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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. This research is reported in detail in the University of Nebraska Beef Research Report for 1996. Other data that may shed some light on this subject, comes from OSU
in 1990. Birth weights of 172 fall born calves and 242 spring born
calves were compared. These calves were the reult 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; p<.05). 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
towerd 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 measureable difference.
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