Beyond EPDs: Other factors affecting calf birth weight
April 26, 2018
Photo cutlines-Deanna Nelson-Licking
IMG-0005- Large new calf
IMG-0012- New pair
IMG-0618- New pair at sunrise
IMG-1725- Cow giving birth
BEYOND EPD'S OF BULLS
Recommended Stories For You
OTHER FACTORS AFFECTING CALF BIRTH WEIGHTS
We have all been there, two in the morning pulling a huge calf out of a cow that really shouldn't have had this problem. The bull had a low birth weight EPD so what went wrong?
Genetic heritability of birth weight only counts for about 45 percent with the remaining 55 percent being caused by environmental conditions. The bull plays a smaller part than the cow, with the general rule being the calf will weigh at birth about seven to eight percent of its mother's body weight. So larger frame cows will on average give birth to a bigger calf than more moderate frame cows will.
Long cold winters also impact birth weight. According to a long term study conducted by the University of Nebraska (Deutscher et al., 1999) the temperature during the last three months of gestation will impact the birth weight. Expect a pound more for each degree below average winter temperature and an increase of calving difficulty by 2.6 percentage points per additional pound. This is due to the fact that the cow's body keeps more of the blood flow to her core during cold times increasing the nutrients being carried to the calf. The same holds true for summer and fall calving herds as the calves will be smaller due to the blood flow being kept near the extremities to keep the cow cooler.
According to this study many other factors also affect calf birth weight and calving difficulty; including cow age, her weight, body condition, nutrition, cow pelvic size, genetics, gestation length and calf sex.
Gene Deutscher, extension beef specialist with the University of Nebraska also authored a beef handbook bulletin. BCH-2130, a product of the Extension Beef Cattle Resource Committee, 'Pelvic Measurements for Reducing Calving Difficulty.' He stated, "Calving difficulty results in a major economic loss to beef producers. This loss is estimated at $750 million annually nationwide."
Calving difficulty increases calf death loss, cow mortality, labor and veterinary costs. It delays the return of estrus to cows and reduces conception rates. It also results in lower calf weaning weights and market values from breeding young heifers to easy calving bulls to reduce calving difficulty. Studies have shown that calf losses of four percent within 24 hours for unassisted births jump to sixteen percent for calves requiring birth assistance. As producers select bulls for more growth, larger calves at birth and more calving difficulties can be expected.
Large framed cows tend to have a wider pelvic area but also have a proportionately heaver calf at birth which offsets any advantage of less calving difficulty. Selecting a cow on size alone seems ineffective. Heifers with small pelvic areas have an 85 percent difficulty rate compared to heifers with large pelvic areas. He recommends obtaining pelvic measurements before breeding; a 600 pound heifer should have a pelvis that measures at least 11 centimeters wide and 12 centimeters high to deliver a 63 pound calf. Heifers with smaller measurements should be considered for culling. Pelvic size has been found to be a heritable trait and may be higher than the 45 percent heritability for calf birth weight. Selecting only for pelvic size could result in an increased mature cow size. The pelvic size of prospective bulls should be another factor one considers when selecting.
The age of the cow has a profound effect on incidence of calving difficulty. A beef cattle handbook bulletin BCH-2120 a product of the Extension Beef Cattle Resource Committee, authored by Harlan D. Ritchie, Michigan State University and Peter T. Anderson, University of Minnesota: 'Calving Difficulty in Beef Cattle: part 1'. says that "Calving difficulty in two year olds is three to four times as high as three year olds. And three year olds have about twice as much difficulty as four year olds. By the time a cow reaches four -five years of age calving difficulties are minimal."
This bulletin cites reports from Montana studies that showed as birth weight increases, the percent of assistance required increases with bull calves being typically heaver and having a higher assistance rate. This is also linked to the fact that on average bull calves tend to have a slightly longer gestational period than heifers.
Kalyn Waters, former SDSU Extension Cow/Calf Field Specialist, in 'Factors Affecting Birth Weight' (3/25/2013) said restricting maternal nutrition to decrease birth weight is not a sound management practice.
Extreme feed reductions such as feeding less than 70 percent of the cow's nutritional requirements will result in a smaller calf but increases calving problems as the cows are too weak and undernourished to deliver the calf and her milk supply and quality are greatly affected.
Pete Anderson, Extension Beef Cattle Specialist, authored a paper for the University of Minnesota Extension: 'Minimizing Calving Difficulty in Beef Cattle' (2012). His conclusions are, "Mate virgin heifers and small cows to bulls that will sire small calves. Feed heifers well enough to weigh at least 85 percent of their expected mature weight at first calving. Use pelvic measurements and do not retain daughters of cows that have a record of calving difficulty. Begin breeding heifers 21 to 30 days earlier than cows so they can be observed, and feed late in the day. Record a calving ease score for all calves that are observed at birth."
Purebred producers record birth weigh on their calves which is something commercial producers might also implement for their own records. Some believe a calf weight tape that takes a measurement around the hoof is fairly accurate while others use a digital scale attached to their vehicle. The sling type scales are a little impractical for out in the pasture as few of us are able to lift high enough.
Our goal as beef cattle producers is to have unassisted births and a high breed back ratio so considering all the factors affecting calf birth weight is important in our business.