Useful Information

Calving Interval & Relative Calving Date…the Ranch Profit indicators everyone has, but very few actually use…

by Christine Mushrush | Published January 29, 2022

Even as we select for faster post-weaning gains, Prime carcass quality, heavier carcass weights and more efficient feed conversion, most of us know our bottom line will be no better than our cow herd’s reproductive success. So, unless our management and marketing strategy prescribes that we purchase all of our replacements and breed them to terminal sires with excellent predictions for post-weaning profitability, most of us yearn for tools that effectively rank our cow herds for reproductive phenotypes and ultimately their impact on our operation’s profitability.  There are two such “Key Success Measures” for cow/calf producers that come around every year, and can be measured without any tool more expensive than a calendar or a calving book. 

For a seedstock operation that is tagging calves and recording individual birth dates. Calving Interval and Relative Calving Date are almost so obvious that most folks forget to look at them.  Yet outside of an “Open” or “Pregnant” diagnosis, all other pre-weaning observations pale in comparison in terms of their impact on a successful, profitable and sustainable outcome for a cow/calf operation.

Most cow/calf operations have a desired time of year that they prefer to calve.  This desired calving season often corresponds to weather, available feed, facilities and marketing strategies.  Breeding dates are selected according to the producers’ desired calving dates – minus the cows’ gestation length. These factors that affect a producers’ decisions about calving dates are generally constant year over year, so most herds have breeding dates that reoccur annually.  For example: a southeastern producer who wants to begin calving around the first of September may “religiously” turn bulls out the day after Thanksgiving.  In the panhandle of Nebraska, a producer with good winter calving facilities may breed heifers on May 5th (for a February 10th calf) and the cow herd 3 weeks later so they’ll start calving around the first of March.  Once a herd has their annual Breeding/Calving plan established where it doesn’t change from year to year, Calving Interval becomes an important data point – both to evaluate an individual animal relative to their herd-mates as well as understanding the cow herd’s fit to its environment as observed through reproductive outcomes. 

Calving Interval refers to the number of days between a cow’s successive calves.   If we want our cows to calve the same time of year and do it over their lifespan they have to breed back within 80-85 days of their last calf’s birth date.  As calving intervals stretch significantly longer than 365 days cows are slipping further back in the calving season.  A cow with a 400-day calving interval is losing roughly one month a year. Her calving dates are sliding from Feb 1st to March 1st to April 1st and soon she’s sliding right into the sale barn and causing you to prematurely replace her with a heifer you could have cash cropped not to mention the time and feed resources she leaves unpaid.  

Given the prior paragraph’s discussion of set calving seasons, it would be logical to expect a best-case scenario to be a herd of 365-day calving intervals.  That best case gets even better when cows maintain that throughout a long and productive life of 10-12 years.  However, some management scenarios make 365-day calving intervals improbable if not impossible.  For example, if a herd calves it’s heifers 30 days ahead of the cow herd which is set to begin calving on April 1st, then a “perfect score” for a 6-year-old cow would be about 373 days.  See the table 1 below:


Cow Age (yrs)Calving DateCalf NumberCalving Interval (days)
2 -yr-oldMarch 1st1st calf 
3-yr-oldApril 1st2nd calf395
4-yr-oldApril 1st3rd calf365
5-yr-oldApril 1st4th calf365
6-yr-oldApril 1st5th calf365
Average Calving Interval373

Also, isn’t uncommon to find calving intervals less than 365 days, and they are not necessarily better.  Table 2 below looks at a different female in a different herd that utilizes a 60-day breeding season designed to start calving on March 1st.   This herd breeds their heifers to calve at the same time as the cow herd.  If the female in question conceives at the very end of her first breeding season, she’s probably 3 heat cycles behind her contemporaries and might not calve until the last week of April.  She only has about a month before bulls are turned out, and only 90 days before bulls are pulled.  She’s in danger of falling out before she turns three.  But, let’s say she breeds back by the first of July; she’ll calve earlier in April the following year.  If she moves up again by breeding in Mid-June she’s maintaining a 350-day calving interval.   Yes, her great calving-interval number is keeping her from falling out of the herd and she’s moving in the right direction.  But after three years, she’s still calving three weeks behind the earlier calving cows and has accumulated a lot of pounds and dollars left on the table when compared to her earlier calving herd mates.  That’s why when we look at Calving Interval, we must also look when in the calving season those cows calve. 


Cow Age (yrs)Calving DateCalf NumberCalving Interval (days)
2 -yr-oldApril 25th1st calf 
3-yr-oldApril 10th2nd calf351
4-yr-oldMarch 25th3rd calf349
Average Calving Interval350

Relative Calving Date refers to the date a cow calves, relative to the rest of the herd.  We tend to measure this in days “plus” or “minus” the average calving date for the contemporary group.  If a herd’s first possible due date is March 1st, then cows that calved during the third or fourth week of March were bred a heat cycle later.  Cows that were two heat cycles later getting bred won’t calve until mid-April.  Three heat cycles later and they are approaching the first of May…you get the picture.  If your average calf weighed 75 lbs. at birth and 500 pounds at weaning (205 days of age), that means every day is worth just over 2 pounds, or at current feeder calf prices – every day later translates to $3.20 in lost calf value.  Think back to January, that April calving cow was probably eating just as much as the early March calving cow standing beside her, so it doesn’t take your CPA to tell you which cow is returning more to your operation.  

Also, note that Contemporary Group integrity is just as important to good calving interval/calving date data as it is when trying to measure within-herd variation for weaning weight, ADG or Rib Eye Area, etc.  Contemporary group strategy is generally the first line of defense when it comes to removing bias from phenotypic data.  If you breed your heifers 30 days earlier than your running age cows and your 2-yr-olds two weeks behind the heifers, then each of those three groups need to be compared independently.

Like any reproductive measure, there is the potential for “noise” in calving interval data.  Noise could be management or environmental factors which impact the number of days between a cow’s successive calving observations.   Fortunately, most good cow herd managers understand the importance of getting cows pregnant, so they are already implementing practices that have the perhaps unintended, but beneficial effect of minimizing noise in calving interval data: BSE testing bulls before you turn them out, making sure cows have equal opportunity at nutrition and mineral supplementation, using the same artificial insemination and/or synchronization protocols across the herd – and that also goes for using an experienced A.I technician(s).  And randomizing technicians across cows if multiple technicians are used.  Donor cows (or any cows) that have been removed from the herd’s normal reproduction routine must be eliminated from the data pool, and their calving interval data is not evaluated.  These protocols have long been standard operating procedure at our ranch – not necessarily because we wanted clean calving interval data, but because we wanted pregnant cows. We also realize these same practices will go a long way to remove management related bias in calving interval data.

When Ranchers buy bulls, they are bombarded with information: EPDs, Indices, weights, ratios, ultrasound measures.  Some seedstock operators share the age of their sale bulls’ dam and possibly her MPPA (probability of that cow producing heavier/lighter weaning weights).  Fewer yet provide those dams’ mature weight and/or body condition score (BCS).  Wouldn’t a measure of reproductive performance be equally or even more important – especially if the bull buyer is retaining replacements?  Wouldn’t you prefer some indicator of how reliable that cow had been from a reproductive standpoint?  How she’d impacted her own herd’s profitability – before you breed your cow herd to her son(s)?  We’ll dig deeper into that aspect of this data in our next newsletter.  We will look at what calving interval and relative calving date data looks like and how we present it so that our customer can make the best-informed bull selection decisions.  In the meantime, if you have any questions about this article, please call or email us at info@mushrushranches.com.