There is conflicting information out there.

On the one hand we often find simple claims like

The majority of milk in industrialized countries is obtained from pregnant cows, which contains increased levels of estrogen and progesterone compared to non-pregnant cows.

A. R. Gilman,corresponding author W. Buckett, W. Y. Son, J. Lefebvre, A. M. Mahfoudh, and M. H. Dahan: "The relationship between fat and progesterone, estradiol, and chorionic gonadotropin levels in Quebec cow’s milk", J Assist Reprod Genet. 2017 Nov; 34(11): 1567–1569. Published online 2017 Aug 24. doi: 10.1007/s10815-017-1025-0 PMCID: PMC5699997 PMID: 28840413

Doubting whether this is widely believed or not: What Dairy Can Do To Your Skin:

The reason? Hormones, Bella says. “Most of the cows used in farming are actually pregnant cows. The hormones such as progesterone and insulin growth factors make their way into the milk,” she explains.

This is slightly different angle in counting the cows, not measuring the volume of milk they actually produce, as that is not constant over time.

Which seems quite counter-inituitive, as pregnancy is usually concluded when a cows goes into lactation. And certainly, it has to be true that a first time pregnant cow does not give any milk at all. But with modern indutrialised farming techniques and practices the above claim might be true.


Does Milk come from Pregnant Cows?

There is a myth online that says milk only comes from pregnant cows and so should be loaded with female hormones. Not only is this ridiculous logic, but being a dairy farmer I can answer this question

The cows that could have the highest amount of hormones in their milk would be in their last trimester. The common practice on farms is to dry up the cow (since she is giving little milk by that time) and give her a vacation for the last two months of her pregnancy. So cows are only milked 7 months of their pregnancy- until day 217 of her pregnancy out of 280 days.

I broke the data down by trimester because a cow’s hormone level goes up slowly over the pregnancy. Her hormones are highest in the last trimester- but cows are usually dry at that time.

The data showed that about 89% of the milk came from cows that were not pregnant or in their first trimester. Only 9% of the milk came from cows in their 2nd trimester and only 2% of the milk came from cows in their 3rd trimester because they are only milked for 1 more month in that trimester before they are dried and sent on vacation. The cows in the later periods of their pregnancy gave less milk and are the smallest portion of the total milking herd.

About 90% of the milk comes from cows that are not pregnant or in an early pregnancy. The remaining amount does not contribute to a significant increase in hormones in milk. Milk has been tested to show the average amount of estrogen.

Now this is from a very interested side and imprecise at that.
This is not about any level of hormones potentially found in milk, nor about any health effects. Although if this 'levels of hormones' could be used to reverse calculate 'pregnancy rates when milked', fine.

Given that first timers are certainly not pregnant and the average cycle of 'life' for an industrialised cow is somewhere:

Dairy cows may continue to be economically productive for many lactation cycles. In theory a longevity of 10 lactations is possible. The chances of problems arising which may lead to a cow being culled are high, however; the average herd life of US Holstein is today fewer than 3 lactations.

This is solely about the sheer volume of total amount of milk produced.

Is the majority of milk in industrialized countries obtained from pregnant cows?

  • The claim doesn't quite check out unless there's some need for dairy cows to be pregnant more than not. I'm not familiar with industrial farming, but the small levels I am familiar with says cows are not pregnant more than half the time, while lactation seems pretty consistent. – fredsbend Feb 1 '19 at 17:53
  • @fredsbend Not for the cow. There is a need for the dairy farmer that his cows are often pregnant… I wonder whether this is standard or malpractice. – LаngLаngС Feb 1 '19 at 18:02
  • 1
    I would guess cows with a calf would be more often to be milked. I have goats, and it is only when they have a kid is when you can milk them, without meds to force them to lactate. – The Mattbat999 Feb 2 '19 at 17:16
  • Milk cows have been having a calf every year since cow husbandry was invented some millennia ago. The exact overlapping between milking season and pregnancy season might have incurred some changes, but this is not exclusive from industrialized countries or farms. – Pere May 31 at 11:36

Definitely true

As specified by the question, I'll concentrate on the amount of milk produced instead of the number of cows in industrialized countries.

In conventional, industrialized dairy farms the Life Cycle of a Dairy Cow looks like this:

It is necessary to mate a heifer with a bull (mature male cow) in order for her to become pregnant and consequently start producing milk. A female cow must calve once a year in order to maintain maximum milk production.

The lactation of a dairy cow (as of any mammal) starts naturally with the birth of the first calf.

The heifer (which is now known as a cow, because it has calved) will now start her period of milk production (lactation) which she would naturally do in order to feed her calf. This period lasts from the time of calving to the time of drying off; naturally, without human intervention, it would last around 10-12 months but is usually forcibly stopped at around 10 months of lactation in order for the cow to go into a dry period before her next calving.
A cow can be ready to calve again around 3 weeks after giving birth and at every 3 weeks after this point (as this is when the cow is ‘in heat’). Generally it is aimed for cows to calve 12 months after from their parturition. If a cow is put back into calf at the earliest opportunity (approx. 1 month after a calf) then she will be dried off after only 7 months of the lactation period (which can last up to 12 months) meaning that production of milk is prematurely halted. To increase the milk yield duration of the cow, the cow can be put into calf at around 2/3 months (around 50 days) after her previous calving date, which will extend the amount of time she can be milked for while she is still lactating.
I.e. 3 months after her calving, becomes pregnant again which will last 9 months. 3 months of being milked + 9 months of gestation while being milked would give 12 months of possible milk yield duration, but the drying period must be taken into account, which is 2 months before calving … therefore 10 months of milking can be achieved.

(emphasis mine)

This makes it very clear that a cow is pregnant for the majority of time in which she yields milk. But how much milk is it in quantity? For this calculation I took the numbers from On the analysis of Canadian Holstein dairy cow lactation curves using standard growth functions because the Holstein-Friesian dairy cow is the breed with the highest milk production and therefore the most common dairy breed in industrialized countries.

The data analyzed consisted of approximately 91,000 daily milk yield records, corresponding to 122 first, 99 second, and 92 third parity individual lactation curves (for a total of 313 curves) of Holstein dairy cows collected at the Elora Dairy Research Centre (University of Guelph). Lactation length varied from 180 to 575d, but longer lactations were truncated to 305 d for this analysis.

Days in Milk | Daily Yield*  | Daily Yield*  
             | 1st Lactation | Subsequent Lactation  
0            |  6.0          | 13.5  
25           | 17.5          | 38.0  
50           | 30.5          | 40.0  not pregnant
75           | 30.5          | 38.0  pregnant
100          | 30.0          | 37.0  
150          | 28.0          | 33.0  
200          | 26.5          | 29.5  
250          | 24.5          | 24.5  
300          | 23.0          | 20.5

Avrg. Yield* | 1st Lactation | Subsequent Lactation  
not pregnant |   900.0       | 1,525.0  
pregnant     | 6,093.75      | 9,656.25

* yield measured in liters

These numbers mostly apply to conventional dairy cattle farming. In organic farming the situation might be very different, but the vast majority of milk is produced conventionally. In 2018 the global market share of organic dairy products was 0.9%.

The conclusion is:

Yes, The majority of milk in industrialized countries is obtained from pregnant cows.

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