By Travis Peardon
Regional Livestock Specialist, Outlook
Saskatchewan Ministry of Agriculture
I recently read an article that pointed out that calf birth weights are often higher in winters where daytime temperatures have been lower that the historical average. According to Environment Canada the five year average temperature for the month of February in the west central area of Saskatchewan is minus 12 degrees Celsius. The average temperature for this area in February for 2014 was minus nineteen degrees Celsius. This spurred me to do some further digging – was this an old wives tale, or could it be true?
In the 1990’s researchers from the University of Nebraska conducted a study on birth weight and calving difficulties as impacted by winter weather. For six years data was collected from March calving heifers of similar breeding that were all bred using artificial insemination to the same calving-ease Angus bull (same bull used over the six year study). Average and wind chill temperatures from December – February of each year were recorded, all calves were weighed at birth and any heifers requiring assistance were noted. Interestingly, calving weights were heaviest and calving difficulty was greater in the colder years.
This group found almost a 1:1 relationship between decrease in average temperature and increase in birth weight of the calf. For every one degree Fahrenheit drop in average or wind chill temperature, there was roughly a one pound increase in calf birth weight. A one degree Fahrenheit drop would be equivalent to a ~0.56 degree Celsius drop. The theory behind the increase in weight lies in the premise that mammals shift blood flow from the extremities to major internal organs during extreme cold. As a result of this shift in blood flow, more blood and more nutrients flow to the fetus. In addition, voluntary feed intake increases during bouts of cold weather and there are changes in various hormones that regulate fetal growth.
Given that the average temperature in February was seven Celsius degrees below the historical average, this could equate to calves that are ~12.5 pounds heavier than we might expect. It is important to note that under no circumstances should we reduce feed levels or amount of energy in late pregnancy diets in an attempt to reduce birth weight. Reducing energy and feeding will cause weak calves and thin cows, and thin cows will have poor conception rates during the breeding season.
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