to increased dystocia also existed when
birthweight was constant. The NZHA office will be open limited hours during the
There was a close genetic correlation between birthweight and conference. Kirsty will be in the office Mondays and
dystocia score, ranging from 0.90 to 0.98. This high genetic Wednesdays 9am – 2pm, alternatively contact Lindy Lawrence
correlation means that selection for decreased dystocia score who is willing to assist where she can (DDI 06 323 0742).
would be associated with decreased birthweight in the selected If you have a registry enquiry contact
animals. Megan (DDI 06 323 0747)

10 NZ HEREFORD ASSOCIATION NEWSLETTER