Dr. Kent Weigel, University of Wisconsin
Unfortunately, about 8% of dairy calves are stillborn or die shortly after birth. Because genetics plays a role in calf survivability, USDA Agricultural Research Service (ARS) scientists added calf survival to the calculations used to determine an AI sire’s Lifetime Net Merit index.
Dairy calf births are scored as 1, alive; 2, dead at birth; or 3, alive at birth, but dead by 48 hours of age, explained Kent Weigel, who works with the University of Wisconsin and the National Association of Animal Breeders. Categories 2 and 3 are combined for computing genetic evaluations.
Like calving ease evaluations, there is a “direct” component of stillbirths that is called service sire stillbirth rate (SSB), as well as a “maternal” component that is called daughter stillbirth rate (DSB). The genetic component of DSB is greater than that of SSB (the heritability is about 4% for SSB, and 7% for DSB – indicating that there are more genetic differences among families for DSB than SSB). The range in predicted transmitting abilities (PTAs) for service sire stillbirths among active AI Holstein sires is about 5% to 11%; whereas, the range for daughter stillbirths is about 5% to 15%.
About half of the genetic variation in stillbirth rate can be attributed to calving ease, meaning that about half of stillborn calves are dead due to calving complications. The other half are born without difficulty and die for other reasons.
Weigel said stillbirth PTAs are published as the expected percentage of stillborn calves from cows of any age, including both first and later calvings. This is in contrast to PTAs for calving ease, which are expressed as the expected percentage of difficult births in first calvings only, which represents a “worst-case scenario”. Weigel noted that the stillbirth rate for first-calf heifers is about 12%. For second- and third-lactation cows, the rate is a little less than 5%; it’s a little more than 5% for fourth-, fifth- and later-lactation cows.
As would be expected, service sire calving ease and service sire stillbirth rate are highly correlated, such that bulls that sire large calves that are born with difficulty are also more likely to sire stillborn calves. Similarly, daughter calving ease and daughter stillbirth rate are highly correlated, such that bulls whose daughters tend to have calves without difficulty also tend to have daughters that produce more live calves.
“Like calving ease, there is a slight positive relationship between the direct and maternal components,” Weigel remarked. “That is, bulls that have low service sire calving ease evaluations (those that produce small calves) are a bit more likely to have low daughter calving ease evaluations as well (their daughters calve easily and tend to produce small calves of their own). In the case of stillbirths, bulls that tend to have a lot of stillborn calves when used as a mating sire will also tend to have daughters that have more stillborn calves when they grow up.”
To determine the calving ability index, ARS attributes 25% to service sire calving ease, 15% to daughter calving ease, 15% to service sire stillbirth rate, and 45% to daughter stillbirth rate. To put this all in perspective, milk production accounts for 46% of the Lifetime Net Merit score, while calving ease and stillbirth are combined into a calving ability index, which is valued at 6%.
While this brief genetics and statistics lesson may be interesting, what’s important is the impact of stillbirths on dairy farm profitability. Stillbirths have a direct and indirect economic value, because there is an economic loss associated with a calf being dead, and thereby not becoming a herd replacement or salable bull calf. Plus, there’s an economic loss in the cow that delivers a stillborn calf. A cow that starts her lactation with a “dead on arrival” calf is more likely to battle various ailments, such as retained placenta, metritis, ketosis and displaced abomasum. “This cascade of misfortune also leads to significant increases in days open and higher culling rates,” Weigel concluded.
