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What Happens to Pregnancy Rates and Calf Survival With Changes in Calving Season?

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A four year study of ranchers in Western Canada indicates that shifting from winter calving to spring calving increases cow pregnancy rates and calf survival.

Due to market pressures, Canada’s cow-calf sector has consolidated into fewer, larger herds and producers have begun calving later, and in pasture, to avoid increases in labor, equipment and facilities that might otherwise go along with their larger herd sizes. About 70% of western Canadian producers have now shifted from calving in late February to calving in March or April. Researchers Cheryl Waldner, Sarah Parker and John Campbell of the Western College of Veterinary Medicine were interested in the impact of this change. So from 2013 to 2017 they worked with 105 cow-calf producers in Alberta, Saskatchewan and Manitoba to gather data on pregnancy and calf survival rates. They looked at the impact of herd size (less than 300 breeding cows vs. 300 or more) and the start of the breeding season.

Rates of pregnancy didn’t differ by herd size, though larger herds tended to have more open heifers than smaller herds. What did change were pregnancy rates based on the start of the breeding season:

April or Earlier May/June July/August
Cows 93% 94% 92%
Heifer 92% 92% 87%


Calf death losses in the first 24 hours after birth were lowest on average among cows bred in July/August. Death loss among heifers was the same regardless of breeding date. There were no differences by herd size. The top 25% of producers lost the fewest calves – 1% of calves from cows and no heifer calf losses.

Breed April or Earlier
Calve in Jan/Feb
Breed May
Calve late Feb
Breed June
Calve Mar
Breed July/August
Calve April/May
Cows 2.5% 2.1% 1.9% 1.7%


Calf death losses from 24 hours to weaning were lowest for cows bred in July/August, and increased the earlier breeding started.

April or Earlier May June July/August
Cows 3.5% 2.7% 2.3% 1.9%
Heifers 4.4% 2.5% 2.3% 3.1%

Photo courtesy of the Beef Cattle Research Council

What Does This Mean?

Calving in late spring avoids the cold and snowstorms that are hard on new born calves. Spring pasture also reduces disease spread, and fresh grass makes it easier for cows to produce enough milk. Both those things improve survival rates for calves. The downside of the shift to later breeding dates is by late summer, pastures begin to mature and nutritional value declines and this can reduce pregnancy rates. This is less heifers, this is a bigger challenge since they are still growing and have higher nutritional needs than mature cows. So you can see it in the decline in rates of pregnancy.

What Can You Do With This?

The colder your winters, the more closely this research relates to you. It means you might look at your herd’s pregnancy and survival rates and consider if a change would improve things for you. It also reminds us of the importance of good nutrition when rebreeding first calf heifers. If you plan to delay breeding until July/August, consider what kinds of changes you might make to your grazing management. Keeping forage vegetative, rather than allowing it to dry and head out is one way to ensure your cows and heifers are getting the nutrition you need. You might also need to supplement them, especially in drought years. Reach out to animal nutritionists, forage specialists, and grazing experts at you local Natural Resources Conservation Service or Conservation District office, or consult with an Extension Specialist in your state for help.

This story is based on an article originally published in Canadian Cattlemen and on the Beef Cattle Research Council website.

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Kathy Voth
Kathy Voth
I am the founder, editor and publisher of On Pasture, now retired. My career spanned 40 years of finding creative solutions to problems, and sharing ideas with people that encouraged them to work together and try new things. From figuring out how to teach livestock to eat weeds, to teaching range management to high schoolers, outdoor ed graduation camping trips with fifty 6th graders at a time, building firebreaks with a 130-goat herd, developing the signs and interpretation for the Storm King Fourteen Memorial trail, receiving the Conservation Service Award for my work building the 150-mile mountain bike trail from Grand Junction, Colorado to Moab, Utah...well, the list is long so I'll stop with, I've had a great time and I'm very grateful.


  1. The biggest $ difference for later calving is the $ saved in fed feed costs. Far greater than the change in pregnancy and calf survival.

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