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.
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?