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 difficulty, 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 1,200 pound cow having a 84 pound calf where a 1,000 first calf heifer would have a 70 pound calf. So to put it 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 conceived in the right horn during gestation have greater 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 direct 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 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.