How February’s cold weather, which followed December’s and January’s above-normal temperatures, may affect birth weights of calves in March and April, remains to be seen.
Will Record Cold Temperatures Bring Bigger Calf Birth Weights?
Research shows colder-than-average weather during the third trimester leads to heavier calves.
The relationship between average daily air temperatures during the last trimester a cow is pregnant and calf birth weights has been documented in research done by the University of Nebraska from 1992 to 1998. During six years, 285 2-year-old cows and their subsequent calf birth weights and calving difficulty were recorded. These data were then compared with year-to-year variation in average temperatures in the months leading up to calving. The heifers were genetically similar and artificially inseminated to the same Angus sire each year. The bred heifers were managed nutritionally the same, each year being grazed on native range pasture in the fall and fed bromegrass hay free-choice with alfalfa hay provided at 3-4 pounds (lb.) per head per day as a protein supplement leading up to calving. Precalving cow weights and body condition scores were similar across the six years. The calving season began in mid-February and ended at the first of April. Calving data collected included calf birth weight, birth date, calf sex, and a calving difficulty score of 1 to 5, in which 1 = no assistance, 3 = mechanical pull and 5 = C-section.
For each 1° colder average temperature for the three-month period leading up to calving, there was a 1-lb. increase in average calf birth weight.
Weather data were collected and analyzed for December, January and February, which were the months right prior to and at the start of the calving season. This time period is when the bred heifers were in their last trimester of pregnancy, and also when the majority of fetal calf growth occurs. The winter of 1992-1993 was the coldest for this six-year period with an average daily temperature for December-January of 20° F, which was 11° colder than the warmest winter of 1994-1995 at 31°.
Average calf birth weight after the 1992-1993 time period was 77.6 lb., with 58% of the heifers requiring assistance. The average calf birth weight after the 1994-1995 time period was 66.4 lb., with 29% of the heifers requiring assistance. Interestingly, across all six years when averaging temperatures and birth weights, for each 1° colder average temperature for the three-month period leading up to calving, there was a 1-lb. increase in average calf birth weight.
For producers, there are many factors that affect calf birth weight, including cow age, weight, body condition, nutrition, genetics, gestation length and calf sex. The environmental factor of average daily temperature is just one factor that contributes to calf birth weight.
As we look back at the months of December and January in Nebraska, according to the Nebraska State Climate Office for the state as a whole, December was 5.3° warmer than the long-term average, and January was 4.2° warmer. The month of February had periods of record-low temperatures for much of Nebraska. How this period of cold weather, which followed two months of above-normal temperatures, may affect birth weights of calves as we move on into March and April remains to be seen.
For more information on the research project cited in this article, see the 1999 Nebraska Beef Report article, “Climate Affects Calf Birth Weights and Calving Difficulty.”
Editor’s note: Aaron Berger is a University of Nebraska Extension beef educator. Photo by Troy Walz.
