Does the severity (coldness or mildness) of the winter have an impact on spring-born calf birth weights? Ranchers have asked that question during many springs and veterinarians have speculated for years. The debate rages on! This is obviously a difficult subject to research because you cannot have a "control" group of cows to compare to a "treatment" group that is exposed to a cold winter while still running on the same pasture. Therefore research data on this subject is limited.
University of Nebraska researchers have done the next best thing. They have monitored the birth weights of genetically similar calves across three different winters and have related average winter temperatures to birth weights. A 3-year study was conducted to evaluate effects of high and low air temperatures and wind chills during winter months on subsequent calf birth weights and calving difficulty of spring-born calves. Records on approximately 400 2-year-old heifers and their calves were used. Heifer and calf genetics were the same each year. Heifers were fed similar quality hay free-choice each year before calving. High temperatures during the 1994-95 winter were 9 degrees higher than during the 1992-93 winter. The low temperatures were five degrees higher for 1994-95 compared to 1992-93. The greatest differences in monthly temperatures between years were found during December, January and February. Average temperatures for these three months increased 11 degrees F over the three years. Average calf birth weights decreased 11 pounds (81 to 70) from 1993 to 1995. A 1:1 ratio was observed. Although calving difficulty was high due to the research design, it also decreased from 57% to 35% from 1993 to 1995. Results indicate that cold temperatures influenced calf birth weight. Weather cannot be controlled; however, if we have below average winter temperatures, larger birth weight calves and more calving difficulty may be expected in the spring. (Source: Colburn, et al. 1996 Nebraska Beef Cattle Report.)
Other data that may shed some light on this subject, comes from Oklahoma State University in 1990. Birth weights of 172 fall born calves and 242 spring born calves were compared. These calves were the result of AI matings using the same bulls and bred to similar crossbred cows. The fall born calves averaged 4.5 pounds lighter at birth than their spring born counter parts (77.7 vs 82.2). Source: Selk and Buchanan, 1990 OSU Animal Science Research Report.) One possible explanation for this phenomenon, the changing of blood flow patterns of cows gestating in hot weather versus cold weather. During hot weather blood is shunted away from internal organs toward outer extremities to dissipate heat, while the opposite is the case in very cold weather with blood flow directed toward internal organs in an effort to conserve heat and maintain body temperature. This change in maternal blood flow may impact fetal growth in a small way, but result in a measurable difference.