This article was
written by Kalyn Waters, former SDSU Extension Cow/Calf Field Specialist.
Calving difficulties
account for a tremendous amount of economic loss in the beef cattle industry.
The highest correlated factor contributing to calving difficulties is the size
and or body weight of the calf. Of all cases of calving difficult, 60-90% of
them can be attributed to the birth weight of the calf.
While it is well
understood that genetic selection for birth weight is critical for a successful
calving season, there are several factors, aside from genetic influence, that
impact the birth weight of a calf.
·
Sex: The average birth weight of calves is 63.6
pounds, with male calves weighing about 5-8% more at birth than females, a
partial effect of the testosterone that begins being produced by day 45 of
gestation.
·
Cow
Size: Research has shown
that the birth weight of the calf will be approximately 7% of the dam’s body
weight. This would calculate out to a 1200 pound cow having a 84 pound calf
where a 1000 first calf heifer would have a 70 pound calf. So to put is simply,
bigger cows will have heavier birth weight calves.
·
Horn
Location: In general the
right horn of the uterus in beef cows is larger, in addition a greater
percentage of the ovulations, that result in pregnancy come from the right
ovary. Some research suggests that calves are conceived in the right horn
gestation are have great birth weights due to the larger uterine horn.
·
Temperature: Interestingly one factor that contributes
to the birth weight of calves is the weather. It is well established that as
the environmental temperature increases, the cow will director a greater
portion of her blood flow to her extremities for cooling. Therefore there is
less blood flow to the core of the cow, which results in a decrease in the
amount of nutrients being carried to the fetus through the maternal blood,
resulting in decreased birth weights when environmental temperatures are
increased during gestation. In contrast, cold temperatures will result in
increased birth weights, as blood flow is directed to the core of the body.
·
Maternal
Nutrition: Restricting maternal
nutrition to decrease birth weights is not a sound management practice. Extreme
reductions in feed, such as feeding less than 70% of the cow’s nutrient
requirements will result in decreased birth weights. However it often times
results in an increase in calving difficulties because the cows are weak and
undernourished. Slightly restricting the nutrient requirements of the cow
will result in decreases in energy reserves (body fat) of the cow before
limiting the nutrient flow to the fetus. In partitioning of nutrients, the cow
puts her pregnancy at the top of the list, right below keeping herself alive,
therefore her body will work overtime to metabolize stored nutrients to allow
the fetus to grow. This is why restricting feed, unless in an extreme case, has
little impact on birth weight.
While it is well
understood that the dam and sire of a calf play a role in the genetically
predicted birth weight of a calf, other factors do come into play. It is
important to keep the other factors in mind that impact the birth weights of
your calves to help ensure a successful and prosperous calving season.
