Sandy Johnson, Beef Specialist, Colby
Over the years I have strongly encouraged producers to spend time each year to assess calving distribution. It provides an excellent score card for how well a given operation matches the genetics and management system with the environment. However, I have learned that I need to clarify some details for you on calculating calving distribution.
When the standardized performance analysis (SPA) definitions were first established well over 20 years ago, two ways were given to mark the starting point for calculating calving distribution: 1) after the third mature cow calves or 2) 285 days after bull turnout. So, if the bull turn-out date wasn’t recorded, an avenue was still available to make the calculation.
My assumption had always been that both calculation methods provided similar data, but I had never actually compared the two until recently. I was looking for examples of calving distributions where nutrition was not a limiting factor, so I asked Bob Cushman at the Meat Animal Research Center in Clay Center, NE to share some of their data. In working with that data set, I calculated calving distribution by both methods listed above. Table 1 has data from two MARC herds and there is considerable difference between the two methods in the proportion that calve the first 21 days (48.5 vs 76.4% ; herd 1). If you start counting after the third mature cow calves (all mature cows in data shown), the number is much lower than starting 285 days after bull turnout. Keep in mind that gestation length does vary by breed and the 285 days was selected to represent the range of breeds in the US.
|Table 1. Calving distribution calculated from two starting points in two groups of mature cows in a single calving season.|
|after 3rd mature cow calves||+ 285 d after bull turnout|
|Herd 1 (n=709)|
|Day 1- 21 d (%)||48.5||76.4|
|Day 22 to 42 (%)||41.3||19.3|
|Day 43 – 63 (%)||4.3||4.3|
|Herd 2 (n=349)|
|Day 1 – 21 d (%)||55.3||72.2|
|Day 22 – 42 (%)||36.7||26.4|
|Day 43 – 63 (%)||8.0||1.4|
If you have been calculating your calving distribution and only comparing it to your own herd from year to year this is a non-issue for you, as long as you do it the same each year. However, if you have been comparing your data to some of the benchmark data I have shared in the past, you most likely need to use the third mature cow method of calculation. This is the method that CHAPS database uses since it does not require the additional data point of bull turn out date. I know at least one of the cow/calf software companies uses this same method and suspect others do as well
So do summarize your calving distribution data each year and use the same calculation method as any benchmark data set to which you compare and from year to year.